Entertaining experiments: some possibilities of a field-effect transistor

Magazine "Radio", number 11, 1998

It is known that the input resistance of a bipolar transistor depends on the load resistance of the cascade, the resistance of the resistor in the emitter circuit and the base current transfer coefficient. Sometimes it is relatively small, making it difficult to match the stage with the input signal source. This problem completely disappears if you use a field-effect transistor - its input resistance reaches tens and even hundreds of megaohms. To get to know the field effect transistor better, do the suggested experiments.

A little about the characteristics of the field effect transistor. Like the bipolar, the field has three electrodes, but they are called differently: gate (similar to the base), drain (collector), source (emitter). By analogy with bipolar field-effect transistors, there are different "structures": with a p-channel and an n-channel. Unlike bipolar ones, they can be gated in the form of a p-n junction and with an insulated gate. Our experiments will concern the first of them.

The basis of the field-effect transistor is a silicon plate (gate), in which there is a thin area called a channel (Fig. 1a). On one side of the channel is the drain, on the other - the source. When the positive transistor is connected to the source, and the negative terminals of the GB2 power battery (Fig. 1, b) are connected to the drain, an electric current appears in the channel. The channel in this case has a maximum conductivity.

It is worth connecting another power supply - GB1 - to the source and gate terminals (plus to the gate), as the channel "narrows", causing an increase in resistance in the drain-source circuit. The current in this circuit immediately decreases. By changing the voltage between the gate and the source, the drain current is regulated. Moreover, there is no current in the gate circuit, the drain current is controlled by an electric field (that is why the transistor is called a field effect transistor), created by the voltage applied to the source and gate.

The above applies to a transistor with a p-channel, but if the transistor is with an n-channel, the polarity of the supply and control voltages is reversed (Fig. 1, c).

Most often, you can find a field-effect transistor in a metal case - then, in addition to the three main conclusions, it may also have a case terminal, which, during installation, is connected to a common wire of the structure.

One of the parameters of the field-effect transistor is the initial drain current (I from the beginning), i.e., the current in the drain circuit at zero voltage at the transistor gate (in Fig. 2, a variable resistor slider in the lower position according to the circuit) and at a given supply voltage .

If you smoothly move the resistor slider up the circuit, then as the voltage at the transistor gate increases, the drain current decreases (Fig. 2, b) and, at a voltage determined for a given transistor, will drop to almost zero. The voltage corresponding to this moment is called the cut-off voltage (U ZIots).

The dependence of the drain current on the gate voltage is quite close to a straight line. If we take an arbitrary increase in the drain current on it and divide it by the corresponding increase in the voltage between the gate and the source, we get the third parameter - the slope of the characteristic (S). This parameter is easy to determine without removing the characteristics or searching for it in the directory. It is enough to measure the initial drain current, and then connect, say, a galvanic cell with a voltage of 1.5 V between the gate and the source. Subtract the resulting drain current from the initial one and divide the remainder by the cell voltage - you will get the slope of the characteristic in milliamps per volt.

Knowledge of the features of the field-effect transistor will complement the acquaintance with its stock output characteristics (Fig. 2, c). They are removed when the voltage between the drain and the source changes for several fixed gate voltages. It is easy to see that up to a certain voltage between the drain and the source, the output characteristic is non-linear, and then, over a significant voltage range, it is almost horizontal.

Of course, a separate power supply is not used in real designs to supply bias voltage to the gate. The bias is formed automatically when a constant resistor of the required resistance is included in the source circuit.

And now pick up several field-effect transistors of the KP103 (with p-channel), KP303 (with n-channel) series with different letter indices and practice determining their parameters using the diagrams given.

Field effect transistor - touch sensor. The word "sensor" means feeling, sensation, perception. Therefore, we can assume that in our experiment, the field-effect transistor will act as a sensitive element that reacts to touching one of its outputs.

In addition to the transistor (Fig. 3), for example, any of the KP103 series, you will need an ohmmeter with any measurement range. Connect the ohmmeter probes in any polarity to the drain and source terminals - the ohmmeter needle will show a small resistance of this transistor circuit.

Then touch the shutter release with your finger. The ohmmeter needle will deviate sharply in the direction of increasing resistance. This happened because the induction of electric current changed the voltage between the gate and the source. The channel resistance increased, which was recorded by the ohmmeter.

Without removing your finger from the gate, try touching the source terminal with another finger. The ohmmeter needle will return to its original position - after all, the gate turned out to be connected through the resistance of the arm section to the source, which means that the control field between these electrodes has practically disappeared and the channel has become conductive.

These properties of field-effect transistors are often used in touch switches, buttons and switches.

Field effect transistor - field indicator. Change the previous experiment a little - bring the transistor with the gate terminal (or body) as close as possible to the mains socket or the wire of a working electrical appliance included in it. The effect will be the same as in the previous case - the ohmmeter needle will deviate in the direction of increasing resistance. It is understandable - an electric field is formed near the outlet or around the wire, to which the transistor reacted.

In a similar capacity, a field-effect transistor is used as a device sensor for detecting hidden electrical wiring or a wire break in a New Year's garland - at this point, the field strength increases.

Holding the transistor-indicator near the mains wire, try turning the appliance on and off. A change in the electric field will be recorded by an ohmmeter needle.

The field effect transistor is a variable resistor. After connecting the bias voltage adjustment circuit between the gate and the source (Fig. 4), set the resistor slider to the lower position according to the diagram. The ohmmeter needle, as in previous experiments, will record the minimum resistance of the drain-source circuit.

By moving the resistor slider up the circuit, you can observe a smooth change in the ohmmeter readings (increase in resistance). The field effect transistor has become a variable resistor with a very wide range of resistance changes, regardless of the value of the resistor in the gate circuit. The polarity of connecting the ohmmeter does not matter, but the polarity of switching on the galvanic cell will have to be changed if an n-channel transistor is used, for example, any of the KP303 series. Field effect transistor - current stabilizer. To conduct this experiment (Fig. 5), you will need a DC source with a voltage of 15 ... 5 mA, yes field effect transistor. First, set the resistor slider to the lower position according to the diagram, corresponding to the supply of a minimum supply voltage to the transistor - about 5 V, with the values ​​\u200b\u200bof the resistors R2 and R3 indicated in the diagram. By selecting the resistor R1 (if necessary), set the current in the drain circuit of the transistor to 1.8 ... 2.2 mA. By moving the resistor slider up in the circuit, observe the change in drain current. It may happen that it generally remains the same or increases slightly. In other words, when the supply voltage changes from 5 to 15 ... 18 V, the current through the transistor will be automatically maintained at a given level (by resistor R1). Moreover, the accuracy of maintaining the current depends on the initially set value - the smaller it is, the higher the accuracy. An analysis of the stock output characteristics shown in Fig. 1 will help confirm this conclusion. 2, in.

Such a cascade is called a current source or current generator. It can be found in a wide variety of designs.

Switching buck regulators

Y. SEMENOV, Rostov-on-Don

The article brought to the attention of readers describes two pulse step-down stabilizers: on discrete elements and on a specialized microcircuit. The first device was designed to supply automotive equipment with a voltage of 12 V to the 24-volt on-board network of trucks and buses. The second device is the basis for the laboratory power supply.

Switching voltage regulators (step-down, step-up and inverting) occupy a special place in the history of the development of power electronics. Not so long ago, every power supply with an output power of more than 50 watts included a step-down switching regulator. Today, the scope of such devices has been reduced due to the reduction in the cost of power supplies with a transformerless input. Nevertheless, the use of switching step-down stabilizers in some cases turns out to be more economical than any other DC-DC converters.

A functional diagram of a buck switching regulator is shown in rice. 1, and the timing diagrams explaining its operation in the mode of continuous current of the inductor L, ≈ on rice. 2. At time t on, the electronic switch S is closed and the current flows through the circuit: the positive terminal of the capacitor C in, the resistive current sensor R dt, the storage inductor L, the capacitor C out, the load, the negative terminal of the capacitor C in. At this stage, the inductor current l L is equal to the current of the electronic switch S and increases almost linearly from l Lmin to l Lmax .

According to a mismatch signal from the comparison node or an overload signal from a current sensor, or a combination of them, the generator switches the electronic switch S to an open state. Since the current through the inductor L cannot change instantly, then under the action of the self-induction EMF, the diode VD opens and the current l L will flow along the circuit: the cathode of the diode VD, the inductor L, the capacitor C VX, the load, the anode of the diode VD. At time t lKl, when the electronic switch S is open, the inductor current l L coincides with the diode current VD and decreases linearly from

l Lmax to l L min . During the Period T, the capacitor C out receives and gives an increment of charge ΔQ out. corresponding to the shaded area on the time diagram of the current l L . This increment determines the amplitude of the ripple voltage ΔU Cout on the capacitor Cout and on the load.

When the electronic switch is closed, the diode closes. This process is accompanied by a sharp increase in the switch current to the value of I smax due to the fact that the resistance of the circuit ≈ current sensor, closed switch, recovery diode ≈ is very small. To reduce dynamic losses, diodes with a short reverse recovery time should be used. In addition, the buck regulator diodes must be able to handle large reverse current. With the restoration of the closing properties of the diode, the next conversion period begins.

If the switching buck regulator operates at a low load current, it can switch to the intermittent inductor current mode. In this case, the inductor current stops by the moment the switch is closed and its increase starts from zero. The intermittent current mode is undesirable at a load current close to the nominal one, since in this case increased output voltage ripple occurs. The most optimal situation is when the stabilizer operates in the continuous current mode of the inductor at maximum load and in the intermittent current mode, when the load decreases to 10 ... 20% of the nominal.

The output voltage is regulated by changing the ratio of the closed state time of the switch to the pulse repetition period. In this case, depending on the circuitry, various options for implementing the control method are possible. In devices with relay control, the transition from the switched on state to the switched off state determines the comparison node. When the output voltage is greater than the set value, the switch is turned off, and vice versa. If you fix the pulse repetition period, then the output voltage can be adjusted by changing the duration of the switched on state of the switch. Sometimes methods are used in which either the time of the closed or the time of the open state of the switch is fixed. In any of the control methods, it is necessary to limit the inductor current at the stage of the closed state of the switch to protect against output overload. For these purposes, a resistive sensor or a pulse current transformer is used.

The choice of the main elements of a pulsed step-down stabilizer and the calculation of their modes will be carried out using a specific example. All the ratios that are used in this case are obtained on the basis of the analysis of the functional diagram and timing diagrams, and the methodology is taken as the basis.

1. Based on a comparison of the initial parameters and the maximum allowable current and voltage values ​​​​of a number of powerful transistors and diodes, we first select a bipolar composite transistor KT853G (electronic switch S) and a diode KD2997V (VD).

2. Calculate the minimum and maximum fill factors:

γ min \u003d t and min / T min \u003d (U VyX + U pr) / (U BX max + U s on ≈ U RdT + U pr) \u003d (12 + 0.8) / (32-2-0.3 + 0.8)=0.42;

γ max \u003d t and max / T max \u003d (U Bvyx + U pp) / (U Bx min - U sbkl -U Rdt + U pp) \u003d (12 + 0.8) / (18-2-0.3 + 0.8)=0.78, where U pr =0.8 V ≈ direct voltage drop across the diode VD, obtained from the direct branch of the I–V characteristic for a current equal to I V in the worst case; U sbcl \u003d 2 V ≈ saturation voltage of the KT853G transistor, which acts as a switch S, with a current transfer coefficient in saturation mode h 21e \u003d 250; U RdT = 0.3 V ≈ voltage drop across the current sensor at rated load current.

3. Select the maximum and minimum conversion frequency.

This item is performed if the pulse period is not constant. We choose a control method with a fixed duration of the open state of the electronic switch. In this case, the following condition is fulfilled: t=(1 - γ max)/f min = (1 - γ min)/f max =const.

Since the switch is made on the KT853G transistor, which has poor dynamic characteristics, we will choose the maximum conversion frequency relatively low: f max =25 kHz. Then the minimum conversion frequency can be defined as

f min \u003d f max (1 - γ max) / (1 - γ min) \u003d 25╥10 3] (1 - 0.78) / (1-0.42) \u003d 9.48 kHz.

4. Calculate the power loss on the switch.

Static losses are determined by the effective value of the current flowing through the switch. Since the current shape is ≈ trapezoid, then I s \u003d I out where α \u003d l Lmax / l lx \u003d 1.25 ≈ the ratio of the maximum inductor current to the output current. Coefficient a is chosen within 1.2 ... 1.6. Switch static losses P Sstat =l s U SBKn =3.27-2=6.54 W.

Dynamic losses on the switch Р sdyn =0.5f max *U BX max (l smax *t f +α*l lx *t cn),

where I smax ≈ the switch current amplitude due to the reverse recovery of the VD diode. Taking l Smax =2l ByX , we obtain

R sdin \u003d 0, 5f max * U BX max * I out (2t f + α ∙ t cn) \u003d 0.5 * 25 * 10 3 * 32 * 5 (2 * 0.78-10 -6 +1.25 -2-10 -6) = 8.12 W, where t f =0.78 * 10 -6 s ≈ the duration of the front of the current pulse through the switch, t cn = 2 * 10 -6 s ≈ the duration of the decline.

The total losses on the switch are: P s \u003d P scstat + P sdin \u003d 6.54 + 8.12 \u003d 14.66 W.

If static losses prevailed on the switch, the calculation should be carried out for the minimum input voltage when the inductor current is maximum. In the case where it is difficult to predict the predominant type of losses, they are determined both at the minimum and at the maximum input voltage.

5. We calculate the power loss on the diode.

Since the shape of the current through the diode ≈ is also a trapezoid, we define its effective value as Static losses on the diode P vDcTaT =l vD ╥U pr =3.84-0.8=3.07 W.

The dynamic losses of the diode are mainly due to reverse recovery losses: *0.2*10 -6 \u003d 0.8 W, where t OB \u003d 0.2-1C -6 s ≈ diode reverse recovery time.

The total losses on the diode will be: P VD \u003d P MDstat + P VDdin \u003d 3.07 + 0.8 \u003d 3.87 W.

6. Choose a heat sink.

The main characteristic of a heat sink is its thermal resistance, which is defined as the ratio between the temperature difference between the environment and the heat sink surface to the power dissipated by it: R g = ΔT / P rass. In our case, it is necessary to fix the switching transistor and the diode on the same heat sink through insulating spacers. In order not to take into account the thermal resistance of the gaskets and not complicate the calculation, we choose a low surface temperature, approximately 70╟С. Then at ambient temperature 40╟СΔТ=70-40=30╟С. The thermal resistance of the heat sink for our case R t \u003d ΔT / (P s + P vd) \u003d 30 / (14.66 + 3.87) \u003d 1.62╟С / W.

Thermal resistance during natural cooling is given, as a rule, in the reference data for the heat sink. To reduce the size and weight of the device, you can apply forced cooling with a fan.

7. Calculate the throttle parameters.

Calculate the inductance of the inductor:

L= (U BX max - U sbkl -U Rdt - U Out) γ min /= (32-2-0.3-12) * 0.42 / = 118.94 μH.

As the material of the magnetic core, we choose pressed Mo-permalloy MP 140. The variable component of the magnetic field in the magnetic circuit in our case is such that hysteresis losses are not a limiting factor. Therefore, the maximum induction can be chosen on the linear section of the magnetization curve near the inflection point. Work on a curved section is undesirable, since in this case the magnetic permeability of the material will be less than the initial one. This in turn will cause the inductance to decrease as the inductor current increases. We choose the maximum induction B m equal to 0.5 T and calculate the volume of the magnetic circuit:

Vp \u003d μμ 0 * L (αI outx) 2 / B m 2 \u003d 140 * 4π * 10 -7 * 118.94 * 10 -6 (1.25-5) 2 0.5 2 \u003d 3.27 cm 3, where μ=140 ≈

initial magnetic permeability of material MP140; μ 0 \u003d 4π * 10 -7 H / m ≈ magnetic constant.

According to the calculated volume, we select the magnetic circuit. Due to the design features, the MP140 permalloy magnetic circuit is usually performed on two folded rings. In our case, rings KP24x13x7 are suitable. The cross-sectional area of ​​the magnetic circuit Sc=20.352 =0.7 cm 2, and the average length of the magnetic line λc=5.48 cm. The volume of the selected magnetic circuit is:

VC \u003d SC * λc \u003d 0.7 * 5.48 \u003d 3.86 cm 3 > Vp.

We calculate the number of turns: We take the number of turns equal to 23.

We determine the diameter of the wire with insulation based on the fact that the winding must fit in one layer, turn to turn along the inner circumference of the magnetic circuit: where d K \u003d 13 mm ≈ the inner diameter of the magnetic circuit; k 3 \u003d 0.8 ≈ filling factor of the magnetic circuit window with a winding.

We select the wire PETV-2 with a diameter of 1.32 mm.

Before winding the wire, the magnetic core should be insulated with a PET-E film 20 µm thick and 6...7 mm wide in one layer.

8. Calculate the capacitance of the output capacitor: C Vyx \u003d (U BX max -U sVkl - U Rdt) * γ min /= (32-2-0.3) * 0.42 / \u003d 1250 μF, where ΔU Сvyx \u003d 0, 01 V ≈ peak-to-peak ripple on the output capacitor.

The above formula does not take into account the influence of the internal, series resistance of the capacitor on the ripple. With this in mind, as well as a 20% tolerance for the capacitance of oxide capacitors, we select two K50-35 capacitors for a nominal voltage of 40 V with a capacity of 1000 microfarads each. The choice of capacitors with an overestimated rated voltage is due to the fact that with an increase in this parameter, the series resistance of the capacitors decreases.

The scheme developed in accordance with the results obtained during the calculation is shown in rice. 3. Let's consider the stabilizer in more detail. During the open state of the electronic switch ≈ transistor VT5 ≈ a sawtooth voltage is formed on the resistor R14 (current sensor). When it reaches a certain value, the transistor VT3 will open, which, in turn, will open the transistor VT2 and discharge the capacitor C3. In this case, the transistors VT1 and VT5 will close, and the switching diode VD3 will also open. The previously open transistors VT3 and VT2 will close, but the transistor VT1 will not open until the voltage across the capacitor C3 reaches a threshold level corresponding to its opening voltage. Thus, a time interval will be formed during which the switching transistor VT5 will be closed (approximately 30 μs). At the end of this interval, transistors VT1 and VT5 will open and the process will repeat again.

Resistor R. 10 and capacitor C4 form a filter that suppresses the voltage surge at the base of the transistor VT3 due to the reverse recovery of the diode VD3.

For silicon transistor VT3, the base≈emitter voltage at which it switches to active mode is about 0.6 V. In this case, relatively large power is dissipated on the current sensor R14. To reduce the voltage on the current sensor, at which the transistor VT3 opens, a constant bias of about 0.2 V is applied to its base along the VD2R7R8R10 circuit.

A voltage proportional to the output voltage is supplied to the base of the transistor VT4 from a divider, the upper arm of which is formed by resistors R15, R12, and the lower arm is ≈ resistor R13. The HL1R9 circuit generates a reference voltage equal to the sum of the direct voltage drop across the LED and the emitter junction of the transistor VT4. In our case, the exemplary voltage is 2.2 V. The mismatch signal is equal to the difference between the voltage at the base of the VT4 transistor and the exemplary one.

The output voltage is stabilized due to the summation of the mismatch signal amplified by the transistor VT4 with the voltage based on the transistor VT3. Assume that the output voltage has increased. Then the voltage at the base of the transistor VT4 will become more exemplary. Transistor VT4 opens slightly and shifts the voltage at the base of transistor VT3 so that it also starts to open. Consequently, the transistor VT3 will open at a lower level of the sawtooth voltage across the resistor R14, which will lead to a reduction in the time interval at which the switching transistor will be open. The output voltage will then decrease.

If the output voltage decreases, the regulation process will be similar, but occurs in the reverse order and leads to an increase in the open time of the switch. Since the current of the resistor R14 is directly involved in the formation of the open time of the transistor VT5, here, in addition to the usual output voltage feedback, there is a current feedback. This allows you to stabilize the output voltage without load and provide a quick response to a sudden change in current at the output of the device.

In the event of a short circuit in the load or overload, the stabilizer switches to current limiting mode. The output voltage begins to decrease at a current of 5.5 ... 6 A, and the closing current is approximately equal to 8 A. In these modes, the on-time of the switching transistor is reduced to a minimum, which reduces the power dissipated on it.

If the stabilizer does not work properly, caused by the failure of one of the elements (for example, a breakdown of the transistor VT5), the voltage increases at the output. In this case, the load may fail. To prevent emergency situations, the converter is equipped with a protection unit, which consists of a trinistor VS1, a zener diode VD1, a resistor R1 and a capacitor C1. When the output voltage exceeds the stabilization voltage of the zener diode VD1, a current begins to flow through it, which turns on the trinistor VS1. Its inclusion leads to a decrease in the output voltage to almost zero and blown fuse FU1.

The device is designed to power 12-volt audio equipment, designed mainly for passenger vehicles, from the on-board network of trucks and buses with a voltage of 24 V. Due to the fact that the input voltage in this case has a low ripple level, capacitor C2 has a relatively small capacitance. It is insufficient when the stabilizer is powered directly from the mains transformer with a rectifier. In this case, the rectifier should be equipped with a capacitor with a capacity of at least 2200 microfarads for the corresponding voltage. The transformer must have an overall power of 80 ... 100 W.

The stabilizer uses oxide capacitors K50-35 (C2, C5, C6). Capacitor SZ ≈ film K73-9, K73-17, etc. of suitable sizes, C4 ≈ ceramic with low self-inductance, for example, K10-176. All resistors, except R14, ≈ C2-23 of the corresponding power. Resistor R14 is made of a 60 mm long piece of PEC 0.8 constantan wire with a linear resistance of approximately 1 ohm/m.

A drawing of a printed circuit board made of one-sided foil-coated fiberglass is shown in rice. 4.

Diode VD3, transistor VD5 and trinistor VS1 are attached to the heat sink through an insulating heat-conducting gasket using plastic bushings. The board is also fixed on the same heat sink. The appearance of the assembled device is shown in rice. 5.

REFERENCES 1. Titze U., Shenk K. Semiconductor circuitry: a reference guide. Per. with him. ≈ M.: Mir, 1982. 2. Semiconductor devices. Transistors of medium and high power: a Handbook / A. A. Zaitsev, A. I. Mirkin, V. V. Mo-kryakov, etc. Ed. A. V. Golomedova. ≈ M.: Radio and communication, 1989. 3. Semiconductor devices. Rectifier diodes, zener diodes, thyristors: Handbook / A. B. Gitsevich, A. A. Zaitsev, V. V. Mokryakov, etc. Ed. A. V. Golomedova. ≈ M.: Radio and communication, 1988. 4 http:/ /www. ferrite.ru

Stabilized single-ended voltage converter

Magazine "Radio", number 3, 1999

The article describes the principles of construction and a practical version of a simple pulse stabilized voltage converter that provides operation in a wide range of input voltage changes.

Among the various sources of secondary power supply (SEP) with a transformerless input, a single-cycle self-oscillating converter with a "reverse" switching on of the rectifier diode is distinguished by its utmost simplicity (Fig. 1).

Let us first briefly consider the principle of operation of an unstabilized voltage converter, and then - a method for stabilizing it.

Transformer T1 - linear choke; the intervals of energy accumulation in it and the transfer of accumulated energy to the load are separated in time. On fig. 2 shows: I I - current of the primary winding of the transformer, I II - current of the secondary winding, t n - interval of energy accumulation in the inductor, t p - interval of energy transfer to the load.

When the supply voltage U pit is connected, the base current of the transistor VT1 begins to flow through the resistor R1 (the diode VD1 prevents the passage of current through the base winding circuit, and the capacitor C2 shunting it increases the positive feedback (PIC) at the stage of formation of voltage fronts). The transistor opens slightly, the POS circuit closes through the transformer T1, in which the regenerative process of energy accumulation takes place. Transistor VT1 enters saturation. A supply voltage is applied to the primary winding of the transformer, and the current I I (collector current I to the transistor VT1) increases linearly. The base current I B of a saturated transistor is determined by the voltage on the winding I II and the resistance of the resistor R2. At the stage of energy accumulation, the diode VD2 is closed (hence the name of the converter - with the "reverse" switching on of the diode), and power consumption from the transformer occurs only by the input circuit of the transistor through the base winding.

When the collector current I to reaches the value:

I K max \u003d h 21E I B, (1)

where h 21E is the static current transfer coefficient of the transistor VT1, the transistor leaves the saturation mode and the reverse regenerative process develops: the transistor closes, the VD2 diode opens and the energy accumulated by the transformer is transferred to the load. After reducing the current of the secondary winding, the stage of energy accumulation begins again. The time interval t p is maximum when the converter is turned on, when the capacitor C3 is discharged, and the voltage at the load is zero.

B shows that the power supply, assembled according to the circuit in Fig. 1, - functional converter of the supply voltage U supply to the load current source I n.

It is important to note: since the stages of energy accumulation and its transfer are separated in time, the maximum collector current of the transistor does not depend on the load current, i.e. the converter is completely protected from short circuits at the output. However, when the converter is turned on without load (idle mode), a voltage surge on the transformer winding at the moment the transistor closes can exceed the maximum allowable collector-emitter voltage and disable it.

The disadvantage of the simplest converter is the dependence of the collector current I K max and, consequently, the output voltage on the static current transfer coefficient of the transistor VT1. Therefore, the power supply parameters will vary significantly when using different instances.

A converter using a "self-protected" switching transistor (Fig. 3) has a much more stable performance.

The sawtooth voltage from the resistor R3, proportional to the current of the primary winding of the transformer, is applied to the base of the auxiliary transistor VT2. As soon as the voltage across the resistor R3 reaches the opening threshold of the transistor VT2 (about 0.6 V), it will open and limit the base current of the transistor VT1, which will interrupt the process of energy accumulation in the transformer. Maximum current of the primary winding of the transformer

I I max \u003d I K max \u003d 0.6 / R3 (2)

turns out to be little dependent on the parameters of a particular instance of the transistor. Naturally, the current limiting value calculated by formula (2) must be less than the current determined by formula (1) for the worst value of the static current transfer coefficient.

Now consider the issue of the possibility of regulation (stabilization) of the output voltage of the power source.

B shows that the only parameter of the converter that can be changed to regulate the output voltage is the current I K max , or, what is the same, the energy accumulation time t n in the transformer, and the control (stabilization) unit can only reduce the current compared to the value calculated by formula (2).

Formulating the principle of operation of the converter stabilization unit, it is possible to determine the following requirements for it: - the constant output voltage of the converter must be compared with the reference voltage and, depending on their ratio, generate a mismatch voltage used to control the current I K max ; - the process of current growth in the primary winding of the transformer should be controlled and, when it reaches a certain threshold, determined by the mismatch voltage, stop; - the control unit must provide galvanic isolation between the converter output and the switching transistor.

The control units that implement this algorithm are shown in the circuits and contain a K521SAZ comparator, seven resistors, a transistor, a diode, two zener diodes and a transformer. Other well-known devices, including television power supplies, are also quite complex. Meanwhile, using a self-protected switching transistor, it is possible to build a much simpler stabilized converter (see the circuit in Fig. 4).

The feedback winding (OS) III and the VD3C4 circuit form a feedback voltage proportional to the output voltage of the converter.

The exemplary stabilization voltage of the zener diode VD4 is subtracted from the feedback voltage, and the resulting mismatch signal is fed to the resistor R5.

From the engine of the trimmer resistor R5, the sum of two voltages is supplied to the base of the transistor VT2: a constant control voltage (part of the mismatch voltage) and a sawtooth voltage from the resistor R3, proportional to the current of the primary winding of the transformer. Since the opening threshold of the transistor VT2 is constant, an increase in the control voltage (for example, with an increase in the supply voltage U pit and, accordingly, an increase in the output voltage of the converter) leads to a decrease in the current I I at which the transistor VT2 opens, and to a decrease in the output voltage. Thus, the converter becomes stabilized, and its output voltage is regulated within small limits by resistor R5.

The stabilization coefficient of the converter depends on the ratio of the change in the output voltage of the converter to the corresponding change in the constant component of the voltage based on the transistor VT2. To increase the stabilization coefficient, it is necessary to increase the feedback voltage (the number of turns of the winding III) and select the VD4 zener diode for the stabilization voltage, which is less than the OS voltage by about 0.5 V. The widespread Zener diodes of the D814 series are practically quite suitable at an OS voltage of about 10 V.

It should be noted that in order to achieve better temperature stability of the converter, it is necessary to use a VD4 zener diode with a positive TKN, which compensates for the decrease in the voltage drop at the emitter junction of the VT2 transistor when heated. Therefore, the zener diodes of the D814 series are more suitable than the precision zener diodes D818.

The number of output windings of the transformer (similar to winding II) can be increased, i.e., the converter can be made multichannel.

Built according to the scheme in Fig. 4 converters provide good stabilization of output voltages when the input voltage changes over a very wide range (150 ... 250 V). However, when operating on a variable load, especially in multichannel converters, the results are somewhat worse, since when the load current changes in one of the windings, the energy is redistributed between all windings. In this case, the change in the feedback voltage reflects the change in the output voltage of the converter with less accuracy.

It is possible to improve stabilization when operating on a variable load if the OS voltage is generated directly from the output voltage. The easiest way to do this is using an additional low-power voltage transformer assembled according to any of the known schemes.

The use of an additional voltage converter is also justified in the case of a multichannel IVEP. The high-voltage converter provides one of the stabilized voltages (the largest of them - at high voltage, the capacitor filter at the output of the converter is more effective), and the remaining voltages, including the feedback voltage, are generated by an additional converter.

For the manufacture of a transformer, it is best to use an armored ferrite magnetic circuit with a gap in the central rod, which provides linear magnetization. If there is no such magnetic circuit, to create a gap, you can use a gasket 0.1 ... 0.3 mm thick made of textolite or even paper. It is also possible to use ring magnetic circuits.

Although it is indicated in the literature that for the diode-reversed converters considered in this article, the output filter can be purely capacitive, the use of LC filters can further reduce the output voltage ripple.

For safe operation of the IVEP, a trimmer resistor (R5 in Fig. 4) with good motor insulation should be used. The transformer windings, galvanically connected to the mains voltage, must be reliably isolated from the output. The same applies to other radio elements.

Like any IVEP with frequency conversion, the described power supply must be equipped with an electromagnetic shield and an input filter.

The safety of establishing the converter will be provided by a network transformer with a transformation ratio equal to one. However, it is best to use a series-connected LATR and an isolation transformer.

Turning on the converter without load will most likely lead to a breakdown of the powerful switching transistor. Therefore, before proceeding with the adjustment, connect the equivalent load. After turning on, you should first of all check the voltage across the resistor R3 with an oscilloscope - it should increase linearly at stage t n. If the linearity is broken, this means that the magnetic circuit enters saturation and the transformer must be recalculated. Using a high-voltage probe, check the signal at the collector of the switching transistor - the pulse drops should be steep enough, and the voltage across the open transistor should be small. If necessary, adjust the number of turns of the base winding and the resistance of the resistor R2 in the base circuit of the transistor.

Next, you can try to change the output voltage of the converter with resistor R5; if necessary, adjust the number of turns of the OS winding and select the VD4 zener diode. Check the operation of the converter when the input voltage and load change.

On fig. Figure 5 shows the scheme of the IVEP for the ROM programmer, as an example of using a converter built on the basis of the proposed principle.

Source parameters are given in Table. 1.

When the mains voltage changes from 140 to 240 V, the voltage at the output of the 28 V source is in the range of 27.6 ... 28.2 V; source +5 V - 4.88 ... 5 V.

Capacitors C1-C3 and inductor L1 form an input mains filter that reduces the radiation of the high-frequency interference converter. Resistor R1 limits the charging current pulse of capacitor C4 when the converter is turned on.

The R3C5 circuit smooths out voltage spikes on the VT1 transistor (a similar circuit is not shown in the previous figures).

On transistors VT3, VT4, a conventional converter is assembled, which generates two more from the output voltage +28 V: +5 V and -5 V, as well as the OS voltage. In general, IVEP provides a stabilized voltage of +28 V. The stability of the other two output voltages is ensured by the additional converter being powered from a +28 V source and a fairly constant load of these channels.

The IVEP provides protection against exceeding the output voltage of +28 V to 29 V. When exceeded, the VS1 triac opens and closes the +28 V source. The power supply emits a loud squeak. The current through the triac is 0.75 A.

Transistor VT1 is installed on a small heat sink made of aluminum plate with dimensions of 40 (30 mm). Instead of the KT828A transistor, other high-voltage devices for a voltage of at least 600 V and a current of more than 1 A can be used, for example, KT826B, KT828B, KT838A.

Instead of the KT3102A transistor, you can use any KT3102 series; transistors KT815G can be replaced with KT815V, KT817V, KT817G. Rectifier diodes (except VD1) must be used high-frequency, for example, the KD213 series, etc. It is desirable to use oxide filter capacitors of the K52, IT series. Capacitor C5 must be at least 600 V.

Triac TS106-10 (VS1) is used solely because of its small size. Almost any type of trinistor that can withstand a current of about 1 A is suitable, including the KU201 series. However, the trinistor will have to be selected according to the minimum control current.

It should be noted that in a particular case (with relatively small current consumption from the source) it would be possible to do without a second converter by building a converter according to the scheme of Fig. 4 with additional windings for +5 V and -5 V channels and linear stabilizers of the KR142 series. The use of an additional converter is caused by the desire to conduct comparative studies of various IVECs and make sure that the proposed option provides better stabilization of the output voltage.

The parameters of transformers and chokes are given in Table. 2.

Table 2
Designation	Magnetic core		Number of turns
	B26 M1000 with a gap in the central rod			PEV-2 0.18 PEV-2 0.35 PEV-2 0.18
	К16x10x4,5 М2000НМ1		2x65 2x7 2x13 23	PEV-2 0.18 PEV-2 0.18 PEV-2 0.35 MGTP 0.07
	К16x10x4,5 М2000НМ1	MGTF 0.07 in two wires before filling
	K17,5x8x5 М2000НМ1
	К16x10x4,5 М2000НМ1
	К12x5x5.5 М2000НМ1

The magnetic circuit for the T1 transformer is used from the filter inductor of the power supply of the drive on replaceable magnetic disks of the EC series of computers.

Types of magnetic circuits of chokes L1-L4 are not critical.

The source is established according to the above method, but first the overvoltage protection should be turned off by moving the slider of the resistor R10 to the lower position according to the diagram. After establishing IVEP, it is necessary to set the output voltage to +29 V with resistor R5 and, slowly rotating the slider of resistor R10, reach the opening threshold of the triac VS1. Then turn off the source, turn the slider of the resistor R5 in the direction of decreasing the output voltage, turn on the source and set the output voltage to 28 V with the resistor R5.

It should be noted: since the voltages at the outputs +5 V and -5 V depend on the voltage +28 V and are not regulated separately from it, depending on the parameters of the elements used and the current of a particular load, it may be necessary to select the number of turns of the windings of the transformer T2.

Literature

1. Bas A. A., Milovzorov V. P., Musolin A. K. Sources of secondary power supply with transformerless input. - M.: Radio and communication, 1987.

Hello. I bring to your attention a review of the integrated linear adjustable voltage (or current) stabilizer LM317 at a price of 18 cents apiece. In a local store, such a stabilizer costs an order of magnitude more, which is why I was interested in this lot. I decided to check what is sold at such a price and it turned out that the stabilizer is quite high quality, but more on that below.
In the review, testing in the mode of a voltage and current stabilizer, as well as checking protection against overheating.
Interested please...

A little theory:

Stabilizers are linear And impulse.
Linear stabilizer is a voltage divider, the input of which is supplied with an input (unstable) voltage, and the output (stabilized) voltage is taken from the lower arm of the divider. Stabilization is carried out by changing the resistance of one of the divider arms: the resistance is constantly maintained so that the voltage at the output of the stabilizer is within the established limits. With a large ratio of input / output voltages, the linear stabilizer has a low efficiency, since most of the power Prass = (Uin - Uout) * It is dissipated in the form of heat on the control element. Therefore, the regulating element must be able to dissipate sufficient power, that is, it must be installed on a radiator of the required area.
Advantage linear stabilizer - simplicity, no interference and a small number of parts used.
Flaw- low efficiency, high heat dissipation.
Switching stabilizer voltage is a voltage stabilizer in which the regulating element operates in a key mode, that is, most of the time it is either in cut-off mode, when its resistance is maximum, or in saturation mode - with a minimum resistance, which means it can be considered as a key. A smooth change in voltage occurs due to the presence of an integrating element: the voltage increases as it accumulates energy and decreases as it is returned to the load. This mode of operation can significantly reduce energy losses, as well as improve weight and size indicators, however, it has its own characteristics.
Advantage pulse stabilizer - high efficiency, low heat dissipation.
Flaw- more elements, the presence of interference.

Review hero:

The lot consists of 10 chips in the TO-220 package. The stabilizers came in a plastic bag wrapped with polyethylene foam.






Comparison with probably the most famous 7805 5 volt linear regulator in the same package.

Testing:
Similar stabilizers are produced by many manufacturers, here.
The location of the legs is as follows:
1 - adjustment;
2 - exit;
3 - entrance.
We collect the simplest voltage stabilizer according to the scheme from the manual:


Here is what we managed to get with 3 positions of the variable resistor:
The results, frankly speaking, are not very good. It does not turn out to be called a stabilizer.
Next, I loaded the stabilizer with a 25 ohm resistor and the picture completely changed:

Next, I decided to check the dependence of the output voltage on the load current, for which I set the input voltage to 15V, set the output voltage to about 5V with a trimmer resistor, and loaded the output with a variable 100 Ohm wire resistor. Here's what happened:
It was not possible to obtain a current of more than 0.8A, because the input voltage began to drop (the PSU is weak). As a result of this testing, the stabilizer with a radiator heated up to 65 degrees:

To test the operation of the current stabilizer, the following circuit was assembled:


Instead of a variable resistor, I used a constant one, here are the test results:
Current stabilization is also good.
Well, how can a review be without burning the hero? To do this, I assembled the voltage stabilizer again, applied 15V to the input, set the output to 5V, i.e. 10V fell on the stabilizer, and loaded it by 0.8A, i.e. 8W of power was allocated on the stabilizer. Removed the radiator.
The result is shown in the following video:

Yes, overheating protection also works, it was not possible to burn the stabilizer.

Outcome:

The stabilizer is fully operational and can be used as a voltage stabilizer (subject to a load) and a current stabilizer. There are also many different application schemes for increasing output power, using it as a charger for batteries, etc. The cost of the subject is quite acceptable, given that offline I can buy such a minimum for 30 rubles, and for 19 rubles, which is significantly more expensive than the monitored .

On this, let me take my leave, good luck!

The product was provided for writing a review by the store. The review is published in accordance with clause 18 of the Site Rules.

I plan to buy +37 Add to favorites Liked the review +59 +88

The LM2596 steps down the input (up to 40V) voltage - the output is regulated, the current is 3A. Ideal for LEDs in the car. Very cheap modules - about 40 rubles in China.

Texas Instruments produces high-quality, reliable, affordable and cheap, easy-to-use DC-DC controllers LM2596. Chinese factories produce ultra-cheap stepdown converters based on it: the price of a module for an LM2596 is about 35 rubles (including delivery). I advise you to buy immediately a batch of 10 pieces - there will always be a use for them, while the price will drop to 32 rubles, and less than 30 rubles when ordering 50 pieces. Read more about the calculation of the strapping of the microcircuit, adjusting the current and voltage, its application and some of the disadvantages of the converter.

A typical method of use is a stabilized voltage source. Based on this stabilizer, it is easy to make a switching power supply, I use it as a simple and reliable laboratory power supply that can withstand short circuits. They are attractive due to the consistency of quality (it seems that they are all made at the same factory - and it is difficult to make mistakes in five details), and full compliance with the datasheet and the declared characteristics.

Another area of ​​application is a switching current stabilizer for power supply of high-power LEDs. The module on this chip will allow you to connect a 10-watt automotive LED matrix, additionally providing short circuit protection.

I highly recommend buying a dozen of them - they will definitely come in handy. They are unique in their own way - the input voltage is up to 40 volts, and only 5 external components are required. This is convenient - you can raise the voltage on the smart home power bus to 36 volts by reducing the cross section of the cables. We install such a module at consumption points and set it to the required 12, 9, 5 volts, or as much as you need.

Let's consider them in more detail.

Chip characteristics:

• Input voltage - from 2.4 to 40 volts (up to 60 volts in the HV version)
• Output voltage - fixed or adjustable (from 1.2 to 37 volts)
• Output current - up to 3 amperes (with good cooling - up to 4.5A)
• Conversion frequency - 150kHz
• Enclosure - TO220-5 (hole mount) or D2PAK-5 (surface mount)
• Efficiency - 70-75% at low voltages, up to 95% at high voltages

1. Stabilized voltage source
2. Converter circuit
3. datasheet
4. USB charger based on LM2596
5. current stabilizer
6. Application in homemade devices
7. Adjustment of output current and voltage
8. Improved analogues of LM2596

History - Linear Stabilizers

To begin with, I will explain why standard linear voltage converters like LM78XX (for example 7805) or LM317 are bad. Here is his simplified diagram.

The main element of such a converter is a powerful bipolar transistor, included in its "original" meaning - as a controlled resistor. This transistor is part of a Darlington pair (to increase the current transfer ratio and reduce the power required to operate the circuit). The base current is set by the operational amplifier, which amplifies the difference between the output voltage and that set using the ION (reference voltage source), i.e. it is included according to the classical error amplifier circuit.

Thus, the converter simply includes a resistor in series with the load, and controls its resistance so that, for example, exactly 5 volts is extinguished at the load. It is easy to calculate that when the voltage drops from 12 volts to 5 (a very common case of using the 7805 microcircuit), the input 12 volts are distributed between the stabilizer and the load in the ratio “7 volts at the stabilizer + 5 volts at the load”. At a half-amp current, 2.5 watts are released on the load, and at 7805 - as much as 3.5 watts.

It turns out that the "extra" 7 volts are simply extinguished on the stabilizer, turning into heat. Firstly, because of this, there are problems with cooling, and secondly, it takes a lot of energy from the power supply. When powered from a power outlet, this is not very scary (although it still harms the environment), but when using battery or rechargeable batteries, one cannot help but remember this.

Another problem is that it is generally impossible to make a boost converter with this method. Often such a need arises, and attempts to solve this issue twenty or thirty years ago are striking - how complicated was the synthesis and calculation of such schemes. One of the simplest circuits of this kind is a 5V->15V push-pull converter.

It must be admitted that it provides galvanic isolation, but it uses the transformer inefficiently - only half of the primary winding is involved at any time.

Let's forget it like a bad dream and move on to modern circuitry.

Voltage source

Scheme

The microcircuit is convenient to use as a step-down converter: a powerful bipolar switch is inside, it remains to add the rest of the regulator components - a fast diode, an inductance and an output capacitor, it is also possible to put an input capacitor - only 5 parts.

The LM2596ADJ version will also require an output voltage setting circuit, these are two resistors or one variable resistor.

Step-down voltage converter circuit based on LM2596:

The whole scheme together:

Here you can download datasheet for LM2596.

How it works: A PWM controlled high power switch inside the device sends voltage pulses to an inductor. At point A x% of the time the full voltage is present and (1-x)% of the time the voltage is zero. The LC filter smooths out these fluctuations by extracting a DC component equal to x * supply voltage. The diode closes the circuit when the transistor is off.

Detailed job description

An inductor opposes a change in current through it. When voltage appears at point A, the inductor creates a large negative self-induction voltage, and the voltage across the load becomes equal to the difference between the supply voltage and the self-induction voltage. The inductance current and the load voltage gradually increase.

After the voltage disappears at point A, the inductor seeks to maintain the same current flowing from the load and the capacitor, and closes it through the diode to the ground - it gradually drops. Thus, the voltage at the load is always less than the input voltage and depends on the duty cycle of the pulses.

Output voltage

The module is available in four versions: with a voltage of 3.3V (index -3.3), 5V (index -5.0), 12V (index -12) and an adjustable version LM2596ADJ. It makes sense to use the custom version everywhere, since it is in large quantities in the warehouses of electronic companies and you are unlikely to encounter a shortage of it - and it requires an additional two penny resistors. And of course, the 5 volt version is also popular.

Quantity in stock is in the last column.

You can set the output voltage as a DIP switch, a good example of this is shown here, or as a rotary switch. In both cases, you will need a battery of precise resistors - but you can adjust the voltage without a voltmeter.

Frame

There are two housing options: TO-263 planar mount housing (model LM2596S) and through-hole mount TO-220 housing (model LM2596T). I prefer the planar version of the LM2596S because the heatsink is the board itself and there is no need to buy an additional external heatsink. In addition, its mechanical resistance is much higher, unlike TO-220, which must be screwed to something, even to the board - but then it is easier to install the planar version. I recommend using the LM2596T-ADJ chip in power supplies, because it is easier to remove a large amount of heat from its case.

Smoothing input voltage ripple

Can be used as an effective "intelligent" stabilizer after rectifying the current. Since the IC monitors the output voltage directly, fluctuations in the input voltage will cause the IC's conversion ratio to change inversely, and the output voltage will remain normal.

It follows from this that when using the LM2596 as a step-down converter after the transformer and rectifier, the input capacitor (i.e. the one that stands immediately after the diode bridge) can have a small capacitance (about 50-100uF).

output capacitor

Due to the high conversion frequency, the output capacitor also does not have to have a large capacitance. Even a powerful consumer will not have time to significantly plant this capacitor in one cycle. Let's carry out the calculation: take a capacitor of 100uF, 5V output voltage and a load that consumes 3 amperes. The total charge of the capacitor q \u003d C * U \u003d 100e-6 uF * 5 V \u003d 500e-6 uC.

In one conversion cycle, the load will take dq = I * t = 3 A * 6.7 μs = 20 μC from the capacitor (this is only 4% of the total charge of the capacitor), and a new cycle will immediately begin, and the converter will put a new portion of energy into the capacitor.

Most importantly, do not use tantalum capacitors as input and output capacitors. They write right in the datasheets - “do not use in power circuits”, because they do not tolerate even short-term voltage surges very well, and do not like high impulse currents. Use regular aluminum electrolytic capacitors.

Efficiency, efficiency and heat loss

The efficiency is not so high, since a bipolar transistor is used as a powerful key - and it has a non-zero voltage drop, of the order of 1.2V. Hence the drop in efficiency at low voltages.

As you can see, the maximum efficiency is achieved with a difference between the input and output voltages of the order of 12 volts. That is, if you need to reduce the voltage by 12 volts, the minimum amount of energy will go into heat.

What is converter efficiency? This is a value that characterizes current losses - for heat generation on a fully open powerful key according to the Joule-Lenz law and for similar losses during transients - when the key is open, say, only half. The effects of both mechanisms can be comparable in magnitude, so we should not forget about both ways of loss. A small amount of power is also used to power the “brains” of the converter itself.

In the ideal case, when the voltage is converted from U1 to U2 and the output current is I2, the output power is P2 = U2*I2, the input power is equal to it (ideal case). This means that the input current will be I1 = U2/U1*I2.

In our case, the conversion has an efficiency below unity, so part of the energy will remain inside the device. For example, with efficiency η, the output power will be P_out = η*P_in, and losses P_loss = P_in-P_out = P_in*(1-η) = P_out*(1-η)/η. Of course, the converter will be forced to increase the input current in order to maintain the specified output current and voltage.

We can assume that when converting 12V -> 5V and an output current of 1A, the losses in the microcircuit will be 1.3 watts, and the input current will be 0.52A. In any case, this is better than any linear converter, which will give a minimum of 7 watts of losses, and will consume 1 ampere from the input network (including for this useless business) - twice as much.

By the way, the LM2577 chip has a three times lower frequency of operation, and its efficiency is slightly higher, since there are less losses in transients. However, it needs three times the inductor and output capacitor ratings, which is extra money and board size.

Increasing the output current

Despite the already rather large output current of the microcircuit, sometimes even more current is required. How to get out of this situation?

1. You can parallel multiple converters. Of course, they must be set exactly to the same output voltage. In this case, you cannot do with simple SMD resistors in the Feedback voltage setting circuit, you must either use resistors with an accuracy of 1%, or manually set the voltage with a variable resistor.

If there is no confidence in a small voltage spread, it is better to parallel the converters through a small shunt, on the order of several tens of milliohms. Otherwise, the entire load will fall on the shoulders of the converter with the highest voltage, and it may not be able to cope. 2. Good cooling can be used - large heatsink, large area multi-layer PCB. This will make it possible to [raise the current](/lm2596-tips-and-tricks/ "Using the LM2596 in devices and wiring the board") up to 4.5A. 3. Finally, you can [take out the powerful key] (#a7) outside the microcircuit case. This will make it possible to use a field effect transistor with a very small voltage drop, and will greatly increase both the output current and the efficiency.

USB charger on LM2596

You can make a very convenient camping USB charger. To do this, you need to set the regulator to a voltage of 5V, provide it with a USB port and provide power to the charger. I'm using a radio model lithium polymer battery purchased from China that delivers 5 amp-hours at 11.1 volts. That's a lot - enough to 8 times charge a regular smartphone (not taking into account efficiency). Taking into account the efficiency, it will turn out at least 6 times.

Don't forget to short the D+ and D- pins of the USB socket to tell the phone that it is connected to the charger and that the transmitted current is unlimited. Without this event, the phone will think that it is connected to a computer and will be charged with a current of 500mA - for a very long time. Moreover, such a current may not even compensate for the current consumption of the phone, and the battery will not charge at all.

You can also provide a separate 12V input from a car battery with a cigarette lighter socket - and switch sources with some kind of switch. I advise you to install an LED that will signal that the device is on, so as not to forget to turn off the battery after a full charge - otherwise the losses in the converter will completely drain the backup battery in a few days.

Such a battery is not very suitable, because it is designed for high currents - you can try to find a less high-current battery, and it will be smaller and lighter.

current stabilizer

Output current adjustment

Only available in configurable output voltage version (LM2596ADJ). By the way, the Chinese also make such a version of the board, with voltage and current adjustment and all kinds of indications - a ready-made current stabilizer module on the LM2596 with short circuit protection can be bought under the name xw026fr4.

If you do not want to use a ready-made module, and want to make this circuit yourself - nothing complicated, with one exception: the microcircuit does not have the ability to control current, but it can be added. I'll explain how to do it, and I'll explain the tricky points along the way.

Application

A current stabilizer is a thing needed to power high-power LEDs (by the way - my microcontroller project high power LED driver), laser diodes, electroplating, battery charging. As with voltage stabilizers, there are two types of such devices - linear and switching.

The classic linear current regulator is the LM317, and it's quite good in its class - but its 1.5A current limit is not enough for many high-power LEDs. Even if this stabilizer is powered by an external transistor, the losses on it are simply unacceptable. The whole world rolls a barrel on the power consumption of standby power bulbs, and here the LM317 works with an efficiency of 30% This is not our method.

But our microcircuit is a convenient driver of a pulsed voltage converter, which has many operating modes. Losses are minimal, since no linear operating modes of transistors are used, only key ones.

It was originally intended for voltage stabilization circuits, but several elements turn it into a current regulator. The fact is that the microcircuit relies entirely on the “Feedback” signal as feedback, but what to apply to it is already our business.

In the standard switching circuit, voltage is supplied to this leg from a resistive output voltage divider. 1.2V is equilibrium, if Feedback is less - the driver increases the duty cycle of the pulses, if more - it decreases. But you can apply voltage from the current shunt to this input!

Shunt

For example, at a current of 3A, you need to take a shunt with a nominal value of not more than 0.1 Ohm. At such a resistance, this current will release about 1W, so this is a lot. It is better to parallel three such shunts, getting a resistance of 0.033Ω, a voltage drop of 0.1V and a heat dissipation of 0.3W.

However, the Feedback input requires 1.2V - and we only have 0.1V. It is irrational to set more resistance (150 times more heat will be released), so it remains to somehow increase this voltage. This is done using an operational amplifier.

Non-inverting op-amp amplifier

The classic scheme, what could be simpler?

We unite

Now we combine the usual voltage converter circuit and an LM358 op-amp amplifier, to the input of which we connect a current shunt.

A powerful 0.033 ohm resistor is the shunt. It can be made from three 0.1 ohm resistors connected in parallel, and to increase the allowable power dissipation - use SMD resistors in the 1206 package, put them with a small gap (not close) and try to leave as much copper as possible around the resistors and under them. A small capacitor is connected to the Feedback output to eliminate possible transition to generator mode.

Adjustable current and voltage

Let's connect both signals to the Feedback input - both current and voltage. To combine these signals, we use the usual circuit of the mounting "AND" on the diodes. If the current signal is higher than the voltage signal, it will dominate and vice versa.

A few words about the applicability of the scheme

You cannot adjust the output voltage. Although it is impossible to regulate both the output current and the voltage at the same time - they are proportional to each other, with a "load resistance" factor. And if the power supply implements a scenario like “constant output voltage, but when the current is exceeded, we begin to reduce the voltage”, i.e. CC/CV is already a charger.

The maximum supply voltage of the circuit is 30V, since this is the limit for the LM358. It is possible to extend this limit to 40V (or 60V with the LM2596-HV version) if the op amp is powered by a zener diode.

In the latter version, it is necessary to use a diode assembly as summing diodes, since both diodes in it are made within the same technological process and on the same silicon wafer. The spread of their parameters will be much less than the spread of the parameters of individual discrete diodes - thanks to this we will get a high accuracy of tracking values.

You also need to carefully monitor that the circuit on the op-amp is not excited and does not go into generation mode. To do this, try to reduce the length of all conductors, and especially the track connected to pin 2 of the LM2596. Do not place the op-amp near this track, but place the SS36 diode and filter capacitor closer to the LM2596 case, and ensure the minimum area of ​​the ground loop connected to these elements - it is necessary to ensure the minimum length of the return current path "LM2596 -> VD/C -> LM2596".

Application of LM2596 in devices and self-layout of the board

I spoke in detail about the use of a microcircuit in my devices not in the form of a ready-made module in another article, which discusses: the choice of a diode, capacitors, inductor parameters, and also talked about the correct wiring and a few additional tricks.

Opportunities for further development

Improved analogues of LM2596

The easiest way after this chip is to switch to LM2678. In fact, this is the same stepdown converter, only with a field-effect transistor, thanks to which the efficiency rises to 92%. True, it has 7 legs instead of 5, and is not pin-to-pin compatible. However, this chip is very similar, and will be a simple and convenient option with improved efficiency.

L5973D- a rather old microcircuit, providing up to 2.5A, and a slightly higher efficiency. It also has almost twice the conversion frequency (250 kHz) - therefore, smaller inductor and capacitor values ​​are required. However, I saw what happens to her if you put it directly into the car network - quite often it knocks out with interference.

ST1S10- Highly efficient (90% efficiency) DC-DC stepdown converter.

• Requires 5-6 external components;

ST1S14- high-voltage (up to 48 volts) controller. High operating frequency (850 kHz), output current up to 4A, Power Good output, high efficiency (no worse than 85%) and overcurrent protection circuit make it probably the best converter for powering a server from a 36V source.

If maximum efficiency is required, you will have to turn to non-integrated stepdown DC-DC controllers. The problem with integrated controllers is that they never have cool power transistors - a typical channel resistance is no higher than 200mOhm. However, if you take a controller without a built-in transistor, you can choose any transistor, even AUIRFS8409-7P with a channel resistance of half a milliohm

DC-DC converters with external transistor

Next part

To effectively overcome various interference in the network, it is necessary to use simple current stabilizers. Modern manufacturers are engaged in the industrial production of such devices, so that each model is distinguished by its functional and technical characteristics. In the household industry, there are no great requirements for current stabilizers, but high-quality measuring equipment always needs a stable voltage.

Short description

Experienced craftsmen are well aware that the simplest current limiters are presented in the form of conventional resistors. Such aggregates are often called stabilizers., which is not true, since they are not able to remove all interference when the voltage fluctuates at their input. The use of a resistor in the power supply circuit of a device is possible only if the entire input voltage is stabilized.

In a different situation, even the smallest power surges are perceived as an increased load, which negatively affects the operation of the entire device. The efficiency of resistive current limiters is rather low, since the energy they consume is dissipated in the form of heat.

Those designs that are made on the basis of ready-made integrated circuits of linear stabilizers have a higher level of efficiency. The circuits of such devices are distinguished by a minimal set of elements, ease of configuration and the absence of interference. To avoid unwanted overheating of the regulating element, the differences between the input and output voltages must be kept to a minimum. Otherwise, the microcircuit package will be forced to dissipate all unclaimed energy, which reduces the final efficiency by several times.

Circuits with pulse-width modulation have the greatest efficiency. Their production is based on the use of universal microcircuits, where there is a feedback circuit and special protective mechanisms, which significantly increases the reliability of the entire device. The use of a pulse transformer leads to circuit retention, which positively affects the level of efficiency and the duration of the operating life. It is worth noting that craftsmen often make such stabilizers with their own hands, using special parts for this.

Functionality

Only the master who knows well the principle of operation of the current stabilizer will be able to effectively use this device in various fields. The main difficulty is that the power grids are saturated with various interferences that adversely affect the performance of equipment and devices. To effectively overcome the sources of negative impact, experts everywhere use voltage and current stabilizers.

Each of these products contains indispensable element - transformer, which ensures stable and trouble-free operation of the entire system. Even the most elementary circuit is necessarily equipped with a universal rectifier bridge, which is connected to various resistors, as well as capacitors. The main performance characteristics include the limiting level of resistance and individual capacitance.

Qualified specialists note that a simple current stabilizer operates according to the most elementary scheme. The thing is that the electric current is supplied to the main transformer, due to which its limiting frequency changes. At the input, it always coincides with this indicator in the mains, being within 50 hertz. Only after the current conversion has occurred, the limiting frequency will be reduced to the optimum level.

It is worth noting that in the traditional circuit there are powerful high-voltage rectifiers that help determine the polarity of the voltage. But the capacitors are involved in the high-quality stabilization of the current, the resistors eliminate the existing interference.

Making a simple converter for LEDs

Experienced craftsmen will agree that it is not so difficult to assemble a high-quality and durable stabilizer. The main feature is that a whole system of low-voltage 20-volt capacitors can be installed on the unit, and a pulse microcircuit can have an input of up to 35 V. The simplest do-it-yourself LED stabilizer is the LM317 variant. You only need to correctly calculate the resistor for the LED used using a specialized online calculator.

An important fact remains that for the coordinated operation of such a unit handy food is great:

• Standard unit for 19 volts from a laptop.
• At 24 V.
• A more powerful 32 volt unit from a conventional printer.
• Either 9 or 12 volts from some kind of consumer electronics.

The main advantages of such a converter always include its availability, the minimum number of elements, a high degree of reliability, as well as the availability in stores. It is very irrational to assemble a more complex circuit on your own. If the master does not have the necessary experience, then it is better to buy a pulse current stabilizer ready-made. It can always be improved if necessary.

The duration of the LED without losing brightness depends on the mode. The main advantage of the simplest stabilizers (drivers), such as the LM317 stabilizer chip, is that they are quite difficult to burn. The LM317 connection diagram requires only two parts: the microcircuit itself, which is included in the stabilization mode, and a resistor. The assembly process itself consists of several main stages:

1. You will need to buy a variable resistor with a resistance of 0.5 kOhm (it has three leads and an adjustment knob). You can order it via the Internet or buy it at the Radio Amateur.
2. The wires are soldered to the middle terminal, as well as to one of the extreme ones.
3. Using a multimeter, switched on in the resistance measurement mode, the resistance of the resistor is measured. It is necessary to achieve a maximum reading of 500 ohms (so that the LED does not burn out with a low resistance of the resistor).
4. After carefully checking the correctness of the connections, the circuit is assembled before connection.

For any device, you can achieve a supply of 10 A (set by low resistance). For these purposes, you can use the KT825 transistor or install an analogue with better technical characteristics and a cooling system. The maximum power of the LM317 is 1.5 amps. If there is a need to increase the current, then a field-effect or ordinary transistor can be added to the circuit.

Universal adjustable model

Many masters are faced with the need to use a high-quality stabilizer that would allow network settings to be made in a wide range. Some modern circuits are distinguished by the fact that they provide for the presence of a current-setting resistor with reduced characteristics. Experts themselves note that such a device allows you to amplify the voltage in another resistor. This condition is commonly referred to as amplified error voltage.

The parameters of the reference and error voltages can be compared using a reference amplifier, thanks to which the master adjusts the state of the field effect transistor. It should be noted that such a circuit requires additional power, which must be supplied to a separate connector. The thing is that the supply voltage must ensure the coordinated operation of absolutely all components of the circuit used. The permissible level should not be exceeded, as this is fraught with premature equipment failure.

In order to correctly adjust the operation of the adjustable current stabilizer, you must use a special slider. It is the tuning resistor that allows the master to set the maximum current value. The network setup is more flexible, since all parameters can be independently adjusted depending on the intensity of operation.

Multifunctional device

Drivers for 220 V LEDs have an average complexity of manufacturing. They can take a lot of time to set up, requiring experience in setting up. Such a driver can be extracted from LED lamps, spotlights and fixtures with a faulty LED circuit. Most of them can also be modified by learning the converter controller model. The parameters are usually set by one or more resistors.

The datasheet indicates the level of resistance required to obtain the desired current. If you install an adjustable resistor, then the number of amperes will be adjustable (but without exceeding the specified rated power).

Until recently, the XL4015 universal module was very popular. According to its characteristics, it is suitable for connecting LEDs with high power (up to 100 watts). The standard version of its case is soldered to the board, which acts as a heatsink. In order to improve the cooling of the XL4015, the circuit must be modified with a heatsink installed on the box of the device.

Many users simply put it on top, however, the effectiveness of such an installation is rather low. It is desirable to place the cooling system at the bottom of the board, opposite the soldering of the microcircuit. For optimal quality, it can be unsoldered and installed on a full-fledged heatsink using thermal paste. The wires will need to be extended. Additional cooling can be mounted for diodes, which will significantly increase the efficiency of the entire circuit.

Among the drivers, the adjustable one is considered the most versatile. Be sure to install a variable resistor that sets the number of amperes. These characteristics are usually specified in the following documents:

• In the accompanying documentation for the microcircuit.
• in datasheet.
• In the standard wiring diagram.

Without additional cooling of the microcircuit, such devices can withstand 1-3 A (in accordance with the model of the pulse-width modulation controller). The main disadvantage of these drivers is excessive heating of the diode and inductor. Above 3 A, cooling of a powerful diode and controller will be required. The inductor is replaced with a more suitable one or rewound with a thick wire.

An indispensable DC device

Even a novice master knows what the unit works on the principle of double integration. In absolutely all models, converters are responsible for this process. Universal two-channel transistors are designed to increase the existing dynamic characteristics. It is important to remember that to eliminate heat losses, you need to use capacitors with a large capacity.

It is possible to make an indicator of rectification only thanks to the exact calculation of the required value. As practice shows, if 12 amperes is obtained with a DC output voltage, then the limit value should be 5 V. The device will be able to stably maintain an operating frequency of 30 Hz. Regarding the threshold voltage - it all depends on the blocking of the signal that comes from the transformer. But the pulse front should not exceed 2 µs.

Only high-quality current conversion allows for the smooth operation of the main transistors. In this circuit, only semiconductor diodes are allowed. If the resistors are ballast, then this is fraught with large heat losses. That is why the scattering coefficient increases significantly. The master can see that the amplitude of the oscillations has increased, but the inductance process has not occurred.

Modern scheme based on KREN

Such a device will work stably only with LM317 and KR142EN12 elements. This is due to the fact that they act as universal voltage regulators, coping well with current up to 1.5 A and output voltage up to 40 volts. In the classical thermal mode, these elements are capable of high-quality dissipation of power up to 10 watts. The microcircuits themselves are distinguished by their low own consumption, since this figure is only 8 mA. The main thing is that this indicator remains unchanged even if the voltage fluctuates.

The LM317 microcircuit deserves special attention, which is capable of holding a constant voltage across the main resistor. This unit with a constant resistance ensures maximum stability of the current passing through it, due to which it is often called a current-setting resistor. Modern stabilizers on KREN differ from their counterparts in relative simplicity, due to which they are actively used as a charger for batteries and for electronic loads.

The microcircuit considered today is an adjustable DC-DC voltage converter, or simply a step-down adjustable current regulator of 40 volts at the input and from 1.2 to 35 V at the output. The LM2576 requires an input power of about 40-50V DC. Since it can handle currents up to 3 amps, the LM2576 acts as a switching regulator capable of driving a 3 amp load with a minimum of components and a small heatsink. The price of the LM2576 chip is approximately 140 rubles.

Schematic diagram of the stabilizer

Circuit Features

• Output regulated voltage 1.2 - 35 V and low ripple
• Potentiometer for smooth adjustment of the output voltage
• The board has an AC bridge rectifier
• LED indication of input power
• PCB dimensions 70 x 63 mm

The circuit is intended for desktop power supplies, battery chargers, as an LED driver. Further 2 versions - in standard and planar form:

Why can't simple parametric stabilizers like LM317 be used in such stabilized power supplies? Because the dissipated power at a voltage of 30 V 3 A will be several tens of watts - a huge radiator and cooler will be required. But with pulse stabilization, the power allocated on the microcircuit is almost 10 times less. Therefore, with the LM2576 we get a small and powerful, universal adjustable voltage regulator.

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