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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Laboratory switching power supply
Encyclopedia of radio electronics and electrical engineering / Power Supplies A feature of the bipolar power supply offered to the attention of readers is the presence of pulse and linear control stages in each arm, which made it possible to reduce the voltage drop and power on the control transistor and, accordingly, to reduce the size of the heat sink. The device, which the author has been successfully operating for more than five years, turned out to be, perhaps, not quite optimal, but we hope that radio amateurs will be able to modify it using the available element base to suit their tasks. The main problem that arises in the manufacture of a power supply operating in a wide range of output voltages and with a large load current is to ensure the minimum power dissipation on the regulating element and, accordingly, to obtain the maximum efficiency of the device as a whole. One way to solve this problem is to use a transformer with a multi-section winding [1]. The main disadvantages are the need to manipulate the switch, which is very inconvenient, and the complexity of manufacturing a transformer. The most successful solution is a pulsed regulated source with subsequent filtering of ripples by a compensation stabilizer. The complication of the device is offset by the small size of the heat sinks, since the voltage drop, and hence the power released on the control transistor of the linear stabilizer, can be made minimal and independent of the load voltage. The laboratory power supply described in [2] was taken as a basis. Its main drawback is a very bulky choke, which dramatically increases the weight and dimensions of the device. In the proposed version of the source, the primary voltage regulation is carried out at a high frequency (15 ... 50 kHz), so the inductor is made on a ferrite magnetic circuit, which significantly reduced the dimensions and weight of the device Main Specifications
The power supply circuit is shown in fig. 1. The dash-dotted line marks the same nodes in both arms. Consider the operation of the device using the example of a positive voltage source.
The alternating voltage of the secondary winding of the mains transformer T rectifies the diode bridge VD1-VD4 and filters the capacitor Sat. Then a constant voltage is supplied to the switching transistor VT4 of the switching regulator and to the Schmitt trigger, assembled on transistors VT5, VT6, the supply voltage of which is stabilized by the parametric stabilizer R13VD18. At the initial moment after turning on the power supply, the voltage sensor - the transistor VT7 is closed, the transistor VT5 of the Schmitt trigger is open, and the transistors VT1 and VT2 are closed. Transistor VT3 is open current flowing through its emitter junction and resistors R6 R7. Therefore, the switching transistor VT4 is also open. Capacitor C8 begins to charge. The voltage on it increases until it becomes close to the set output. A further increase in the voltage across the capacitor C8 will open the voltage sensor VT7 and trigger the Schmitt trigger. As a result, transistors VT1 and VT2 will open, and transistors VT3 and VT4 will close. Then the L1 choke is switched on. The self-induction voltage opens the VD17 diode, and the energy accumulated in the choke is transferred to the load. After the energy reserve in the inductor is exhausted, the VD17 diode closes, and the current flows into the load from the capacitor C8. The voltage across it begins to decrease, and at some point the voltage sensor VT7 closes. The Schmitt trigger will switch (transistor VT6 will be closed and transistor VT5 open), transistors VT1 and VT2 will close and transistors VT3 and VT4 will open. Capacitor C8 will start charging again. Diode VD16 protects the switching transistor VT4 in emergency situations, for example, when the diode VD17 fails or the capacitor C6 loses capacitance. The compensation stabilizer on transistors VT8, VT9, VT11 is assembled according to a simple scheme and has no features. Elements R19, VD20, C10 are used to smoothly increase the output voltage after turning on the power supply and prevent the protection from tripping under a significant capacitive load. At the moment of switching on, capacitor C10 is charged in two circuits: through resistor R19 and resistor R21, diode VD20. The voltage on the capacitor (and the base of the transistor VT9) slowly increases over about 0,5 s. Accordingly, the output voltage also increases until the stabilizer enters a steady state. Next, the VD20 diode closes, and the capacitor C10 is recharged through the resistor R19 and does not affect the operation of the stabilizer in the future. The VD19 diode is needed to quickly discharge the capacitor C10 after the power supply is turned off and when the output voltage decreases. In this case, the voltage on the capacitor C8 decreases faster than on C10, the VD19 diode opens and the voltage on both capacitors decreases simultaneously. In addition, relay K1 is used to quickly reduce the output voltage when the power supply is turned off. After the unit is connected to the network, relay K1 is energized through resistor R1 from a diode rectifier VD7 VD8. The rectified voltage filters a small capacitor C3. The relay is activated, its contacts K1.1 open and do not affect the operation of the stabilizer. When the unit is turned off, the voltage on capacitor C3 disappears faster than on C6, so relay K1 almost immediately releases its contacts K1.1 closes and capacitor C10 quickly discharges through resistor R20. At this moment, the VD20 diode opens and the voltage at the base of the VT9 transistor decreases almost to zero. The voltage at the output of the stabilizer disappears. The R26VD23 circuit serves to accelerate the discharge of the capacitor C13 and the capacitors in the load when lower voltage values are set. In this case, the voltage at the collector of transistor VT11 becomes less than the voltage at the output of the unit, diode VD23 opens and capacitor C13 is discharged through the circuit: resistor R26, diode VD23, collector-emitter section of transistor VT11 and diodes VD21, VD22. In steady state, the R26VD23 circuit does not affect the operation of the unit. Capacitor C12 prevents self-excitation of the stabilizer. Capacitors C14 and C23 are connected directly to the output terminals of the power supply to reduce high-frequency ripple. The R6C7 circuit is needed to reduce the closing time of transistors VT3, VT4. If the transistor VT3 is open, a voltage drop is created across the resistor R6, plus applied to the base of the transistor. Capacitor C7 is charged in the same polarity. When the transistor VT2 opens, through its collector-emitter section, the lower capacitor plate according to the circuit will be connected to the emitter of the transistor VT3. Thus, a closing voltage will be applied to the emitter junction of the transistor VT3, which contributes to its forced closing, and hence the closing of the switching transistor VT4. When the protection is triggered (during overload or short circuit in the load), the base of the transistor VT10 through the divider R22R23 receives the voltage that opens it. As a result, the base of the transistor VT9 is connected to a common wire through the collector-emitter section of the open transistor VT10. The voltage at the output of the block disappears. Note the features of the construction of the negative channel of the power supply. Switching stabilizer and Schmitt trigger remained unchanged. The compensation stabilizer is made on transistors of a different conductivity, and the VT21 control element is included in the negative power line circuit. This simplified the connection of the compensation stabilizer with the protection unit. The Schmitt trigger (on transistors VT17, VT18) is connected directly to the transistor VT20. The function of the voltage sensor is performed by the transistor VT18 of the Schmitt trigger. So that when the power supply is turned off, the output voltages disappear synchronously in both arms, a common relay K1 is used (contacts K1.2). The protection node is fed from a bipolar voltage source. This makes it very easy to control both arms of the power supply [3]. The negative voltage forms a multiplier on the diodes VD5, VD6 and capacitors C1, C2 and at the level of -5 V stabilizes the parametric stabilizer R2VD10. The scheme of the protection node is shown in fig. 2.
When the load current reaches the set value, the voltage drop across the resistor R30 (see Fig. 1) will be sufficient to open the transistor VT12. The input S (pin 14) of the trigger DD1 receives a high level, and it switches to a single state. A low level will appear at the output of the inverter DD2.1, which, through the diode VD1 and the resistor R50, acts on the transistor VT19 (see Fig. 1), which will lead to the opening of the latter and the closing of the composite transistor VT20VT21. The voltage at the output of the negative source will disappear. At the output of the inverter DD2.3, a single signal will appear, acting through the diode VD5 and the resistor R22 (see Fig. 1) on the transistor VT10, which generally leads to the closing of the positive shoulder. The HL1 "+" LED indicates the presence of an overload in the positive arm of the power supply. Similarly, the protection unit works in the event of an overload of a negative source. Thus, wherever an overload occurs, both arms of the stabilizers are turned off, and this state will remain indefinitely until the SB1 "Return" button is pressed. In this case, a high level affects the R inputs (pins 3 and 15) and switches the flip-flops to the zero state. The performance of the stabilizers will be restored. Capacitor C3, shunting the contacts of the SB1 button, is needed to set the triggers to zero at the moment the unit is connected to the network. Resistors R1, R2 are used to set the protection sensitivity level. Capacitors C1, C2, shunting the inputs S of triggers, prevent false triggering of the protection unit against impulse noise induced in the connecting conductors. Diodes VD1-VD6 are needed to decouple the outputs of microcircuits. You can use any mains transformer in the power supply that provides the necessary power. In the author's version, a ready-made transformer TS-180-2 was used. The primary winding is left unchanged. It contains 680 turns of wire PEV-1 0,69 All secondary windings are removed, and new windings II and III are wound in their place, containing 105 turns of wire PEV-1 1,25 each. The transformer can be made independently on the basis of the PL21 x45 magnetic circuit. Inductors L1 and L2 are wound on B-30 armored magnetic cores made of M2000NM ferrite. The windings contain 18 turns of a bundle made up of nine PEV-2 0,4 wires. The gap between the halves of the magnetic conductor is 0,2 .. 0,5 mm. Diodes KD202R (VD1-VD4, VD12-VD15), which are placed on small heat sinks, can be replaced by others designed for direct current of at least 3 A and the required reverse voltage. Instead of diodes KD105B (VD5-VD9) and D223A (VD19-VD23, VD27-VD31), it is permissible to use any of the KD208, KD209 series. Diodes D9B (VD1-VD6, Fig. 2) are replaceable by any of the KD521, KD522 series. Relay K1 - RES48A version RS4 590 202 for an operating voltage of 12 V. It is better to choose a relay for a higher voltage, for example, RES48A version RS4.590.207 with a voltage of 27 V. In this case, you should use a current-limiting resistor R1 of lower resistance and power. Transistors KT644B (VT3, VT15) are interchangeable with KT644A, KT626V, in extreme cases, with KT816V, KT816G or KT814V, KT814G. In place of transistors VT1, VT10, VT13, it is permissible to use any silicon with an allowable collector-emitter voltage of at least 60 V. Instead of MP26A transistors (VT7, VT12, VT19, VT22 and VT1, Fig. 2), you can use any of the MP25, MP26 series; instead of KT3102A (VT5, VT6, VT11, VT17, VT18) - KT315V-KT315E, KT3102B. We replace the KT827A (VT8) transistor with any of this or from the KT829 series, as well as KT908A, KT819G, the KT825A (VT21) transistor - with any of this or from the KT853 series, as well as KT818G maximum collector current. The MP37B transistor (VT23) should be selected according to the maximum collector-emitter voltage, since it operates at the limit of the permissible value. Transistors VT4, VT8, VT16, VT21 and diodes VD17, VD25 are installed on small heat sinks with dimensions of 50x50x5 and 40x30x3 mm, respectively. Microcircuits of the 564 series are interchangeable with the corresponding analogues of the K561 series. Oxide capacitors C6 and C15 are made up of two K50-24 1000 microfarads each and two K52-1B 100 microfarads each, all for a voltage of 63 V, connected in parallel. Capacitors C1, C2, C10, C11, C19, C20 - K50-6, C3, C4, C5, C13, C22 - K50-16, C12, C14, C21, C23 - K73-17. Microammeters RA1, RA2 - M4205 for a current of 100 μA. All parts of the device are checked in advance. In the author's version, the power supply is assembled on several boards by surface mounting. When setting up a block, it is best to use an oscilloscope. It is connected to the emitter of the transistor VT4. The engine of the resistor R28 is set to the middle position, and the resistor R22 is temporarily soldered. Turn on the power supply to the network. Rectangular pulses should appear on the emitter of transistor VT4. If there is no voltage, first of all, you should make sure that relay K1 has worked. Otherwise, by selecting the resistor R1, they ensure that the relay operates at the minimum mains voltage (190 V). After that, the collector-emitter voltage of the transistor VT8 is measured. It should be within 1,5 ... 2 V and be maintained when the output voltage changes. Switching of the switching regulator occurs when the collector-base voltage of the VT9 transistor is approximately equal to 0,9 V. If it needs to be increased, one or more diodes in the forward direction should be connected to the emitter circuit of the VT7 transistor. The switching frequency depends to a small extent on the resistance of the resistors R17 (with its decrease, the frequency decreases) and R15 (with its increase, the frequency decreases). Resistors R27 and R29 select the minimum and maximum values of the output voltage (3 and 30 V). Now, a load (or its equivalent) with a resistance of about 3 ohms with a power of at least 27 W is connected to the output of the stabilizer, having previously set the output voltage to approximately 5 V. Gradually increasing the output voltage, make sure that the current in the load does not exceed 3 A. In addition, one should control the shape of the impulses. If the duration of the pauses between pulses becomes less than 1/5 of the period, oscillations may stall. In this case, it is necessary to increase the inductance of the inductor by using a large magnetic circuit or by increasing the number of turns. Then the microammeter is calibrated to measure the load current. To measure the voltage at the output of the power supply, you can turn on a microammeter with an additional resistor with a resistance of about 300 kOhm. Next, solder the resistor R22. The engine of the resistor R32 is set to the upper (according to the scheme) position, and the resistor R28 is the minimum voltage. A 40 ohm resistor is connected to the output of the stabilizer. Turn on the power supply to the network and, increasing the output voltage, set the load current to 250 mA. Then, using the resistor R1 (see Fig. 2), they ensure that the protection works and the HL1 LED turns on. For a negative voltage source, the minimum protection operation current is set by resistor R2. After that, the slider of the resistor R32 is moved to the lower (according to the diagram) position. The load resistance is reduced and the current is set to 3 A. By moving the slider of the resistor R32 up (according to the diagram), they notice the moment the protection is triggered. Now you should measure the resistance of the output part of the resistor R32, put a resistor of close rating and calibrate it according to the protection trip current. The negative voltage shoulder is adjusted in the same way. In conclusion, the ripple voltage is measured with an oscilloscope at the maximum load current. If the ripple exceeds 30 mV, install additional capacitors C11 and C20 (shown in dashed lines in the diagram in Fig. 1). It may turn out that when the engine of the resistor R28 (R56) is quickly turned, the output voltage still changes, although the engine is already stationary. In this case, the upper terminal of the resistor R21 must be unsoldered and connected to the collector of the transistor VT4 (shown by a dashed line). The lower terminal of the resistor R49 should also be unsoldered and connected to the connection point of the elements R2, C2, VD6 (see Fig. 1). The resistance of resistors R21 and R49 must be increased to 20 kOhm. The efficiency of the compensation stabilizer can be increased if, in place of VT8 and VT21, transistors with a lower collector-emitter saturation voltage are used, taking into account the recommendations [4]. Instead of MP37B (VT23), it is better to use a germanium transistor with a large allowable collector-emitter voltage, for example, GT404V, GT404G. Literature
Author: G. Balashov, Shadrinsk, Kurgan region.
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