Network switching power supply, 50 watts. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The main purpose of the device described here is to power a personal computer. But not only. It is suitable for powering many other high-power amateur radio developments, for example, UMZCH.

The principle of operation of the proposed power supply (Fig. 1) is the same as that of third-generation color TV power supplies. It also operates in a mode close to that of discontinuous currents and is therefore a self-oscillating device. But there is also a fundamental difference: it uses "emitter switching" of a powerful switching transistor, which allows it to be used in a wider frequency range and, in addition, the probability of failure of a high-voltage transistor is reduced.

The experiments carried out confirmed that the KT839A transistor with the KT972A switching transistor in its emitter circuit works well even at a frequency of 120 kHz. Another advantage of the power supply is the possibility of using it in a wide range of output current.

(click to enlarge)

The device is a single-ended voltage converter with reverse switching of the rectifier diode. The output voltage of the unit channels is stabilized by changing the duration of the open state of the transistors of the electronic switch.

The main components of the power supply unit: a mains voltage rectifier with a filter, a single-ended converter with output filters, a pulse-width controller, a mismatch amplifier and an auxiliary switching regulator.

The mains voltage passes through the noise filter formed by the chokes L1, L2 and capacitors C1, C2, is rectified by the diode bridge VD1 ... VD4 and through the resistor R1 the rectified voltage is supplied to the smoothing capacitor C7. Capacitors C3 ... C6 weaken the penetration of interference into the network, and the resistor R1 limits the inrush of the input current at the moment the power supply is turned on. The converter starts approximately 0,1 s after connecting the unit to the network, which somewhat facilitates the work of the rectifier.

The main components of the converter are a T1 pulse transformer, a powerful high-voltage switch based on KT839A (VT1) and KT972A (VT2) transistors, rectifiers and output filters. The KT839A transistor (with a high maximum allowable collector-emitter voltage) opens and closes by closing and opening its emitter circuit with a high-speed KT972A transistor, which prevents secondary breakdown and reduces the switching duration of the emitter transistor. This is what allows you to change the output voltage in a wide range without altering the pulse transformer.

Resistors R11 and R12, the total resistance of which is 0,5 ohms, serve as a converter current sensor. When the transistor VT1 closes, its collector current through the diode VD6, the zener diode VD5 and the capacitor C8 is closed to the negative terminal of the rectifier bridge VD1 - VD4.

Diodes VD13-VD15 - pulse voltage rectifiers of the secondary windings 3, 4 and 5 of the transformer T1. The output voltage ripple of the rectifiers is smoothed out by capacitors C13-C18 and LC filters L5C21, L6C22.

Resistor R15, connected to the +5 V channel output, prevents excessive voltage rise on it when the +12 V channel is loaded. Thanks to this resistor, the voltage at the +5 V channel output without load does not exceed 6 V, which is safe for computer chips, at the channel load current +12 V to 2,5 A. The -12V channel voltage is stabilized by the DA2 microcircuit stabilizer.

The mismatch amplifier is connected to the +12 V channel output. The reference voltage source is the output of the DA2 stabilizer. Transistor VT4 amplifies the error signal. The load of the transistor is the LED of the optocoupler U1, and the VD17 diode protects its emitter junction. When the voltage at the +12 V channel output is more than 12 V, the optocoupler LED turns on and thereby increases the current flowing through the optocoupler phototransistor.

The open state of the transistor VT1 of the switch is determined by the duration of the charging of the capacitor C11 (from about 4 to +1 V) by the current of the phototransistor of the optocoupler. The greater the current value of the phototransistor of the optoron, the faster the capacitor charges. From 11 and the less time the transistor VT1 is in the open state.

After connecting the power supply to the network, the capacitor C8 also starts charging (through the resistor R2 and the diode VD6). When the voltage across it reaches 4,5 V, the current flowing through resistor R6, Zener diode VD12, emitter junction of transistor VT2, resistors R11, R12, and also through resistors R6, R5, emitter junction of transistor VT1, transistor VT2 and resistors R11, R12, switches the switching transistors to the active mode of operation. The positive feedback signal between the windings I and II of the transformer T1 through the diode VD7, capacitor C10 and resistors R5, R7 quickly opens the switching transistors. The accumulation of energy of the magnetic field in the magnetic circuit of the transformer T1 begins. After a certain period of time, the transistor VT3 opens and closes the transistor VT2, and therefore the transistor VT1. In this case, the transistor VT3 summarizes the voltages supplied to its base from the current sensor R11, R12 and capacitor C12.

At the time of startup or in the event of an overload of the converter, when the voltage drop across the resistors R11, R12 exceeds 1 V, the transistor VT3 opens with a current flowing through the resistor R10 and the diode VD11, due to which the device withstands short-term overloads. When any of its channels is shorted to a common conductor, the power supply automatically switches to power limiting mode without failure. In the normal mode of operation of the converter, the moment of closing the switching transistors is determined by the duration of the charging of the capacitor C11.

After closing the powerful transistors, the polarity of the voltage on the windings of the pulse transformer is reversed, and the diodes VD13 ... VD15 are turned on in the forward direction and charge the capacitors of the LC filters with a rectified current. When the value of this current is close to zero, electrical oscillations occur in the oscillatory circuit formed by the winding / transformer T1, its parasitic capacitance and capacitor C9. The first of them opens the powerful transistors of the switch - and the described process is repeated.

While the transistors VT1 and VT2 are closed, the voltage at the lower terminal of the winding II of the transformer relative to the negative terminal of the capacitor C7 is negative and, through the resistor R8 and the diode VD8, reliably holds the transistor VT2 in the closed state. The minimum voltage at the base of this transistor is determined by the stabilization voltage of the VD12 zener diode and the voltage across the VD10 diode. Capacitor C8 A is also charged through the R9VD11 circuit. Since the cathodes of diodes VD8 and VD9 are combined, the voltage across capacitor C12 cannot be less than that on the base of transistor VT2 (i.e., about -4 V).

The voltage at the +12 V channel output is stabilized by the method of pulse-width regulation. This simultaneously stabilizes the +5 V channel voltage.

However, since the pulse transformer, diodes and some other elements of the device are by no means ideal, the voltage stability at the output of this channel is not high. Therefore, an auxiliary switching regulator was used, which performs two functions: it provides the +5 V channel with part of the load current to increase the voltage stability on it and loads the +12 V channel if it is not loaded.

The auxiliary stabilizer includes a microcircuit stabilizer DA1, chokes L3, L4, capacitor C19, diode VD16, resistor R14. In it, the DA1 microcircuit serves as an electronic switch, a reference voltage source and an error signal amplifier. The L4 inductor and the VD16 diode are the necessary attributes of a switching regulator. The excitation of the DA1 microcircuit is provided by the inductor L3 and capacitor C19, and the resistor R14, which reduces the quality factor of the L3C19 circuit, prevents the occurrence of high-frequency oscillations.

All elements of the power supply are mounted on a printed circuit board with dimensions of 205x105 mm (Fig. 2) made of one-sided foil fiberglass 1 mm thick.

The main parameters of resistors and capacitors are indicated on the circuit diagram of the device. The transistor KT839A (VT1) can be replaced with KT838A, KT872A, KT846A, KT81148, and KT972A - with KT972B. Instead of transistors KT645B (VT3) and KT342BM (VT4), similar transistors with a base current transfer coefficient of at least 50 can work. We will replace the AOT101AC (U1) optocoupler with AOT101BS, AOT127A or AOT128A.

Diodes KD212A (U06, VD7) can be replaced with KD226 or KD411 with any letter index, and KD2999V (VD13, VD14) - with others with similar characteristics, for example, series KD2995, KD2997, KD2999, KD213. Instead of diodes VD1-VD4 of the rectifier bridge, KD226G or, in extreme cases, the KD243 series for a reverse voltage of at least 400 V are suitable.

A significant current flows through the Zener diode D814B (VD5), which should be taken into account when replacing it - the current allowed for it must be at least 40 mA. Significant currents also flow through capacitors C16-C18, so it is desirable that they be of the K50-29, K50-24 series. The rated voltage of capacitors C1-C6 (KD-2, K78-2, K73-16, etc.) must be at least 400 V, they must allow operation with a variable component of at least 350 V at a frequency of 50 Hz. Capacitor C9 - K78-2 for a rated voltage of 1600 V. The remaining parts are not critical for replacement.

Transistor VT1 is installed on a heat sink with a surface area of ​​about 200 cm2, diodes VD13 and VD14 - on heat sinks with an area of ​​45 and 35 cm, respectively, and a stabilizer DA2 - on a heat sink with an area of ​​70 cm2.

Transformer T1 is made on a magnetic circuit. W 12x15 from ferrite 2000NM, with a non-magnetic gap of 0,5 mm. Winding I contains 160 turns of wire PEV-2 0,47, folded in half. Winding II - 4 turns of the same wire, but folded three times. To improve the magnetic coupling, the windings III and IV are made with a copper tape 0,2 thick, 27 mm wide and contain 3 turns each. The copper tape can be replaced with a PEV-1 0,8 wire folded in three. Winding V contains 8 turns of wire PEV-1 0,4, folded four times.

Inductors L1 and L2 are wound on a common magnetic circuit of size K20x10x5 made of ferrite 2000NM and contain 35 turns of wire PEV-1 0,4 each. The magnetic circuits of the chokes L5 and L6 are pieces of a ferrite rod M400NN with a diameter of 8 and a length of 20 mm; each of them contains 15 turns. The L4 choke, made in the BZO armored magnetic circuit made of 2000NM ferrite (with a non-magnetic gap of 0,5 mm), contains 35 turns of PEV-1 0,8 wire.

An error-free mounted power supply, as a rule, starts working without prior adjustment. But, as an insurance policy, it is desirable to make the first connection to the network through an incandescent lamp with a power of 15 ... 25 W, designed for a voltage of 220 V. As soon as the converter starts up, a variable resistor R18 must be set at the output of the +12 V channel corresponding to it.

If the requirements for the supply voltage of the +5 V channel are more stringent (or a larger output current is required), the mismatch amplifier should be connected to the +5 V channel output. for example, to the positive terminal of the capacitor C16, and also reduce the resistance of the resistor R17 to 5 Ohms, and the resistor R17 to 16 kOhm. Stabilizer DA300, chokes L17 and L1,5, resistor R1, capacitor C3 and diode VD4 are excluded. However, after such an alteration, the voltage at the +14 V channel output will also increase with an increase in the +19 V channel current, so the voltage of this channel will have to be additionally stabilized (for example, using the KR16EN12B microcircuit).

An undesirable voltage increase at the +5 V channel output can be prevented by connecting the second LED of the optocoupler U17 in parallel with the capacitor C1 through the KS156A zener diode and a resistor with a resistance of 180 ... 200 Ohms. In this case, conclusions 6 and 7, as well as conclusions 5 and 8 of the optocoupler, must be combined. This will not only protect the power supply from exceeding the output voltage, but also increase the reliability of its operation, since in this case the feedback circuit will be duplicated.

The described device is applicable for powering many other amateur radio structures, for example, AF power amplifiers. It is only necessary, taking into account the features of a particular radio engineering device, to rebuild the secondary part of the power supply, and a 1,5-fold change in the output voltage is achieved by adjusting the level of the feedback signal of the winding of the transformer T1. Specific example. To power the power amplifier based on the K174UN19 chip, a bipolar voltage source of ±15 V is required. In this case, the secondary part of the described power supply can be assembled according to the circuit shown in fig. 3.

The windings III and IV of the T1 transformer contain 7 turns of copper tape 0,1 thick and 27 mm wide or wire PEV-1 0,8, folded three times. The winding of both windings is performed simultaneously. Conclusions 6 and 7, as well as 5 and 8 of the optocoupler U1 must be combined.

Literature

1. Polikarpov A. G., Sergienko E. F. Single-cycle voltage converters in power supply devices for REA. - M.: Radio and communication, 1989.
2. Sergeev B. S. Circuitry of functional units of secondary power sources. - M: Radio and communication, 1992

Author: D.Bezik

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