Switching network power supply. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Switching network power supply. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The power supply is designed to power portable television and radio equipment. Its rated output power is 20 W, and the efficiency at rated power is at least 85%. Working conversion frequency - 68 kHz.

The characteristics of the unit are optimized for a load within 0,5...1 of the rated power. It works stably when the mains voltage changes in the range from 170 to 240 V, withstands short-term output circuits (the circuit current measured by the V7-35 avometer is 6 A).

(click to enlarge)

The schematic diagram of the block is shown in fig. 1. The inverter master oscillator is assembled on an operational amplifier DA1, covered by a positive feedback circuit. The load of the op-amp is the primary winding of the pulse transformer T1. The differentiating circuit R7C6 creates a forced switching front of transistors VT2 and VT4.

The node that accelerates the process of resorption of minority charge carriers in these transistors consists of elements VT1, VT3, VD8, VD9, R8. R9, C7-C10 and additional windings III, IV of the transformer T2. Consider the operation of the node on the example of the upper arm of the half-bridge inverter according to the scheme.

Let the transistor VT2 is open and saturated. In this case, the transistor VT1 is closed and a voltage of about 6 V is applied to it from the winding III of the transformer T2. Capacitor C9 is charged.

At the end of the switching half-cycle, the polarity of the voltage at the output of the master oscillator changes abruptly and, therefore, on all windings of the transformer T1; LI¦ - voltage on the primary winding of the transformer T1. Transistor VT1 opens, and thanks to the differentiating circuit R8C7, this process is accelerated.

Under the influence of the total voltage from the windings III of the transformer T2 and II of the transformer T1, forced absorption of minor charge carriers from the base of the transistor VT2 begins along the circuit: winding III of the transformer T2, collector junction of the transistor VT1, winding II of the transformer T1, emitter junction of the transistor VT2, diode VD8. After a time tpac, a sharp decrease in the collector current of the transistor VT2 begins. In this case, the transistor VT4 is in the opening stage.

The voltage on the windings of the transformer T2 changes its polarity. The charge accumulated by capacitor C9 maintains the closing current of the base of the transistor VT3 until it is completely closed. From this moment, the reverse voltage at the emitter junction VT2 decreases to a value not exceeding 1 V, which has a positive effect on the reliability of the inverter.

As switching transistors, in addition to those indicated in the diagram, you can use transistors KT704B, KT704V, as well as any of the KT809 series; other high-voltage low- and mid-frequency transistors are also suitable. It is desirable to install them on heat sinks with an effective dissipation area of about 50 cm2. Transistors VT1, VT3 - any silicon with a maximum allowable collector pulse current of at least 500 mA and a cutoff frequency of at least 100 MHz.

Oxide capacitors C1, SZ, C4 - K50-12. K50-27; C11, C12-K50-29, K53-25, K53-28; the rest - any ceramic, glass or glass-ceramic. Capacitor C5 must be of the TKE PZZ, MZZ or M47 group. Resistor R1 - C5-16, the rest - MLT.

Transformer T1 is wound on an annular magnetic circuit of size K10x6x4,5 made of ferrite 2000NM. Winding I is made with PEV-2 0,1 wire and contains 52 turns, windings II and III - 8 turns of the same wire each. Transformer T2 is also wound on an annular magnetic circuit of size K20x12x6 made of ferrite 2000NM 1-A. Winding I contains 120 turns of wire PEV-2 0,25, winding II - 2x12 turns of wire PEV-2 0,8, windings III and IV - 5 turns of wire PEV-2 0,25 each. Choke L1 is made on the armored magnetic circuit Sh5x5 of 700NM ferrite. The winding contains 50 turns of PEV-2 0,8 wire. Between the halves of the magnetic circuit of the inductor, it is necessary to insert a gasket made of thick paper 0,3 mm thick.

The overall power of the transformer T2 at a conversion frequency of 68 kHz is 42 W, which allows you to increase the output power of the power supply to the specified value. To do this, its windings I, III, IV must be wound with PEV-2 0,41 wire, winding II, as well as the inductor winding L1 - with PEV-2 1 wire. capacitors C2, C4, C1 and the inductance of the inductor L11 in accordance with the recommendations set out in [12]; the resistance of the resistor R1 must be reduced to 3 ohms, R7 - to 680 ohms.

In this case, there may be a need to "power" the operational amplifier DA1 with an additional transistor current amplification stage, as well as to increase the capacitance of the capacitors C2, Sat.

The assembled structure must be placed in a shielding box. The author's version of the block design has dimensions of 100x63x33 mm, which corresponds to a power density of about 105 W/dm3. With error-free installation and serviceable parts, the source of adjustment does not require and starts working immediately.

Literature

1. Biryukov S. Power supply for "Radio-86RK". - Radio, 1990, No. 7, p. 58-61.
2. Tsvetaev S. Powerful power supply. - Radio, 1990. No. 9,0.59-62.
3. Eranosyan S. A. Network power supplies with high-frequency converters. - L .: Energoatomizdat, 1991.

Author: V. Dorozhinsky, Gelendzhik; Publication: N. Bolshakov, rf.atnn.ru

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