Miniature switching power supply, 220/5 volts 3 watts. Scheme, description

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Miniature switching power supply, 220/5 volts 3 watts. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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This unit can be used to power an electronic clock, a microcalculator, an electronic thermometer, and other low-power electronic equipment (Fig. 5.3).

(click to enlarge)

Main technical characteristics:

alternating voltage of the power supply ..... 220 ± 15% V;
maximum load power ..... 3 W;
conversion frequency ..... 35 kHz;
efficiency ..... 75%;
specific power.....115 W/dm3

The main node of the block is the so-called half-bridge voltage converter, made on transistors VT1, VT2 and transformer T1. The supply voltage is rectified by the diode bridge VD1. Zener diodes VD3...VD5 and resistor R1, connected by a parametric stabilizer, together with capacitors C2...C4 form a rectified voltage divider. LED VD2 serves as an indicator of the power supply. The voltage on the symmetrical zener diode VD3 is used to power the master oscillator. Resistor R1, which is included as a ballast in the parametric stabilizer, provides voltage boost on capacitor C8.

In the event of an output short circuit, this resistor limits the current drawn by the power supply from the mains. In addition, it and capacitor C8 form a filter that suppresses high-frequency noise generated by the converter. The master oscillator is assembled on an operational amplifier DA1 according to the multivibrator circuit. Its peculiarity is the use of a unipolar voltage for power supply, taken from the VD3 zener diode, which reduces the total number of block elements.

The galvanic isolation of the generator and the high-voltage transistor VT1 in direct current is provided by the capacitor C6. The way to control the transistor VT2 is also unusual. It opens the current flowing through the resistor R9 when the transistor VT1 is closed. At the moment of opening the transistor VT1, a reverse voltage is applied to the emitter junction of the transistor VT2, causing the effective absorption of minority carriers in the base, which leads to its closing without the appearance of a through current. The diode VD11 limits the reverse voltage at the emitter junction of the transistor VT2, without preventing the current through the primary winding of the transformer T1 and the open transistor VT1. Such a construction of this converter unit makes it possible to exclude the through current through the transistors and does not require the use of a transformer in the control circuit. Closing the transistor VT1 leads to the opening of the transistor VT2 also without the appearance of a through current. Transistors VT1, VT2 operate in light mode, which provides increased reliability of the power supply.

Transformer T1 is wound on a ring of size K12x8x2000 made of 500NM ferrite. The primary winding contains 2 turns of PEV-0,15-5 wire; secondary, designed for an output voltage of 50 V, - 2 turns of PEV-0,31-162 wire, with a tap from the middle. Zener diode KS156A can be replaced by KS604A. Transistors can be KT605B or KT630B, but then the KS620A zener diodes will have to be replaced with KS1A. In this case, the efficiency of the unit will deteriorate somewhat. Establishing a power supply comes down to a selection of resistors R9 and RXNUMX for a specific load power value.

Resistor R9 is selected from the saturation condition of the transistor VT2, which is determined using an oscilloscope. The resistance of the resistor R1 must be such that at a rated load and a supply voltage of 220 V, a current of at least 4 mA flows through the zener diodes VD5, VD5. To reduce the ripple of the output voltage, the capacitance of the capacitors C3 and C4 must be increased by 2 ... 3 times. In addition, the ripple level can be further reduced by connecting an oxide capacitor with a capacity of 7 ... 50 microfarads for a nominal voltage of 100 V in parallel with capacitor C10.

Author: Semyan A.P.

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