Low power switching power supply. Scheme, description

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Low power switching 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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A distinctive feature of the proposed low-power switching power supply is a small number of parts and their low cost. It can be used to power low-power electronic equipment.

Technical specifications

Input voltage, V......220
Output voltage, V ...... 5
Maximum load current, A.....1,2
Conversion frequency, kHz......50

A miniature network power source was taken as a basis (see the article by Tsypushtanov A. "Miniature network" in Radio No. 4, 1986, p. 48). The device is made according to the scheme of a half-bridge voltage converter (see figure). One arm of the bridge is formed by capacitors C4 and C5, the other by transistors VT1 and VT2. Transformer T1 is included in the diagonal of the bridge.

Low power switching power supply

The generator on the op-amp DA1 is assembled according to the multivibrator circuit. Its supply voltage is stabilized by zener diodes VD2 and VD3. The oscillator frequency depends on the capacitance of the capacitor C6 and the resistance of the resistor R8 and for the values indicated in the diagram is approximately equal to 50 kHz.

Capacitor C7 provides DC decoupling of the generator and transistor VT1. Resistor R10 limits the base current, and diode VD7 - the reverse voltage at the emitter junction of the transistor VT1.

One of the disadvantages of a half-bridge converter is the need for an additional transformer to drive the transistors. The proposed power supply managed to solve this problem in a different way.

The opening of the transistor VT2 occurs at the moment when the transistor VT1 closes. The positive feedback voltage from the winding III of the transformer T1 is supplied to the emitter junction of the transistor VT2, opening it. The base current of the transistor VT2 is limited by the resistor R9. The reverse switching of transistors occurs when the transistor VT1 is opened by a pulse from the master oscillator and the emitter junction of the transistor VT2 is shunted by an open diode VD8. At this point, a through current flows through the transistors, so to reduce it to a minimum, resistors R10 and R9 should be selected to prevent deep saturation of the transistors.

The output voltage from winding II of transformer T1 is rectified by diodes VD4 and VD5. The rectified voltage is supplied to the stabilizer DA2. The converter does not have voltage stabilization and short circuit protection nodes, therefore, these functions are performed by the output linear voltage regulator DA2.

The device uses capacitors K73-11 (C1, C2).

Transformer T1 is wound on a K20x10x5 ring magnetic circuit made of M1000NM ferrite. The primary winding contains 300 turns of PEV-2 0,2 wire, the secondary - 2x22 turns of PEV-2 0,5, winding III - 11 turns of PEV-2 0,2.

Establishing the device is reduced to the selection of resistors R9, R10 for a specific value of load power based on the condition of saturation of transistors VT1, VT2. To do this, the oscilloscope is connected through a voltage divider in parallel with the collector-emitter sections of the transistors. The selection should start with small resistance values, for example, R10 = 300 ohms and R9 = 100 ohms for maximum output power. After that, the resistance is increased until the transistor starts to exit saturation. The measured resistance is halved and a resistor of the nearest suitable value is installed. With the optimal selection of resistors, the transistors heat up very slightly and it is enough to install them on small heat sinks, each with an area of \u5b\u2bapproximately XNUMX cmXNUMX.

The output voltage can be set differently by changing the number of turns of the winding II and applying the DA2 chip to the desired voltage.

Author: S.Gorshenin, Kazan

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