Advanced economical power supply, 220/2x25 volts 3,5 amps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The journal "Radio" published the design of network transformerless power supplies with voltage conversion at a frequency of several tens of kilohertz [1]. Another similar unit with higher power is described below. It is designed for joint (care with a powerful stereo AF amplifier.

The power of the power supply is about 180 watts. output voltage 2x25 V at a load current of 3,5 A. The range of ripples at a load current of 3,5 A does not exceed 10% for a conversion frequency of 100 Hz and 2% for a frequency of 27 kHz. The output impedance does not exceed 0,0 ohm. Block dimensions - 170x80x35 mm, weight - 450 g.

(click to enlarge)

After rectification by the diode bridge VD1, the mains voltage is filtered by capacitors C1-C4. Resistor R1 limits the charging current of the filter capacitors flowing through the rectifier diodes when the unit is turned on. The filtered voltage is fed to a voltage converter built according to the half-bridge inverter circuit on transistors VT1, VT2. The converter is loaded with the primary winding of the transformer T1, which converts the voltage and galvanically isolates the unit output from the AC mains. Capacitors C3 and C4 prevent RF interference from the power supply from entering the network. The half-bridge inverter converts DC voltage into a square wave AC voltage at a frequency of 27 kHz. Transformer T1 is designed so that its magnetic circuit is not saturated.

The self-oscillating mode of operation is provided by a feedback circuit, the voltage of which is taken from the winding III of the transformer T1 and fed to the winding I of the auxiliary transformer T2. Resistor R4 limits the voltage on winding I of transformer T2. The conversion frequency depends on the resistance of this resistor within certain limits (see note at the end of the page). Details on the operation of converters with a non-saturable transformer can be found in [2].

To ensure a reliable start of the converter and its stable operation, a start-up unit is used, which is a relaxation generator based on a VT3 transistor operating in an avalanche mode [3]. When the power is turned on through the resistor R5, the capacitor C5 starts charging and when the voltage across it reaches 50 ... 70 V, the VT3 transistor opens like an avalanche and the capacitor is discharged. The current pulse opens the transistor VT2 and starts the converter.

Transistors VT1 and VT2 are installed on heat sinks with an area of ​​50 cm2 each. Diodes VD2-VD5 are also equipped with plate heat sinks. The diodes are clamped between five duralumin plates 40x30 mm each (three middle plates 2 mm thick, two outer ones 3 mm thick). The whole package is tightened with two screws M3x30, passed through the holes in the plates. To prevent the plates from closing with screws, pieces of a PVC tube are put on them.

Winding characteristics of transformers are summarized in the table.

Winding wire - PEV-2. Winding I is placed evenly along the length of the ring. To facilitate the start of the converter, winding III of transformer T1 should be located in a place not occupied by winding II (see figure). Interwinding insulation in transformers is performed with varnished fabric tape. Between the windings I and II of the transformer T1, the insulation is three-layer, between the remaining windings of the transformers it is single-layer.

Capacitors C3, C4 in the block - K73P-3; C1, C2 - K50-12; C5 - K73-11; C8, C9 - KM-5; C6, C7 - K52-2. KT812A transistors can be replaced with KT812B, KT809A, KT704A-KT704V, KD213A diodes - with KD213B. A properly assembled power supply usually does not need to be adjusted, but in some cases it may be necessary to select a VT3 transistor. To check its performance, temporarily turn off the emitter output and connect it to the negative output of the network rectifier. On the oscilloscope screen, the voltage across the capacitor C5 is observed - a sawtooth signal with a swing of 20 ... 50 V with a frequency of several hertz. If there is no sawtooth voltage, the transistor must be replaced.

The use of this power supply does not eliminate the need to block the output power circuits of the amplifier with large capacitors. Connecting such capacitors further reduces the level of ripple.

When turning on the power supply, measure the conversion frequency (at the terminals of the winding II) - it can be much lower than 27 kHz (for example, 9 - 12 kHz). And although the device will work, the power transistors will fail from overheating. Frequency adjustment is carried out by resistor R4. Moreover, the denomination may differ from that indicated on the diagram by tens of ohms.

A properly configured power supply works fine, with a load of 50 - 70%, the power transistors remain cold.

References:

1. V. Tsibulsky. Economical power supply. - Radio, 1981. No. 10, p. 56.
2. Romash EM Sources of secondary power supply of radio electronic equipment. - M.: Radio and communication, 1981.
3. Biryukov S. Digital frequency meter power supply. - Radio, 1981, No. 12, p. 54,55.

Author: D. Baraboshkin

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