Compact transformerless power supply, 220/9 volts 0,2 amperes. Scheme, description

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Compact transformerless power supply, 220/9 volts 0,2 amperes. 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 construction of secondary power sources using rectified mains voltage converters (without a mains transformer) attracts the attention of developers not only due to their compact design. In some cases, such a block turns out to be the most rational in terms of ensuring the electromagnetic compatibility of nodes in portable equipment.

The power supply described below is of high performance, which makes it suitable for use in high-quality equipment.

(click to enlarge)

Main technical characteristics:

Mains voltage, V.......220±15%
Current consumed from the network, nA.......25
Conversion frequency, kHz.......20
Output stabilized voltage at a load current of up to 200 mA and a total ripple coefficient (peak value) of 0,02%, V ....... 2x9
Stabilization factor......2000

The power supply unit consists of a converter, an isolating transformer and a compensation bipolar voltage stabilizer. The mains voltage rectified by the diode bridge VD2 is subjected to preliminary stabilization before being supplied to the converter. The stabilizer is assembled on transistors VT1, VT2. The exemplary source VD3R2, included in the emitter circuit of the transistor VT2 of the comparison element and the error signal amplifier, is powered from the winding III of the transformer T1 through a rectifier on the diode VD5.

The converter is built on a half-bridge circuit on a transformer T1 (windings I, II and IV), capacitors C3, C4 and Transistors VT3, VT4. RC circuits R9C6, R10C8 - timing. The emitter junction of transistors VT3 and VT4 is protected by a diode (VD4, VD6). The converter start-up unit is a relaxator on a VT5 transistor, the avalanche-like operation mode of which and the parameters of the triggering pulse are set by the elements R6-R8, C5.

After the unit is connected to the network, capacitor C5 is charged through resistors R6, R7, a zener diode VD1 and an open transistor VT1. When the voltage at the collector of the transistor VT5 reaches 70 ... 90 V, it opens like an avalanche and the capacitor C5 is discharged through the transistor VT5, the emitter junction of the transistor VT4 and the resistor R7. As a result, the transistor VT4 opens and the converter starts to work. Capacitor C5 is periodically discharged through transistor VT5, which is opened by pulses from winding IV of transformer T1.

After starting the converter, the parametric stabilizer on the zener diode VD3 comes into operation and the transistor VT1, together with the opened transistor VT2, goes into voltage stabilization mode. When the mains voltage changes, for example, when it increases, the voltage on the winding III increases, which leads to an increase in the current through the transistor VT2 and a decrease in the collector current of the transistor VT1 and, consequently, to a decrease in the supply voltage of the converter (voltage across the capacitor C1). When the load current decreases, the auto-regulation process occurs similarly.

When the mains voltage changes by ±15% and the load current from 0 to 200 mA, the voltage change at the input of the secondary stabilizers (at the output of the VD7 bridge) does not exceed ±1 V at a nominal voltage of 12,5 V (set by resistor R2). In this case, the voltage drop across the control element of the network stabilizer (between the positive terminals of capacitors C1 and C2) varies from 15 to 80 V. The introduction of a network stabilizer makes it possible to ensure an almost nominal operating mode of the op-amp in the output stabilizers and get by with a small heat sink for the control transistor VT7 (about 4 cm2; while the heat sink area for the transistor VT1 is approximately 8 cm2).

The output compensation regulator contains a regulating transistor VT7, an emitter follower on a transistor VT6, a DC amplifier on an op amp DA1, DA2 and a parametric stabilizer on a zener diode VD8. Each op-amp is covered by negative feedback (through resistors R11, R12 and R14, R16), which increases the speed of the control element. The use of a high-speed direct current amplifier in the control loop makes it possible to improve the transient response of the stabilizer and eliminate the high-capacity capacitor at its output. For the block under consideration, such a technical solution using a relatively small capacitor C11 is equivalent to turning on a 1000 microfarad capacitor at the output of the stabilizer.

It should be noted that one op-amp can be dispensed with with a slight deterioration in the characteristics of the stabilizer, however, the capacitance of the capacitor C11 must be increased (work was tested with a capacitor with a capacity of 68 microfarads). With this simplification, the output of the op-amp DA2 is connected directly to the base of the transistor VT6, and all elements related to the op-amp DA1 are excluded.

The device uses fixed resistors MLT; R2, R18 - SP3-27; capacitors C1, C2 - K50-7 for 300/345 V; C3, C4 - K73-17 for 250 V; C5, C6, C8, C12, C13 - KM-5a; C7, C9, C11 - KM-6; C10 - K53-1. Transistors KT704A can be replaced by KT940, KT605 with any letter index, KT312V - by KT315B with a decrease in the resistance of the resistor R8 to 10 kOhm, KT646A - by KT602, KT503, GT404 with any letter index.

Transformer T1 is wound on a ring of size K28x16x9 made of ferrite 3000NN. Windings I and IV contain 12 turns of PELSHO 0,15 wire, winding II - 240 turns of wire PEV-2 0,25, winding III - 15 turns of PELSHO 0,15 wire, windings V and VI - 34 turns of wire PEV-2 0,35.

Elements related to the converter itself must be shielded.

A properly mounted source usually starts working immediately. In the event that the converter does not start, it is necessary to check the correct connection of the windings and the start-up unit, the operation of which can be checked by monitoring on the oscilloscope screen (with a closed input) the signal shape on the collector of the VT5 transistor - the signal should look like a sequence of sawtooth pulses with a frequency of several hundreds of hertz.

The establishment of a network stabilizer consists (at the rated mains voltage and load current) in setting the voltage drop between the positive terminals of the capacitors C2, C1 by resistor R2 to 40 ... 45 V, the voltage at the collector of the transistor VT5 should be about 12,5 V. voltage 2x9 V is set by resistors R18.

Author: V.Karlashchuk, S.Karlashchuk

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