Stabilized network voltage converter, 220/20 volts. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Stabilized network voltage converter, 220/20 volts. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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When developing the device described below, the task was to create a small-sized network power supply with high efficiency, capable of delivering a power of 1 ... 3,5 W to a load that is galvanically unconnected to the network. These requirements are fully met by a single-cycle pulse stabilized voltage converter, which transfers energy to the secondary circuit in the pauses between current pulses in the primary winding of an isolation transformer. One of the options for such a device is offered to the attention of readers (Fig. 4.3).

(click to enlarge)

Main technical characteristics:

Output voltage, V, ±2.....20;
Total output power, W ..... 3,5;
Conversion frequency, kHz.....20;
Limits of mains voltage change, at which the output voltage changes by no more than 1%, V.....210...250.

The device includes a mains voltage rectifier (VD1) with a smoothing filter (R4, C3, C4), a master oscillator (DD1.1 ... DD1.3) with a start circuit (R17, C7), a rectangular pulse shaper (DD1.4 ...DD1.6, VT2, VT4), electronic key (VT3), pulse transformer (T1), adjustable current source (VT5), load short circuit protection device (R10, VT1), three rectifiers (VD2... VD4) and the same number of filter capacitors (C9 ... C11). Capacitors C1, C2 prevent interference from the conversion frequency from entering the network.

With the inclusion of the device in the network, capacitors C3, C4 and C7 begin to charge. After the voltage on the last of them reaches approximately 3 V, the master oscillator (DDL1 ... DD1.3) is self-excited. The repetition frequency of its pulses (depends on the time constant of the circuit R7, C5) is about 20 kHz, the shape resembles a sawtooth. The shaper (DD1.4...DD1.6, VT2, VT4) converts them into square waves. Since the pulse sequences at the bases of transistors VT2 and VT4 are out of phase, they open strictly alternately, which ensures the minimum opening and closing time of the transistor VT3. When this transistor is open, a linearly increasing current flows through the I winding and the transformer T1 accumulates energy, and when it is closed (there is no current through the primary winding), the energy accumulated by the transformer is converted into the current of the secondary windings III ... V.

After several cycles of operation of the generator, a voltage of 7 ... 8 V is set on the capacitor C10. The output voltage of the converter stabilizes an adjustable current source made on transistors of the VT5 assembly (VT5.2 is used as a zener diode). When the voltage fluctuates in the network or on the load, the voltage on the winding II changes and the adjustable current source, acting on the shaper, changes the duty cycle of the rectangular pulses based on the transistor VT3.

When the pulse current through the resistor R10 increases above a certain threshold value, the transistor VT1 opens and discharges the capacitor C6 (which serves to prevent false operation of the protective device from short current surges that occur when the converter is turned on, as well as during switching of the transistor VT3). As a result, the pulses of the master oscillator stop coming to the base of the transistor VT3 and the converter stops working. When the overload is eliminated, the device starts again in 0,8 ... 2 s after the capacitors C6 and C7 are charged.

The windings of the T1 pulse transformer are wound on a polystyrene frame with PEV-2-0,12 wire and placed in a B30 armored magnetic circuit made of 2000NM ferrite. Windings 1.1 and 1.2 contain 220 turns each, windings II, III, IV and V - respectively, 19, 18, 9 and 33 turns. Winding 1.2 is wound first, then windings II, IV, III, V, and finally winding 1.1. Between the windings II, IV, V and 1.1, electrostatic screens are placed in the form of a single layer (approximately 65 turns) of PEV-2-0,12 wire. When assembling the transformer between the ends of the central part of the ferrite cups, a gasket made of varnished fabric with a thickness of 0,1 mm is inserted. The transformer can also be made on the basis of a ferrite (of the same brand) B22 armored magnetic circuit. In this case, the PEV-2-0,09 wire is used, and the number of turns of windings 1.1 and 1.2 is increased to 230. The KT859A transistor can be replaced with KT826A, KT838A, KT846A.

Setting up the device is not difficult. Having set the engine of the tuning resistor R15 to the upper (according to the scheme) position, turn on the converter to the network and set the required output voltage values with this resistor. To reduce interference in secondary circuits with a conversion frequency (20 kHz), it is necessary to experimentally select the connection point of the electrostatic screens with one of the wires of the primary circuit, as well as the connection points of the capacitor C8. To do this, it is enough to connect one of the conclusions of any secondary winding through an alternating current milliammeter to the primary circuit and determine the named points according to the minimum readings of the device.

The converter, assembled according to the described scheme, was tested to power a load that consumes 10 W of power. In this version, the number of turns of windings 1.1 and L2 was reduced to 120 (with magnetic circuit B30), capacitors C3, C4 were replaced by one oxide capacitance of 10 μF (nominal voltage 450 V), the resistance of the resistor R10 was reduced to 2,7 Ohm, and the resistor R18 - up to 330 Ohm.

Author: Semyan A.P.

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