Voltage converter with SHI modulation. Encyclopedia of radio electronics and electrical engineering

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

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This pulse-width stabilized converter (Fig. 4.7) can be used in portable tape recorders and other similar battery-operated equipment. In particular, the converter is able to maintain the normal operation of the Vesna-202 tape recorder when the battery voltage drops to 3 V.

Such a converter is most suitable for battery-powered equipment. Stabilizer efficiency - not less than 70%.

Stabilization is maintained when the power supply voltage drops below the output stabilized voltage of the converter, which a traditional voltage regulator cannot provide. When the converter is turned on, the current through the resistor R1 opens the transistor VT1, the collector current of which, flowing through the winding II of the transformer T1, opens the powerful transistor VT2. Transistor VT2 enters saturation mode, and the current through the winding I of the transformer increases linearly. Energy is stored in the transformer. After some time, the transistor VT2 goes into active mode, self-induction EMF occurs in the transformer windings, the polarity of which is opposite to the voltage applied to them (the magnetic circuit of the transformer is not saturated). Transistor VT2 closes like an avalanche, and the self-induction EMF of the winding I through the diode VD2 charges the capacitor C3. Capacitor C2 contributes to a clearer closing of the transistor. Then the cycles are repeated.

After some time, the voltage across the capacitor C3 increases so much that the zener diode VD1 opens and the base current of the transistor VT1 decreases, while the base current also decreases, and hence the saturation current of the transistor VT2. Since the energy accumulated in the transformer is determined by the saturation current of the transistor VT2, a further increase in the voltage across the capacitor C3 stops. The capacitor is discharged through the load. Thus, the feedback maintains a constant voltage at the output of the converter. The output voltage sets the zener diode VD1. The change in the conversion frequency lies within 20...140 kHz.

The voltage converter, the circuit of which is shown in fig. 4.8 differs in that in it the load circuit is galvanically isolated from the control circuit. This allows you to get several stable secondary sources with any voltage. The use of an integrating link in the feedback circuit makes it possible to improve the stabilization of the secondary voltage.

The disadvantage of the converter is some dependence of the output voltage on the load current. The conversion frequency decreases almost linearly as the supply voltage decreases. This circumstance deepens the feedback in the converter and increases the stability of the secondary voltage. The voltage on the smoothing capacitors of the secondary sources depends on the energy of the pulses received from the transformer. The presence of resistor R2 makes the voltage across the storage capacitor C3 also dependent on the pulse repetition rate, and the degree of dependence (slope) is determined by the resistance of this resistor. Thus, the trimming resistor R2 can be used to set the desired dependence of the change in the voltage of the secondary sources on the change in the supply voltage. Field effect transistor VT2 - current stabilizer. The maximum power of the converter depends on its parameters.

Key Features:

• Converter efficiency, %.....70...90;
• Output voltage instability, %, no more than.....0,5;
• Maximum load power, W.....2.

When setting up the converter, resistors R1 and R2 are set to the minimum resistance position and load equivalents are connected. A supply voltage of 12 V is supplied to the input of the device and a voltage of 1 V is set at the load with resistor R15. Next, the supply voltage is reduced to 4 V and the previous voltage is achieved with resistor R2. By repeating this process several times, a stable output voltage is achieved.

Windings I and II and the magnetic circuit of the transformer are the same for both versions of the converter. It is wound on a B26 armored magnetic core made of 1500NM ferrite. Winding I contains 8 turns of PEL-0,8 wire, and winding 11-6 turns of PEL-0,33 wire (each of windings III and IV consists of 15 turns of PEL-0,33 wire).

Author: Semyan A.P.

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