Transformerless voltage converters. Encyclopedia of radio electronics and electrical engineering

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

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With the help of transformerless converters, it is possible to obtain bipolar voltages and increase the voltage of the power source several times. Due to the fact that in transformerless converters the voltage increases due to the summation of the voltages on the capacitors, it is advisable to manufacture them for small load currents that do not exceed 0,5 A.

On fig. 64a shows a schematic diagram of a low-current, half-wave, transformerless voltage converter. For load current up to 10 mA, which allows you to get double or triple the voltage of the power source, as well as reverse polarity voltage. The converter operates from a DC source with a voltage of 3...12 V and has an efficiency of about 50%.

The device consists of a master oscillator assembled on a transistor VT1 and VT2 according to the multivibrator circuit, and two voltage doublers on diodes VD1-VD4 and capacitors C2 and C5.

When the transistor VT1 is open, the capacitor C1 is charged through the diode VD2 to the power supply voltage. After closing this transistor, the negative plate of the capacitor C2 is connected through the resistor R1 to the positive wire of the power source. At the same time, a positive voltage is formed on the positive plate of the capacitor C2 with respect to the positive electrode of the power source, which charges the capacitor C2 through the diode VD1. Thus, at the output + Uout, twice the voltage of the power source is obtained with respect to the common wire.

When transistor VT2 is closed, capacitor C4 is charged through resistor R3 and diode VD5 to the power supply voltage. When the transistor VT2 is opened, the positive lining of this capacitor is connected to the common wire of the device. On the negative plate of the capacitor C5, a negative voltage is formed in relation to the common wire of the converter. Capacitor C4 is charged from this voltage through diode VD6.

In this case, the -Uout2 output will have a negative voltage with respect to the common wire, the value of which corresponds to the voltage of the power source. Three times the power supply voltage will be applied between outputs + Uout1 - Uout2.

(click to enlarge)

Fig. 64

To obtain a full-wave conversion, which doubles the current load capacity, it is necessary to additionally connect a doubling node to the transistor VT1, similar to that connected to the transistor VT2 (C5, C6, VD3, VD4), and to the transistor VT2 - a doubling node connected to the transistor VT1 ( C2, C2, VD1, VD2), and connect the outputs of these minds accordingly. Filter capacitors C1 and C6 in this case will be common for two conversion half-cycles. On fig. 64b shows a diagram of a full-wave transformerless voltage conversion with transistor switches, designed for a load current of up to 0,5 A. Double or triple the voltage of the power source can be removed from the outputs of the converter, similarly to the first version of the device.

The master oscillator G is assembled according to the multivibrator circuit on transistors VT3 and VT4. Transistors VT1, VT2 and VT5, VT6 are used to amplify the current of the multivibrator transistors and operate in the key mode. In one half-cycle of the multivibrator, transistors VT1, VT3 VT6 are open. At this time, capacitors C2 in C5 are charged, and C1 and C6 are discharged. In the other half-cycle, these transistors close, and transistors VT2, VT4, VT5 open, capacitors C1 and C6 are charged, and C2 and C5 are discharged. Capacitors are charged through diodes VD2, VD4, VD5, VD7, discharged through VD1, VD3, VD6, VD8.

The converter can be assembled with a capacitor voltage multiplier according to the circuit shown in fig. 64, in. From the + Uout1 output, almost three times the power supply voltage is supplied at a load current of about 200 mA. With an increase in the voltage multiplication steps, the permissible load current of the converter decreases.

A transformerless converter can be assembled with a master oscillator on a microcircuit, as shown in fig. 65. Diode VD1 sets the duty cycle of the multivibrator on the elements DD1.1 and DD1.2, equal to 2. When the voltage is high, the transistors VT1.3, VT1.4 are open at the outputs of the elements DD2 and DD4 and the capacitor C2 is charged through the diode VD2. After switching the multivibrator to another state, in which a low-level voltage is set on the output elements, transistors VT1, VT3 open, and capacitor C3 is charged through the diode VD3 to the power supply voltage. The total voltage on the capacitors C2, C3 corresponds to twice the voltage of the power source.

Fig. 65

The efficiency of converters with transistor switches is about 50%. Unproductive losses in the converter occur mainly during the switching of transistors. To increase the efficiency of converters, high-frequency transistors and diodes should be used in them. Transistors should operate in a shallow saturation mode and have a static current transfer coefficient of at least 50. It is advisable to use germanium diodes with a low supply voltage, since they have a lower forward voltage drop compared to silicon ones.

When establishing converters, it is necessary to temporarily disable the positive feedback in the multivibrator by disconnecting one of the capacitors: in fig. 64b - C3 or C4; in fig. 65 - C1. Then, by selecting resistors in the base circuits of the transistors, set them to a mode in which the collector-emitter voltage does not exceed 0,5 V.

Author: Drobnitsa N.A.

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