Improving the performance of the voltage converter. Scheme, description

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Improving the performance of the voltage converter. 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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Converter DC voltage 12 V to AC 220 V 50 Hz is described in [1]. It is more complicated than similar devices, however, it allows you to do without additional generators, single vibrators to obtain the required sequence of pulses and eliminates inconsistent operation of the driving stages when their operating parameters change. This guarantees the necessary pause between control pulses in different operating conditions.

According to this scheme, the control circuit of the converter 24 V / 220 V 50 Hz is assembled. Power transistors are the same as in the circuit [1]. The converter worked normally with the load, and when switching to idle mode, one of the transistors (KT827) of the key stage failed (the converter was tested from the power supply).

Improving the performance of the voltage converter

When analyzing the reasons for the failure of transistors, it was found that on the collectors of the output transistors with a loaded converter, there are short pulses with an amplitude of up to 450 V (oscillogram in Fig. 1, a).

This is what we managed to fix with an oscilloscope, the real value may be higher. When switching to the idle mode, the shape of the pulses was distorted and the previously short voltage surges became longer. In this case, the converter consumed a current (at idle) of 2,5-3 A. In this mode, the KT827A transistors failed. To observe the described processes, high-voltage transistors were specially used.

By shunting the high-voltage winding of the transformer with a 2,5 μF capacitor, it was not possible to achieve satisfactory operation of the converter. With the converter loaded, the high-voltage surges disappeared, but at idle, the converter still consumed 2,5-3 A current.

To facilitate the operation of power transistors, the output stage is made according to the scheme of Fig. 2. It shows a fragment of the converter circuit from [1] and newly introduced elements (the numbering of elements continues, started in [1]).

With closed power transistors, capacitors C3, C4 and C5, C6 are charged to the power supply voltage. The high-voltage pulse that appears when the transistor VT3 (VT4) is closed partially charges the capacitors C3, C4 (C5, C6) and partially closes to part of the T1 winding through VD1 and R10, R13 (VD2, R14, R15). At the end of charging, capacitors C3, C4 (C5, C6) are discharged through R10, R13 (R14, R15) to the power supply voltage. The shape of the pulses on the collectors of the transistors is shown in Fig. 1, b in the load mode, in Fig. 1, c - in the idle mode.

As you can see, in the worst mode, the voltage amplitude on the collector does not exceed 60 V. This allows you to use the KT827B transistor (100 V) instead of the KT827A transistor (80 V), and even KT12V (220 V) in the 50 V / 827 V 60 Hz converter. After such refinement, the current consumed by the converter in idle mode is about 1 A.

Any diodes VD1, VD2 with a maximum pulse current of more than 10 A. Capacitors C3-C6 for 100 uF x 63 V. Since a significant variable component acts on the capacitors, it is not necessary to replace C3, C4 and C5, C6 with one capacitor. The operating voltage of the capacitors should also not be less than 63 V. The recommended type is K50-29 and K50-31. The total capacitance of each group of capacitors (C3, C4 and C5, C6) is at least 47 microfarads. It is possible to use capacitors of types K73-16 and K77-1 of the required capacity. In this case, the series connection of capacitors is not required, and the resistors R11, R12 and R16, R17 can be replaced with a single resistance of 2-2,2 kOhm with a power of 2 W.

The described refinement can also be carried out in a 12 V / 220 V converter without changing the ratings of the circuit elements (Fig. 2).

Improving the performance of the voltage converter

References:

Shepotko A.D. Voltage converter // Radioamator.-1999.-№4.-S.53.

Author: A.N. Karakurchi

See other articles Section Voltage converters, rectifiers, inverters.

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