Economical switching power supply, 220/2x27 volts 600 milliamps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Reducing the weight and dimensions and increasing the efficiency of power sources is one of the urgent tasks in the design of modern electronic equipment. The easiest way to solve this problem is to replace the traditional rectifier (with a bulky mains transformer and capacitive filter) with a high-frequency converter, followed by rectification of the high-frequency voltage.

A schematic diagram of such a power supply is shown in fig. 5.7. At the output of the block, a bipolar voltage of 2x27 V is obtained at a load current of up to 0,6 A. The amplitude of the output voltage ripple at the maximum load current does not exceed 30 mV.

(click to enlarge)

The mains voltage rectifier is assembled on diodes V1 ... V4. The rectified voltage converter is made on transistors V6, V7 and transformers T1 and T2, and the increased frequency voltage rectifier is made on diodes V8 ... V11. The operating frequency of the voltage converter is 22 kHz. Capacitors C1 and C2 are necessary to protect the power supply from interference that occurs during the operation of the converter. Resistors R1 and R2, together with capacitors C3, C4, are the primary filter and at the same time a voltage divider for the converter. The chain V5, R3, C5, R5 serves to facilitate the start of the converter generator. Filter rectified high-frequency voltage are capacitors C6, C7.

The use of two transformers in the voltage converter made it possible to increase its efficiency. In conventional converters with one transformer, the latter operates in saturation mode. In a converter with two transformers, the output transformer operates in linear mode at much lower inductances than in a single-transformer converter. This makes it possible to reduce losses in the core and, consequently, to increase the efficiency of the converter.

Saturable transformer T2 is designed only for the power consumed by the base circuits of transistors V6 and V7 and therefore has a small size. The disadvantages of converters with a single transformer include the fact that at the moment of switching transistors there is a significant surge of the collector current. In a converter with two transformers, this surge is practically absent, which significantly reduces the so-called dynamic losses and increases the overall efficiency of the converter.

The presence of a connection between the transformers through the windings III leads to the fact that at the right time the transformer T2 enters the saturation mode. This is necessary in order to fulfill the operating conditions of the converter, which were mentioned above. Transformer T2 is a switching element included in the base circuits of transistors V6 and V7. When the transformer T2 is saturated, its magnetizing current increases rapidly, as a result of which the voltage drop across the resistor R4 increases and the voltage across the winding III decreases, and consequently, on the windings I and II, which leads to a decrease in the base current and the exit of an open transistor into the active region and switching transistors. The switching frequency is determined by the magnetization reversal time of the core of the saturable transformer T2.

Inductors L1 and L2 provide a delay in opening one transistor until the other is completely closed. This is necessary to eliminate through currents and reduce losses when switching transistors. The data of transformers and chokes are placed in table. 5.1.

A properly assembled power supply does not require adjustment.

Author: Semyan A.P.

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