The switching regulator is increasing in popularity because it offers the advantages of higher power conversion efficiency and increased design flexibility multiple output voltages of different polarities can be generated from a single input voltage. The most commonly used switching converter is the Buck, which is used to down-convert a DC voltage to a lower DC voltage of the same polarity. This is essential in systems that use distributed power rails (like 24 V to 48 V), which must be locally converted to 15 V, 12 V or 5 V with very little power loss.
This circuit is the variable output switching regulator for general purpose applications. An LM105 positive regulator is used as the amplifier-reference for the switching regulator. Positive feedback to induce switching is obtained from the LM105 at pin 1 through an LM103 diode. The positive feedback is applied to the internal amplifier at pin 5 and is independent of supply voltage. This forces the LM105 to drive the pass devices either “on” or “off,” rather than linearly controlling their conduction. Negative feedback, delayed by L1 and the output capacitor, C2, causes the regulator to switch with the duty cycle automatically adjusting to provide a constant output. Four LM195’s are used in parallel to obtain a 6 amp output since each device can only supply about 2 amps.
Note that no ballasting resistors are needed for current sharing. When Q1 turns “on” all bases are pulled up to V+ and no base current flows in the LM195 transistors since the input PNP’s are reverse biased.
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