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Transistor Filter for Switching Regulator

IP.com Disclosure Number: IPCOM000052711D
Original Publication Date: 1981-Jul-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 25K

Publishing Venue

IBM

Related People

Stadler, EE: AUTHOR

Abstract

A transistor filter is described which reduces considerably the ripple voltage at the output of a switching regulator despite the use of low value capacitors.

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Transistor Filter for Switching Regulator

A transistor filter is described which reduces considerably the ripple voltage at the output of a switching regulator despite the use of low value capacitors.

The figure is a circuit diagram of the transistor filter and its connection to the switching regulator represented by block 1.

The DC voltage drop delta U = U(1)out - U(2)out of about 1 V occurring at transistor T is sufficient for operating this transistor in the unsaturated mode. The latter mode is required, since in the saturated mode parts of the ripple voltage are directly transferred from the collector to the emitter. The collector/emitter voltage of transistor T is made up of the base/emitter voltage drop and the voltage drop generated at resistor R1, which is caused by the base current forced by the emitter current. The ripple voltage amplitude, the transistor filter is capable of handling, must not be set by resistor R1 but by the voltage drop at the base/emitter diode of transistor T.

If the amplitude of the ripple voltage exceeds the voltage drop at the base/emitter diode of transistor T, diodes D1 and D2 are series-connected to resistor R1. If the switching regulator is to supply higher DC currents, transistor T is designed as a Darlington circuit. A lower cut-off frequency of this transistor prevents high-frequency oscillations occurring during switching from reaching the output of the transistor filter. The control voltage necessary for the switching r...