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Low Power Start Circuit

IP.com Disclosure Number: IPCOM000040903D
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Driscoll, CD: AUTHOR [+3]

Abstract

This article describes a startup circuit for a switching power supply. The circuit utilizes a chain of series-connected, active circuit elements to deliver power from a capacitor to the power supply control circuit. The circuit supplies the energy required for startup without a large steady state power dissipation.

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Low Power Start Circuit

This article describes a startup circuit for a switching power supply. The circuit utilizes a chain of series-connected, active circuit elements to deliver power from a capacitor to the power supply control circuit. The circuit supplies the energy required for startup without a large steady state power dissipation.

The startup circuit is enclosed with broken lines in the accompanying figure. An incoming AC voltage is converted first to a high voltage DC and then to a desired low voltage DC output level. Also, the transformer labeled T1 and its rectifier CR1 supply operating voltage during steady state operation. During startup there is no voltage on the transformer winding, so the startup energy must come from capacitor C1. When the high voltage DC (derived directly from the AC line) is applied, capacitor C1 begins to charge through resistor R1. During this time Q1 and Q2 are off so that there is no voltage at the power supply control circuit and the power supply does not attempt to start. The voltage in C1 increases until zener diode CR2 begins to conduct. When this happens, gate current is supplied to SCR Q1 which turns on. The transistor Q2 receives base current through resistor R4 and Q1, and it also turns on. This connects C1 to the power supply control circuit, and the power supply begins its startup sequence.

The voltage in C1 falls until voltage is established on the control winding of T1. As soon as the voltage on C1 begins to dro...