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High Speed Power Transistor Base Drive

IP.com Disclosure Number: IPCOM000051238D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Hatcher, SC: AUTHOR

Abstract

With this circuit a power transistor operating under high current conditions is switched quickly and efficiently without a dedicated transformer in the base drive circuit or a high current negative bias supply.

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High Speed Power Transistor Base Drive

With this circuit a power transistor operating under high current conditions is switched quickly and efficiently without a dedicated transformer in the base drive circuit or a high current negative bias supply.

In order to drive a high current in the collector of a switching transistor, a large positive base drive is required to keep power dissipation low by keeping collector-emitter saturation low. This, however, increases the stored base-emitter charge and, under fixed driving conditions, the storage time of the transistor. If this stored charge is not swept out quickly, the collector voltage rises slowly out of saturation with the collector current remaining high such that very high instantaneous power is dissipated. Thus, in order to turn off the transistor efficiently, a considerable negative base current must be applied.

In the figure, high speed switching transistor Q4 is turned on by dropping the signal input to the base of transistor Q1. This allows the collector voltage of Q1 to rise which turns on transistor Q2 and provides an AC pulse through capacitor C1 to turn on Q4. Saturation in Q4 is maintained by the current flow out of winding 1 of transformer T1, through diode CR2 and transistor Q3 to the base of Q4.

To turn Q4 off, the signal to the base of Q1 is switched to a high level. This brings the cathode of zener diode VR1 to near ground potential. The anode of VR1 falls to its zener voltage below that of the ca...