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Transistor Proportional Base Drive Circuit

IP.com Disclosure Number: IPCOM000042928D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 39K

IBM

Related People

Azzis, D: AUTHOR [+2]

Abstract

To improve speed dissipation in power switching transistors, it is highly desirable to keep the ratio IC/IB relatively constant over a wide IC operating range. The problem to be solved is therefore to assure that IB tracks IC independently of load variations. The standard solution implies the use of transformers or self-transformers in parallel with the base-emitter junction. It is a good, but rather expensive solution. The proposed solution is rather simple and cheap. It has been demonstrated to be both accurate and fast. Turning now to the circuit depicted in the figure: T1 is the power transistor, the base current of which is to be made proportional to the collector current. It is loaded by the windings of a half transformer.

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Transistor Proportional Base Drive Circuit

To improve speed dissipation in power switching transistors, it is highly desirable to keep the ratio IC/IB relatively constant over a wide IC operating range. The problem to be solved is therefore to assure that IB tracks IC independently of load variations. The standard solution implies the use of transformers or self-transformers in parallel with the base-emitter junction. It is a good, but rather expensive solution. The proposed solution is rather simple and cheap. It has been demonstrated to be both accurate and fast. Turning now to the circuit depicted in the figure: T1 is the power transistor, the base current of which is to be made proportional to the collector current. It is loaded by the windings of a half transformer. T2 and T3 are a current mirror circuit so that: IE (T3) Z IE (T1) x R1 Z 2 10-2 IE (T1) R4 D1 and T4 are another current mirror circuit in which: IC (T4) Z IC (T3) x R5 Z 5 IC (T3) R6 T5 is a switch driven by an ON pulse applied to the driver module (such as SN 75450, not represented). When T5 is turned ON, we consequently obtain: IE (T5) Z IB (T1) Z 5 x 2 10-2 IE (T1), and IB (T1) Z 0.1 IE (T1) This circuit was experimented in a push-pull converter, and chronograms which were obtained are shown in the figure. It can be seen that this circuit is perfectly operative for 5 ms pulses. Particularly, it may be understood that IB perfectly copies IC without any delay at the turn-off. Resistor R7 has be...