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Driver Bypass Circuit for Linear Control of Inductive Loads

IP.com Disclosure Number: IPCOM000039254D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Hedman, RL: AUTHOR [+2]

Abstract

This circuit makes use of the fact that the base and collector voltages of a transistor are approximately equal when the transistor is near saturation. A control signal can thus be generated to prevent the transistor and the amplifier that drives it from saturating. Fig. lb shows a typical driver circuit with the bypass circuitry applied, while Fig. 1a shows the typical driver circuit before the bypass circuitry is applied. In Fig. 1b, bypass circuitry to generate the control signal to prevent saturation consists of PNP transistor Q2 and diode D1. During normal operation when the driver transistor Q1 is operating well away from saturation, the PNP (Q2) in the bypass circuitry is shut off, and the circuit functions the same as the unprotected circuit in Fig. 1a.

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Driver Bypass Circuit for Linear Control of Inductive Loads

This circuit makes use of the fact that the base and collector voltages of a transistor are approximately equal when the transistor is near saturation. A control signal can thus be generated to prevent the transistor and the amplifier that drives it from saturating. Fig. lb shows a typical driver circuit with the bypass circuitry applied, while Fig. 1a shows the typical driver circuit before the bypass circuitry is applied. In Fig. 1b, bypass circuitry to generate the control signal to prevent saturation consists of PNP transistor Q2 and diode D1. During normal operation when the driver transistor Q1 is operating well away from saturation, the PNP (Q2) in the bypass circuitry is shut off, and the circuit functions the same as the unprotected circuit in Fig. 1a. However, as the driver transistor approaches saturation, its collector voltage will drop to approach the base voltage. As the collector and base voltages become equal, the bypass transistor (Q2) begins to bypass the base drive current away from the drive transistor and feed it into a summing junction at the input of the control amplifier M1. This saturation condition will exist when an input transition is sufficiently fast and the load current di/dt becomes large enough to cause the required load voltage to exceed the available supply by the relationship V = L * di/dt (where V is the required voltage across the inductive load L). When the base dri...