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Predriver for a Power Transistor

IP.com Disclosure Number: IPCOM000075627D
Original Publication Date: 1971-Oct-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 42K

Publishing Venue

IBM

Related People

Scotto, VA: AUTHOR

Abstract

This circuit utilizing a unique feedback circuit to control the constant current output operates as follows: With A1 in an up level, T7 will be conducting providing a down level of 5 amps. The base current for T7 is provided by T5 and T6 whose output is determined by the voltage drop across R8. With the feedback arrangement of T3, T4, R3 and R4 the voltage drop across R8 is forced to equal the voltage drop across R3. The voltage drop across R3 is equal to R3 times the current provided by the current source comprising T1, T2, R1 and R2. The currents through T1 and T2 are made equal, since the Vbe's of T1 and T2 are matched within +10 mv. For equal currents the voltage across R2 equals -Vs2. Thus the current source provides a current, I, equal to -Vs2/R2.

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Predriver for a Power Transistor

This circuit utilizing a unique feedback circuit to control the constant current output operates as follows: With A1 in an up level, T7 will be conducting providing a down level of 5 amps. The base current for T7 is provided by T5 and T6 whose output is determined by the voltage drop across R8. With the feedback arrangement of T3, T4, R3 and R4 the voltage drop across R8 is forced to equal the voltage drop across R3. The voltage drop across R3 is equal to R3 times the current provided by the current source comprising T1, T2, R1 and R2. The currents through T1 and T2 are made equal, since the Vbe's of T1 and T2 are matched within +10 mv. For equal currents the voltage across R2 equals - Vs2. Thus the current source provides a current, I, equal to -Vs2/R2. Since Ib3<<1, Vr3 equals (-Vs2/R2)R3 and since T3 and T4 are matched within +/-10 mv for equal currents Vb3=Vb4. Therefore, V8 = Vb3 + Vr3 V8 = Vb3 + Vs2(R3/R2) and Vr8 = V8 - Vb4 = (Vb3 + R3/R2 Vs2) - Vb4 and Vb3 = Vb4 therefore Vr8 = (R3/R2) Vs2 + Vb3 - Vb3 or Vr8 = (R3/R2) Vs2 = Vr3. Since Vb3 always equals Vb4, any change in Vb4 will be translated to Vb3, and with Vr3 being constant this same change in Vb4 will be transferred to V8; thus maintaining a constant voltage across R8 which maintains a constant current into T7 independent of changes in Vs1. Changes in Vsl are only seen at the collectors of T3, T5 and T6, while the emitter of T6 is always maintained at Vb4 + Vr3. T7 i...