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Negative Bias Source for Switching Transistor(s)

IP.com Disclosure Number: IPCOM000085276D
Original Publication Date: 1976-Mar-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Knickmeyer, KH: AUTHOR

Abstract

When switching inductive loads with a transistor, such as in a switching power supply, it is highly desirable to provide both a snubber circuit and a negative bias to minimize transistor turnoff stresses and storage time, and to allow the transistor to withstand maximum blocking voltage.

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Negative Bias Source for Switching Transistor(s)

When switching inductive loads with a transistor, such as in a switching power supply, it is highly desirable to provide both a snubber circuit and a negative bias to minimize transistor turnoff stresses and storage time, and to allow the transistor to withstand maximum blocking voltage.

Using the transistor switching circuit of Fig. 1 as an example, the snubber is composed of capacitor C1, resistor R1, and diode D1. Capacitor C1 is charged through resistor R1 while transistor TX is on, and discharged through diode D1 during the turnoff operation, absorbing switching losses. Resistor R2 provides a dissipative negative bias source proportional to the load current through lead L.

The circuit of Fig. 2 utilizes snubber current in a transistor switching circuit to generate negative bias for the transistor as a byproduct of the snubber operation.

As shown in Fig. 2, capacitor C2, diode D2 and zener diode Z1 are utilized in place of resistor R2 to provide bias current derived from the snubber in an essentially nondissipative manner. Capacitor C2 is large compared to capacitor C1, for example, 2 ufd vs 2 nfd. When transistor TX is on, both C1 and C2 are charged through diode D2 and resistor R1. Since C2 is large in value compared to C1, most of the voltage appears across C1 at the end of the RC charging cycle. Zener diode Z1 may be added to precisely control the voltage to which C2 is charged, i.e., regulating the negativ...