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Dual Voltage Clamp Circuit

IP.com Disclosure Number: IPCOM000035357D
Original Publication Date: 1989-Jul-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 31K

Publishing Venue

IBM

Related People

Covi, KR: AUTHOR

Abstract

This article describes a clamp circuit for use in high-power dual- switch converters that provides effective clamping of the voltage across the switches at turn-off while avoiding the formation of a bulk tank circuit which often arises with the addition of a traditional "bulk decoupling" capacitor. The associated large circulating currents are therefore eliminated, allowing the selection of a bulk capacitor with significantly lower ripple current ratings than would otherwise be required.

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Dual Voltage Clamp Circuit

This article describes a clamp circuit for use in high-power dual- switch converters that provides effective clamping of the voltage across the switches at turn-off while avoiding the formation of a bulk tank circuit which often arises with the addition of a traditional "bulk decoupling" capacitor. The associated large circulating currents are therefore eliminated, allowing the selection of a bulk capacitor with significantly lower ripple current ratings than would otherwise be required.

The figure shows a diagram of a diagonal half bridge (dual switch) which is conventional except for the particular clamp circuit 10. As in other converters, a function of the clamp is to provide a path for primary current (ip) to continue to flow after turn-off of the switches S1 and S2, allowing decay of primary current (due to leakage and magnetizing), resetting of core flux in T1, and limiting the peak voltages on the switches to the bulk voltage (Vb).

As switching speeds and output power have increased, the high di/dt imposed on the stray inductances (Lb1 and Lb2) associated with the bulk capacitor C1 and wiring from C1 to the switches and clamp circuits becomes significant.

A simple decoupling capacitor across points 12, 14 would create the problem of forming a tank circuit with C1, Lb1 and Lb2, resulting in large circulating currents. However, in the circuit shown, the decoupling capacitor is replaced by C3 and C4 and a diode network which breaks up the tank into complementary one-way paths.

R1 and R2 are present only in the charge C3 and C4 to the bulk voltage prior to the start of switching. Thereafter, they serve no purpose, and there...