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Into the valley

Exploring quasi resonant – or valley – switching in flyback power supply design. By Carl Walding.

When developing off line power supplies, designers invariably have two major
objectives: creating a device which offers high reliability, as well as one which can be manufactured at low cost. Quasi resonant designs offer engineers a viable method by which to achieve these goals.
Quasi resonant technology is a technique that allows switching losses in a mosfet to be reduced, thereby improving reliability. Moreover, the softer switching which the technique brings improves the emi characteristics of the power supply, which could, in turn, allow designers to reduce the amount of filtering employed. This would then allow cost reduction.
This article discusses the theory behind quasi resonant architecture, its implementation and demonstrate the value of using this type of flyback power supply.

The basics
The term ‘quasi’ means somewhat or partial. In implementing a quasi resonant design, an existing L-C tank circuit is used strategically in a pulse width modulated power supply. The result is that the resonant effects of this L-C tank circuit can ‘soften’ the transitions of the switching device. The softer transitions will, in turn, reduce the switching losses and emi that is associated with hard switching converters. Because the resonant circuit is only used during the switching transition to an otherwise conventional square wave converter, this gives rise to the term ‘quasi resonant’.

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Vanessa Knivett

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