Fairchild Design Guidelines for RCD Snubber in Flyback Converters

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Application Note AN-4147 Design Guidelines for RCD Snubber of Flyback Converters

Abstract is derived from a buck-boost converter by replacing filter inductors with coupled inductors, such as gapped core trans-

This article presents some design guidelines for the RCD

formers. When the main switch turns on, the energy is stored

snubber of flyback converters. When the MOSFET turns off,

in the transformer as a flux form and is transferred to output

a high-voltage spike occurs on the drain pin because of a res-

during the main switch off-time. Since the transformer needs

onance between the leakage inductor (Llk) of the main trans-

to store energy during the main switch on-time, the core

former and the output capacitor (COSS) of the MOSFET. The

should be gapped. Since flyback converters need very few

excessive voltage on the drain pin may lead to an avalanche

components, it is a very popular topology for low- and

breakdown and eventually damage the MOSFET. Therefore,

medium-power applications such as battery chargers, adapt-

it is necessary to add an additional circuit to clamp the volt-

ers, and DVD players.

Figure 1 shows a flyback converter operating in continuous conduction mode (CCM) and discontinuous conduction mode (DCM) with several parasitic components, such as pri- Introduction mary and secondary leakage inductors, an output capacitor of MOSFET, and a junction capacitor of a secondary diode.

One of the most simple topologies is a flyback converter. It

id id

i D diode reverse recovery current

Vds Vin+n Vo

resonance between

Vin Llk1 and Coss

Lm im Vo

(b) CCM operation

Llk1 id

id id

Vds i D Coss id

(a) Configuration with parasitic components

resonance between Lm and Coss

Vds Vin+n Vo

resonance between Llk1 and Coss

(c) DCM operation

Figure 1. Flyback Converter; (a) Configuration with Parasitic Components, (b) CCM Operation, (c) DCM Operation

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