Synchronous rectifier control for switched mode power supplies and method therefor

Abstract

A power converter includes a transformer, a first primary side transistor coupled to the primary winding, a primary side controller that provides a gate signal to a gate of the first primary side transistor, a first synchronous rectifier (SR) transistor, and an SR controller. The first SR transistor has a drain coupled to a secondary winding of the transformer, a gate for receiving a first SR gate signal, and a source coupled to a first output terminal of the power converter. The SR controller is coupled to the gate and drain of the first SR transistor, for activating the first SR gate signal when a voltage on the drain falls below a turn-on threshold, and deactivating the first SR gate signal when a voltage on the drain rises above a variable turn-off threshold, wherein the variable turn-off threshold increases over an expected on-time of the first SR gate signal.