Method and apparatus for generating control signal and charging dc supply in a secondary synchronous rectifier

Abstract

A multi-function switch has a Junction-Field Effect Transistor (JFET) that outputs a voltage for both charging and timing/sense the synchronous rectifier controller. Current is provided to a synchronous rectifier controller from a JET having its drain conductively coupled to the drain electrode of the secondary side rectifying MOSFET, wherein the current from the source of the JET is used for both timing/sense and powering the synchronous rectifier controller. The JFET is biased for a fixed output with its source to gate voltage at a turn on threshold voltage of the JFET for charging. The JFET is fully conducting from the secondary side of transformer with a small voltage drop across the drain to source electrode of the secondary side rectifying MOSFET for timing/sense. An input voltage is applied to timing and charging inputs of the synchronous rectifier controller, at low voltage across the drain to source electrode of the secondary side rectifying MOSFET or an input voltage is applied to a charging input through the first OR-ing MOSFET at high voltage across the drain to source electrode of the secondary side rectifying MOSFET.