CONTROLLING POWER UP AND POWER DOWN OF THE SYNCHRONOUSMOSFETS IN A HALF-BRIDGE CONVERTER

摘要

The half-bridge converter has several major advantages over other topologies, but in the past italso had the disadvantage of providing no way for the designer to control the secondarysynchronous MOSFETs during dwell time or to predict when they need to turn off. This paperoutlines the problems associated with driving the synchronous MOSFETs using generic halfbridgecontrollers and demonstrates how they were overcome by the introduction of a primaryside half-bridge controller that incorporated the driving the logic for the synchronous MOSFETs.However, even with this controller and the correct logic signal outputs, there remained arequirement for application circuits to handle start-up and shut-down, due to the need for thesecondary side synchronous MOSFETs to be on when the primary MOSFETs are off. If notmanaged correctly the MOSFETs being inherently on can cause undesirable conditions duringstart-up and shutdown. This paper describes a new integrated circuit that has been developed tocontrol the synchronous MOSFETs during start-up and shutdown, even under pre-biasedconditions.This paper will demonstrate how the half-bridge converter can be implemented with two Ics(one primary and one secondary) to provide an alternative to existing topologies in small,efficient, dc-to-dc converters. It shows that the problems of the past, which may have preventeddesigners from using this topology, can be overcome.