Introduction

Introduction

In recent years there have been increasingly urgent demands for higher efficiency of AC-DC power supplies (reference article). As one approach to boosting the efficiency of AC-DC converters, synchronous rectification can be used for the method of diode rectification. In examples for DC-DC converters, it is a known fact that efficiency is raised dramatically through adoption of synchronous rectification designs. Similar improvements are possible in AC-DC converters as well; as indicated in the diagram below, by changing the secondary-side rectifying diode to a MOSFET, losses in this part of the circuit are greatly reduced, and it is easy to see that efficiency can be improved.

Well then, why do many current AC-DC converters continue to use diode rectification?

For example, many AC-DC converters extending up to medium power using PWM flyback designs, and continuous-mode operation is employed depending on the input/output conditions and transformer specifications. This is because simple use with a synchronous rectification design makes normal control impossible in continuous-mode operation; the primary-side switching element (MOSFET) and secondary-side rectification element (MOSFET) turn on simultaneously, and there is the possibility that a through-current may cause element failure. Hence it has been necessary to add a circuit preventing simultaneous turn-on, or to use a Quasi-resonant method that differs from continuous mode operation, or to accept constraints such as operation only in non-continuous modes. Thus there has been the problem that synchronous rectification cannot be introduced without complications.

However, improvement of efficiency in AC-DC converters is essential, and because diode and other component technologies have reached their limits, secondary-side synchronous rectifying controllers are being developed. Here we present an example of synchronous rectification design of an AC-DC converter that normally uses diode rectification, employing the BM1R001xxF series of secondary-side synchronous rectifying controller ICs.

Discussion of the following topics is planned.