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.
・Improvement of AC/DC converter efficiency is essential given the increasingly rigorous standards being adopted in various countries.
・Use of secondary-side synchronous rectification in flyback AC/DC converters is plagues with various problems such as avoiding through-current states.
・Controller ICs are being developed to enable secondary-side synchronous rectification.