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2019.08.07 AC/DC


Design of a Secondary-Side Synchronous Rectifying Circuit to Improve the Efficiency of an AC/DC Converter


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.

In one simple comparative example, if the current IF in a diode is 3 A and the voltage across it VF is 1 V, the loss is 3 W. By replacing the diode with a MOSFET having an on-resistance of 10 mΩ, the I2R law means the loss is 0.09 W, or 1/30 the loss when using the diode.

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.

  • Design procedure
  • Design conditions
  • Controller IC selection
  • Component selection
  • Adjustment of circuit operation
  • PCB layout
  • Characteristic evaluation

Key Points:

・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.

Design Example for PWM Flyback Converter

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