Synchronous Rectifying Circuit Section: Selection of Synchronous Rectifying MOSFET

Keeping in mind the “Example of Power Supply Specifications and Replacement Circuit” of the previous article, we begin actual circuit design. The secondary side of the original circuit is a diode rectifying circuit; as the procedure to modify this for synchronous rectification, 1) a MOSFET is selected to replace the rectifying diode; 2) various conditions are confirmed and settings are decided; and, 3) the optimum IC for use in the design is selected from the BM1R001xxF series. In this article, we first select the MOSFET that will replace the diode.

Synchronous Rectifying Circuit Section: Selection of Synchronous Rectifying MOSFET to Replace the Secondary-Side Rectifying Diode

The MOSFET M2 for synchronous rectification to replace the secondary-side rectifying diode DOUT is selected. In order to replace the diode, the currents, voltages, waveforms and the like of the existing circuit are determined, and a MOSFET with suitable specifications is selected. Initially, the reverse voltage V_R and forward current I_F for the rectifying diode DOUT are determined. Please refer to the diagram on the right.

Using the measured rectifying diode reverse voltage V_R and forward current I_F as guides, the maximum drain-source voltage V_DS and drain current I_D for the MOSFET to replace the diode are determined.

When selecting the MOSFET, losses due to Ron (the on-resistance) and the maximum allowable dissipation PD of the package must also be considered. If the Ron of the MOSFET M2 is too high, abnormal heating of the MOSFET becomes a concern. After considering these matters carefully during design, and confirming operation with the product incorporated, a heat sink or other means of heat dissipation is added as necessary.

From the above-shown observed waveforms, the MOSFET M2 may be selected as follows. Here adequate margins are provided for V_R and I_F relative to peak values.

As a matter to be noted, the absolute maximum rating of the DRAIN pin of the BM1R001xxF series of power supply ICs is 120 V (Ta=25°C). It is necessary to ensure that the voltage applied to the DRAIN pin of the IC does not exceed the absolute maximum rating. In future we intend to describe “troubleshooting in flyback applications” and measures to take “when the effect of a surge causes V_DS2 to reach or exceed the V_DS voltage of a secondary-side MOSFET” if a voltage exceeding the absolute maximum rating is applied to the DRAIN pin.