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 VR and forward current IF for the rectifying diode DOUT are determined. Please refer to the diagram on the right.
Using the measured rectifying diode reverse voltage VR and forward current IF as guides, the maximum drain-source voltage VDS and drain current ID 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 VR and IF relative to peak values.
＜MOSFET M2 Selection Example＞
VDS＝60V (from VR_PEAK＝40V)
ID=50A (from IF_PEAK＝26A)
Other parameters: Ron＝4mΩ、PD＝120W
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 VDS2 to reach or exceed the VDS voltage of a secondary-side MOSFET" if a voltage exceeding the absolute maximum rating is applied to the DRAIN pin.
・In this design example, a diode-rectification AC/DC converter is modified for synchronous rectification.
・At the beginning of the design process of modification for synchronous rectification, a MOSFET to replace the output-rectifying diode is selected.
・To determine the specifications of the replacement component, the currents, voltages, waveforms, and the like in the existing circuit are determined.