SLUSCM5A August 2017 – February 2018 UCC24612
UCC24612 can be paired with an appropriate MOSFET to replace the diode rectifier on existing designs and demonstrate significant conduction loss reduction. The SR MOSFET selection should consider the tradeoff between cost and performance. Lower on-state resistance gives lower conduction loss, while it reduces the efficiency at light load. Due to the unique implementation of proportional gate drive, the benefit of lower on-state resistance is diminished. It is recommended to select the MOSFET on-state resistance so that the proportional gate drive operates for less than 50% of the full load SR conduction time.
According to UCC28740 datasheet, for 3-A output DCM Flyback design, the secondary side peak current should be about 14 A. To allow the proportional gate drive operating less than 50% of the SR conduction time, SR MOSFET Rdson should be more than 7 mΩ , according to .
The MOSFET breakdown voltage should be higher than the maximum voltage the SR MOSFET sees under maximum input voltage. For this design, the transformer turns ratio is 3.5, the voltage stress on the SR can be calculated as in Equation 2.
In this EVM, a 150-V, 19-mΩ MOSFET is used to get a balance between the cost and performance.