SLUSBL5A February 2015 – June 2019 UCC28730
The transformer turns ratio selected affects the MOSFET VDS and secondary rectifier reverse voltage VREV, these should be reviewed.
The secondary rectifier reverse voltage stress is determined by the equation below. A snubber around the rectifier may be necessary to suppress any voltage spike, due to secondary leakage inductance, which adds to VREV.
For the MOSFET VDS peak stress, an estimated leakage inductance voltage spike, VLK, should to be included.
In the high-line, minimum-load condition, the UCC28730 requires a minimum on-time of the MOSFET (tON(min)) and minimum demagnetization time of the secondary rectifier (tDMAG(min)). The selection of fMAX, LP and RCS affects the actual minimum tON and tDMAG achieved. The following equations are used to determine if the minimum tON is greater than tCSLEB and minimum tDMAG target of >1.2 µs is achieved.