MOSFET body diodes in phase shift smps


2008-01-23 9:22 pm
Hi all,
I have designed a 24v 100A smps based on the phase shift topology using ucc3895 contrller.
I have seen on some app notes that the mosfet of the H-brigde needs to have a fast body diode
to avoid secondary breakdown at very light load.
In my design I am using STW25NM50N mosfet in which the body diode is not too fast...
The efficiency of my smps is good (>93%) and the mosfets stays at about 50°C at full load.
I have also tried to keep the smps on at no load for the whole day to see if there are some problems at no load but nothing happened.

The question is now: how can I judge if the STW25NM50N body diodes are fast enough for my application? I can easily measure the current flowing in the transformer primary winding trough a current probe but it is not as easy for me tho check the current flowing trough the mosfet drains.

I have checked for other mosfets with fast diode but the majority are much bigger than STW25NM50N and much more expensive. I don't think I need a reduction of the rds_on since my efficiency is good and the thermal behavior is ok.

I have found a nice mosfet from Fairchild FQA28N50F which has an rds_on and Qg comparable with STW25NM50N but with a much faster body diode. Is it worth to switch tothe Fairchild part?

Does anyone of you know a mosfet suitable for my application?

Other data:
fsw=125kHz (250kHz oscillator)
transformer ratio: 21:4
input voltage: 385V from pfc
output stage: current doubler with sync. Rectifier
primary current: around 11Arms.

Thank you very much