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Old 30th August 2010, 06:49 PM   #1
mag is offline mag  Switzerland
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Default MOSFET body diodes in phase shift smps

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 50C 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

-marco
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Old 11th May 2011, 03:51 AM   #2
savu is offline savu  Romania
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Question related to you're post:

Have you used synchronous rectification or not? because i am designing in Lt Spice something similar using LTC1922-1.

Reards,
savu
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Old 11th May 2011, 09:21 AM   #3
hesener is offline hesener  Germany
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can you check the voltage spikes at the primary side MOSFETs?
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Old 18th May 2011, 12:05 AM   #4
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Quote:
Originally Posted by hesener View Post
can you check the voltage spikes at the primary side MOSFETs?
With phase shift there can't be any spikes , hey are clamped by the antipararel diode , the only problem is the primary curent ringing due the output rectifier capacitance resonating with the leak inductance....
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Old 18th May 2011, 12:55 AM   #5
TechGuy is offline TechGuy  United States
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You can add a parallel set of fast recovery diode to address the short comings of the body diode. Using a external fast diode also reduces the thermal stress on the mosfet.
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Old 7th February 2014, 08:14 AM   #6
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Quote:
Originally Posted by TechGuy View Post
You can add a parallel set of fast recovery diode to address the short comings of the body diode. Using a external fast diode also reduces the thermal stress on the mosfet.
I disagree because the forward voltage drop for internal body diode tends to be less than the external connected diode.
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