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Old 8th September 2010, 01:14 AM   #11
TechGuy is offline TechGuy  United States
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Quote:
Originally Posted by kall View Post
is this really a good approx?, i found this very simple...

-------------

i have a case with 100kHz 20V 210A (i guess MOSFETs are the only option here?)

any rule of thumb what "size" to choose to have a good "safety margin", go up to 40V, 60V??
Which such a low input voltage and high frequency MOSFETs are the practical way to go. Safety margin will depend on the topology. for instance for Push-Pull the switching transistors need to have at least twice the device voltage rating. It also depends on how much inductance leakage your transformer (or inductor) will create and how aggressive your snubbers are. It might make sense to use higher voltage rated MOSFETs during your initial testing until you are able to measure inductance leakage spikes in a working circuit.

I think your first big problem will be laying our your circuit that is capable of sustaining 210 Amps and not having layout problems picking up excessive switching noise. 210A is going to require a lot of copper. FWIW: I would recommend doubling or tripling the input voltage to reduce the input current demand. If you haven't noticed, Copper is pretty expensive these days. Its going to cost you a small fortune in copper to pass 210 amps!
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Old 8th September 2010, 09:17 AM   #12
kall is offline kall  Sweden
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Quote:
Originally Posted by TechGuy View Post
Which such a low input voltage and high frequency MOSFETs are the practical way to go. Safety margin will depend on the topology. for instance for Push-Pull the switching transistors need to have at least twice the device voltage rating. It also depends on how much inductance leakage your transformer (or inductor) will create and how aggressive your snubbers are. It might make sense to use higher voltage rated MOSFETs during your initial testing until you are able to measure inductance leakage spikes in a working circuit.

I think your first big problem will be laying our your circuit that is capable of sustaining 210 Amps and not having layout problems picking up excessive switching noise. 210A is going to require a lot of copper. FWIW: I would recommend doubling or tripling the input voltage to reduce the input current demand. If you haven't noticed, Copper is pretty expensive these days. Its going to cost you a small fortune in copper to pass 210 amps!
thx for answering...

im using a fullbridge topology with input 400V, i have 20V on the low-voltage side... i was thinking of using sync. rect.(MOSFETs) instead of diodes for rectifying to reduce my losses..
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Old 13th September 2010, 06:01 PM   #13
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The approximation is rather bad, especially for synchronous rectification.
Note that with synch rect. the switching is nearly soft, i.e. mosfets work at near zero voltage.
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