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Voltage dimensioning MOSFET in Synchronous Buck ?

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Jadeit

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I use Ti tools for create Synchronous Buck Converter and this tool suggested 60V mosfet in converter where in max input voltage 48V.

My question is what is right VDS for Q1 and Q2 in Synchronoust Buck converter as this

M SB,.png
 

Akanimo

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Hi,
The tool left a safety margin of 25%. I would leave 33% safety margin.
It's a matter of choice. The higher the margin, the safer but most likely, the higher the cost. You can choose a safety margin you want buy I recommend not to go below 25%.
 

Jadeit

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Thanks for information. I have nominal voltage 48V, 55V as maximum with +15% and plan use 80V MOSFET.
 

KlausST

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Hi,

The better the layout, the less is stray inductance --> the less overshot. Thus you need less headroom.
The slower the voltage rise and fall rate --> the less overshot. Thus you need less headroom.

With a good layout 5V of headroom is enough, with a bad layout even 100% may not be sufficient.

I think the voltage rating of Q1 is more critical as Q2.

Klaus
 

Akanimo

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Hi,

The better the layout, the less is stray inductance --> the less overshot. Thus you need less headroom.
Klaus

It's not just about the layout. You can have a very good layout and hope it will be fine but with a high closed-loop Q-factor, you'll still get large overshoots and lots of ringing.

I think the voltage rating of Q1 is more critical as Q2.
Klaus
Partially. It depends more on the conversion factor. It's better to make the selection at worst-case operating point and to use "identical" switches too.
 

Akanimo

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Thanks for information. I have nominal voltage 48V, 55V as maximum with +15% and plan use 80V MOSFET.

It's okay. But components go bad in a circuit. You don't want the entire system dead as soon as one component goes bad. I think you need some more safety margin.
 

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