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Choose Vds max for buck-boost converter

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noisepic

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I'm designing buck boost converter with Vin max spec is 50V. Which minimum VDS max i should choose? 1.5x or 2x or 3x or just 60V is enought. Please advise what is the best value.
 

Vds seen by the FET is vin+vout+diode drop. So you choose the fet which can take that taking the worst case for all and make sure to include any Vin surge conditions. After that add some margin.
 

Hi,

It depends on the circuit and it depends on the PCB layout.
The circuit shows the "ideal" values that you might expect.
But the real PCB layout with it's stray inductances is a somehow unknown parameter.
For a very good PCB layout I'd say a 20% headroom might be sufficient.
For mediocre PCB layout you may need 100% headroom.
For a breadboard or any other "just wired all together" you maybe 500% of headroom isnot sufficient. -- therefore we call a breadboard "not suitable" for switching power circuits.

Klaus
 

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