MOSFETs near their voltage limit

Electric Motors and Controllers
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1N4001
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MOSFETs near their voltage limit

Post by 1N4001 » Dec 19, 2017 9:27 am

Hey, a quick question.

When selecting FETs for a motor controller, would it be a good idea to keep some breathing room between VDS and battery voltage?

I am considering a 14s battery pack which, fully charged, comes very close to the 60V absolute maximum rating of most common MOSFETs. I could select 80V ones, but will have to deal with higher RDS(on) and QG.

Thanks for any advice.

Addy
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Re: MOSFETs near their voltage limit

Post by Addy » Dec 19, 2017 11:12 am

It's absolutely a good idea to keep some breathing room. 14s x 4.2V = 58.8V, very close to the limit.

When a MOSFET in your controller turns off and stops the current flow, the inductance of the battery wires will try to keep the current flowing by increasing the voltage. This means that your controller will see a voltage spike which is higher than the battery's voltage. If you went with 60V MOSFETs you would have almost no margin to tolerate this.

Punx0r
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Re: MOSFETs near their voltage limit

Post by Punx0r » Dec 19, 2017 4:44 pm

I have heard a rough guide to allow at least 20% for voltage transients. However, I cannot substantiate this and there's a continuous sliding scale between conservativeness in design and reliability offset by increased cost & lower power density. Hobbyists will sometimes run 0% margin while someone design a critical system might want more like 50%...

A lot of ebike controllers rated up to 48V nominal are 60V max and use 75V FETs.

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Gregory
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Re: MOSFETs near their voltage limit

Post by Gregory » Dec 19, 2017 5:58 pm

As an amateur I use 20% for MOSFETs and 50% for caps.


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1N4001
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Re: MOSFETs near their voltage limit

Post by 1N4001 » Dec 20, 2017 3:59 am

Thanks, that helps. I'll be going with 80V ones then. Cheers!

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