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How to size a flyback diode?

M

Michael

Jan 1, 1970
0
Hey there - I'm putting together a board that will have a number of
FETs switching the low side of some electromagnets. I do not know what
these electromagnets will be - but I've been told to design to be able
to switch half an amp across each.

So, though I don't know anything about the electromagnets - I think
it's pretty safe to say that they'll be at least somewhat inductive.
So how should I size the flyback diode? Or will the body diode of the
FET I plan on using (On Semiconductor NTJS4405NT1G,
http://www.onsemi.com/pub_link/Collateral/NTJS4405N-D.PDF ) be enough?

Thanks!

-Michael
 
S

Spehro Pefhany

Jan 1, 1970
0
Hey there - I'm putting together a board that will have a number of
FETs switching the low side of some electromagnets. I do not know what
these electromagnets will be - but I've been told to design to be able
to switch half an amp across each.

Then the flyback current won't exceed half an amp, and you can use a
1A (preferably fast) diode
So, though I don't know anything about the electromagnets - I think
it's pretty safe to say that they'll be at least somewhat inductive.
So how should I size the flyback diode? Or will the body diode of the
FET I plan on using (On Semiconductor NTJS4405NT1G,
http://www.onsemi.com/pub_link/Collateral/NTJS4405N-D.PDF ) be enough?

Thanks!

-Michael

The body diode is not in the correct place, so it won't work. You want
to put the diode across the load.
Best regards,
Spehro Pefhany
 
1N4001, or any cheap power diode, unless mag field dropout time has to
be fast.

Or will the body diode of the


Nope, wrong direction.

Some fets will let you skip the clamp and just absorb the flyback in
avalanche mode; some will die, instantly, first cycle. Lots of
darlingtons are avalanche rated.

John

lots of FETs that are avalanche rated, with active clamping, current
limiting, overtemp shutdown etc.
in a few amps range in sot223, many with logic level input

-Lasse
 
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