How to size a flyback diode?
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Posted by Michael on July 3, 2008, 9:57 am
Please log in for more thread options 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 | |||||||||||||||||||
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Posted by Spehro Pefhany on July 3, 2008, 11:00 am
Please log in for more thread options 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 -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com | |||||||||||||||||||
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Posted by John Larkin on July 3, 2008, 12:05 pm
Please log in for more thread options wrote:
>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? 1N4001, or any cheap power diode, unless mag field dropout time has to be fast. Or will the body diode of the >FET I plan on using (On Semiconductor NTJS4405NT1G,
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 | |||||||||||||||||||
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Posted by on July 3, 2008, 12:49 pm
Please log in for more thread options On 3 Jul., 18:05, John Larkin
> wrote:
> > >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? >
> 1N4001, or any cheap power diode, unless mag field dropout time has to > be fast. > > Or will the body diode of the > > >FET I plan on using (On Semiconductor NTJS4405NT1G,
>
> 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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>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.