How to size a flyback diode?

Question

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,

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) be enough?

Thanks!

-Michael

Spehro Pefhany · Accepted Answer

Then the flyback current won't exceed half an amp, and you can use a1A (preferably fast) diode The body diode is not in the correct place, so it won't work. You wantto put the diode across the load. Best regards, Spehro Pefhany-- "it's the network..."                          "The Journey is the reward"             Info for manufacturers:  software/hardware/analog  Info for designers:

John Larkin · Answer

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

langwadt · Answer

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

Jim Thompson · Answer

Monolithic CMOS processes have no diode suitable for the flybackfunction because parasitic effects will kill the chip.So I turn the control device back "on" (linear analog) as long as theoutput voltage is a few volts above VDD.                                        ...Jim Thompson-- | James E.Thompson, P.E.                           |    mens     || Analog Innovations, Inc.                         |     et      || Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    || Phoenix, Arizona  85048    Skype: Contacts Only  |             || Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  || E-mail Icon at  |    1962     ||                                                                ||        Vote Barack... Help Make America an Obama-nation        ||                                                                ||  Due to excessive spam, googlegroups, UAR & AIOE are blocked!  |

How to size a flyback diode?

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