MOSFETs and PWM
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Posted by Randy Day on February 24, 2007, 1:05 pm
Please log in for more thread options I've built some TTL motor/relay controllers out of NPN transistors in a Darlington configuration, and I'm considering using mosfets instead. +V |_ - )| ^ )| relay |_)| | _R_ |/ TTL in -|___|--| |>
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GND (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de) My experience with mosfets is zero. Is an n-channel mosfet pretty much a drop-in replacement for an NPN device, aside from gate/base resistor values? If so, how do I calculate an appropriate value for R on a mosfet? I'm looking for full-off/full-on switching, not voltage regulation. The Mouser website lists the FDN5630 and FDC655BN, which seem to have the current capacity (and low price) I want. Are there any other things I need to consider when designing for mosfets, aside from ESD? TIA | ||||
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Posted by John Larkin on February 24, 2007, 1:49 pm
Please log in for more thread options wrote: >I've built some TTL motor/relay controllers
>out of NPN transistors in a Darlington >configuration, and I'm considering using >mosfets instead. > +V > |_ > - )| > ^ )| relay > |_)| > | > _R_ |/ > TTL in -|___|--| > |> > | > GND >(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de) > >My experience with mosfets is zero. Is an >n-channel mosfet pretty much a drop-in >replacement for an NPN device, aside from >gate/base resistor values? If so, how do I >calculate an appropriate value for R on a >mosfet? I'm looking for full-off/full-on >switching, not voltage regulation. You don't need a gate resistor at all. Just run your logic level to the fet's gate. There are some second-order reasons to use a gate resistor (kill rf oscillations during the slew, slow down the edges, protect the driver if the fet dies) so 1K, close to the fet, wouldn't hurt. >
>The Mouser website lists the FDN5630 and >FDC655BN, which seem to have the current >capacity (and low price) I want. Are there >any other things I need to consider when >designing for mosfets, aside from ESD? Make sure your logic high is enough to turn the fet on hard, not merely "gate threshold" voltage; that's on the datasheets. Be aware that an open-gate fet could be on or off, so make sure your driver circuit keeps the gate low when it should. Disconnected cables can get interesting. Most fets like this are fairly esd hard, so don't go overboard on handling. I take no esd precautions when working in my lab, and have never blown a mosfet. My manufacturing people do all the strap/mat stuff, and they don't blow fets either! John | ||||
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Posted by Randy Day on February 24, 2007, 5:28 pm
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John Larkin wrote: [snip] Wow! Lots of really excellent advice from everyone! > You don't need a gate resistor at all. Just run your logic level to
> the fet's gate. There are some second-order reasons to use a gate > resistor (kill rf oscillations during the slew, slow down the edges, > protect the driver if the fet dies) so 1K, close to the fet, wouldn't > hurt. I think I'll put the 1k in for driver protection. For peace of mind, if nothing else. ;) > Make sure your logic high is enough to turn the fet on hard, not
> merely "gate threshold" voltage; that's on the datasheets. The FDC655 lists 33 mOhms Rds(on) at 4.5v, which seems more than adequate. I think I'll go with it. >
> Be aware that an open-gate fet could be on or off, so make sure your > driver circuit keeps the gate low when it should. Disconnected cables > can get interesting. | ||||
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Posted by maxfoo on February 25, 2007, 3:23 pm
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>John Larkin wrote:
> >[snip] >The FDC655 lists 33 mOhms Rds(on) at 4.5v, which
>seems more than adequate. I think I'll go with it. > I find it hard to believe that this little super sot-6 package can handle 6.5Amps, Hell I've blown mosfet in soic-8 packages rated a 4amps. too bad digikey doesn't sell it, I like to try to see where it explodes. ;^D http://www.fairchildsemi.com/ds/FD/FDC655BN.pdf | ||||
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Posted by Randy Day on February 25, 2007, 7:34 pm
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maxfoo wrote: >
> >>John Larkin wrote:
>> >>[snip] >
> >>The FDC655 lists 33 mOhms Rds(on) at 4.5v, which
>>seems more than adequate. I think I'll go with it. >> >
> > I find it hard to believe that this little super sot-6 package can handle > 6.5Amps, Hell I've blown mosfet in soic-8 packages rated a 4amps. too bad > digikey doesn't sell it, I like to try to see where it explodes. ;^D > http://www.fairchildsemi.com/ds/FD/FDC655BN.pdf That was my impression too. We'll see if it lives up to its billing. ;) | ||||
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