Hi Spehro, On Tue, Jan 7, 2014 at 4:36 AM, Spehro Pefhany wrote: > IMHO, the best thing is to look for guaranteed Rds(on) at your minimum > drive voltage or lower, AND at the maximum junction temperature rating > (Rds(on) may increase by >50% at Tjmax). Calculate the temperature > rise from this > (I^2*R, theta junction-ambient) and verify it's okay at worst-case ambien= t. Given the part that Sergey recommended ( http://www.fairchildsemi.com/ds/FQ/FQP13N06L.pdf ) I believe I'm well within the limits based on a load of under 1A. I might be well under 1A though. At 5Vgs the datasheet is quoting an Rds(on) of 0.11R, but that's with an Id of 6.8A. > BTW, it's not a terrible idea to put a relatively high value pulldown > resistor from the gate to source, especially during development. Otherwis= e, > if the driver goes high-Z for some reason, the gate voltage could wander > around into 'undesirable' regions. Something like 100K will often suffice= .. I was considering a 10K, but I can up it easily. > Don't (unnecessarily) drive the gate too hard- it may be a good tradeoff = to > have more switching dissipation and less EMI (keep within the SOA =3D > Safe Operating Area) curve though). If your PWM is at 100Hz, you don't > need to drive it like a 100kHz PWM. Cheaper too, if you can avoid drivers= .. In this case the PWM would be to dim the indicator (an LED array of some kind, still trying to figure out what). This is just an indicator, so I don't think I need to go too nuts on the frequency, it won't appear on camera or anything like that. It just has to not be too flicker-y for a person looking at it. > Unlike BJTs, there is a real cost to having a much higher Vds rating than > you need, so don't go nuts on the voltage rating (or cut it too close!). The part referenced above has a max Vds of 60V. If I end up switching 5V, it'll be rated for 12 times what it's actually switching. Too far? Thank you! Josh --=20 A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .