On Thu, 25 Jun 1998 11:35:06 +1000 James Cameron writes: >G'day, > >Amateur wants to learn. > >My MOSFETs are getting hot and I'm wondering why. Is my PWM frequency >too high? Am I simply pushing them too hard? Ron Fial's answer was excellent, but I'll go over the same points in a different way. They could be becoming hot either from DC resistance (too much current or incomplete turn-on) or AC effects (slow turn on or turn off, or oscillation). The DC is easy to check. First look at the ratings, specifically the on-resistance and the load current. The DC power dissipation is load current squared times on-resistance times duty cycle. A TO-220 device with no heat sink will become noticeably warm at about 0.5W of power dissipation and generally too hot around 1 Watt. If the load current is too much for the on resistance of the device, either you need a larger device (lower on resistance) or a heat sink. To meet the 0.5W requirement with a 100% duty cyle of 1.75A, the on-resistance needs to be less than 0.16 ohms. If the theory looks good, try it in practice. Set the PWM for 100% and measure the drain-source voltage. If it is more than expected (load current times on resistance), you likely have incomplete turn-on due to not enough gate-source voltage. Make sure the FET is a "logic level" type specified for 5V drive. Another possibility is that the FET and driver are oscillating rather than staying on. A small resistor in series with the gate helps prevent that. Finally, consider that an incandescent bulb draws more than its rated current when it is at less than full brightness. Of course the duty cycle is lower then too so the worst case is probably still with full brightness. The remaining possibilty is slow turn-on or turn-off, or maybe incomplete turn-off (which is unlikely in this case). An oscilloscope is really essential to examine this phenomena. Of course using a slower PWM frequency will make the transistion times less important. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]