Crist=F3v=E3o Dalla Costa wrote: > Hello > > My mosfets are misteriously short circuiting. I had the circuit working up > to yesterday using Microchip=B4s TC4427 mosfet gate driver then when it > overheated (due to me failing to account for increased switching losses > under pwm and insufficient heat sink) it took my last TC4427 chip with it. > However manual switching was OK many repeated times so I know up to then = the > circuit worked. > > Now while I wait for more TC4427s to arrive in a couple of days I wanted = to > continue working so I copied the discrete driver circuit from here: > http://users.tkk.fi/~jwagner/tesla/tv-driver/flyback-mosfets-large.gif > > However using that circuit caused the mosfet to immediately short-circuit > when power is applied (with 0 gate voltage). Using a 1u cap between gate = and > source cured this first problem and when I turn on the transistor it works > ok (gate voltage is mantained) until the point I turn it off when I can h= ear > a small bang and the mosfet short-circuits. > > The load is inductive, using 15 A 120VDC, I have the necessary anti-paral= lel > diode and also experimented with 100n and 50u polyester capacitors in > parallel to attempt to control the kick back voltage, to no avail. The > transistor is a IRPF460 20A, 500V. > > = But I can assure you, kickback IS the problem. You will need a very FAST = spike-killing device; this will be a varistor or TVS. I recommend the PK1E6 series, with the voltage rated = at the voltage that the MOSFET will handle without breaking down. People here will argue with me about = this, but to protect a MOSFET a diode will NOT work as well as a TVS. To placate people, I put a diode = across the line as WELL as a TVS. With that problem solved, then you can examine other things. MOSFETS are = very sensitive to pulse noise, and inductive pulses can be as high as 500-600V. The reason is that most diodes have a lengthy switching time. TVS and = varistors will switch in a few picoseconds. The voltage across a diode will continue to rise until it decides to = switch; TVS devices switch so fast the pulse has no chance to rise. --Bob A --Good luck. > However the fact it switched on and off repeatedly with Microchips mosfet > gate driver leads me to believe the problem is not in kick back voltage b= ut > rather in the gate control circuit I built. Would anyone have any idea why > the switch off failure occurs? > > Thanks. > = -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist