Taming the FET destroyer-
I have been running some tests over the last few months, wind and time permitting, trying to get a handle on what is going on with my controller.
First, easy and most logical, calm down the wild AC to DC with more capacitance across the bridge rectifier output. Added 6 ea 1000 uF of 450v electrolytic capacitors. Very weird side note here, see below.
put a 0.1 uF across the DC buss.
Put the RF filter from a computer power supply across the drain to source on the FETs.
Built the Fast diode, 0.47 uF 15 ohm filter suggested by Joestue & FLUX from drain to source.
put 1/2 of CSD20060 600V 20A Silicon Carbide Schottky Diode across the load.
modified FET driver board so it would be easy to select different gate drive resistors
10, 100, 470 ohm.
Took some 9N90 FETs 9 amp 900v with 470 ohm gate drive, thought I would test with one at a time at low wind /amps, it would be a good opportunity to look for spikes. As the genny came up to speed I was ready with scope to see the PWM start.
Volts built to 80v the PWM turned on with 1% and the FET shorted within 1 second didn't get a chance to see anything.
Blew a couple more instantly on successive tries.
I had a couple of IRFK3DC50 600v 24a half bridge hex packs I had been reluctant to try ( destroy) but decided to try one while waiting for some more 11N90Cs. Had all the filters on, 470 ohm gate drive. Another low wind event, volts rising, pwm started 1 -8%, controller was loading the genny, things were looking good. Could not see any spikes on scope across FET, signal was quite rounded on turn on and off, maybe 10% of pulse height and on time. ( scope will hold but doesn't
have storage or down load and I could not take a decent picture with the glare on the screen) After about 5 minutes of 0.5 to 1 amp flowing the FET was getting real hot. I didn't notice it in the specs but on the drawing it showed internal gate resistors so I removed mine and the FET pack ran cool. Could see no spikes, had sharper turn on and off, the RC filters stopped the 400 hz buzzing I used to hear.
I received some more 11N90s so decided to see how they would work with the new filters and higher gate resistors. Would try it with one FET, after all they are rated at 11 amps and it takes a very good wind to get that. With the IRFK3DC50 in the water heater side and a 11N90 on the space heater, 470 ohm gate resistor, same filters on each. Had the genny running with 1.5 to 2 amps going through the IRFK3DC50 and switched it over to the 11N90. It ran about 5 seconds then shorted out, did not get a chance to see anything on the scope.
Connected two 11N90s in parallel, individual 470 ohm gate resistors, ran for a few seconds and one FET shorted. Got to thinking, in parallel they may not turn on at the same time and one is taking all the surge, remembered OperaHouse tied the gates to common bus with one gate drive resistor.
Connected two more 11N90s in parallel, gates connected, one 470 ohm gate drive. Ran it for a few hours this way, could see no spikes, on and off edges were quite rounded. With 1 to 2 amps it started to get hot so reduced gate resistor to 100 ohms, the on off edges were squarer and the FETs ran cold. I let it run for several hours over the next few days less than 4 amps. One day the wind picked up with the opportunity to test at higher loads. Scope signals showed no spikes,a little jittery at the on and off corners. The amps picked up to 5 to 6 amps and one FET shorted after a few minutes.
Getting discouraged and tired of all the running back and forth testing I just took the other IRFK3DC50 and connected it to the space heater circuit. Have been running this way for two weeks, mostly less than 1000 watts but there have been a couple of high wind events pushing close to 10 amps at 220 volts, genny furling nicely at 20 mph.
Still not sure what was/is causing the problems, I can't help thinking it is the surge current, one of the paralled FETs turning on a fraction before the rest and taking all the surge. The 96 amp pulse current of the IRFK3DC50 verses 46 amps for the 11N90.
We hung a lot of parts on things without definite proof that they had a beneficial effect but in the end the system is working a lot better and the time and money has been spent. Question is, do I leave well enough alone or let my curiosity lead me to further testing, maybe later.
that's my story and I'm sticking to it! ( unless my wife tells me different )
Thanks again for all the help!
tale of capacitors
At first I left the existing 200 uF of the AC motor run capacitors in parallel with the new electrolytic s. The digital scope was hooked across the FET, volts and time settings set for the anticipated signals, and my good the digital volt meter on the DC buss. There was a mild wind, genny putting out about 150 volts. As the PWM started to turn on the FET I was watching the scope to see if I could catch any spikes and he scope's screen filled up a solid black, the volt meter was beeping and the reading jumping all over. I quickly unhooked them and thought Oh crap have I destroyed my instruments. I tested them on a battery and they seemed to function properly. Each time I attempted to reconnect the volt
meter to the wind DC buss it would beep and freak out, actually I didn't even need to connect it just get the lead close to the buss. Tried a different DVM, same freaky readings. The scope would react the same just getting the probe close the the FET. Went outside and the genny was making some weird humming noise. There was some super high frequency being generated somehow. As it turned out I took the motor run capacitors off and everything returned to normal. Didn't dare put them back on to see if the problem returned, thought better leave well enough alone.