Fairchild 11N90s

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Fairchild 11N90s

Postby frank carcia » Wed Mar 28, 2007 8:48 pm

Well this week I was asked to make a pulse generator for power quality testing of aircraft equipment. We need to place a 600 volt transient spike on a 28 VDC power bus.
The old circuit had a crappy old mercury relay that crapped out.
I suggested we go solid state. I used this as a chance to really beat on some 11n90s. I figured 3 in parallel should do the job of discharging an 8 uf cap charged to 400 volts through an air core transformer and LRC pulse shaping network.
Our tech built up the circuit and mounted it in a nice box ready for testing.
We fire it up and start making pulses. We get up to 200 volts and we measure a 100 amp peak current through the three FETs. the saturation voltage was about 40 volts meaning 4 KW dissipation in 3 FETs. It only lasted 5 us and we were pulsing at 1 HZ.
I figured we might not want to go higher but WTH. We ran it up another 100 volts. Fets hung in there but I was getting chicken.
Today we built a second board with 3 more FETs and put both boards in parallel.
Whit a fresh pair of underwear we started our way up. by the time we made the 600 volt level we were pulling 200 amps peak through both boards with a saturation voltage of about 30 volts. 6 KW peak dissipation.
Everything looked fine so we let in run for an hour with no problems.
We neded faster then 2us rise time but we were sitting just over 1 us. 0 to 600 volts. Each FET sitting there dissipating 1 KW peak and stone cold with a tiny little heat sink.
Went and got the boss and gave hima little demo. He wanted a slight pulse shape adjustment but was happy.
Then he told me about the next generator he wants. We will be jumping up to 10 hz at 600 volts peak but lower power but it has to be fast and pretty for customer inspectors.
It is fun to play in the sand box and get paid. It kind of makes you wonder how guys get these parts to fail. Our layout was only fair. I actually wanted a long drain lead off each fet to guarantee they would share current and it was 6 inches long. 1 HZ you can cheat a lot.
the drain signal was a nice clean fall 400 to 30 volts. Gate drive was fair with a 12 volt signal. The one thing I did do was put a 10 ohm resistor in series with each gate. There was no ringing on the gate. my drive source was a 50 ohm wavetek generator with a 30 ohm series resistor on each board. nothing special.
This peak power dissipation level is about two times higher than the last time I played with pulse drivers.
I think the 11N90 will be a hard FET to beat for the price. fc
frank carcia
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Location: Enfield Ct. / Niantic Ct.

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