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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
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Posts: 329
I have a nice 50x 330V photoflash capacitor bank for an induction coil launcher that I'm switching with a SCR/diode module(diode wired as a catch/anti-reverse charge diode) . It had worked fine even with single turn coils, so I figured the bank wasnt capable of providing the 1600A for 8.3mS (60hz half cycle) the SCR was rated for. Either this was wrong, or there was another failure mode for the SCR because after putting a short single turn coil of ~28AWG wire on it to watch it vaporize, the SCR seems to have failed short.
reading online, I suspect turn on di/dt was the cause of failure, but I would expect any sort of current related fault to cause it to fail open instead of short. Any wisdom on SCR failure modes to share? Do they tend to fail short also like mosfets like to go?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
What are you using to trigger the thyristor? In addition to too high of a di/dt, they can fail by not turning on completely if you don't give it a long enough trigger pulse. What's the part number of your thyristor?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
afaik ALL semiconductors initially fail short-circuit whether due to excessive voltage, current or heat, if there is enough current or energy available to the short-circuit then the internal connections and/or the semiconductor can blow open-circuit.
I think that excessive di/dt causes 'spot' heating of the junction before the entire junction has a chance to turn on.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Sulaiman wrote ...
I think that excessive di/dt causes 'spot' heating of the junction before the entire junction has a chance to turn on.
Thats exactly what happens, only worse. The localised heating drops the forward voltage at that point on the die, resulting in current focusing and destruction.
This can be mitigated to some extent by having a powerful gate drive, but your results may vary.
SCRs designed for pulsing have carefully designed gate structures intended to turn on the die as evenly as possible.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
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Thanks! I'm using a new 9V battery with a 30 ohm current limiting resistor on an IXYS MCD-72
Hopefully switch bounce isnt fouling stuff up (given it was fine all afternoon during testing with a proper coil, and didnt die till I had the small coil on it, but I know that doesnt rule it out) I guess in future revisions I'd have to look at switch debouncing and something like a FET/IGBT gate drive IC to drive the gate hard for a clean turn-on?
Also, seems reasonable enough on most semiconductors failing short until they turn into a fuse. Would the fact that the terminals the wire was attached to ended up with copper vapor condensed on them give any clue to the risetime of the pulse, or just that it put enough energy into it while the plasma channel was still conducting.
Registered Member #2939
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10 seconds reading that datasheet and you find the max di/dt @ 500A/us, but under conditions of .45A gate drive. 1: can you even get .5 A out of your 9V battery? 2: your resistor should be less than 18R to drive the gate hard enough, especially since Vg seems to be about 2.5 V. At a guess I think you should be around 10 ohms. I couldn't see a limit for gate current, but the turn-on delay graph is characterized to ~1.5A gate current.
You can limit di/dt with a suitable inductor.
When I was designing electric fence energisers we used a PUT to dump a 10uF cap straight into the gate of the main thyristor. You might like to try something along those lines.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
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2Spoons wrote ...
10 seconds reading that datasheet and you find the max di/dt @ 500A/us, but under conditions of .45A gate drive. 1: can you even get .5 A out of your 9V battery? 2: your resistor should be less than 18R to drive the gate hard enough, especially since Vg seems to be about 2.5 V. At a guess I think you should be around 10 ohms. I couldn't see a limit for gate current, but the turn-on delay graph is characterized to ~1.5A gate current.
You can limit di/dt with a suitable inductor.
When I was designing electric fence energisers we used a PUT to dump a 10uF cap straight into the gate of the main thyristor. You might like to try something along those lines.
My main issue has been reading the datasheet and knowing what certain values mean. units obviously give clues, and I didnt notice the conditions under the di/dt spec. I was working off of Vgt and Igt, but I assume those are threshold values then? I'll throw in a cap post resistor to dump into the gate for now, but the 30 ohm should keep the constant power dissipation in the gate under the 0.5W for Pgav (average gate power?) and ditch the pushbutton switch as the actual switch for the gate drive as soon as I can.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Has a list of all the terms and what they mean. Pgm is maximum gate power, quoted as 10W for 30us. So you could theoretically drive 4 amps into the gate for 30us, which would be good for getting the device turned on fully, fast. Thats around 300uJ, which would be a ~7uF cap charged to 9V. I done a little pic for you. this is how I would trigger that thyristor. Maybe add a couple of ohms gate resistor, no more.
Incidentally, I calculate a series inductor of just 700nH would be enough to limit di/dt to less than 500A/us, given a 330V source.
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SCR failure modes
