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Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
when you guys build your bridges do you think that you must excied the safe operating area of the mosfets to get large spark? In order to make a DRSSTC that is reliable does it have to be in the SOA. I thought tha te flip flop is used to keep the circuit synced to the zero crossing and any over current shut downs should also be synced to the zero. please correct me if I am rong about this N.B.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You don't necessarily have to exceed any ratings to get big sparks. The only reason to exceed ratings is so you can get big sparks without spending much money on silicon.
On my DRSSTC, I decided to oversize the devices somewhat so I could get nice sparks without exceeding the safe operating area at any time, even if the driver screwed up completely and hard switched everything. So I used a full bridge of miniblock IGBTs (that others have got 60-80" from IIRC) to drive a small 14" tall coil with a fairly weedy 0.05uF tank capacitor. It turned out that it would flash over and set fire to its own secondary before any of the device ratings was exceeded.
Later I added more toploads and increased the tank cap to 0.1uF, and turned the primary current limiter up to 475A (somewhat out of the safe operating area) which gave me 60" sparks.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I debated this a lot with Richie a couple of years back. As far as I know, the only way that overvoltage causes failure is by overheating due to the power dissipation in avalanche mode. The only difference between avalanche rated and non-avalanche rated devices, is that in non-avalanche rated devices, the heating can occur very unevenly over the die, and current hogging can make little patches of the die go into thermal runaway and burn out. So you can kill a big device with relatively little energy. In avalanche rated devices they try to keep those bad effects under control.
Well, i sure hope your right about that. Right now i cant find a decent solution to my problem. I have some half-dead bricks that i was thinking of testing the avalanche ability on. Its kinda funny to think about trying to kill an IGBT intentionally, but its better then losing 4 of them all at once .
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Just remember, that even if you kill one IGBT within a half-bridge or full-bridge, in all likelihood the other IGBTs have becomed damaged in some way as well, although you may not see it right away.
The reason you see so many of these large IGBT bricks on the surplus market, which appear to be "good" is because they do replace all the devices on a single device failure event, and the extras they sell as surplus, because they do measure "okay" with a multimeter etc...
I verified with this one two occasions with two of the more popular surplus vendors that frequent Ebay.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey, don't knock the walking wounded! They have given hobbyists like us a lot of fun for not much money. The only sad thing is that if a cheap surplus IGBT brick fails on your Tesla coil, you'll never know if it died because you were abusing it, or because of a bad experience it had in a past life.
As regards testing the avalanche ability: I was once curious to see how much "headroom" there was on the 1200V spec of the CM600HA-24H IGBT brick. I used voltage from a MOT, current limited by a 22k power resistor, to overvolt one on purpose with a low current of a few tens of mA. The breakdown voltage turned out to be about 1350V, and it didn't seem to suffer any ill effects. I used that brick in my OLTC2 coil and it worked fine until "Retirement".
At the other end of the spectrum, I once was trying to repair a PFC front end (basically a big boost converter with an IRFP460.) When I put it back together, I forgot to put back the diode in the boost converter bit, so it fired up at its maximum duty cycle, with all the power drawn from the line going into avalanching the MOSFET. So it was basically acting as a 600V, 200 watt zener. It actually stood that for a second or two, getting incredibly hot, before failing short.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
does anybody have any comments about the internal diode in mosfets. According to Richie Burnett the diode is a leftover from the manufacturing prosses and that it is the cause if some failures. As a matter of practice does anybody try to bypass this with FRED doides or are they ignored for the moast.
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DRSSTC: Importance of soft-switching on interrupter turn-off?
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