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Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
In commercial applications I've seen TO-247 parts clamped directly (with thermal compound) to copper heat spreading plates (which are much larger than the transistor case), with the plate itself then clamped to the heatsink with a very large silpad for insulation. This is done to try and defeat the problem Steve mentioned - large thermal resistance due to the electrical insulator. There is added thermal resistance there due to the extra junction, but at least the worst one with the silpad is much lower resistance due to the large area.
Edit: of course the most effective option is to have isolated heatsinks so the transistors can be mounted directly on them. This has a whole different set of issues however!
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Steve Ward wrote ...
Bigger chips clearly can dissipate more heat. Your IRG4PC50UD has a Rjc of 0.64 *C/W, and the STGW80V60DF has Rjc = 0.32. The ST part likely has about 2X the silicon area compared to the IR part, and yet they have nearly the same peak collector current spec. The difference will be how quickly the IR part heats up during a "burst" vs the ST part, having twice the thermal capacity makes a big difference, hence it survives. If you shop around you can find TO-247 devices with Rjc < 0.2 or even .1 (though they dont have mounting holes by that point, you need to clamp them).
Thats interesting information and I'll take a look at those specs in some other parts and see how they compare. I did notice that the ST part has a lot larger continuous current capacity so this suggests that its a better performer too. The data also states its can handle 120A continuous at 25*C although I'm not sure that the junction could ever be kept that low in practice!
My operating frequency is 85KHz with my secondary just above this at around 90KHz so not too high. My coil is deliberately bigger so I can use the same system with bricks once I understand things a little better. I'll also be using phase lead on those once I put together an updated driver.
Let see if I can keep these ST parts alive for a while. I may push them a little harder when I get closer to building the updated bridge and see what they can do without being too bothered about the loss!
Hi, I think another thing you should check is gate drive waveform and particularly amplitude: this should be close to the maximum rating of the IGBT (or even a bit higher, if you want to take some risk ...) to prevent desaturation, especially when the IGBT is operated at very high peak currents.
For comparison, I'm also using a full bridge of TO-247 IGBTs, IXGR60N60C2D1, mounted on a PCB, never blown any (I cross fingers ...); they have slightly better current characteristics then your IR type (Icm=300A) and isolated case which makes mounting them on a common heatsink quite straightforward (even so they have a lower junction to case thermal resistance, 0.5C/W, then your IR type). I use Steve's old driver (no phase lead), my OCD is set to 450A (i.e. 50% higher than Icm rating!, limiting most of the time), bus cap is 3200uF, voltage is rectified 230V, tank cap about 90nF, resonant frequency about 130kHz, max spark length (up to now ...) 120cm. My interrupter generates over 300us pulses, although OCD limits the burst length. Gate drive amplitude is around 30V, which is the transient max rating of my IGBTs.
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
My gate waveform looks a healthy voltage just over the top spec of the IGBT. I am concerned that the rise / fall times are a bit slow but have noticed that the primary feedback has a habit of dropping out and the interrupter signal restarting the switching when the bus is at low voltage with and running short pulse widths. My scope is only a PC based one and really doesn't like it when the power goes up so getting measurements at higher power is impossible.
I think I'll spend some time trying to record all waveforms across the system to check all is OK.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
If at all possible try and use a faster scope to record your waveforms.
Your gate drive screenshot only seems to be sampled at 1MHz (despite saying 16MHz), this is not nearly fast enough to accurately capture ringing, risetime etc on fast signals such as gate drive and the (voltage) output of the bridge (may be quick enough for tank current however).
Edit: if you persist in using that scope, try and wrap the USB and probe leads through ferrites to block some of the common mode noise. USB is extremely susceptible to common mode noise (despite mostly using differential signalling), and I had some similar problems with my scope before I had it converted to ethernet.
Also, you mentioned that the rise/fall times were slow - I wouldn't draw any conclusions about that until you get better measurements, as you're not sampling fast enough to actually tell what they are.
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Well I discovered something today that seems to have been holding my coil back a lot. One of the back to back zener diodes protecting an IGBT gate had gone bad and finally short circuit causing my gate resistor to cook! I replaced my diodes and now I'm getting better results with under 100us on that I was previously getting with 350us on times. I suspect that this diode may have always been dodgy because the coil is now hitting 75cm arcs with this much lower on time, not even lighting my OCD protection set at 220A and I havent tried moving the ground further yet so will probably do more. I was only getting around 60cm tops before whilst running at the 250A mark.
Would it be feasible that this zener could have been faulty before I blew two IGBT's and may have contributed in some way? And would the coil have been working in some kind of half bridge mode hence the shorter arcs?
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Hi all, I have now got access to a better scope for a couple of days and got some waveforms taken with a 1000:1 current transformer (home made with 22R burden) over the connection between the tank cap and primary. From what I can see these look fine but would appreciate any feedback if anyone notices anything?
Also how quickly should I expect the OCD protection to cut in? Mine is set to come in at 6V which would be 272A but on the waveform it peaked at 341A for one cycle! The OCD is however working I think as the on time doesn't increase after this point despite increasing the interrupter.
Which driver are you using? Most DRSSTC drivers use a flip flop to make sure the IGBTs only switch at zero current, so if the OCD trips as the current is rising up, it will not turn off your driver until the next zero crossing, so the current (in that cycle) can continue to ramp up evern more. Just something to take note.
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Thanks for the replies. That makes sense as my driver is Steve Wards original which does use a flip flop as part of the current limiting circuit. My current when using the original IRG4PC50UD parts must have been massive then so no surprise two blew up.
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Quick IGBT failure in DRSSTC
