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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi all, I am just now starting to experiment with the classic DRSSTC As I've always built CW/QCW coils, which involve relatively small peak currents, I have no experience with overloading IGBTs in pulse duty.
I'm mainly looking at the "smaller" IGBTs, like the famous HGTGxxN60A4D series. Let's assume I don't exceed the switching SOA and peak junction temperature. Is there any "rule of thumb" how much the peak current rating can be exceeded? I would say it has something to do with the transistor going out of saturation? It would be great if some of the DR "gurus" could provide a bit of info from experience
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
The tried and true method has been run until it blows and then back it off a little. I however I moved to a more scientific approach. Generally I grab several values off of the thermal impedance curve on the datasheet and extrapolate a RC thermal equivalent circuit that I then put in SPICE.
From SPICE you are able to put any size, type, shape of power dissipation waveform into your projected IGBT and look at how its die temperature will change in response to a change in dissipated power. This will tell you if you are going to exceed the junction temp under the simulated conditions.
As far as SOA goes, I take it as a guideline, I have in many cases exceeded the SOA many times over without issue.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for the reply, Eric Yes I am aware of the thermal model, and I think the output V-A charactersitics can be extrapolated beyond the datasheet maximum, as the curves seem to be very close to a straight line for high currents.
However, what does the datasheet not say, is where (and if) the de-saturation occurs, and I think this is the major limiting factor. You can see what I am talking about if you look at the curves for lower gate voltages (eg. 10V) where the curve suddenly goes flat for higher currents. I think that if the transistor got into this region even for a fraction of a second, it would get cooked instantly.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
As far as SOA goes, I take it as a guideline, I have in many cases exceeded the SOA many times over without issue.
I would recommend you never violate the switching SOA, that can make them go splat immediately.
I also work along the same lines as Eric described when designing any power conversion stuff. Its key to understand what the switching and conduction losses are for the parts, as well as the thermal resistance/impedance of the part. On a "pulse" basis, the higher the change in die temp, the lower the reliability for long term use. My former supervisor suggested limiting thermal pulses to just a 3*C rise, but this is for equipment designed to last >10 years in continuous pulsed applications. Some of my designs run the IGBTs with 20*C (worst case, calculated) temp spikes. Going any higher than that seems scary.
De-saturation of the IGBT junction is one way to instantly vaporize some silicon. You should be sure to operate well below this limit! I once tested CM300DY-24H to 5200A (Vge = 30V) where signs of desaturation were detected, the IGBT failed within 1 RF half-cycle of the de-sat event. Over-driving the gates to 20V should be very safe, 25V is what i prefer. This should push the de-sat current way outside of the expected operating zone and make it a non-issue. Just base conduction losses off of extrapolated data (like your pic) and you should get the right answer.
Before reliability was a big concern for my coiling, i used to think running IGBTs at ~2X the peak Icm rating was OK. I think the most extreme i ever went was running those old 40N60 mini blocks from fairchild at 1000A peak (they have a 300A pulse rating).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If reliability isn't a concern, I go with twice the datasheet peak pulse rating ("Icm"). If it is, I go with 1x the rating. I've built some perfectly good performing DRSSTCs this way by just using somewhat more silicon than usual. They are all still running on their original silicon after several years.
A big IGBT datasheet will say something like "Self-limiting to 6x Icm". This is the current at which desaturation really happens (a gate voltage is always specified along with it)
In industrial applications, the user is expected to sense desaturation and shut the gate drive off within a few microseconds. The device is only rated to survive maybe 100 events like this over the course of its life.
You can take the desaturation current as one extreme where it will survive for 100 1us shots, and you can take Icm as the other end of the curve, where it might run all day every day for 10 years. In between there is about a 6:1 range where you can choose the right cost-reliability tradeoff for your coil.
Most IGBTs nowadays have a very good switching SOA. They should be able to hard switch at least 1x Icm without damage, even if it does cause a lot of switching losses. The industrial ones need an excellent turnoff SOA to withstand the worst-case event when the device gets desaturated by a short-circuited load and then shut down by the protection circuit.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
What kind of role does gate voltage have in this problem? IGBT datasheets use generally gate voltages from +-12 to +-15V for typical operation. In this quite untypical application we are usually pushing the gate voltage up to 30V. There are quite few big coils around that are running their silicon almost 4x Icm and still seem to be reliable (=as reliable as hobby coil needs to be.)
Edit:
I kind of like the method of pushing things untill they pop and then back off a little bit :D
Surplus igbts are dirt cheap and explosions are good for ratings...
On a "pulse" basis, the higher the change in die temp, the lower the reliability for long term use. My former supervisor suggested limiting thermal pulses to just a 3*C rise, but this is for equipment designed to last >10 years in continuous pulsed applications. Some of my designs run the IGBTs with 20*C (worst case, calculated) temp spikes. Going any higher than that seems scary.
The transient thermal response curves seem to imply, that you can heat up the junction to max temperature during a pulse. That's a deltaT of more than 100 degrees for a cold transistor. Manufacturers should really specify, how long their device holds up to the thermally induced mechanical fatigue. Transient thermal response would be very misleading.
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IGBT peak currents
As I've always built CW/QCW coils, which involve relatively small peak currents, I have no experience with overloading IGBTs in pulse duty.
