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Registered Member #18516
Joined: Sat May 18 2013, 09:09AM
Location: Lancashire, UK
Posts: 38
I've been slowing working towards building my first DRSSTC, and i'm trying to get my head round IGBT selection.
I was planning on using a full bridge of IXYS 60n60c2 bricks, as it seems like a tried and tested approach, however the 60n60 appears to be EOL. IXYS recommends the IXGN72N60C3H1 as a replacement, but i've been looking at the datasheet and trying to understand the differences.
The 72N60C3 appears to have slightly slower t(don), t(ri) and t(fi) but faster t(doff)
Can anyone explain how these times relate to a DRSSTC, and how to figure out what values you would want for a given coil?
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Its a complicated matter when talking about DRSSTC, I do not fully understand the theory behind it all, but here is a few guide lines.
If the switching times all added together is slower than a period at your resonant frequency, you are likely to have big trouble.
A IGBT that switches a resonant circuit can be pushed over its limits.
A DRSSTC has so short pulses that, in most cases, the thermal properties of the die never comes in effect.
The newer IGBTs, the less you can push them, as technology gets better, the producers can make dies closer to their specified ratings compared to older IGBTs where they were made with a good overhead to make sure they would hold up to specifications.
Some coilers stay within SOA for heavy duty coils and others go for 2-3 times Icm.
It all depends on how adventurous you are and if you are willing to blow some silicon in the quest for finding the limit :)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Mads Barnkob wrote ...
If the switching times all added together is slower than a period at your resonant frequency, you are likely to have big trouble.
even if they were 1/5 the switching period, you would have big trouble
Mads Barnkob wrote ...
A IGBT that switches a resonant circuit can be pushed over its limits.
No transistor should be pushed over its limits, only when you are switching a resonant circuit, you can set the switching transitions to occur near zero current so the switching losses are greatly decreased.
Mads Barnkob wrote ...
A DRSSTC has so short pulses that, in most cases, the thermal properties of the die never comes in effect.
They do come into effect, and quite drastically.
Mads Barnkob wrote ...
The newer IGBTs, the less you can push them, as technology gets better, the producers can make dies closer to their specified ratings compared to older IGBTs where they were made with a good overhead to make sure they would hold up to specifications.
Well this is usually true, the old transistors had bigger dies for the same ratings and held up better to overloading.
Mads Barnkob wrote ...
Some coilers stay within SOA for heavy duty coils and others go for 2-3 times Icm.
The switching SOA and peak current rating are two different things, the first one should never be exceeded, the second one can be exceeded without ill effects.
The proper answer would be something like this. Select a transistor, calculate the peak power dissipation (conduction + switching losses combined) and from the transient thermal impedance graph, read out the temperature rise of the die. It should probably not exceed 20 degrees Celsius.
The switching SOA and peak current rating are two different things, the first one should never be exceeded, the second one can be exceeded without ill effects.
I've some difficulty understanding SOA ratings. AFAIK the SOA is a rectangle defined by Vcemax and Icm. Compliance with the SOA would mean, never to exceed Icm.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Uspring: The Switching SOA defines the current, at which the transistor can turn off or turn on reliably, without latching. The peak collector current is the current the transistor holds up in the conduction state. So, for example with sine wave current, you can have a 500 amp peak but turn the transistors off at 100 amps.
Thank you for the explanation. This implies, that you shouldn't switch at large currents, even if Vce is low. Eric Goodchild is sort of not doing that, though, when using the parasitic capacitance and hard switching. He turns off, which is at a Vce of saturation voltage or near there, depending on how fast he can switch, then waits until the voltage hits the other rail and then turns on the other IGBT. Looks like a very special case.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well, if there is a capacitor in parallel to the collector-emitter, the transistor can turn off at a larger current without latching. But how large - you can only guess, the data sheet will not tell you this.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I can see that I should not have made a reply covering so many subject when I was also about to leave out the door :)
Dr. Dark Current wrote ...
Mads Barnkob wrote ...
If the switching times all added together is slower than a period at your resonant frequency, you are likely to have big trouble.
even if they were 1/5 the switching period, you would have big trouble
Mads Barnkob wrote ...
A IGBT that switches a resonant circuit can be pushed over its limits.
No transistor should be pushed over its limits, only when you are switching a resonant circuit, you can set the switching transitions to occur near zero current so the switching losses are greatly decreased.
Mads Barnkob wrote ...
A DRSSTC has so short pulses that, in most cases, the thermal properties of the die never comes in effect.
They do come into effect, and quite drastically.
Mads Barnkob wrote ...
The newer IGBTs, the less you can push them, as technology gets better, the producers can make dies closer to their specified ratings compared to older IGBTs where they were made with a good overhead to make sure they would hold up to specifications.
Well this is usually true, the old transistors had bigger dies for the same ratings and held up better to overloading.
Mads Barnkob wrote ...
Some coilers stay within SOA for heavy duty coils and others go for 2-3 times Icm.
The switching SOA and peak current rating are two different things, the first one should never be exceeded, the second one can be exceeded without ill effects.
The proper answer would be something like this. Select a transistor, calculate the peak power dissipation (conduction + switching losses combined) and from the transient thermal impedance graph, read out the temperature rise of the die. It should probably not exceed 20 degrees Celsius.
The switching times added together should ofcourse make it able to switch as many primary cycles as you want, in the on-time, and some overhead is always good.
What I said about switching resonant current, I meant the same as you said, that with zero current switching we can switch much higher current with IGBTs made for hard switching much lesser currents.
Running a IGBT at 200uS ontime, we are often in the lower end of the Zth graph if not even outside of the window, so just by the very low on-times the die will not rise as fast as with specified examples of normal circuits. A CM600 with Zth(j-c) 0,045 at 200uS and Rth(j-c) 0,081, that with maximum permissible power dissipation of 1540W we only see a delta T of 5 degrees Celsius. Lets say we are running 200BPS, after the 5ms break the die temperature have fallen almost back down to case temperature.
I am sorry that I mixed up SOA and Icm, what I meant was that the low on-times of a DRSSTC makes us able to run on the very edge of the SOA. Icm is again limited back to subject on die temperature rise.
I hope I understood it right or else there is room for learning :)
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IGBT Selection
