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Odin The All-Fragger: a large DRSSTC

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Artlav

Fri May 23 2014, 11:38AM

Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94

On a slightly off topic - did that module popped open on it's own, or have you dissolved the glue somehow?

I have a dead module of a similar construction that i'm curious to take a look inside, preferably without having to resort to a dremel.

Steve Conner

Fri May 23 2014, 11:44AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I "dissolved" the glue with a hammer and screwdriver. The Semikron bricks are quite a flimsy construction and the plastic top comes loose after a few good wallops. The older Mitsubishi/Powerex devices seem to be much more solid.

Hydron

Fri May 23 2014, 12:51PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

Steve, is there any chance it's hard switching due to the PLL not having time to lock into ZCS before the current gets large?

Doesn't look like you have many cycles before hitting the limit with your low-Z primary, is it possible you're managing to get outside the switching SOA of the IGBTs on one of the early switching transitions? Even once could be enough to make it pop if it latches up or runs into second breakdown or something (I'm not sure exactly how they fail, but I do know that you need to be much more careful with the switching SOA than the general current limits).

It may be useful to run a frequency sweep of the primary when the secondary is in place to look at where it's in resonance, and check against what you get from the PLL at the start of the burst. I just ran a quick sweep on mine and got the graph below:

Edit: impressive pics of the dead IGBT, never seen the dies quite so uniformly black!

Steve Conner

Fri May 23 2014, 01:27PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Neither of the datasheets I have even specifies the switching SOA. In other similar devices (CM300DU-24H etc) it is square up to the device's rated voltage and Icm.

The switching transitions look reasonably accurate in the first few cycles. However I think I spotted some weird behaviour when the gate drive comes back on after tripping the current limit. I'll add it to the list of things to check.

I swept the resonator with a signal generator at home before rolling it out for testing, and set the PLL up accordingly. I will have to test it again in situ, as the long wires connecting it to the power electronics probably added some inductance.

Intra

Mon May 26 2014, 02:06PM

Registered Member #2694 Joined: Mon Feb 22 2010, 11:52PM
Location: Russia, Volgograd (Stalingrad).
Posts: 97

Steve Conner wrote ...

Another possibility is that the fast rise time of my gate drivers produced some very high gate voltage transients at the dice. These bricks have quite high inductance internal gate wiring, and no internal gate resistors or TVS.

Exactly.

If I saw the few your last videos right, you did control gate drive voltage manually, and if you won't use Zeners on the gates, your bricks will blow up even on 30Vbus. Zeners should defend your gates from spikes and overvoltage at the gate at all.

Sigurthr

Tue May 27 2014, 01:26AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I'm fond of high inductance primaries. Should you end up going that route I'd love to see lots of documentation on your progress and experiments.

Steve Conner

Wed May 28 2014, 12:46PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Intra wrote ...

If I saw the few your last videos right, you did control gate drive voltage manually, and if you won't use Zeners on the gates, your bricks will blow up even on 30Vbus. Zeners should defend your gates from spikes and overvoltage at the gate at all.

The problem is with the inductance of the gate wiring inside the brick. Even with a perfect clamp at the gate terminals, this inductance would cause overshoot at the dice.

The SKM300GB12T4 claims to have built-in gate resistors which should help. I also installed 24V TVS directly on the gate terminals this time (the TVS were mounted on the gate driver board before)

I don't understand what you mean by "control gate voltage manually".

For more info on primary inductance, see the thread I started on "What is a low impedance DRSSTC" where we develop some guidelines for choosing the primary impedance.

Avalanche

Wed May 28 2014, 03:55PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

There's some info on SOA's for Semikron devices in the application manual (page 181 onwards) which you can get here -

Not sure if this is any use... there's nothing really device specific, but you can calculate some things from datasheet values.

For anyone intersted, that link is a brilliant application manual on power devices in general. I'm lucky to have a hard copy, I just need to read it

Intra

Thu May 29 2014, 02:29PM

Registered Member #2694 Joined: Mon Feb 22 2010, 11:52PM
Location: Russia, Volgograd (Stalingrad).
Posts: 97

Steve Conner wrote ...

I don't understand what you mean by "control gate voltage manually".

oh, no, I thought you see your gate voltage before gate. you probably saw it from after the gate, from bridge directly. that's explains rise of meander.

Steve Conner

Thu May 29 2014, 05:14PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, the square waveform on the scope is the output of one side of the H-bridge, using a x100 probe.

I built a new tank capacitor of 0.3uF, connected it up, and retuned the system with a signal generator. The new tuning point is about 4.4 turns. Ready for another high voltage test run tomorrow

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