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Odin The All-Fragger: a large DRSSTC

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LAN_403

Goodchild

Thu May 22 2014, 02:09PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Hey steve, how are you getting +24/ -12 out of a GDT? Are you dong some fancy trickery with the GDT driver?

EDIT:

After looking at your link up top I see what you did. Very clever I may add.

In addition to the comments made above about gate drive voltages, I also like driving at the higher voltages (both on and off) I feel that the high voltage helps to force the IGBT into and out of saturation much faster than with lower voltages. This helps minimize the time spent in the linear portion of the switching cycle. Although this addition accounts for only maybe 10's of nS it does seem to help reduce losses.

Steve Conner

Thu May 22 2014, 09:30PM

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

Oh dear... :(

Two issues, the first is why it popped at less than 1200A peak current. (The SKM200GB128D datasheet doesn't suggest a particularly good peak current capability, but other coilers claim to have used them.) The second is why the coil hardly produced any sparks at 1200A peak current.

Kizmo

Thu May 22 2014, 10:01PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Odin The All-Fragger was such a hardcore fragger it fragged itself.

Avalanche

Thu May 22 2014, 10:39PM

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

I'm wondering where all the energy was going then if there was hardly any output?

Is the checker-plate below the primary getting hot from induced eddy currents?

Very clean exploded devices though

Thomas W

Fri May 23 2014, 12:11AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Hmmm, pity~ gonna scale up to some larger IGBTs then?

Where is that you are testing it at?

Mads Barnkob

Fri May 23 2014, 06:09AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

What was the second issue? That all energy was just dissipated in the brick?

Steve Conner

Fri May 23 2014, 07:19AM

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

The energy isn't really being dissipated anywhere. If the coil fails to break out, it gets returned to the DC bus. You can see that the mains current meter hardly moves until breakout begins (or until some current is needed to blow the silicon to smithereens :/ )

The impedance of the primary circuit is probably somewhat too low for the current limit. This is due to an unfortunate incident involving some large and sexy GTO snubber capacitors. One of the experiments I'll try later is to use a smaller tank capacitor and more primary turns: it's currently tapped at 3 out of 5. But today's test will involve installing some larger IGBTs (the SKM300GB12T4) and trying a few different primary tapping points. It blew up before I had a chance to start tuning.

The test location is the university high voltage lab.

Uspring

Fri May 23 2014, 09:38AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Sorry to hear about the IGBTs demise. Return them for repairs

Any idea, why they survived the dummy load test and not the one with the secondary?

3 primary turns don't make much of a field. Could it be, that the PLL ramped up at the wrong pole, i.e. the lower one?

Steve Conner

Fri May 23 2014, 09:54AM

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

I have no idea. The current limit was set the same in both cases, and the actual peak currents should have been lower with the secondary in place, due to energy transfer to the secondary. For all I know they might have been weakened during the dummy load test, or in the equipment they were salvaged from. 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.

The ringup of the primary circuit is so quick that the concept of separate poles doesn't even seem to make sense. As far as I can tell, it is hitting the current limit within 1 cycle, before it even gets a chance to resolve itself into two separate pole frequencies and pick one of them. After hitting the current limit, the primary current decays away to zero and when the gate drive comes back on it is back where it started.

This makes it all very tempting to hit it with a bigger bridge that can supply say 3000A instead of 1000.

Uspring

Fri May 23 2014, 10:44AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

The scope shots seem to indicate a somewhat longer rampup. Might be due to the delayed response of the OCD. I wouldn't expect a secondary loading effect for the primary current on the first cycle. Even if perfectly tuned, energy transfer takes place on the time scale of the beats.

More primary inductance looks like a good idea.

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