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Re: [TCML] DRSSTC ring-up

Hi there!

Do you have any pictures of your setup? Seeing your primary/secondary
assembly would definitely be helpful, and it would be nice to see a picture
of your bridge layout just to confirm that it's routed in a way
that won't cause IGBT failures once you turn the power up.

A lot of the primary current envelopes you're probably looking at are of
streamer-loaded coils with higher bus voltages and higher OCD limits. Based
on my experience, with only 75A peak and 57V on the bus, you're simply not
developing enough voltage at the topload to break out properly. DRSSTCs
tend to have weak output until you start turning the power past some
minimum level.

What IGBTs are you using that require such a low current limit setpoint? If
you're basing your maximum setpoint on the IGBT's rated peak current, then
you need to go higher! For example, the FGH60N60SMD IGBTs I use are rated
to 60A at the top of their datasheet (elsewhere the datasheet lists the
absolute maximum pulse pulsed Ic-e as 180A), but are completely reliable up
to 300A peak current in a DRSSTC.

A better way to test your coil might be to set your OCD
less conservatively at, say, 200A, and start with full or half bus voltage
but with the on-time all the way down. Then, you could slowly start turning
the on-time up to adjust the current ring-up. Finally, you could slowly
start turning up the OCD until you're either satisfied with the results, or
you get an IGBT failure, in which case you'd reassemble the bridge, turn
the OCD back down by 50A or so, and call it a working DRSSTC.

Hope that helps!

On Thu, May 23, 2013 at 7:15 AM, Herwig Roscher <herwig.roscher@xxxxxx>wrote:

> Hello all,
> I'm still tussling with my DRSSTC:
> In the list archives and the internet there are very few pictures showing
> the ring-up of
> a DRSSTC. Sometimes the amplitude rises linearly, sometimes it goes up
> exponentially. My
> DRSSTC showes linear ring-up, but with a strong sinusoidal modulation, if
> the natural
> frequencies of primary and secondary coil are identical. As this
> modulation extends the
> time-to-breakout, I have tried to avoid it by altering the coupling
> between the coils -
> but without avail. The only possibility to get a smoother envelope was to
> tune the
> primary coil lower.
> My circuit:
> half-bridge with TO-247-IGBTs - primary coil - MMC - GND
> ON-time of burst 100 µs, OFF-time of burst 8 ms
> peak current limited to 75 A to keep the IGBTs alive
> natural frequency of primary and secondary coil: 299 kHz
> after coupling the lower pole is 264 kHz and the upper pole is 313 kHz
> MMC for natural frequency is 75 nF and for tuning low (see above) it is 94
> nF
> inductance of primary coil is constant
> rail voltage is +57 Vdc at the begin of current limitation
> no streamers, only strong corona from breakout pin (distance to grounded
> rod 2")
> What's wrong with my system? Any advice is appreciated very much!
> Herwig
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