Saturday, December 31, 2011

Musings about a 600A QCW coil

Stuck at home thinking about things at 6 in the morning => Terrible Things...
Goodchild's and Ward's coils both run at about 300 KHz, and neither pushes over 200A of primary current.
200A is real wimpy for a DRSSTC! QCW's are apparently forced to run at high frequencies - the long, sword-like sparks fail to appear when the resonant frequency is < 300 KHz. No brick will do 300 KHz happily, so we're limited to minibricks and the like, which can't cope with more than a couple hundred amps.
Ideally, we'd like to push more amps; it gives us more room to play with (and one of my goals with this project was to figure out whether the bus-voltage ramping trick scales well with coil size and primary current).  We'd also like to not use minibricks - as gorgeous as they are, $25 a die for the exact same silicon found in a $4 IGBT is not really sane or affordable.
Well, it turns out Fairchild is pushing a new IGBT. It's essentially the beloved 40N60A4D, only twice as fast and significantly cheaper (only $2.40 from Arrow!). It can't handle as much current as a minibrick can (a bit of staring at the transient thermal impedance charts tells us that if we can hold the case at 25C [via water-cooling or some other means] we should be able to push 120A for 10 mS), but a trick pioneered by Steve Ward's "Gigantor" coil helps us here - instead of paralleling multiple devices per leg, we can instead use multiple single H-bridges, synchronized to the same secondary, to drive the resonator.  Five bridges gives us 600A to play with, and for only $50. Even nicer, we can hope that if and when it goes bang, it only takes out one bridge instead of all six.
Finally, a trick used by Steve Conner's OLTC II lets us conveniently connect the coil to the bus modulator - 10  parallel coax cables will carry 1000+ pulsed amps without adding very much stray inductance.

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