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Re: Recent s.s.t.c. work



Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

Yes, you're right about that. But my design incorporates what I've been calling a "pilot oscillator", which runs all the time and whose purpose is to keep the floating IGBT drivers' supply-capacitors charged up via the gate-driving transformer. So that does much the same as your PLL: I can press my "fire" button & get the bursts of gate drive just as if there were feedback from the primary current. But still, that's only an approximation of what occurs when drain voltage is applied to the IGBTs, since that condition will strongly affect the shape of the gate signals due to Miller effect.

The frequency of the pilot oscillator becomes entrained with that from the primary current when the latter becomes sufficiently large; as I say, operating much the same as with a PLL.

Plus, of course, any adverse effects due to EMI from the secondary will not be present absent IGBT drain voltage.

KCH

Tesla list wrote:
Original poster: Steve Conner <mailto:steve@xxxxxxxxxxxx><steve@xxxxxxxxxxxx>

> With a
> feedback circuit dependent on primary current to
> operate, it's a
> little tough to do otherwise.

That's another reason why I chose to build my driver
around a PLL. If the feedback signal is missing or
bad, it runs as a plain oscillator and still drives
the IGBTs at a sensible frequency. That means I can
check out gate signals etc. with the main DC bus
turned off, and then bring the DC bus voltage up
slowly and verify that it locks on.

To test your circuit, you can of course substitute the
current transformer signal with a sine wave from a
function generator. But that will only get you so far.

Steve Conner