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Re: Snubbing an IGBT in a sstc

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

Steve Ward & all-

The p.s. to my prior posting on this topic:

I'll ask Chip to post http://www.pupman.com/current/kcherrick/s-e_nov-snub.jpg. Steve's notion of "recycling" the charge on the snubbing capacitor got me thinking & that schematic is the (simulated) result. Notice the mains capacitor (waveform at the left labeled TX1-11) being recharged every half cycle!

D6 and C3 are the conventional capacitive snub-circuit elements. (R5, R7 & R1 simulate circuit-element resistances.) Q4, D11, Q1 (not used) and D6 would be the elements of a 1/2-H IGBT pair. And the "snub primary" is what's new--at least, to me.

When Q4 turns off, C3, conventionally, both delays the collector-voltage rise until Q4's current goes to zero and absorbs the inductive rise from the main primary, snubbing the collector voltage to ~970 V. Since D6 is then on, it, in effect, shorts out the snub primary. When Q4 next turns on, D6 stops conducting (a forward voltage now appearing across not-used Q1). Now, main-primary current starts to flow, and snub-primary current also starts, with C3's voltage acting as that primary's power-source.
C3 becomes discharged to ~0 V and the cycle repeats.

At least...that's what one might initially think. In the simulation-reality, the main primary current appears as a somewhat-distorted sine wave ranging between +800 and -1000 A peak. The snub primary's current swings between ~+300 and ~-1100 A...and, it's 180 degrees out of phase from the main current!

I'd tried this at first with a diode in series with the snub primary, to avoid having it act as shorted turns. And I've also tried it with the snub primary's connections interchanged--and with and without the diode--so that the currents are (more-or-less) in-phase. All changes result in less output voltage together with more current drawn from mains-capacitor C4.

So go figure... It would seem hard to beat the circuit's efficiency, in terms of power input, but...just what's going on there? Who'll comment on this? Steve, I suspect you will...

Ken Herrick