Re: Snubbing an IGBT in a sstc

["Tesla list" <tesla@xxxxxxxxxx>]

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