[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
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
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
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...