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Re: Snubbing an IGBT in a sstc
Original poster: "Steve Ward" <steve.ward@xxxxxxxxx>
Ken,
Be sure not to exceed the avalanche energy rating for the MOSFET. Its
typically in the mJ, so its not a whole lot. 1600W for 1uS is about
16mJ, so it might be OK.
I did extensive R & D on snubber design for my H-bridge drives. I
eventually found that my snubbers just couldnt overcome the internal
package inductance of my relatively "old" IGBTs (they have some
50-60nH from each lead). I found my voltage spikes to oscillate in
the 10Mhz region, meaning that your snubbers have to have exceedingly
small inductance to work effectively on this noise. In the end, i
ended up leaving the snubbers off, and the coil works just fine. But,
this was after creating an extremely low inductance buss structure
(laminated plate conductors). I also slowed down the IGBT turn on a
little, since the voltage spikes were caused by the opposing
free-wheel doide's not-so-soft recovery.
There is something i dont understand about your snubber design. It
has to turn the main IGBT back ON to clamp the spike. But, the IGBT
is supposed to be off because the other IGBT is now ON. I realize
that if this were a push-pull arrangement, driving a center-tapped
primary, then there is no real harm in having both switches ON at the
same time. But, at some point you have to turn this IGBT OFF... so
wouldnt there still be a transient when that happens? Otherwise, why
not just have overlapping gate drive signals? Anyway, i think i might
be missing the essence of your design, so perhaps either i need to
spend some time and look at it myself, or maybe you can shed some more
light on things.
Finally, i worry that this active snubber simply wont be fast enough
to turn the device back on in time. Be sure your model includes all
the main buss inductances as well as the delays inside of the
switches.
There is always the RCD snubber...
Steve Ward
On 1/21/07, Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>
I'll ask Chip to post
<http://www.pupman.com/current/kcherrick/TCH_p-p4.jpg>http://www.pupman.com/current/kcherrick/TCH_p-p4.jpg,
a simulation-schematic illustrating what I have in mind.
I'd like to ask if anyone has had experience with using a
power-MOSFET in its avalanche mode in an active snubber circuit. In
my posted circuit, Q4 is a power-IGBT prospectively driving an
untuned Tesla-coil primary, with secondary-return feedback to sustain
oscillation. In hardware, I would implement a push-pull
configuration, with a center-tapped primary and a pair of IGBTs.
The snubber circuit involves Q1, Q3 and the associated
components. Absent snubbing, negative-going Q4 drive passes through
D9 and turned-on Q3 while positive-going drive passes through R1,
C4/R4 and D5. (In simulation, C4/R4 was necessary to preclude
oscillation during the snubbing.) Thus negative-going drive is
relatively fast while positive-going drive is relatively slow--as
necessarily implemented in push-pull or H-bridge configurations. But
when Q4 cuts off every half-cycle, TX1's resultant overshoot reaches
~800 V and Q1 avalanches. Q1's current then passes thru D8, C4/R4
and D5, shutting off Q3 and thus the negative-drive, and rapidly (and
briefly) turning on Q4 to snub the overshoot.
The circuit of D6, C3, D1 and R9 acts to divert Q4's current during
each turn-off so as to diminish the power dissipation in Q4, and also
to slow the rise of TX1's overshoot to give time for the Q1 circuit
to operate. In effect, turn-off power dissipation is transferred
into R9 each time Q4 turns on, easing the stress on Q4. (In hardware
for D6, I'd plan to utilize the otherwise-unused reverse-diode of a
half-H-bridge "brick" of which Q4 is a part. The diode needs to be
fast and it needs to have a large reverse-voltage-withstanding.)
Simulation shows that Q1, in avalanche, needs to conduct ~2 A--which
goes thru R1--while the voltage across it is 800 V. That's 1600 W,
but only for ~1 us out of 8 (for a ~120 KHz Fr). That cuts the mean
dissipation to 200 W during the pulse burst, and with a 1% sparking
duty-cycle, that's cut further to only 2 W. So the question is...can
a IRFBE30 MOSFET or the like withstand the 800 V and at the same time
the 2 A--even at only a 12% x 1% = 0.12% duty cycle?
I'd thought of using an MOV in the same circuit, or perhaps a string
of them, instead of an avalanched MOSFET, but I'd be worried about
the capacitance of the MOV(s). Any ideas about that, as well?
Ken Herrick