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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> I assumed the cap was bolted across the DC bus terminals, not one or other of the switches. > (In which case "snubber" is the wrong word for it.)
The capacitors placed across MOSFETs in the phase-shifted ZVT full-bridge converter are often referred to as "lossless snubber" capacitors. (They supliment the MOSFET's Cds and reduce turn-off losses by controlling dv/dt. The energy stored in them is ultimately returned to the DC bus, so in that respect they are "lossless".)
In general, in power electronics you can only get away with putting a capacitor directly across a switch if the voltage across the switch falls to zero naturally before the switch is turned on. The phase-shifted ZVT bridge, and the Class-E amplifier are two examples that come to mind. Both use the free-wheeling current of an inductive load to drive the switch voltage to zero during the dead-time before the switch is turned on.
But I agree that in the DRSSTC instance "firecracker" is probably a better word for it instead of "snubber"
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
if i understand correctly, RCD snubber, in normal running, don't give big difference than using just C snubber.. But if i have spark from toroid to primary, i believe it can protect IGBT from very high voltage spike, but he can't take lot ( cap storing energy) off energy.It much faster than TVS, So should i use it, or not ??
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Actualy that spike will be connected to the mains all the time, either through an open switch or via a diode. However this also requires a properly grounded bridge as a spike in the cables in case it's just rigged to the mains terminals without any additional ground may be enough to destroy your couplers (I never had alike problems on sstc that now is just fed off-wall with only the secondary and box that also serves as a common logic minus grounded, even when it had a flashover - but again, it's just an ISSTC, with a pretty high power and dv/dt, but still it is nothing compared to a brick-based drsstc). If you have a differential protection, better just rig a cap between one of the bridge input terminals and common ground.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
+1, I also put this cap between DC bus negative and ground on my DRSSTC.
Richie, I think I worded my last post wrong. I meant to say that I think Linas is using these big blue bricks as DC bus bypass capacitors, and confusing us by calling them snubbers. If you look, they're bolted across the DC bus terminals on his halfbridge bricks, not across any single switch. (and yes, those bolts are carrying the current, so they'd better not be steel!)
DC bus bypass caps are all very well, but if you're unlucky they can go into parallel resonance with the busbar inductance at a harmonic of your switching frequency. The result is worse overvoltage than if you'd just used the main electrolytics and nothing else.
LithiumLord, thanks for your input on the forum lately
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
Steve McConner wrote ...
I meant to say that I think Linas is using these big blue bricks as DC bus bypass capacitors, and confusing us by calling them snubbers. If you look, they're bolted across the DC bus terminals on his halfbridge bricks, not across any single switch. (and yes, those bolts are carrying the current, so they'd better not be steel!
sorry , my English is bad, also i am bad in electronics ...
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Apologies Steve, After looking at the picture in detail I can now see that the big cap is indeed connected across the DC bus.
I can also see an RCD snubber, which is of course acceptable across the IGBTs because the resistive part limits the current at which the capacitor discharges.
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Snubber for IGBT
, my English is bad, also i am bad in electronics ...