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Registered Member #2422
Joined: Tue Oct 06 2009, 02:41AM
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Posts: 85
Does it work to put several MOSFETs in series instead of using a single one with a higher D-S breakdown voltage? If so, is there a specific way to do this?
you can, some are avalanche rated for dong specifically this. that means they are rated for handling a certain amount of jolt (non tech term) since they might not switch at the same time
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
The term is snubbers ... they limit speed and waste power though.
An alternative is gate side techniques, there are passive ones for instance :
Also this one, I'm not too sure how well it works ... but it's worth trying out since you can do it without isolated gate drives :
And active ones, for instance the scheme I described in this post :
To use that for switching you would use a ramp guaranteed slower than the slowest device as input.
I've also seen papers just switch a stack of MOSFET simultaneously and not care about the consequences (supposedly the manufacturing variance of MOSFETs is pretty low). Or simply put MOV's across them. To me either seems better than snubbers. Not build any of these schemes yet though, just trawled the web.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
The problem with putting mosfet's or IGBTs (or really any semiconductor) in series with the total voltage above its rated breakdown voltage is simultaneous turn-on.
Take for example, a 0v to 10kV rail with 10 mosfets rated at 1200V each, theoretically they should be able to stand off that 10kV just fine. If just 2 adjacent mosfet's turn on, the other mosfets are exposed to a greater voltage across them (10000/8 = 1250V). Once one mosfet breaks down, the next mosfet is exposed to even more, etc. You could put TVSs across each mosfet, or a small capacitor as a snubber, though you would have to compare the RC time constant of the cap + Rds(on) of the mosfet against the mosfet's turn on time.
I do recall that TDU made a stack of SCR pucks, and blew it up because of voltage standoff issues some time ago. You would also want to use some decent drivers for the mosfets (either with high side isolation and bootstrapping, or, a GDT and proper driver) to force fast, hard switching to prevent a mosfet from dieing and killing the rest of the stack.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Hard switching is not necessarily the best way, the gate side techniques all keep the MOSFET in the active region so they can use feedback one way or another.
For hard switching the easiest way seems to use a transformer and simply use a larger output voltage than the threshold and connect it to the MOSFET through a resistor to limit current and a zener to limit gate source voltage ... avoids having to use any active components which will need their own floating power supply, which is a head ache.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
If you make sure the turn on/turn off current is below the maximum avalanche current (which is typically the same as the DC continuous current), then this should work without snubbers. And of course you need isolated gate drive for each transistor...
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MOSFETs in series?
