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Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
I have been reading up on switching power supplies and a question popped into my head that didnt seem to be answered anywhere else. I know the practice of paralleling switches is common and that transistors share current well if they are kept at even temperatures, assuming they are the same transistor. But what if you need more voltage at the same current? Can you use multiple IGBTs or MOSFETs in series to get the desired voltage, and might switching topology (hard switched PWM vs. ZVS/ZCS) make a difference in this answer?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
It does exist. Probably the safest route to go is ZVS switching. The reason being that you have a wider window before the collector voltage is higher than the rating of any single device.
So just like ZCS is nice for paralleling devices, ZVS is nice for seriesing them, and the reason is simply that it provides a wider window for the transitions to take place before any single device gets blasted with excessive current or voltage.
Hard switching is tough for series devices, because even nS timing can be critical. I think the schemes ive seen put TVS diodes or MOVs across each device to briefly take over if it doesnt turn on in time. Not a desirable option.
Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
I've wondered this very same thing.... If I understand, what you're saying is that the design would always be primary in this concept?
My original thinking was that the reason the 2N3055 drivers were such a failure was that they reduced them to one unit...(?) I had thought that the original designs using two 2N3055's brought the needed levels to the deign (via multiple units) but you're saying that would not be the case: that circuit design is primary?
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Aren't ZCS and ZVS the almost the same thing? I mean, you cannot have ZCS without ZVS, because if there was a potential difference, current would want to flow (V=IR), correct?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
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Posts: 3384
Firefox wrote ...
Aren't ZCS and ZVS the almost the same thing? I mean, you cannot have ZCS without ZVS, because if there was a potential difference, current would want to flow (V=IR), correct?
The problem is tha zcs is never perfectly zcs, even a little current always flows. And this is enough to force the full voltage across very high impedance switch in off state. That said, most today MOSFets even IGBTs are avalanche rated, so seriesing them should not present very big problems.
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Using Transistors in Series
