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Registered Member #8743
Joined: Thu Dec 13 2012, 04:18PM
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Posts: 12
I never quite understood why everyone uses MOSFETs/IGBTs rated for the full bus voltage on half-bridge (DR)SSTCs. Theorethically the voltage divider caps should make sure the transistors only see half the bus voltage, right?
If that were the case I could feed a half-bridge of IRFP260Ns (200V) with a voltage doubled bus at 340V and each of them would only see 170V... would that work or am I missing something?
The voltage divider caps should also see only half the bus voltage, which would make 250V caps OK for use on such bridge...
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
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Posts: 600
ScotchTapeLord wrote ...
When one switch turns on, the other one sees the full supply voltage. A bridge's voltage is limited by a single switch.
Stl is correct. While the potential across the load is half the full voltage, that's not the only potentially available path to ground for the bridge +(or inverse). Vb volts across a bridge where one of the two igbts is closed results in Vb-Vce across the other.
Registered Member #8743
Joined: Thu Dec 13 2012, 04:18PM
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Thanks, I should have seen that.
The voltage divider caps see only half the voltage all the time, right?
A somewhat related question: on a DRSSTC the primary cap/MMC is always rated for a voltage much higher than the IGBTs, usually on the thousands of volts. From what I understand that's due to resonant rise... but doesn't that voltage show up on the IGBTs and divider/snubber caps?
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Atomic wrote ...
A somewhat related question: on a DRSSTC the primary cap/MMC is always rated for a voltage much higher than the IGBTs, usually on the thousands of volts. From what I understand that's due to resonant rise... but doesn't that voltage show up on the IGBTs and divider/snubber caps?
The switches only feed energy to the RLC circuit that the primary and its capacitor really is, with its very low resistance the current rings up rapidly and thereby a much higher voltage is seen by the primary capacitor. As this current only flows between the primary coil and primary capacitor, only some transients are seen by the switches and that is why you have snubber capacitors, TVS diodes etc.
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Half-bridge voltage
