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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That's not really true.
Doubling the on-time doesn't double the voltage, because the spark loading limits the ringup.
If you took a halfbridge DRSSTC, and replaced the halfbridge with a fullbridge without changing anything else, then because of the extra drive voltage, the primary current would increase and the sparks would get bigger.
If you paralleled extra devices in the halfbridge, the spark length wouldn't change, and you'd have to decrease the number of primary turns and increase the tank capacitance to get the benefit of the extra devices.
Registered Member #1637
Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
>and you'd have to decrease the number of primary turns
Ok, I expressed my question incorrectly.
Lets imagine that we have two DRSSTC's with optimal primary/capacitor bank combination. First DRSSTC is driven by full brigde, and second one by halfbridge with paralleled devices.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Neither is better, they both perform the same because you reoptimized the primary coil.
In practice, the fullbridge is slightly better for the following reasons:
You don't have to worry about current sharing of parallel IGBTs. In theory they will share current, as was said, but it's VERY easy to destroy the sharing with a bad layout that adds too much inductance in the emitter connections.
A higher drive voltage makes it somewhat more convenient to optimize the primary coil and capacitor bank.
And if you tried to build a very large DRSSTC with a halfbridge, the optimal primary might end up less than one turn, which is impossible.
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What is better for DRSSTC: full-bridge or double half bridge?
