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4hv.org :: Forums :: Tesla Coils
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What is better for DRSSTC: full-bridge or double half bridge?

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BSVi
Tue Feb 23 2010, 07:27AM Print
BSVi Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
 		I'm going to redesign my DRSSTC (Link2 and I'm in doubt whitch is better for DRSSTC - full-bridge or half-bridge with two paralleled IGBTs.

Half-bridge with paralleled devices gives double curent, and full-bridge halfes rise time.
In Terry's pdf "Steve's vs. Terry's coils..." he says that rise time is critical.

So what is better?
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Luca
Tue Feb 23 2010, 08:46AM
Luca Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
 		I don't think that paralleling IGBTs is a good idea... Conduction mechanisms in IGBTs are a mix of MOS and bipolar mechanism (actually they are bipolar devices), and it is well known that it is not a good idea to parallel bipolar devices (BJT), unless you put resistors to equally divide the current in the devices.
I'm not totally shure but I think that this applies also to IGBT.

Regards,

Luca
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mmt
Tue Feb 23 2010, 10:12AM
mmt Registered Member #2333 Joined: Thu Sept 03 2009, 03:04PM
Location: Slovakia
Posts: 80
 		i have never see a paralled igbt's..
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Steve Conner
Tue Feb 23 2010, 10:22AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		Full bridge is better!
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BSVi
Tue Feb 23 2010, 10:22AM
BSVi Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
 		There are several IGBTs especially designed for parallelling. They have slightly positive temperature coefficients and well matched.
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Luca
Tue Feb 23 2010, 11:45AM
Luca Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
BSVi wrote ...

There are several IGBTs especially designed for parallelling. They have slightly positive temperature coefficients and well matched.

I see...

In any case, I would choose full-bridge because losses on resistive paths (windings, cables, ESR of capacitors...) increase with current^2...

Regards,

Luca
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BSVi
Tue Feb 23 2010, 03:42PM
BSVi Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
 		>Full bridge is better!
Would you please explain why?
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GeordieBoy
Tue Feb 23 2010, 03:52PM
GeordieBoy Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
 		> Would you please explain why?

...because you develope twice the peak-to-peak voltage across the load with a full-bridge than you do with a a half-bridge.

Losses can also be spread evenly between 4 switches in a full-bridge which reduces temperatures and improves reliability.

-Richie,
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Sulaiman
Tue Feb 23 2010, 04:48PM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
 		Full bridge requires less capacitance across the dc bus for the same ripple/hummmm.
The caps are not cheap.
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BSVi
Tue Feb 23 2010, 06:04PM
BSVi Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83
 		>..because you develope twice the peak-to-peak voltage

Yeap, but you can double voltage with double current halfbridge too - all you have to do is increase "on" time on your interrupter.

Anouther way to double voltage is to decrease capacitance and increase inductance.

As from my point of view all difference beetwen dullbrigde and double halfbridge is ringup slope (which is doubled in FB design) and maximum current (which is doubled in HB design).
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