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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys, just a quick badger from me:
I need to paralell two irfp450 fullbridges as a SSTC driver. Now, I see some people using DC blocking capacitors on their bridge outputs. I don't see particular purpose in this. Should I split the capacitor between two bridges? (difficult due to mechanical reasons, in my case).
I'm just double minded, forgive me for this kind of thread
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Unless you are certain of 50:50 squarewave the blocking capacitors are required to prevent 'flux walking'/high currents. Would a bi-filar primary help? (assuming wire, not copper tubing) Could you just parallel the mosfets and rely on current-sharing? Otherwise I guess you'll need beefier mosfets (or igbts)
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I dont think splitting the DC blocker will force the current to share evenly because its impedance is small compared to the actual load (unlike a resonant set up where the caps impedance is the load impedance).
I would go for a twisted pair primary and drive each winding from each bridge. You want a heavier gauge wire with that much current anyway. The high coupling between the 2 primaries should provided a substantial equalizing force.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Passing the in-phase wires from the primaries through a current balancing transfomer would help equalise the current. The large yellow toroidal inductors from the output of a PC power supply would probably do the job, or perhaps one half of an E core, again salvaged from an old power supply.
However you probably don't want to introduce too much stray inductance in the primary circuit as this could result in spikes appearing across your transistors.
Steve's idea of twisted pair primaries probably has the most merit. Close thermal coupling of the full bridge circuits would also be useful.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Thanks guys.. design I currently have interconnects the bridges directly, one above another. You can notice the weird orientation of mosfets, but that is for a reason.
I'l separate bridge outputs so I can try all your ideas, and split blockers externally. Speaker cable makes a nice bifilar primary winding...
Thermally: I saw people paralleling mosfets without any thermal connection without problems, relying on their PTC. Problem probably isn't big at all, but I just wanted to be a step forward with it...
PS. If I have the bridges divided like that, I imagine I should use split gate drive too? Interconnecting gate drive just looks wrong in case I use bifilar primary.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi..
This is how the bridge will look like.
boards will be exactly one above another, sharing the same bolts wich also serve as mechanical link. Mosfets will be bolted between four large 200*200 mm aluminium plates for heatsinking. Decoutpling caps and everything else directly above, still keeping a low inductance supply connection. Outputs of both bridges are separate so I can by need connect split primary and DC blocking caps.
Gate drive is supplied by ribbon cable bundled and directly soldered as close as possible to mosfets.
Is there merit in this design? Should I change anything in particular?
I've did some reading regarding mosfet paralelling and got nothing conclusive, leading me think that all that forced current sharing may be overkill. But I'l still do it if you guys recommend so.
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Paralelled H bridges, DC-block caps.
