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Old 01-20-2011, 12:56 PM   #4242 (permalink)
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Originally Posted by MPaulHolmes View Post
Hey DJ Becker! I was just thinking that because I'm limited to 3 and 4 ounce copper right now, that I shouldn't push too much current through too short of a board. I was thinking that 100 amps per leg was asking for trouble with the solder joints and the pcb tracks. I thought if I could just make the board a bit longer and use maybe 14 of the irfp4668's instead, which would keep the current down. Man, there are schottky 200v diodes in a TO-264 package that are rated for 250 amps. An almost perfect match to the TO-264 gigamos mosfets that are rated for 230amps. My main reason for doing SR before was because I couldn't find any diodes that matched the current of the mosfets.
I can see that as an issue. We are wiring directly to the power device pins, which puts extra material high up on the leg of the device. That reduces the resistance and provides a heftier thermal path so I haven't been concerned with the lead heating. The bigger packages have heavier leads, but that doesn't help you if the limit is the foil immediately around the solder pad.

On the plus side, these are hand-assembled boards. It would be easy enough to solder a copper washer to the board as a heat spreader. Or a copper wire with a loop formed at the device end. The tricky part would be writing a description that would let first-timers get it right.

For those that don't know, the reason that circuit boards can carry high current with a tiny copper cross-section is that the large surface area keeps things cool. Increasing the foil thickness doesn't let you get rid of any more heat. Doubling the foil thickness only gets you about a 1.4 (root-2) increase in allowable current because the electrical resistance drops.

Normally a device lead will cool the circuit board. In the case where the lead is generating the heat, there is a critical ring around the solder pad where there is a minimum of surface area and thermal conductivity. Increase the thickness of the wire coming out of the device won't change that ring size. The only thing you can do is move the critical ring to a larger diameter with solder or a reinforcement.

Back to the device topic: the larger devices still seem to have an edge. They have a lower on resistance, and are in a bigger package with heavier leads. You won't be able to take advantage of their full power capability, but you wouldn't want to do that anyway. The "on-resistance per inch" is only slightly better, but the thermal coupling to the heatsink is much better. I think the packages are rated at 520 vs 1670 watts.

Another consideration is the gate drive. The bigger devices are almost 50% more difficult to drive. You probably only want to put 30% more current through them. But at some point the number of individual devices becomes more of a problem then providing enough current in the gate drive pulse. I don't know where that point is, but 14 devices on one side might be getting there.

Finally, the device count is a bit of a hassle for hand assembly. It's more clips, more holes to drill, etc. We are using a "live" heatspreader/bus-bar where we can't put a nut on the back, so every hole has to be tapped. Not a huge deal, but every extra device is a little more work.

BTW, I didn't mean for any of this to sound critical. I was really just interested in what pushed your decision to that side of the trade-off. You never really know the best choice until you build both. And then everything changes the next week when a new device is available.

Last edited by DJBecker; 01-20-2011 at 01:01 PM..
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