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Old 07-22-2009, 02:22 PM   #15 (permalink)
Join Date: Feb 2009
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Originally Posted by MPaulHolmes View Post
There are several things for future versions, like multiple gate drivers to allow for smaller gate resistors. Another thing that would help is if I would have torqued the stupid mosfets'/diodes' clamps all the way down. I'm such an idiot!!! If you ever want to do that, you will have to unsolder each gate resistor from the control board, unsolder the 2 thicker power ground wires from teh control board, and then remove the control board, unbolt the 5 bolts, flip it over, take off the base plate, tighten the clamps, and then re-assemble. I'm sorry about that. Maybe they are OK though.
That does seem like quite a process to get in there! more pressure would probably help. I didn't see the thermal pad material on the BOM - do you know what it is or what the specs are?

I ran the numbers for smaller gate resistors for various design points: The first is the original 30 ohm, the 2nd is 15 ohm, and the last is 5 ohm. On paper at least, it shows up to ~50% less heat with the 5 Ohm.

Syncronous rectification is another option. Another option is to have 2 separate heat spreaders. One for the diodes and one for the mosfets. Clamp teh mosfets' back DIRECTLY to the heat spreader, which makes the heat spreader wired to M-. The heat spreader then would have to be isolated from the base plate. Similarly do the same with the diodes. Must go to bed! haha!

I'm skeptical of synchronous rectification. I think it makes sense if the diode is on for a large portion of the duty cycle, but in our case, we're often above a duty cycle of 0.5. From the chart, the Wdiode is often less than the sum of the Wfet (due to Rds) and Wswitching (indicating that adding another set of mosfets would actually increase losses). Even for the 5 ohm case, there's only a significant advantage at low speeds.

I think sync rectification is most useful for low voltage power supplies that convert 120V down to 12 or something. In that case, the duty cycle is so low that the mosfet is only briefly on and the diode is on a long time, and thus, is the source of a lot of heat.

I do like the idea of eliminating the thermal pad... seems like it creates other problems, huh...

That's really serious about the 80 amp minimum. Maybe I messed up so that the dead zone does nothing, but then the throttle goes from 0% to around 12-13%? I really need to look at the code to see if that is true. I don't have a motor amps measuring thing. I can sort of measure battery amps, though, and I have never noticed that. hmm! I'll check tomorrow.

I felt that the minimum throttle is very noticable. like, cruising at 25mph, i'd back slowly off the throttle, i could feel the torque dropping, and then bam - instantly zero. For me, it's at 80amps, but maybe it's less at 72v and a 6.7" motor.
ReVolt AZ testing thread:
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