Thanks for your response guys!
jack:
I looked on the diy charger thread and your last few posts were not talking about miller capacitance. Perhaps I was looking in the wrong place. I did a search of the site for miller knee and miller capacitance, and it only came up with two hits, both on P+S controller pages. I am not seeing the effect described, my gate is always driven in a nice exponential curve. Could you provide a link to the post you describe please?
The mosfets have never gotten hot, though they are SMT devices directly soldered to 1/4" thick copper buss bars. Perhaps the semiconductor is getting hot but the case is cool?
I am using flyback diodes, 240A worth at the moment.
The diodes anode leads are almost touching the drain of the mosfet and are well-soldered. The wire connections are all made with 6AWG wire; all wire connections are tinned and bolted with 5/16" bolts.
I have run a small ~1A radioshack motor off this controller when I first put it together, and it worked first try. I expected this meant that the principles were sound, but that I was hitting a problem when running high currents on the inductive load.
I have some car window motors lying around but no headlights. Would these be acceptable alternatives for testing do you think?
jyanof:
Yeah, it is fun. Even when it doesn't work, it's still at least a little fun
I've tested my starter's current draw with the same LEM sensor as the ReVolt project. When one of my mosfets failed short, I just tapped the battery with the cable and measured the LEM with my scope. It peaks to around 380A, and within a full second it is at a steady-state 60A.
I was running a very low duty cycle (1/30) at 16kHz. If the load were not inductive I would expect current spikes to 380A when on and 0A when off. However the starter has inductance and I had imagined that the current would instead be pulsating at a lower value (380/30 = 12A), thus within the tolerance of the mosfet.
Is this not a reasonable assumption?
The caps are connected to the outside buss bars, so BM+ and B-. The M- connection is purely mechanical; I cut away the copper clad all around that bar.
Ah yes looking back at that picture I see I chose a poor camera angle, sorry!
I had intended to test the circuit with 1 diode and one mosfet; there is room for 3 diodes and 3 mosfets. If you guys think it could be a current problem, I can try and throw all the parts at it.. it just might cost me $40 if I blow it all up.
Okay, when I try again, what do you think I should measure next? I measured the current and Vgs on the test last night. Vgs was as expected, and the current did nothing visible until the mosfet failed (which I thought was odd).
I guess though if the controller was working correctly, 12A is only 19mV; difficult to see on the scope - with higher resolution vertical scale the 2.5V-3V signal is out of range for my scope. I definitely saw when the mosfet failed the current went to its steady-state 60A, or 0.1V change on the LEM.
I am thinking Vds to ensure I'm not getting inductive kickback (i.e. diodes are broken or something) which is frying my mosfets. Any other suggestions?
I just want to thank you guys again wholeheartedly.