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I don't guess I've explained my position very well. If you would be willing to allow me to explain. From your circuit diagram, it appears you are using diode STTH6002C for the flyback diode. Each is rated at 60 amps and you are using 16 of these total, for a current carrying capacity of 960 amps through the diodes. I may be stretching a bit but you may be thinking that using a capacitor in in parallel with the battery lowers the serial resistance of the battery and allows it to more quickly absorb the transient surges that go from drain, through the flyback diode and then to the positive of the battery. I agree, in theory that this sounds like a reasonable assumption to make. With my testing, I am unfortunately unable to reproduce this symbiotic relationship between flyback diode and capacitors across the battery. As you have stated, you are only receiving 20 volt peak spikes on your mosfet drains @ 500 amps by the use of your configuration. I am unable to reproduce such small voltage spikes on my mosfet drains by using a configuration similar to yours. The spikes that I am seeing are much larger and very transient in nature. I am concerned that, depending on your oscilloscope or other test equipment you are using, you may have missed these transient higher voltage spikes. With your configuration, spikes that rise above V+ should be shunted to V+ by the flyback diodes. Looking at the datasheet for your diodes, if I read it right, the forward recovery time is 350ns and reverse recovery is 40ns max. I am using 20khz clock speed, so each mosfet pulse lasts 1/20000=50 microseconds or 50000 ns. That means the diode can turn on and off a maximum of 125 times per cycle. The diode I used was slower than yours, so our results will vary. If you look at the o-scope traces I showed earlier, you will see that the initial spike is indeed about 1/100th of a cycle and the oscillations are close to this frequency also. This diode recovery time is one possible contributor for the transient spikes and also the oscillation/ringing that I was seeing. In order to dampen the oscillations, I have proposed a 3-pronged approach including voltage-limiting zener diodes, flyback diodes in series with resistors and also adding a capacitor with series resistor from drain to ground (v-). I acknowledge that I will need further testing to obtain the best possible combination of these and to determine what values of capacitors, resistors and diodes that will work best. It sounds like from your post that you are convinced that the simple flyback diode system you are using is the best overall. After some more testing, I should be able to determine which system is most cost-effective and most protective of the mosfets.
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