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Old 07-17-2010, 09:02 PM   #24 (permalink)
saand
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Join Date: Jul 2010
Location: Melbourne, Australia
Posts: 236

bugler - '91 Mazda 626
90 day: 35.89 mpg (US)
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I looked up those polyproplene caps their ESR is about 1 ohm each at the frequencies your interested in (your ringing oscillations are up around 10 MHz). So the ESR would be a problem when drawing high current, you can do some calcs to work out if you are likely to have an issue.
The standard way of dealing with this sort circuit which switches high currents very quickly is to put down multiple types of capacitors,
- some with high capacitance which likely have high ESR so that it keeps up the rails on low frequency transients
- some with mid range capacitance with his average ESR for higher frequencies
- Some very low capacitances that will have low ESR for very high frequencies (above 1 MHz)
If you see a digital circuit design there is always a 100nF capacitor physically right beside every digital chip (if the circuit has been designed properly). This is so that any digital switching takes current from the very close capacitor rather than the power supply and therefore stops ringing in the rails.
You are essentially running a really really big digital switch so you need some more of these very low ESR caps physically close to your mosfet
the wikipedia page might explain it a bit better than i have
Decoupling capacitor - Wikipedia, the free encyclopedia
Also if you ever see a product designer have issues on their rails due to ringing they wont even think about it and just put some more caps on the rails, the caps are cheap and easy and they wont hurt your circuit

you might also want to consider is your oscillation real at these sort of frequencies if your oscilliscope ground is not connected it might cause you to see this. Although from my understanding of your circuit in this instance it seems real.

Your epoxy sounds very interesting, i have never come across this sort of compound before, any plastic/ polymer based electrical connection i would be hestitant about, i think your right on the money thinking it might provide higher resistance at higher frequencies.
Can you put solder on then epoxy.
You mention you have a connection between your capacitor board and your bus bar. I assume your mosfets aren't on the capacitor board in which case you will want to be very careful about making this connection very low inductance and very low resistance. If you have a spare one of those polyproplene caps you should just physically hold it across the rails, one leg on the mosfet connection and one leg on the return of the motor. This might just get rid of or reduce the oscillation due to the physical closeness.

I also appreciate everyones help on this forum, i wouldn't be trying half of the crazy mods to my car without people here. I am currently trying to modify the parameters in my ECU its proving to be very difficult but people on here are helping a lot.
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