Quote:
Originally Posted by Kasmodean
1) Every controller circuit I have looked at either has a capacitor connected to the motor's + & - terminals, or no capacitor, or capacitors around the + & - terminals and from + terminal to ground. In your schematic you have lots of capacitors from Batt + /Motor + to ground. How does that control the voltage spikes hitting the diodes from the motor - terminal while the mosfets are off? Or if that is not the purpose of them, what it?
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Hi Kasmodean;
I've been working on my own controller based on P+S's controller. I have found why the capacitors are needed, and Paul is exactly right:
Quote:
Originally Posted by MPaulHolmes
You want to keep the inductance as small as possible, so the capacitors, mosfets and diodes need to be really close. If you had no caps, the pulses would have to come from the batteries, which would be far away, and the ESR would be high. The large inductance would cause very large voltage spikes as the current tried to change quickly, which would probably destroy the mosfets.
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You can see my controller here:
Homebuilt Electric Vehicle, especially the first post's link back to my ecomodder forum post "Why is my diy controller blowing mosfets?"
I have determined that the 3" long by 1" wide 1oz traces between the buss bars and the capacitor bank has too much inductance. The mosfets have such a low impedance and such a fast turn-on time that the voltage across the B+ and B- rails goes crazy. Measured with an oscilloscope across the rails, the nominal 12V potential oscillates between 0V and 24V at 25MHz.
So when people tell you that layout is important, they *really* mean it.
I thought I could get away with it by making a very short controller, and building on buss bars which should have lower inductance than building on copper PCB. However I learned that you CANNOT get away with bad layout, not for serious currents
I still need to rethink my design; I'm hoping that adding small axial capacitors directly to the mosfet drain and B+ will allow me to continue without scrapping all my work. Just letting you know to be careful!
-William
p.s. you can get away with the bad layout for small currents. My controller is currently built to handle 120A continuous and several hundred intermittently.
Despite the poor layout and the oscillations, my controller works perfectly on a 14A car window motor. The problem is it blows up if I use a car starter motor :P