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Old 05-17-2010, 09:44 AM   #3403 (permalink)
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Originally Posted by squiggles View Post
Given that you have stated that the guide is 20mA/uF is there a rule of thumb for estimating the ripple current of a particular set up?
I don't actually subscribe to the 20mA/uF rule of thumb; I only mentioned it because it is a common shortcut to get a first pass approximation. The input capacitor for a buck converter has a very hard life so it pays to calculate what specs are really needed.

As mentioned before, the worst case for ripple current occurs at 50% duty cycle and can be estimated by simply halving the output current. Thus, if your controller is delivering 500A at 50% duty cycle then the ripple current will be 250A (this is an approximation, too, btw because the ripple current waveform is not a simple square wave, rather, it is a "ramp on a step").

Originally Posted by squiggles View Post
Also is there any wisdom about the best location for the capacitors, it seems they are generally built within a controller but is this any better than say a separate capacitor box located closer to the batteries?
The capacitor(s) absolutely must be placed as close as possible to the switch/freewheeling diode. This is to minimize the spike produced both during switch turn off and diode recovery. Another way of minimizing this spike is by simply slowing down the switching times for the switch (IGBT, MOSFET, whatever). But this results in higher switching losses. Always a balancing act between switching losses and voltage spikes.

The battery pack is, in some respects, already a capacitor, so putting more capacitance next to it doesn't really help. On a related note, though is that it is beneficial to twist the battery cables together as this reduces their inductance (and reduces radiated electromagnetic noise).
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