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Originally Posted by rbmj
Yes, at some point the PWM has to be digitally processed - I think 1-49% is reverse, 50% is neutral, 51-100% is forward (based off of reading the WPILib source a while back, so might be a bit off). The code on the microcontroller must be very tightly optimized if they're generating the pulses on the fly - no interrupt timer will give you the time resolution you need - they must be relying on the actual instruction processing speed of the microprocessor. That manual talks about delays in microseconds...
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Courtesy of timers, purpose built hardware functions and the very sequential nature of code execution itself it's very doable with a microcontroller of this scale.
In point of fact many cheap electronic motor controllers use Microchip PICs and Atmel AVRs to perform this function, just slower and with less features.
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So those diodes are part of the MOSFET?
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Yes.
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That manual says they're arranged in a circle under the fan - those heat sinks must dissipate a lot of power.
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Yes the fan blows directly on the MOSFETs to keep them from overheating.
In other applications where weight and size would not be an issue heatsinks could be used.
In the current design there is no additional headsink beyond the packages of the MOSFETs themselves (obviously the MOSFETs have a little exposed metal on them someone might consider a heatsink).
The Jaguars I've seen have a plastic retainer around the MOSFETs that keeps them apart (generally the exposed metal on these MOSFETs is actually connected to one functional part of the MOSFET, it can carry an electric charge just as easily as the other connections on the MOSFET, for this purpose best to keep them apart).
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I don't quite grok how the synchronous rectification works - seems like black magic to me. The energy in the motor has to go *somewhere*. It's just a matter of where and how quickly (right?). Maybe if they had all that current switched to a capacitor or something, it would make sense, but I don't understand how they get an order of magnitude less dissipated energy without channeling that energy TO somewhere.
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In effect the issue with synchronous rectification is that the body diodes heat up as the energy coming back from the motor dissipates. The MOSFET transistor (so long as the polarity of the current flow is considered) when fully turned on would have less resistance than the MOSFET body diode. Therefore it makes sense when possible to turn on the proper MOSFET transistor to take the current away from the body diode. The body diode would simply get hotter and take longer to allow the same current to flow. The savings is merely what gets hot as the energy moves. Storing the energy instead of letting it dissipate would be difficult because the exact timing and amount of energy flowing is hard to predict, better to let it dissipate.
Would the circuit function without that feature, yes. However, the MOSFETs as a package would get hotter. Obviously as designed getting rid of heat is a good idea so long as it doesn't make anything significantly more expensive.