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Originally Posted by dsirovica
Ether, I assume you got the PN FDP8441 from an actual Jag
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I got it from the schematic in the RDK-BDC24 Rev B Hardware Design Package. The schematic shows a pair of FDP8441's for each of high+, high-, low+, and low- in the H bridge. Perhaps the FETs have changed since then.
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the FDP8441 max switching times are 77ns ON, 147ns OFF that is about 0.3% of the PWM cycle.
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The turn-on rise time is 24ns and the turn-off fall time is 17.9 ns. Which would be the correct numbers to use for this analysis?
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The Jag datasheet has a writup on how the MOSFET is used to sink motor current thus reducing power dissipation compared to a diode based solution.
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That's the alternating ON/OFF of the hi and lo side FET pairs mentioned in my previous post. If the source for that information is correct, then during the OFF portion of the PWM cycle, the motor is shorted through the low-side FETs, providing a low-resistance path for the inductive current in the motor to continue flowing (and producing torque). I assume that if the jumper is in the "coast" position, this behavior is modified for low duty cycles.
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Maybe Victors don't do that so that would be a -ve for the Vic in this debate.
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I've heard the Vics do not do this, and here's my
speculation why they do not: the PWM period of the Vics is so long that even for relatively high duty cycles (say 50%), the motor inductive current would decay before the end of the OFF portion of the cycle, and, for higher motor speeds the motor back EMF would take over and start shoving current in the other direction, effectively reversing the torque. This would reduce the average torque output of the motor. Anyone care to comment?