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I am planning to drive a 24v motor from a 12v battery, using a DC-DC boost circuit to generate the 24v and an H-bridge to PWM drive the motor.

A problem arises when the motor goes into the regen quadrant. The motor will just dump energy into the high-side capacitors of the boost circuit, overcharging them since there is no way to send current back out to the battery.

So here is my idea: Place a P-channel mosfet between the high and low sides of the boost circuit, with a voltage comparator and reference connected to the gate. This way, the mosfet would be programmed to dump from the high-side capacitors to the low-side whenever the output is above 25v.

Obviously this would put a lot of stress on the caps and mosfet, so I'm looking for any advice on how to achieve this more elegantly. Maybe if I threw an inductor in there somewhere it wouldn't be such a harsh dump of current?

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  • \$\begingroup\$ If you do not need to charge the battery from braking the motor, you may get some inspiration from the HERIC architecture used for inverters. \$\endgroup\$ – jippie Aug 22 '14 at 20:06
  • \$\begingroup\$ @jippie what is this HERIC architecture of which you speak? \$\endgroup\$ – Adam Head Aug 22 '14 at 20:08
  • \$\begingroup\$ I was looking for the a reference electronicproducts.com/Power_Products/Invertors/… \$\endgroup\$ – jippie Aug 22 '14 at 20:08
  • \$\begingroup\$ That is interesting. The problem is I don't necessarily just want braking, but the ability to regen normally by controlling the voltage applied to the motor. \$\endgroup\$ – Chriszuma Aug 22 '14 at 20:47
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    \$\begingroup\$ If you want to "regen" I guess this means regenerate therefore in my mind this means a battery to store the excess energy and whatever you call it, it will apply some amount of braking to the motor. \$\endgroup\$ – Andy aka Aug 22 '14 at 21:34
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Hooray, I don't need to add any circuitry! As it turns out, the DC-DC controller chip I am using, the LTC3787, actually has internal logic to dump the high-side caps to the low-side through the inductor when the output voltage is 10% higher than the target.

This is not mentioned anywhere in the datasheet, except if you look very closely at the block diagram, and notice there is an extra comparator connected to the Feedback pin which triggers the top MOSFET. I only discovered this behavior by simulating in LTSpice to see what would happen when output voltage spiked.

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