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I am trying to step up voltage from 24V to 36V using a boost converter, specifically the PI3740-00-LGIZ by Vicor, in order to power a motor. However, I want to be able to do regenerative braking with the motor, which would require reverse current through the step-up converter. Can I simply connect a Zener diode in reverse in parallel with the boost converter, which breaks down at some voltage above 36V, e.g. 37V? If not, how can I achieve this?

Edit: The data sheet for the converter is here: http://www.vicorpower.com/documents/datasheets/ds_pi3740-00.pdf

I am looking to return about 100 watts for 1/4 of a second, once every second.

Thanks!

Edit 2: Thanks everyone for the posts. After looking through all the answers, I think the easiest thing to do is use the LT8708 controller from Analog Devices, and build a circuit around that. https://www.analog.com/media/en/technical-documentation/data-sheets/LT8708.pdf

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    \$\begingroup\$ Add a link to the data sheet and estimate the amount of energy delivered back from the motor and over what duration in time. \$\endgroup\$
    – Andy aka
    Mar 20, 2020 at 17:56
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    \$\begingroup\$ I've done something similar to this before. We were actually trying to capture and store the energy from braking, but the principle is the same. We ended up designing our own discrete transistor solution. \$\endgroup\$
    – Matt Young
    Mar 20, 2020 at 21:15
  • \$\begingroup\$ I was under the impression that most/all synchronous fixed-frequency DC-DC converters were "bidirectional" ("regenerative braking"). You may find these helpful: "Bidirectional DC-DC Converter Reference Design for 12-V/48-V Automotive Systems"; or "Bidirectional DC-DC Power Converter Design". \$\endgroup\$
    – davidcary
    Mar 21, 2020 at 4:02
  • \$\begingroup\$ Thank you for those links davidcarry, the TI sheet is especially helpful. If I can figure it out from these I will come back here and let the forum know. \$\endgroup\$
    – lgabs
    Mar 21, 2020 at 18:04

2 Answers 2

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Can I simply connect a Zener diode in reverse in parallel with the boost converter, which breaks down at some voltage above 36V, e.g. 37V?

You can do that, but it won't have any effect unless the motor is being mechanically driven at higher than normal speed.

To brake the motor you can switch a resistor across it. The lower the resistance the more current the motor will drive though it, which produces the braking torque. The resistor must be rated to take the average power generated.

I am looking to return about 100 watts for 1/4 of a second, once every second.

Not sure what you mean by 'return' but at 100 watts and 36 volts the current would be 100/36 = ~2.8 amps. Some voltage will be lost across the motor's internal resistance, and as it slows down it produces less voltage, so the peak current for an average 100 watts of braking power will be higher.

The braking circuit could simply consist of the resistor and a suitably rated power MOSFET connecting its lower end to ground. However if the brake is turned off before the motor stops a positive voltage spike will be produced by the motor inductance. The Zener diode would help here.

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    \$\begingroup\$ Thanks for the response! I would like to return the energy to a battery instead of losing it to heat in a resistor, which is why I would like to do something like the diode option. In terms of braking the motor, I am driving it through an Elmo motor driver (elmomc.com/product/gold-twitter), so all I really have to worry about is allowing the motor driver to send current in the reverse direction. I assume the voltage on the output of the driver will rise when it brakes the motor, and at some point that rise would trip the diode. \$\endgroup\$
    – lgabs
    Mar 20, 2020 at 22:08
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    \$\begingroup\$ Pity you didn't say that you wanted regenerative braking. What type of battery are you using? \$\endgroup\$
    – Bruce Abbott
    Mar 21, 2020 at 5:01
  • \$\begingroup\$ Good point, I should have said that, I'll edit it up top. I am using a 6-cell 18650 Li-Ion battery. \$\endgroup\$
    – lgabs
    Mar 21, 2020 at 18:00
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    \$\begingroup\$ So the battery is 22.2V nominal, 25.2V max,, right? If your booster is 'bi-directional' (datasheet doesn't say so you would have to test it) then you may not need anything other than a voltage clamp. If the booster won't feed current back into the battery then the simplest solution might be to power the controller directly from the battery, either with a higher voltage battery (eg. 8s) or running the motor on '24V' (does it really need 36V?). Otherwise it gets messy. \$\endgroup\$
    – Bruce Abbott
    Mar 21, 2020 at 22:08
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If you want to have a reversible convertor, imagine a buck or boost topology (no matter which one of them, one is a reverse to the another).

Generally, it consist of two switches and if you don't want the reverse capability, one of the switches is a diode so it drives itself and you have only to drive the other one. If you drive both switches in anti-phase, your convertor will be reversible.

You just have to think twice about how to control it.

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