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We're building a device that would take power from the output of BMS in a 48V Electric Vehicle (EV) (2/3 wheeler) with say a 0.5 to 2 kW motor. We are planning to use a buck regulator with Vin max of about 75V for powering our device.

  1. Usually in such an EV, when BMS disconnects due to any issue, would the motor controller handle and prevent load dump from affecting other devices connected to the battery?

  2. In case of a load dump how do we handle it?

2a. One approach would be to use a bi-directional high power TVS such as ATV50C right at input connection to prevent over voltage. Only issue here is managing that TVS Vbr is higher than max charged battery voltage and Vcl lower than max Vin of the DC-DC buck at input. Would a series resistor be helpful in dissipating load dump's energy and how to get its value?

2b. Another approach would be to sense voltage above a threshold and switch off a MOSFET to protect the rest of the circuitry. For this sensing circuitry and MOSFET Vds needs to have high voltage rating, which would be hard to get or expensive. Any thoughts or references of this design?

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We are planning to use a buck regulator with Vin max of about 75V for powering our device.

That's a bad idea, because you will introduce yet another device, weight, loss of efficiency, lastly you loose the direct connection to the battery which is an important thing.

Usually in such an EV, when BMS disconnects due to any issue, would the motor controller handle and prevent load dump from affecting other devices connected to the battery?

Without a working braking chopper with a large dump resistor, I guess no. But the battery itself could be a good recipient of dumped energy.

One approach would be to use a bi-directional high power TVS

No way.The TVS can sink a large power for very small period of time, few microseconds.

Another approach would be to sense voltage above a threshold and switch off a MOSFET to protect the rest of the circuitry. For this sensing circuitry and MOSFET Vds needs to have high voltage rating, which would be hard to get or expensive.

You didn't provide any detail about the motor driving circuit nor the motor itself. But suppose you will have a BLDC, then it matters how the MOSFET bridge is controlled. The system may turn into a generator when decelerating - if this option exists - i.e. you have 4Q driver. Or the generated voltage from BLDC becomes higher than battery voltage. In this case, it's better to have MOSFETs rated for higher voltage that directly switch the BLDC without an intermediate buck. The dumped energy will charge the battery, but the 48V motor has to spin at insane speed to charge the 75V battery, so you won't get the dumped energy.

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  • \$\begingroup\$ Thanks Marko. Some clarifications. I am not as concerned in scenario where the battery is still connected, I'm more worried about cases where the BMS cuts the battery off due to any issue - say undervoltage, temperature, battery voltage imbalance etc. If this BMS cuts off the battery when the motor is running, load dump happens. As per the installation of our device, it'll happen with the OEM. And its not only one vehicle, but different models where our device would be installed. What would you suggest to protect against load dumps. \$\endgroup\$
    – EarthLord
    Jun 1, 2021 at 11:41
  • \$\begingroup\$ @EarthLord You should clarify what's a load dump and the consequences. Further you should describe the details of motor driver and the motor and what is the cause of this dump, are you sure the motor is the cause? \$\endgroup\$ Jun 1, 2021 at 12:16
  • \$\begingroup\$ maximintegrated.com/en/design/technical-documents/app-notes/7/… this article gives a good explanation of load dump in a 24V system. And most other I have found are for 24V and 12V systems, so this question is specifically for 48V system and challenges that we have. Also as I mentioned, these are for various models of 2-3 wheel EVs with motors of 0.5 to 2 kW motors. Since it is not for one specific vehicles, I don't have further specific details. \$\endgroup\$
    – EarthLord
    Jun 1, 2021 at 12:50
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    \$\begingroup\$ Sounds like you just need to adapt the 24V solutions to a 48V environment. They are not fundamentally different; the specific chips that article are selling may not be 48V rated but either others may be, or you can develop 48V equivalents along the same approach. \$\endgroup\$
    – user16324
    Jun 1, 2021 at 13:38
  • \$\begingroup\$ @EarthLord Then a question is about load dump protection for 48V system, all about 75V and motor and its controller, BMS,...etc is just misleading. \$\endgroup\$ Jun 1, 2021 at 13:40

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