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I am designing a controller pcb for a wall climbing robot, which uses a ~150W suction brushless motor to stick to the walls and two smaller ~30W brushless motors to move around. The system consists of a high level Intel NUC (12V ~50W) and a low level ARM-M4 (3.3V). Everything is powered by a 6 cell 250 W*h drone lipo.

Would you insist on using an isolated DC-DC to power the digital logic? The non-isolated supply can save some money and space.

Some precautious that I took to save the precious digital logic:

  • The Suction motor driver (electronic speed controller) is connected through a TPS1HA01 power switch with over-voltage over-current protection (what if the beast stalls at 150W).
  • Each motor driver is connected through their own LC filter and a TVS diode.

After reading a similar post I am inclined toward the isolated choice. What do you think?

Thanks!

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  • \$\begingroup\$ You don't have to use and isolated converter. But if you don't, you'll have to understand layout, routing, EMI/EMC, and grounding. The isolated converter may allow you to get away with a more non-ideal system. \$\endgroup\$
    – John D
    Commented Jul 13, 2020 at 21:48

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You can, but you don't have to. I would not because for isolation to be of much use, you must isolate all paths to the system. You can't leave direct connections for unpleasantness to go around your isolation. That means that every time your digital logic connects to something you want it isolated from, or connects to something connected to something you want it isolated, you need to isolate those signals too.

For example, if you were concerned enough to use an isolator to protect your logic from noise or other high-power unpleasantness from the motor drivers, you would also have to isolate the signals that interface with all the motor drivers (or use motor drivers where the power-stage and logic stage are isolated onboard the motor driver), as well as signal connections to every other unisolated circuit powered directly off the battery (since the motor drivers are also powered directly off the battery). It can quickly become overwhelming.

You might want to be careful with the LC filter as inputs to your motor driver though...that could cause more problems than it solves. I would have put an LC filter on the digital logic power input instead so you don't have a high-power LC filter in front of the motor drivers.

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  • \$\begingroup\$ Thank you! I realize that if end up using an isolated dc-dc I would direct all mcu control signals through digital isolators! Could you please explain why it is not a good idea to put LC on motor driver inputs. I could create another question for that? \$\endgroup\$
    – lead-free
    Commented Jul 13, 2020 at 22:14
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    \$\begingroup\$ @lead For reference, there are lots of RC model planes (which like your robot also fall if something goes wrong) flying around with brushless motors at much higher power levels that are not isolated. The higher power level you get, the more likely it is to become isolated because reduced incremental effort is involved vs using a DC-DC converter to step down high main battery voltage for the logic. Eventually it's easier and more reliable to provide a a main battery for the motors, a second battery for the logic, and at that point you just throw in an isolated motor driver and you're isolated. \$\endgroup\$
    – DKNguyen
    Commented Jul 13, 2020 at 22:17
  • \$\begingroup\$ RC planes are a great example, thanks again for your answer! \$\endgroup\$
    – lead-free
    Commented Jul 13, 2020 at 22:23

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