I'm designing an H-bridge motor controller with synchronous rectification similar to the Ti Stellaris RDK-BDC24 link here. Synchronous rectification has the promise of handling the back-emf from a motor more efficiently than schottky diodes, but requires a control signal at the transistor gates. If my motor is on and I have a black out, there will be no control signals sent to the H bridge to protect the circuit from the back-emf and poof! I've got some magic smoke.

How should I design my motor controller to have enough power to stay alive long enough for the back-emf spike? The ti motor controller has, what looks like, a 1800uF cap to keep the microprocessor alive. I'm shooting for my circuit to tolerate 30A continuous to a 12V motor.

  • \$\begingroup\$ Wouldn't removing the input power just convert the motor into a generator and start producing some DC on your Vdd bus. That's what occurred on my H bridge stepper design and moving the stepper by hand started the 12V fan at low speed with power off. using DRV8844's. If your motor is 30A then start current is ~300A and your FET's need to be rated much higher than this. \$\endgroup\$ Commented Jul 31, 2020 at 21:31
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 I believe that the OP thinks that when unpowered, the transistors will attempt to block any and all current and flyback spikes from the generating DC motor from getting to the DC rails, thus killing themselves in the process. \$\endgroup\$
    – DKNguyen
    Commented Jul 31, 2020 at 22:46
  • \$\begingroup\$ The BEMF will be absorbed in the Supply Cap and limited by Diode Rs , Cap ESR and coil DCR. \$\endgroup\$ Commented Jul 31, 2020 at 22:48
  • \$\begingroup\$ motirs are mostly like capacitors, they don't produce much more voltage than you put in. \$\endgroup\$ Commented Jul 31, 2020 at 23:43

1 Answer 1


Discrete MOSFETs already have a parasitic anti-parallel body diode. That's why they can only block current in one direction. Not ideal, but should suffice for just one last cycle during a blackout. Of course, you could also just add schottky diodes but not rely on them during regular operation and rely on synchronous rectification instead.


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