I'm trying to sum up all the ingredients for motor control power electronics:

High Inrush current circuit – The short circuit and starting current rating of the DC motor is 7 times higher than the steady state operating state.

Current Sensing circuit - Active current control/feedback is needed as torque (~current) is the only right control parameter.

Current limiting circuit

Protection circuit from transient/spike from power supply

Overheating protection/Thermal shutdown

Overvoltage Clamping

Overvoltage and undervoltage shutdown

Better heat dissipation IC & Board layout

Back EMF protection circuit

Reverse current protection – Charge builds up can cause damage back to the circuit by a reverse current surge when switch is closed. The electrical energy stored in the windings that have inductance in the DC motor. Flyback diode is a solution that dissipate the stored charge.

PWM Interface - Use a PWM, measure current, feedback the PWM with a PI controller from the current measurement

Did miss anything ? I'm doing a re-design from scratch.

load stall torque (650in-lb) and 30A peak current. Voltage 28Volts

  • 1
    \$\begingroup\$ you forgot one thing ... a question ... you did not ask anything \$\endgroup\$
    – jsotola
    Jun 16, 2021 at 17:02

1 Answer 1


You got wrong the inrush current. If you do current control, there is no higher current to the motor on startup, only what you intend to drive, usually a result of friction and acceleration.

Current limiting is largely based on the same current sensing and control circuitry and algorithm. Except you can add fast protection for a real hard failure- to disable the power circuit immediately.

Overheating protection in fact starts with proper cooling. You should be inside a certain performance envelope. Do monitor the temperature, but again, you are not supposed to exceed the normal operating limits.

Overvoltge- why will you get overvoltage? If due to regeneration by the motor, you might need a circuit to not simply clamp the voltage, but to waste excessive energy.

BEMF- you don't need protection, it's normal. You should rather know what to expect of it.

The bottom line- before you rush to a design, find examples. TI, ST and others have ready made servo boards, go see the schematics. Understand all of it. It's easy, but guesswork was done by others many years ago, you don't need to guess anymore.

  • \$\begingroup\$ For BEMF I have 90 Volts being generated, huge problem as I can't send it back to the battery. There is a lot of water pressure adding to extra mechanical force converted to electrical charge by the motor. Such a charge will damage the IC or FETs for sure. \$\endgroup\$
    – Manny
    Jun 16, 2021 at 17:24
  • \$\begingroup\$ I have no idea what you are talking about :) I have systems for 400V and for 24V. It's all about the system. What is your source? What are you even trying to achieve? Be a bit more specific, I'll be happy to help. \$\endgroup\$
    – user76844
    Jun 16, 2021 at 17:28
  • \$\begingroup\$ The DC motor is connected to a valve and I have water in the container which introduces alot of pressure to the valve. This the source of back emf . An inductive pulse from the motor is producing 90V, this back EMF, supplied by mechanical energy, and could last a long time, making it hard to absorb that much energy. Hope its clear. \$\endgroup\$
    – Manny
    Jun 16, 2021 at 17:40
  • \$\begingroup\$ Sorry, something is wrong. Pressure doesn't introduce BEMF. BEMF is only related to the motor speed. If the motor spins due to flow of the water, and reaches speed corresponding 90V, then you get 90V. If it's only a static pressure- no BEMF. \$\endgroup\$
    – user76844
    Jun 16, 2021 at 17:44
  • \$\begingroup\$ On the other hand, i don't know what an "inductive pulse" is, but I guess, you get some movement when your valve opens. If that small movement corresponds to 90V in your motor, you might have a serious problem related to sizing the motor. \$\endgroup\$
    – user76844
    Jun 16, 2021 at 17:45

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