Output voltage of MC33926 H-bridge has permanently dropped

In my application that uses the dual MC33926 H-bridges, one of the two H-bridges now outputs approximately 0.5V less than the other.
Has anyone experienced damage to the H-bridge where it seems to operate normally, but the output voltage is permanently decreased?
I’m trying to determine the root cause; so that I can redesign the motor driver circuit properly.



How are you measuring the voltages? What is your load? Could you tell me more about your setup? If you post pictures of your entire setup, it would be helpful. Were they at the same voltage before?


Hi Jeremy,

Thank you for responding. Below is a photo of the physical circuit with a drawing of the power and motor connection included for reference.

I am measuring the voltages with an oscilloscope and digital voltmeter. The two loads are DC motors. The resistance at the motor terminals was measured as 1.6 ohms.
The setup shown consists of a microcontroller connecting to the H-bridge, and a 6V DC 1A wall adapter supplies the H-bridge.

They were both operating at 6V output initially. During testing, the motors were both stalled momentarily at 100% duty cycle. There were a few incidences where the 6V DC power had to be disconnected while the motors were stalled. After a couple days of testing, I noticed the “braking” physical resistance was less on one of the motors. At that point, the measured output of one H-bridge was 5.5V. The other H-bridge still operates at 6V. And both still output correct PWM pulses.

Something in that H-bridge must be damaged. I read the data-sheet and the section titled Output Avalanche Protection concerns me. Please share your experiences with failures of power MOSFET’s and H-bridges.

I am not sure that the driver is damaged. Could you disconnect the motors, set the duty cycle to 100%, and measure the voltage across the motor output terminals. If you still get the same voltages (i.e. 6.0 and 5.5 V), could you check if it is still able to drive a motor forward and backward with the H-bridge.

- Jeremy

Tests done. The questionable H-bridge outputs 5.5V at 100% duty cycle and drives the motor in both directions.

Just to be absolutely clear, can you confirm that you made the measurement with no load connected? Could you drive the outputs in both directions (still with no load connected) and see if the direction affects the measurement? Could you measure the two driver outputs relative to the ground pad of one of the four big electrolytic caps (the ground side is the one with the black mark on the can)?

- Jeremy

Yes, this is the test sequence that I followed.
With no motors connected:

  1. control H-bridges for freewheeling high output
  2. control H-bridges for 100% duty cycle from one output pin, while other pin is held low
  3. repeat step 2 swapping 100% duty cycle and low output pins–i.e., reversing direction
    With motors re-connected:
  4. repeat steps 1 through 3

Hello, jfl.

Thank you for taking those additional measurements. It seems like something is probably broken, especially if it didn’t behave this way before, but we are not sure what. Can you try raising the supply voltage above 8V and see if that makes any difference? (These drivers have derated performance below 8V.)

- Ben