I have been trying to design this H-bridge circuit as a little project. I cannot seem to get the output I want and I am not sure why. At the output I am getting a constant voltage line instead of a PWM signal that shows switching at 42kHz.

So my questions are:

  1. What is wrong with my circuit for why I am unable to a PWM signal at the output? Full Bridge
  2. How can do we design the H-bridge to make sure it switches at our desired switching frequency?

Any help would be much appreciated.


  1. I have changed the A-side gate drive signals into the MOSFETS

  2. I have connected the DIS pin to ground


2 Answers 2

  1. Your IC does not appear to actually be connected to a power supply. Your Vdd and Vcc pins are certainly connected to stuff. But none of that stuff goes to any of your voltage supplies.

  2. As a consequence of #1, you also have no decoupling capacitors for your IC. You also do not have any for your H-bridge (your H-bridge probably draws much more current than your IC so will require much larger decoupling capacitors).

  3. Your gate drive signals on the A-side are reversed.

  4. Both of your HI input signals tied permanently to Vcc. That's just going to keep both high-side MOSFETs on all the time which will cause a short-circuit whenever the low-side MOSFETs turn on. At least until the bootstrap cap is exhausted of charge since it will never get a chance to refresh (bootstrap caps require the low-switch to turn on periodically so they can recharge).

    Why aren't you driving them like you do with your low-side signals?

EDIT: It seems this IC has shoot-through protection where the low-side input overrides the high-side input such that whenever a low switch turns on, it forces the high-side switch off. That's why you can tie the high-side inputs permanently on and rely just on the low-side inputs.

  • 1
    \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Commented Jun 1, 2021 at 3:19
  1. Your gate drive signals on the A-side are reversed. AHO and BHO go to the high-side MOSFETS, and ALO and BLO go to the low-side MOSFETs. (That's why they are labelled H and L.)

  2. You have left the DIS line floating. It needs to connected to ground.

(3. You have connected the AHI and BHI inputs correctly.)

Recheck your connections against https://www.renesas.com/us/en/document/dst/hip4081a-datasheet

  • \$\begingroup\$ Cheers, I have made the adjustments. \$\endgroup\$
    – Turei
    Commented Jun 1, 2021 at 0:54

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