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LMG3410R070: H-bridge gets /FAULT interrupts

Part Number: LMG3410R070


My design currently gets FAULT trips and there a few things that I want to change, but also a few things that i am not sure about. here is 1/4 of my H-bridge. The other parts of it is identical.

Each power level has a well decouple 2W isolated power supply. I currently switch at around 100 khz. I have around 100ns deadtime. As my bus voltage gets to around 200V one of the FAULT_BL signals triggers and turns of my system. There is hardly any temperature rise on the LMG3410R070 and i have no reason to believe that there is anything wrong with my 12V supplies as the entre system draws less than 2W (that is 3 isolated 2W supplies plus an MCU plus some LEDs).

* I intend to change my RDRV resistor to around 40K

* I have also added a RC filter on PSFB_BL before the isolator

However when i Look in the datasheet, section 9.2 it is not entirely clear what RC filter i should add as there is one filter before the isolator and an identical one before the LMG3410R070. The datasheet is a little unclear in section (signal level shifting) on what filter is needed. 

The name "PSFB" is by the way not valid any more. This is a frequency controlled H-bridge that is regulated to do ZCS.

Can you give me some pointers of what i have missed here.

Best regards


  • Hello Martin,

    Thanks for contacting us. I couldn't see the picture you posted. Would you post the picture again please?

    As for the RC filter for input signal, you are correct about having one filter before the IC and one filter on the source side of the isolator. The value recommended can be found on the sample schematics. 

    What current level were you operating at before fault happens? Are you able to get some waveforms for the current and switch node?

    It would be good if you can share the schematics and waveforms after testing. Also does fault around 200V happens everytime?


  • Hi and thanks for the quick response

    I have updated the original post with the original schematics i mentioned,

    Since i made the changes the fault has stopped to occur. I take it that i had quite alot of ringing with the 15K RDRV resistor. I tis now 43K instead.

    I also added the RC filter on the outside of the isolator but the more i think about it it makes more sense to place it as close to IN as possible. 

    I am doing this for a rather small startup so my testing equipment is not ideal. What testing points would be of interest?

    // Martin

  • Hello Martin,

    Glad to hear that! You can probe the switch node to verify the slew rate, overshoot and ringing. You can also observe the fault signal from the isolator output to verify steady signal output. If you have high common mode rejection differential probe (IsoVu type), you can also look at high side signals such as IN etc.

    Let us know if any other help is needed.