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LM5176: Possible causes of damage

Part Number: LM5176

Hi Team,

My customer designed in LM5176 and found few parts got damaged during tried run phase. Below is SCH.

SCHEMATIC1 _ 01_LM5176-1.pdf

Test condition is Vin=48V, Vout=50V, Iout=1.5A(by electronic load). Please help to check the SCH is there any improper design.

The pins in red frame of damaged unit have different impedance compare to good unit.

Customer probed some waveforms and please let us know what signals you would like to check or any experiments need to do for the investigation.

LM5176-Wavefrm.xlsx

Thanks very much.

Vincent Chen

  • Hello Vincent,

    The overcurrent protection is set extremely high, I wonder if the customer is doing output short tests which could lead to a saturating inductor.

    For me it is difficult to understand the measurements in the Excel file. Please check especially the BOOT1 and BOOT2 voltage with respect to the switch nodes.

  • Hi Brigitte,

    Please find the boot1/2 and SW1/2 waveforms under different conditions.

    LM5176-WavefrmIII.XLS

    Thanks a lot.

    Vincent 

  • Hello Vincent,

    The measurements show -18V on the gate drive signals. Please add gate drive resistors, up to 5 ohm to check if the devices survive then.

    In addition, there is a lot noise on the switch node measurements. How many layers is the design using?

  • Hi Brigitte,

    Regarding SCH portion, customer populated wrong capacitor in the compensation circuit. The original design the value of C26 should be 47nF, but it was populated by 47uF wrongly. Customer used the calculation sheet to run the bold plots for two conditions as following. 

    Could you advise which one is the proper value?

    C26= 47uF

    C26= 47nF

    Btw, the PCB is 4 layers.

    Thanks a lot.

    Vincent Chen

  • Hello Vincent,

    47uF is for sure the wrong value for the compensation. This makes load responses very slow and it could happen that the overshoots happening on the output cause damage to the system.

    The quick calculator is calculating the necessary values for the compensation network. The customer only needs to make sure that he selects a crossover frequency that is low compared to the right have plane zero.

  • Hi Brigitte,

    Thanks a lot for your advice.

    Vincent Chen

  • Hi Brigitte,

    Customer's design spec. is as following, 

    Vin : 10~50V

    Vout : 50V

    Iload : 1.5A

    Could you please comment the values of compensation circuit below is OK or not(C26=47nF)? Or, do you have suggested values for this application?

    compesation-47uF_2D00_1.jpg (318×275)

    Thanks very much.

    Vincent Chen

  • Hello Vincent,

    Please use the Quick Start Calculator on the LM5176 product folder to check if these values are ok. Please add the calculator result here if it is showing issues with the components used, then we can dig more into the details.

  • Hello Vincent,

    I expect that the 47nF are ok, but please again ask the customer to check for overvoltage on the device pins. The measurements show overshoots and undershoots that are outside of the abs max rating.

  • Hello Vincent,

    I expect that this issue is solved and I will close this thread now. If this is not the case, please post additional information below or create a new thread, if this one is already locked.