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LM5118 upper fet driver damage after powerup

Jack Cachel
Prodigy 120 points

Input 18-56V

Output 41V /1.5A

Freq  <300kHz

VCCX supplied externaly with ~13V

Used Quick Start Calculator.  Rechecked already for wrong comp values.

Blows upper fet driver immediately/ or few seconds after power up. I power up at ~52V input to run it in buck mode initially.

I have two layer board.

Boost/Buck diodes Schottky

All components surface mount.

I see quite a bit of ringing at the top of buck diode if it runs long enough for me to get a look.

Also, as soon as boost starts switching,  the circuit breaks out in oscillations, about 3x higher frequency.  If I lower the input more its start hiccuping.

Help will be appreciated.

  • 0
    TI__Intellectual 1435 points

    Jack,

    Can you post a pdf of your schematic?   

    Buck ringing at SW and Vin can be caused by poor bypass capacitance at the Vin pin.   In the buck mode, the input current is discontinuous so bypass is critical to reduce ringing.    What FETs are you usuing.  Is the FET BVdss significantly higher than Vin?   All bucks need some BVdss margin to allow for ringing.  20% margin is a good rule of thumb.

    Best regards

    David

  • 0 in reply to David Pace
    Prodigy 120 points

    0676.BB_ 6800221E06.PDF

    Hi David,

    Thanks for helping me.  Since yesterday I have added a snubber 10 Ohms / 1nF on the buck fet source. I powered uo into no load and some 52V input.  The chip did not blow up, I looked at the buck fet gate:  low duty drive with sinusoidal ringing when the inductor went discontinuous.  The pulse width was not stable though, a lot of jitter.  I increased the load and at some point about 75% and the gate pusle got more unstable. In order to find stability I decreased R3 to give it steeper ramp, at which point it went hiccup, I think. I switched Vin off. Next power up was unsuccessful - the upper driver got damaged.

    I am using a 1uF MLLC cap on Vin, close to the pin. FETs are IRF7854 80V/10A..( previously FDS 3692 100V / 4.5A). FET do not get damaged.

    Rgds.

    Jack

  • 0 in reply to Jack Cachel
    TI__Guru 59331 points

    Hi 

    I am not sure how big was the voltage spike at SW node though, it looks like the voltage spike at switch node is related with your issue since you got an improvement after applying RC snubber.

    Is it working at 18V input voltage ? or you see the same failure at 18V input ? 

    Regards,

  • 0 in reply to EricLee
    Prodigy 120 points

    Hi , Eric.

    I am not sure if it works at 18V.  I tend to power up at higher input volt. I will do the test tomorrow and get back to you.

    Thanks.

    Jack

  • 0 in reply to Jack Cachel
    TI__Guru 59331 points

    Hi 

    Since the high-side driver is being damaged at high input supply voltage, if the board works well at 18V, you can measure the spike and estimate what will be the maximum allowable input voltage with current spike voltage. 

    Regards,

  • 0 in reply to EricLee
    Prodigy 120 points

    Hi, there

    I need to continue this thread.

    Now I am having the same problem - high side driver damage.  This time I have the same circuit on a 4 layer bd: I followed layout guideline of TI eval board. I have 10 Ohm /1nF snubber on HS node,

    I powered up into no load, ~31Vinput. It looked good until I increased the input to ~45V. 

  • 0 in reply to Jack Cachel
    TI__Guru 59331 points

    Hi

    If the RC snubber was not enough, please try to increase R4 gate resistor (Use 3~10ohm)

    This gate resistor will slow down switching and minimize voltage ringing. 

    Regards,