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LM5085: Transistor dies and turns into a short circuit under heavy load

Michael Browning
Prodigy 210 points
Part Number: LM5085

I'm using the LM5085 buck controller to create a 48V to 24V DC Buck Converter with a 8.8A Load

Thus far I have been able to get a 6A load to work but when the load is increased to 7A the transistor shorts out. The circuit went back to working with a 6A load when I replaced the transistor with a new one.

Here is the schematic:

Here's the board layout:

The transistor that is dying is SUD09P10-195-GE3

I know I need to work on shrinking the current loops but I don't believe that is the source of the current problem.

In case it is relevant, I have to switch to using MFA420PS48 (48V 8.8A) instead of a bench top power supply to test the 7A load.

  • 0
    TI__Guru 64897 points
    Hi Michael,

    I recommend checking the inductor current to see if the inductor is saturating. Alternatively, check the inductor datasheet to determine the sat current rating (and derate by 20% for operation at hot).

    Note that the dissipation in the diode and FET are both relatively high at 7A load current. Also, I recommend using a 100V ceramic cap for C2 and increase its capacitance (e.g. 1uF/100V). This provides better decoupling for the high-frequency switching currents and reduces parasitic power loop inductance that may result in SW voltage overshoot and ringing.

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Prodigy 210 points
    Tim

    The inductor's datasheet says it reaches saturation at 19A, so I don't believe that has anything to do with the problem.

    I will order the 1uF/100V capacitor. Can the voltage rating of a ceramic capacitor be too high?

    Thanks
  • 0 in reply to Michael Browning
    TI__Guru 64897 points
    The voltage rating can't be too high, but you can get more capacitance by going with a 100V rating. What is the MOSFET part number?
  • 0 in reply to Timothy Hegarty
    Prodigy 210 points

    The MOSFET is SUD09P10-195-GE3

  • 0 in reply to Michael Browning
    Prodigy 210 points

    The problem is still occuring after switching to a 2.2µF 100V capacitor

    This is what the load (yellow) and switching signal (blue) at the gate of the transistor look like at the load where the transistor dies. It takes about 20 seconds before the transistor dies.

    Same signals but at a smaller load

    Same signals after the transistor died

    I'm going to replace the transistor again and I will capture the voltage at the drain pin of the transistor.

  • 0 in reply to Michael Browning
    TI__Guru 64897 points

    HI Michael,

    This is a very high Rds(on) FET, ~170mOhm at the LM5085's gate drive voltage level. Thus, the conduction losses at 6-7A is excessive, especially given the 50% duty cycle.

    Pcond = D*Iout^2*Rds(on) = 0.5*7^2*170m = 4W. Try a lower Rds(on) FET with the intention of keeping total FET loss (conduction and switching) below 1.5W.

    Regards,

    Tim