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LM5050-1: Gate output is unexpectedly low and FET gets hot

WM5295
Guru 54715 points
Part Number: LM5050-1

Hi TI,

A user has a problem with the GATE output voltage level when driving a buck/boost circuit, needs assistance, and does not know who to contact at TI.

"I connected a DC power supply to the input of it and it turns on FET by creating a Vgs of 3.5V. Below are my voltages when I connect the output of buck regulator to LM5050

Pin4 – 15.6V

Pin5 – 15.9V

With the above condition, the body diode in the FET conducts and the voltage at pin 6 is 15.6V, but its heating to 90C. What would be inhibiting LM5050 form providing a Vgs to enhance the FET when I connect my output of buck regulator to it? "

  • 0
    TI__Genius 16305 points
    Hi

    Can you please provide a waveform showing the faulty behavior.

    Regards,
    Kari.
  • 0 in reply to Karikalan Selvaraj
    Prodigy 15 points

    Hello,

    Attached are the waveforms showing the problem we're experiencing.

    Blue channel is LM5050-1 Pin 4 which is also the source of the mosfet

    Red channel is LM5050-1 Pin 6 which is also the drain of the mosfet

    Green Channel is LM5050-1 Pin 5 which is also the gate of the mosfet

    Black channel is Vgs

    First waveform is captured at power on. Second picture is in run mode. As you can see the gate voltage is almost equal to the source voltage and the mosfet doesn't turn on.

    I did try the same LM5050-1 with Vin from a power supply instead of the buck output and the gate turns on.

    Also, this is the MOSFET we are using: FDMC86340ET80