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Unstable operation, hot MOSFET on TPS23753A following PMP8803

Other Parts Discussed in Thread: CSD19533Q5A, PMP8803


    I followed PMP8803 to design a 48Vin to 12Vout/1A PD. Below is the SCH. It is almost the same as PMP8803, except MOSFET is CSD19533Q5A and one output cap is a 25V/10uF electrolytic capacitor

With 48V input, I found the MOS is very hot if Io>100mA. Below is when Io=100mA, CH2 is Vds, CH3 is CS(current sense). There is big noise on CS, and the MOSFET is hot.

Below is When Io=200mA. It works unstably and there is a lower frequency oscillation. It looks overcurrent comes periodically.

I've changed the gate resistor R10 from 10ohm to 0 and used a 150V MOS, but no any improvement.

Which measure can I do to locate the cause?


  • Hi Di,

    Where is your 48V power supply being placed? Is the output regulating <100mA?

    Is C5 an electrolytic?

    The schematic looks okay (the class resistor should be 90.9 ohms and not k but this shouldn't explain the unusual waveforms with a 48V power supply input). There might be a misconnection or a different component value somewhere on the board itself. So that might be good to check.

    Do you also have a .brd file of the layout?

  • Thank you, Darwin.

                   The issue is the same with 48V over RTN and Vss. I don't think the output regulating with any load. I found sometimes the output was obvious brownout. I also found the board consume 0.5W in no load.

                   C5 is an electrolytic. I've attached the .brd file. Could you recommend any measure I could try?



  • Cause might be the RCD circiut(D4 R2 C6).

    D4 is SS110, which is stated as a schottky, but the reverse recovery time is unknown.

    R2 is 0603 package and C6 is 0805. 

    I changed D4 to ES1D which is a superfast diode. The heat decreased a lot on MOSFET.

    Any recommendation on selection of RCD devices?


  • Hi Di,

    It's best to stick with the reference design components and values. This reference design is a known working solution.

    However, if you'd like to experiment with different components, I recommend you look at the app note below. It shows step by step design equations to the different circuits in the poe design. Page 9 discusses the RCD snubber.