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Original question:

PMP10215: Q12 exploded when applying input voltage (240V AC)

PMP10215: Q11 exploded when applying input voltage (240V AC)

Mark Thompson2
Intellectual 610 points
Part Number: PMP10215
Other Parts Discussed in Thread: LM5021

Hi there,

I finally got around to replacing the parts that blew last time.  They have exploded again.  The second PCB I made was fine and it works.  For some reason the first one, assembled with the same parts, keeps blowing Q11.  What could be causing this?

Many thanks!

  • 0
    TI__Expert 5265 points

    Hello Mark,

    Sorry for the late reply, but I am still on vacation until 18th of August.

    When I designed this power supply, I didn't have any problem so it should work always fine :-).

    If in one board Q11 keeps blowing, then I suggest to take a look to the values of the components R66, R68, C51 (important), D22 and if the diode D21 or the FET Q12 are broken.

    One thing that comes to mind is the part number of U2: this LM5021 comes in two version, -1 and -2. The used version here is the -2, which has 50% duty cycle limit (which is an absolute "must" here, otherwise the two-switch Forward will never work and damage all MOSFET). Can you please double check which version you have on the board?

    If still all components are fine, and the version of U2 is correct, can you please apply the 24V on U2 (but no voltage on "400V" net) and watch all waveforms (pin 4 of U2), and the gate drive of Q11? These waveforms should be very similar, but Q11 turn off should be a bit delayed compared to U2 signal.

    Please let me know.

    Regards,

    Roberto