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ucc28070a: Noise problem with external disable

evs
Genius 5910 points
Part Number: UCC28070A

Hi,

I got the following problem:

I got a SS disable circuit. The circuit test if the mosfet temperature is below a threshold.  Is so the SS is released. Because of noise the PFC is not working properly.

 The SS disable:

This circuit is switched by some comparators.

SS circuit on UCC28070A:

When I remove Q2, everything works fine. So how do I reduce SS introduced noise. My suggestion is to add a 1k resistor between SS and Q2. So that there is some time to empty C7.

Any better suggestions?

EVS

 

  • 0
    TI__Mastermind 25570 points
    Hi EVS

    The problem is that the MOSFET is being turned on when it should not be turned on. I'd suggest the following:

    Double, double check the pinout of the MOSFET you are using - pinouts can get mixed up in the symbols used for PCB layout.
    Short the MOSFET gate to ground - that will turn it off for sure. If the system works then you can suspect that the Comparators driving the MOSFET aren't doing so correctly.
    Take a close look at the comparator outputs and verify that they are able to pull the MOSFET gate to ground. (Probe at the MOSFET gate - not the comparator output while doing this)

    In general this type of circuit is used widely and should work correctly.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Genius 5910 points
    I solved it by adding a small capacitor at the gate of the FET.
  • 0 in reply to evs
    TI__Mastermind 25570 points

    Hi

    I'm glad to hear that -

    Regards

    Colin