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LM3447 PWM generation

Goutham Bondugula
Prodigy 110 points
Other Parts Discussed in Thread: LM3447

Hi,

I'm trying to see the pwm across Gate of lm3447..

I didn't connected any secondary side.

When I'm trying to ON my circuit sometimes the Ccomp is getting failed and sometimes the Qpass transistor is getting failed

Whats the problem with my circuit?

It is similar to PMP8025

  • 0
    TI__Mastermind 25500 points
    It's hard to say with this information. But perhaps the OVP is not set correctly? If you run with the output open the voltage will rise and if the OVP is not set correctly you could over-voltage VCC. But if Qpass is failing it could just be because you are trying to power the circuit through it rather than the winding. The pass transistor in the reference design was not designed for that much power dissipation.
  • 0
    TI__Mastermind 28655 points
    Hello,
    Is the circuit failing when you are trying to look at the waveforms and works correct when not?
    Are you using an oscilloscope to look at these waveforms? If so is it or the LM3447 circuit isolated?

    If the circuit is failing by itself make sure the MOSFET Qpass transistor can handle the SOA when starting up.

    Ccomp? Is that the capacitor on the compensation pin? How is that failing?
  • 0 in reply to Irwin Nederbragt
    Prodigy 110 points

    When the device is at startup it is working...after some time Vbias is going to 17.5V and Qpass is getting failed

  • 0 in reply to Goutham Bondugula
    TI__Mastermind 28655 points

    Hello,

    How similar to PMP8025 is your circuit because PMP8025 should not be able to have Vbias go to 17.5 volts unless the LED string is opening.  At 34 volts across the LEDs the bias voltage should be 14-15 volts.  If the leakage of the coupled inductor is high you can get peak charging on the bias voltage.  As for Qpass getting failed and/or Ccomp I do not have enough information.

    Is your output 34 volts?

    Is the power about 10 watts?

    Is the coupled inductor around 2.33:1?

    Thanks,