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UC2844 output frequency variation

mukesh chaudhary
Prodigy 185 points
Other Parts Discussed in Thread: UC2844

Hello,

I am using UC2844 in flyback topology as PWM controller for power supply of 50W. Whenever I load the power supply to full load, the switching frequency reduces to half say 20khZ and on no-load the frequency is rated i.e 40 khz in my case.

But when I see oscillator frequency it is fine i.e 80kHz and output frequency of gate pulses should be 40kHz, but I am getting 20kHz on load and when I remove load its 40kHz.

I am not getting why the frequency reduces to half on load although oscillator frequency is fine in both the cases.

Thanks,

Mukesh

  • 0
    TI__Mastermind 31580 points
    Hi Mukesh,

    The UCC2844 is a fixed frequency controller so the switching frequency is constant. Are you measuring the frequency on the gate drive? If the measure frequency of the gate drive is varying you could have an instability in the control signals. Take a look at the FB pin voltage and the CS pin. Is the FB pin flat and stable? Is the CS pin hitting peak current limit value and then shutting down the PWN drive.

    Is the Rt/ct pin waveform stable and fixed frequency.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 185 points
    Hello peter,
    Thanks for the information.
    Yes,it was an issue with compensation of the system. By changing the value of compensation resistor at COMP pin, now its stable and i am getting correct waveforms at load also.
    But i am still in search of good document regarding compensation of UC2844 i.e. component selection between COMP pin and VFB pin. Some bode plot analysis or so. or if any PSPICE simulation it would be a good experience, coz right now i have done based on trial and error.

    Regards,
    Mukesh.