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UC2844 compensation tuning

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

Hello,

I am working on a flyback converter of approx. 35W. Input range 250V-850V working in discontinuous mode. 

Its UC2844 based PWM controller. The operating frequency is 50kHz. 

The compensation pin components RC should be tuned at frequency of  approx. 1/10th of output frequency as per literature.

But i am getting stable output at RC values which is tuned at 300Khz which is out of operating frequency band and am not able to get convinced with this RC tuned value although this works perfectly at all load values.

Can somebody comment on this further or some document particularly on COMP pin tuning.

Thanks,

Mukesh

  • 0
    TI__Expert 8210 points

    For closed loop control stability the crossover point of the loop point needs to be below the switching frequency.  Typically this crossover point can be as high as 1/4th or 1/5th of the switching frequency, but more conservative designs can work at 1/10th.  The RC parameters need to be selected based on the power stage components, since the compensator you designed works with the plant (flyback converter power stage) to close the loop.

    Here is some literature describing feedback design with the UC284x/UC384x parts.

    Closing the Feedback Loop

    Design Review: 150W Current-Mode Flyback - page 13 goes through a design example with a DCM flyback

  • 0 in reply to Eric Faraci
    Prodigy 185 points
    Hello Eric,
    Yes right i also agree that for stability the corner frequency of compensation network can be 1/5th to 1/10th of the switching frequency. But when i do that my power supply becomes unstable and output frequency oscillates and changes as low as 5khz from 50khz. But when my compensation network is tuned close to 300kHz(R=1kohms and C=470pf) then the system works perfectly. I really dont understand how it works at such high frequency COMP value tuning frequency and have no relation with switching frequency its almost 6 times of switching frequency.