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Original question:

UCC28740: UCC28740 Flyback converter stability analysis

UCC28740: UCC28740 Flyback converter stability analysis

Yassine DAMOH
Prodigy 70 points
Part Number: UCC28740

Hello All,

In order to better understand the UCC28740 functionalities, this presentation ( Here ) was provided to me. 

I started by deriving the open loop transfer function for all 4 modulation regions. but I still have some points that need clarification:

- Slide 11 :  For the modulator + Plant Transfer function . The Ip/Ipmax ratio is clear for the FM control mode, but not so much for the AM region. and the Vcs_max should be the image of Ipmax on the CS pin?

- Slide 12 : is neglecting the ESR value of the output capacitance could influence the results.

thank you for the support

  • 0
    TI__Expert 6080 points

    Hello Yassine,

    Thank you for your interest in UCC28740.

    1. In the AM region, the frequency is kept constant and Ip is reduced in direct proportion to the load. Ip is the primary peak current at a specific load when the control law is in the AM region and Ipmax is the maximum peak current which corresponds to the max load.

    2. The feedback pin monitors the load and generates a current that is fed to the Control law block. This block converts this current to VCST which is then fed to a comparator with the VCS voltage. The maximum current-sense threshold VCST(max) is 0.773 V for IPP(max), and the minimum current-sense threshold VCST(min) is 0.194 V for IPP(min).

    3. Yes, the ESR of a cap will have an effect on the transfer characteristics. But in a DCM topology, the max phase shift is only 90deg and this makes it hard to make the system unstable. The ESR of the output cap makes for the fixed low frequency-zero, making a high-frequency pole to compensate the ESR zero can achieve a better phase margin but it can not be compensated completely.

    I hope this answers your question.

    Regards,

    Sonal