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UCC2893: Transfer function of flyback topology

Tack
Prodigy 10 points
Part Number: UCC2893
Other Parts Discussed in Thread: LM5025A, UCC28780

Hi there,

I would like to research the active clamp flyback converter using UCC2893. As I found the "Switch-mode power converter compensation made easy" SLUP341 PPT of TI seminar, I designed the Active clamp flyback converter regarding "Isolated current-mode flyback" page. But my circuit doesn't seem to be working as I expected.

Can I consider Active clamp flyback converter's transfer function using UCC2893 as same as single switch topology?

If no, where can I study transfer function appliable for this controller?

  • 0
    TI__Expert 6200 points

    Hello Tack,

    Active clamp is different than single switch topology since there are two switches on the primary side. Have you looked through the documentation listed on the UCC2893 website? Those will be specialized in active clamp topology. 

    We have a training video seminar specifically on active clamp flyback here: https://training.ti.com/active-clamp-flyback-part-1. 

    The UCC2893 is similar to the LM5025A if you are interested in the active clamp forward topology. The LM5025A has additional documentation and an EVM that you can order to test out the active clamp forward topology on hardware. For active clamp flyback topology, you may also be interested in the UCC28780. The UCC28780 also has an EVM you can order so you can test out the active clamp flyback topology hardware. 

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 10 points

    Thanks Jonathan,

    I understand that active clamp topology is totally different from simple flyback, but I am not sure how I apply the loop compensation network. I had watched the video you recommended.
    Now transformer and FETs are working well, but it is not stable.

    I prepared type2 circuit for my design since it is based on current mode topology but I don't have the information to calculate its poles and zeros of active clamp flyback power stage.

    So I would like to know how can I design error amp and its loop compensations. I thought I will be able to design loop compensation network if the power stage transfer function of active clamp flyback has the same poles and zeros.

    For my understanding, I will check UCC28780's documents.

    Thanks,

    Tack

  • 0 in reply to Tack
    TI__Expert 6200 points

    Hello Tack,

    The UCC28780 has the feedback transfer function available here on page 45: 

    This is a newer part with more support and resources so it may be easier to design with the UCC28780 for active clamp flyback. 

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 10 points

    Hello Jonathan,

    Thank you for your help.

    I read this datasheet but I could not find power stage transfer fuction. I thought that the page45 shows the way to design loop compensation network after we know the power stage characteristics very well.

    I would like to understand about difference between simple flyback and active clamp flyback's power stage.

    Could you tell me about above?

    Regards,

    Tack

  • 0 in reply to Tack
    TI__Expert 6200 points

    Hello Tack,

    I will ask my team for more resources on active clamp flyback as I am not an expert in this topology. I can get back to you with a response by Monday.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    TI__Expert 6200 points

    Hello Tack,

    Because the active clamp controller output is D, 1-D, the transformer primary magnetizing current always operates in Continuous Conduction Mode (CCM). The large signal transfer function is the same as any flyback operating in CCM: 

    This E2E post has more information on the active clamp controllers: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/987254/ucc2897a-active-clamp-flyback-converter?tisearch=e2e-sitesearch&keymatch=active%20clamp%20flyback 

    Regards,

    Jonathan Wong