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LM3481-Q1: 2 Output DCM Isolated Flyback Control Design

Anas EL MOULA
Prodigy 30 points
Part Number: LM3481-Q1
Other Parts Discussed in Thread: LM3481

Hello,

I am currently working on the design of a 2 output flyback operating in DCM with primary side regulation for my customer, where:

OUTPUT 1: 15V/43mA_max indirectly regulated.

OUTPUT 2: 12V/200mA_max directly regulated.

My issue is, there is App Notes for control design for CCM only (one oputput flybacks).

Now, I wish to design control myself and check for stability (mathematically speaking) but I am having trouble doing so and i would like to have some support concerning this matter with whatever I can get.

Thanks in advance and Best regards,

Anas.

  • 0
    TI__Guru* 78200 points
    Hi Anas,

    Why do you want DCM? DCM does not have good cross regulation and your OUTPUT1 output voltage would vary a lot. Nor does it have good efficiency. Recommend to design your OUTPUT2 (directly regulated) to stay in CCM.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 30 points
    Hi Youhao,

    Cross regulation is not an issue in here since we are using a buck converter just after OUTPUT1.

    What I would like to know is:

    Firstly, how do I calculate the power stage Transfer Function in DCM.

    Secondly, how open loop TF of the power stage + control (in peak current control mode) are calculated, in order for me to check for worst case phase margin and see if the system will ever go out of stability specifications due to power stage and control components' tolerances.

    Thanks and Best Regards.
    Anas.
  • 0 in reply to Youhao Xi
    Prodigy 30 points
    Hi Youhao,

    Cross regulation is not an issue in here since we are using a buck converter just after OUTPUT1.

    What I would like to know is:

    Firstly, how do I calculate the power stage Transfer Function in DCM.

    Secondly, how open loop TF of the power stage + control (in peak current control mode) are calculated, in order for me to check for worst case phase margin and see if the system will ever go out of stability specifications due to power stage and control components' tolerances.

    Thanks and Best Regards.
    Anas.
  • 0 in reply to Anas EL MOULA
    TI__Guru* 78200 points
    Hi Anas,

    I checked our archives and unfortunately I haven't found doc to share with you. I would recommend you to refer to some power electronics text book like written by Dr. Erickson. or the book by Christophe Basso, in which DCM flyback analysis is detailed. The LM3481 is just a current mode PWM controller governed by the theories in these text books.

    Thanks,
    Youhao Xi, Applications Engineering