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LM5006: Stress voltage calculation for internal/external mosfet for multi-output fly-buck application

KPA
Prodigy 220 points
Part Number: LM5006
Other Parts Discussed in Thread: LM5017, LM5161

Hi,

To decide the comparability/compatibility of the regulator IC for fly-buck application,

How to calculate max stress voltage that can appear across main MOSFET and synchronous MOSFET in multiple output Fly-Buck topology?
In multi-output (on secondary sides) application, whether each output has to be separately added for its reflection on primary side or the only maximum output (among multiple output) only needs to be added for its reflection over primary side?

Regards,

  • 0
    TI__Expert 4051 points

    Hi KPA,

       About your question, I think fly-buck is different from flyback. In flyback design, you need to add reflection voltage to Vin. For example, at primary side Vmos=Vf+Vbulk. But in fly-buck, things are different. When high side MOSFET is turned off. The reflection voltage from secondary side is equal to VL(inductance). So the max stress voltage is nearly same to a normal BUCK regulator.

       About your project, I have to say LM5006 is a Non-Synchronous Buck Regulator, which is not suitable for fly-buck application.  Generally speaking ，when the load current of primary side is low/secondary is high，the  voltage of secondary side will drop. That would be more likely to happen when duty cycle is high. You may take risk if you insist to design a LM5006 fly-buck, although it depend on real load situation. 

       I would like to recommend LM5017 for your flybuck design. LM5017 is a 100V Synchronous Buck Regulator, which is suitable for fly-buck. I think it can work in your project. You can use webench tools to assist you. If you have any other questions, please feel free to reach out. 

    BR,

    Victor

  • 0 in reply to Victor Yu
    Prodigy 220 points

    Hello Victor,

    Thank you very much for your reply.

    I think I got your point for fly-buck regulator. I am designing my application around LM5161.

    And I am still curious about multi-output FLYBACK (not fly-buck this time) to get my knowledge updated.

    In multi-output (on secondary sides) application, whether each output has to be separately added for its reflection on primary side or the only maximum output (among multiple output) needs to be added for its reflection over primary side?

    Regards

  • 0 in reply to KPA
    TI__Expert 4051 points

    Hi KPA，

      In multi-output (on secondary sides) application, we only need to calculate Vf for one time, which is the same as a signal output flyback.

      LM5161 is a good choice for fly-buck, happy to see that.

    BR,

    Victor

  • 0 in reply to Victor Yu
    Prodigy 220 points

    Hi Victor,

    Still I am not getting what I am looking for. Sorry for that.

    Let's say, Vpri = 10V, Vf1 = 5V, Vf2 = 12V Vf3 =18V and turns ratio is 1:0.5:1.2:1.8.

    Now,

    Vmos = 10 + (5/0.5) + (12/1.2) + (18/1.8) = 40V

    OR

    Vmos = 10 + (18/1.8) = 20V

    Which one is true from above?

    Regards,

  • 0 in reply to KPA
    TI__Expert 4051 points

    Hi KPA，

     Later one. In your description, Vmos=20V. It will be better if you could consider a estimated leakage inductance voltage spike Vlk and reserve 20%~50% margin. That is chosen Vmos=1.2*(10+18/1.8+Vlk)

    BR,

    Victor

  • 0 in reply to Victor Yu
    Prodigy 220 points

    Dear Victor,

    Thank you very much for the straight forward answer and valuable information.

    It clarifies my confusion.

    Warm regards,