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UCC28950: Aux System , Synchronous rectification and some magnetics issue.

Mikail
Intellectual 465 points
Part Number: UCC28950
Other Parts Discussed in Thread: PMP8740, UCC28180,

hello team , I'm designing a system again with the ucc28950-Q1 but this time I want it to be similar to pmp8740.

So my questions are 


1- Our magnetic elements are evaluated as loaded and unloaded. Do we have set the value when he is unloaded or loaded? for example my shim inductance value is when there is no load 50uH but with the full load 30uH , which one do we have to take into account?


2-In the flyback converter system used in PMP8740, a line was drawn from the aux winding to feed the PFC. as in the picture below.




So in my case i have designed a flyback converter like the figure below wich has two secondary output(12V). The second output of the converter is supplying the primary devices ,  like  fan , ucc28180 controller and the drivers  and the first output of the controller is suppying the analog controllers like ucc28950 , tlc272c etc. So do you think that this will work?




3-We want to use synchronous rectification in our new system. Is there anything we should pay attention to here?
For example, can we get set up the values directly in the design tool?



Thanks in advance 

Best Regards 


Mikail 

  • 0
    TI__Guru* 85896 points

    Hello,

    1. You do have to account for the shim inductor and magnetics a full load.

    2. Using a flyback controller to power the UCC28950 is acceptable.

    3. There is an excel design tool for the UCC28950 that designs with SRs.  You can find it at the following link. 

    Regards,

  • 0 in reply to Mike O'
    Intellectual 465 points

    Hi expert , Thank you very much for your answer, but I could not understand that you said I apologize, for example, my Ls value was 20uH in the design tool, but the value of the coil in my hand is 50uH at no load and 20uH at 100% load.Should I enter as 20uH?

    My problem with the flyback is essentially that I was going to connect one to the primary and the other to the secondary so I wondered if I would have a problem. That's why I wanted to ask.

    Thank you expert.

    Best Regards 

    Mikail 

  • 0 in reply to Mikail
    TI__Guru* 85896 points

    Hello,

    If your transformer has 50 uH of magnetizing inductance you may not need the Ls.  However, I would leave a place holder for it just in case you have more switch node capacitance than you estimate.

    Is the flyback a separate design using a separate controller?

    Regards,