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PMP20686: PMP20686, UCC28730

Rushi Jani
Prodigy 60 points
Part Number: PMP20686
Other Parts Discussed in Thread: UCC28730

Hi,

I am using the PMP20686 board for the customize solution.

Output Parameters are:

Output Voltage: 100V

Output Current: 0.5A

To achieve valley switching at full load of 0.5A, how to calculate the inductance, turns ration for the auxiliary winding and Vs voltage divider calculation? 

The equations mentioned in UCC28730 for Vs voltage divider is not matching with the current calculation

Please share the Design Calculation / Design Rational and reference document for the Up-side down buck converter using flyback control IC.   

  

  • 0
    TI__Intellectual 2885 points

    Hi Rushi,

    The first thing to consider in a buck application, is your input minimum input voltage.  It must be higher than your 100V output.  Keep in mind that if you have an AC input, the input voltage will sag during the zero crossings of the AC waveform.

    The turns ratio from the main output to the auxiliary winding should be such that it generates around 12V-15V for VCC of the controller.  The VS divider is the same as in a flyback application.

    Calculating the inductance is a little more tricky, as the equations are very different for a DCM buck vs. DCM flyback.  We don't have any officially TI-sanctioned calculators for this application yet.  Here a Mathcad calculator that was used for this design.  It is nothing fancy and there could be some errors in it.  You can open this file with the free version of Mathcad Express.

    Buck.zip

    thanks,

    Brian

  • 0 in reply to Brian King
    Prodigy 60 points

    Hi Brian,

    Thanks for the response.

    I have a question regarding the value of D2 in calculation in valley switching mode.

    The value of D2 considered as 0.425. Is it a assumed value?

    According to my understanding, the value of D2 determine the minimum peak current and inductance.

    What are the considerations for selecting the value of D2?

    Again, thanks for the insightful information.

    -Rushi