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TPS23758EVM-080: Theory of Q1 Operation

Part Number: TPS23758EVM-080

Hi Genius Folks!

In reference to the TPS23758-EVM, I'm making a board that pretty much follows the design.

I'm curious about how to understand the operation of Q1 (what is the role, and how do all those diodes drive the gate?)

I looked for Primary Side Control Flyback info, but usually I mostly only see D8+RC Snubber as part of the secondary side.    But all that other fancy Mosfet/diode gate stuff is new to me. (Can't mentally figure out the direction of current flow/stages of operation)

Is there a similar design I could read into to understand this architecture better?

Thanks a bunch!


  • Schematic of secondary side... seems Q1 body diode is parallel with D8, so... when does Q1 get turned on? Hmm! :)

  • Hi Jeff, 

    Thank you for your question.

    The PoE applications engineering team is currently out of the office and will respond to this post by end of day Monday. 

    Best Regards, 

    Brett Colteaux

    PoE Product Marketing Engineer

  • Hello,

    This is called a synchronous flyback since it uses a FET on the output. You can get rid of all the diodes for simplistic version of the schematic. 

    In short, D3 and D4 are clamping diodes so the gate does not exceed 20V.

    D2 is a part of a pulldown circuit so the FET turns off quickly to maintain synchronous timing. 

    D3 is in parallel with the body diode. When the FET is off, the circuit still needs to provide current to the output. This provides a more robust current path to the output. It also acts as a clamp when the FET turns on and off.

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.




    Michael P.

    Applications Engineer

    Texas Instruments 

  • Michael,

    Thanks a bunch for your insight.  I'm sure it will help me get things working!