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TPS23754: TPS23754 circuit review (follow slvrb41b.pdf )

Arang Liang
Prodigy 90 points
Part Number: TPS23754
Other Parts Discussed in Thread: PMP11254, PMP8812, PMP6812

Hi, 

     We have new project to use TPS23754 ,

     We follow slvrb41b.pdf suggestion schematic of TI , but change secondary rectifier diode to NMOS.

     Please help to check our schematic , especially rectifier NMOS.

     And, in the spec, how much voltage of VGS do we need for Q3206?

     Thanks.

0317.POE 0413.pdfArang

  • 0
    TI__Mastermind 40100 points
    Hi Arang,

    What is trying to be achieved by switching to NMOS? Is it efficiency? If so, note for 12V, a diode flyback will have similar performance as a self-driven synch. This is why we do not have a self-driven synch flyback in our TI design portfolio and do not recommend it.

    For example, in PMP11254 we get ~91% of converter efficiency.
  • 0 in reply to Darwin Fernandez
    Prodigy 30 points
    hi, Darwin:
    thanks for your comment.
    But I don't understand it , if our output is 12V/2A
    diode loss is about 2A * 0.5V (PDS760 VF) = 1W
    NMOS loss is about 2A*2A* 0.02ohm (Rdson) = 0.08W

    What are the possibilities for the similarity of effiency?
    Thanks.
  • 0 in reply to Darwin Fernandez
    Prodigy 90 points

    Hi ,

    update circuit in attached file ,

    please help to check.

    thks.

    Best Regards,

    Arangpoe 0416.pdf

  • 0 in reply to Dylan Sun
    TI__Mastermind 40100 points
    Hi Dylan,

    The loss isn't from the FET. The inefficiency will come from energy loss from the transformer driving the FET for a self-driven flyback. It makes the efficiency comparable to a diode flyback.

    If wanting to implement a synch flyback with higher efficiency, it should be designed around a driven flyback but it uses extra costly components with a gate drive transformer circuit. This is shown in my other post in PMP8812. The improvement is about 1% better because the gate of the PWM controller directly drives the synch FET.
  • 0 in reply to Darwin Fernandez
    TI__Mastermind 40100 points
    Hi Arang,

    Since the 2nd gate driver is not being used, DT should be connected to VB.
    L3002 should not be used and there needs to be a series resistor (4.7 ohms) on the bias winding.
    Since you're using ceramic caps on the C3014 and C3015, I would add two more in parallel because of DC biasing.
    It looks like the feedback network was changed. You should verify stability since the compensation is different. Please see below PMP6812. This is a known proven design as shown by the test report with its stability plot. I recommend using the same design and feedback components as this.

    www.ti.com/.../slvrb41b.pdf