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TPS23754: Request for Schematic Review

ikon
Genius 5355 points
Part Number: TPS23754
Other Parts Discussed in Thread: UCC2897A

Hi Support, 

Currently, designing the TPS23754 with Forward Active Clamp method.

However, the board will heat up without load (Secondary MOS, inductor and transformer all heat up) and cannot be start up when connected to about 200mA load (TPS23754 seem being reset).

Can I send you the schematic offline? Please advise.

Thanks.

  • 0
    TI__Genius 9230 points

    Hi Ikon,

    You may send me a friend request & send the schematic via direct message.

    I have a few questions:

    What output voltage & current are you designing for?

    Are you able to regulate?

    Have you tested with any amounts of load? What was the behavior?

    Regards,
    Tom Amlee
    PoE PD Apps Engineer

  • 0 in reply to Thomas Amlee
    Genius 5355 points
    Hi Tom,
    Sent you the schematic.
    The design is to output 24V @ 1.25A.
    Am able to regulate 24V from PoE.
    However, the board is quite hot (~60C) when there is no load.
    When i tried to connect to a load (usually draw 200mA @24V), the output and T2P LED keep blinking, seem like the TPS23754 keep resetting itself.
    Besides, I dare not to try any others load and long duration test because I burnt quite a lot of secondary MOSFET.
    Thanks.
  • 0 in reply to ikon
    TI__Genius 9230 points
    Hi ikon,

    You are currently set for 1260 nanoseconds of deadtime. During this period, the full output current is conducting through the body diodes of the secondary FETs, so they are getting very hot. I suggested reducing your deadtime to 150nsec. You can try even 50nsec, but you may need the additional deadtime to reduce the likelyhood of shoot through, which is more common at 24V.

    Here is a helpful design guide for Active Clamp Forward: www.ti.com/.../slua535.pdf

    It is for UCC2897A, but is equally applicable to TPS23754.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Genius 5355 points

    Hi Tom,

    According to the TPS23754 datasheet, my designed deadtime is 126.8 nanosecond.

    From my measurements, the deadtime is roughly 140ns, which is close to the design.

    Besides, recently I read an article from TI, http://www.ti.com/lit/an/snva603/snva603.pdf

    Seem like active-clamp forward converter is not suitable for my design (24V @1.25A).

    I might consider change solution to flyback.

    Do you have any recommendations?

    Thanks.

  • 0 in reply to ikon
    TI__Genius 9230 points

    Ikon,

    Regarding the deadtime: you are correct. Thank you for the correction.

    Here is a schematic for a 24V flyback attached.

    TPS23754EVM420_24V-SingleGate-Schem.pdf


    Regards,
    Tom Amlee
    PoE PD Apps Engineer