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Original question:

TPS23756: TPS23756 SCH CHECK

TPS23756: TPS23756 output 12V unable to carry load

xiaowei chen
Intellectual 680 points
Part Number: TPS23756

Dear all:

 1.TPS23756 output 12V, but unable to carry load, load current is about 100mA, the below is the no-load of output waveform:

2. VDD-VSS only is 34V, but isn't 48V, the below is the sch:

2063.PX30 POE _ 39.pdf

  • 0
    TI__Mastermind 21320 points

    Hello Xiaowei Chen,

    This appears to be the exact same as your other post: https://e2e.ti.com/support/power-management/f/196/p/843438/3127733#3127733

    Moving forward, it would help us and the E2E community to keep the same problem in one thread, so we can see all the steps, tests and results in one place. Thank you for understanding. 

    In this scope shot, the voltage is transitioning from 14.3 to 9.8 volts, and the average is 12.05V. This voltage ripple is probably due to light loads and no output capacitance. I would also add that since the input voltage is low, that can be a contributing factor as well. Low load, and low input voltage may be putting the flyback in DCM. Based on the VDS scope shots from the other thread, it appears to turn on for a one pulse, then turn off, instead of continuous switching. This behavior is exactly what DCM mode is. To confirm, you can take a scope shot of the gate of the primary FET to look for this behavior. 

    Our EVM for the TPS23756 uses 2 x 220uF caps and 2 x 22uF caps. Try using these values on your board. Let us know how it goes and we can continue the conversation here. Thank you!

    Regards,

    Michael Pahl

  • 0 in reply to Michael Pahl
    TI__Expert 5615 points

    Xiaowei Chen,

    It has been ~1 week since our reply to you.  Has this resolved your question?  If so, please let us know by clicking the "Resolved" button.

    If we don't hear back from you by the end of this week, I will assume the matter resolved and close this thread.

    Thanks for your local support of PoE products from Texas Instruments,