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TPS23753 PoE + 24VAC used for 12VDC

Joe Rule
Intellectual 330 points
Other Parts Discussed in Thread: TPS23753

Hello,

     I'm trying to use the reference design from "PoE Powered Device for 24 VAC Building Power Applications " application note to allow for both 24V AC as well as 12V DC input on same AC+ line.  This results in about a 1.4V drop for the 12VDC, resulting in a 10.6V input at VDD and VDD1 of the TPS23753.  What I'm finding is that at any input level under 13V DC, I get a stable 12V output of the power managment circuit for about 600ms, then I see a drop to about 6V for a few ms, then repeats.  At first, I assumed I needed to change the values of the APD resistor divider to accomodate a lower start-up voltage.  But I have changed them to 316k and 63.4k and I still see this issue.  The rest of the component values are still that of the app note schematic.

It is my understanding that I should be able to start-up as low as 9.3V with TPS23753, so can anyone help me understand what I'm not doing correctly?

Thanks,
Joe

  • 0
    TI__Mastermind 42795 points

    This design will not operate from a 12V adapter at full load. This is based on the power stage and is primarily dominated by the flyback transformer turns ratio. I'd say you could get ~5W out of it with a 12V adapter but as loading goes up, the duty cycle will rail at 80% and the output voltage will drop.

    The TPS23753 Vc UVLO is ~9.3V and at this voltage GATE will start switching. But if the AUX winding cannot keep Vc above ~ 5.5V the circuit will restart.

  • 0 in reply to Eric Wright
    Intellectual 330 points

    Eric,

         Thanks for the quick response.  Can you tell me if there is another transformer that would allow for full load on 12V DC input adapter?  I'm just trying to get an idea of how much delta design change I would require.

    Thanks,
    Joe

  • 0 in reply to Joe Rule
    TI__Mastermind 42795 points

    Would it be possible to insert the 12VDC adapter at the output side of the flyback instead of the input? You could OR two 12V sources this way. I cannot think of a wide input range XFMR for this application, but I will check around.