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[FAQ] TPS23753: About TPS23753 to 12V output 48V voltage instability

Part Number: TPS23753

I borrowed from the original circuit using TPS23753 to 5V output, I want to change to 12V output, the original circuit is as shown below, the reference circuit is shown in Figure 2 or the link (ie development board: PMP8803)

We borrowed the existing PCB board to replace the corresponding transformer, optocoupler and other materials according to the BOM of PMP8803, but after the replacement, we found that the 12V output is unstable.After testing, we found that the 48V power supply is unstable and the 12V output is unstable. The 48V voltage waveform is as shown below.

This problem has plagued us for a while and has not been resolved. Do you have any ideas?

I have done a few tests:
Confirm that the resistance of the DEN pin is 24.9K.
The resistance of the CLS pin is changed to 1.24K or 90Ω without change.

  • We found that when the board is connected to a non-standard PSE device (no detection and power grading), 48V stable input, 12V output is also stable; when docking standard PSE equipment (with detection and power grading), 48V input is unstable 12V output also has fluctuations, fluctuation range: 8.6V~12.3V.
    Any hints where the problem with this issue might be or what possible solutions would be possible?
  • In reply to bridge:

    Hi Bridge,

    Thanks for sharing the waveform. It looks like the PSE is removing power after successful detection/class. Do you have some load on the output? Can you try adding around 50mA to the 12V load to see if the PSE maintains power?

    Thanks!

    Regards,

    Darwin

  • In reply to Darwin Fernandez:

    Hi Darwin,
    Thank you very much for your reply.I have added a 50mA load to the 12V and the circuit is working,12V output and PSE 48V input are stable.
    I tried to adjust the current to 10mA, or the 5mA circuit can work normally.(But when the current is less than 2mA, it will go wrong.)
    I am wondering why there is such a phenomenon? I want to know how you usually solve this problem in practical applications.

    Thanks a lot!

    Best Regards,

    Bridge
  • In reply to bridge:

    Hi Bridge,

    Per the IEEE802.3at standard, the PSE must have some minimum load to stay connected (maintain its voltage) to the PD. This is 10mA DC through the Ethernet cable (provided by the PD's load). So if the Ethernet cable is ever removed (current goes to 0mA or below the 10mA threshold), the PSE will believe the PD is no longer connected and can therefore remove the high voltage on the RJ45 (for safety). Operating your DCDC converter with no load (also looking at your schematic, the diode recertified flyback in general has a high light load efficiency), the PSE was probably not seeing greater than 10mA to maintain power. Note the spec is called Maintain Power Signature or MPS. This is also explained in the datasheet.

    Thanks!
    Regards,
    Darwin

  • In reply to Darwin Fernandez:

    Hi Darwin

    Thank you very much for your patience and my question has been resolved. At the same time, I will read the datasheet carefully.

    Thanks a lot !
    Regards,
    Bridge

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