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Original question:

TPS23753A: PD off after start

TPS23753A: Output instable after a short time period

Andreas Auer
Prodigy 50 points
Part Number: TPS23753A


Tool/software:

Hello.

I made a PCB with the TPS23753A PoE controller. The design is mainly based on the reference design of the TPS23753AEVM-001 (5V output) board. I changed the transformer because of availability and used one from Wuerth 749119950 with similar specs.

Basically it is working. But at the startup I have a strange behavior. When I connect the PSE the output is regulated very well to the nominal 5V output. But after a few milliseconds (40 to 50ms), the the switching stops and the output voltage is falling. Is this because of light load and the discontinuous mode? Please have a look at the picture below. CH3 is the output voltage and CH4 is the gate output to the Mosfet.

Another strange thing I observed is, that during the first 40-50ms when the switching looks good, the voltage between RTN and VSS is rising. Looks like the hotswap Mosfet would be switched off? Below is the waveform of the output voltage (CH3) and the RTN-VSS voltage (CH4). Shouldn't the voltage between RTN and VSS be close to 0V?

The PoE power supply is powering a CPU module. When the CPU module is booting up, the PoE voltage becomes stable and stays pretty well at 5V. Even with the CPU board, I have a few hundreds of milliseconds before the output voltage becomes stable. So, I'm not sure if this "issue" is because of light load at the beginning or there is a more serious issue?!

Thanks,
Andreas

  • 0
    TI__Mastermind 34160 points

    Hi,

    Please expect delay in response due to Thanksgiving holiday.

    Regards

  • 0 in reply to Tommy Lin
    TI__Mastermind 37111 points

    Hi Andreas,

    Thanks for your patience. PoE team has multiple leaves and travels at the end of 2024. There could be 

    1. The PSE turns off the port due to lose of maintain power signature. This means the PD draw too less current from the PSE, so the PSE will assume this port is disconnected and turn off the port. It is the PD's requirement to keep the maintain power signature, not the PSE. PSE must see PD consume >10mA，duty cycle > 60ms/300ms (af, at PSE)，or 14mA , duty cycle > 6ms/320ms (bt PSE). Otherwise, the PSE - PD will loss maintain power signature, and PSE will turn off the port.

    You can consider to add a dummy load on the PD side. Or if you have a CPU module load, this issue will be bypassed.

    2.  The voltage at VC is not stable due to AUX:SEC winding of POE70P-50LD used in TPS23753AEVM-001 is 0.667:0.262 = 2.54:1, and 749119950's AUX:SEC is 2:1. This will result the VC voltage of 749119950 is lower than the case with POE70P-50LD. If VC is insufficient, it will cause a hiccup mode (as below shown). You can also measure VC-RTN to verify. 

    3. Could you share your circuit schematic? So I can see if there are other potential causes.

    Best regards,

    Diang