TPS23753A: TPS23753A PoE PD – VC start-up does not reach steady operation, no AUX voltage, no output (based on PMP7355 reference)

Hi Servan,

Thanks for reaching out.

I will get back to you in 2-3 business days.

Thank you for your patience.

Best regards,

Zhining

Hi Servan,

Thank you for your patient. 

• Could the different transformer (EP1322S-10 instead of PMP7355’s) prevent start-up?

  A: According to the specs you just send, the transformer might not be the cause.

• Is there a possible issue with my RSN/CSN or RFRS/RBLNK choices?

• A: They are reasonable here. Could you also provide the Rcs value?

• Could CS pin being ~4 V DC indicate an internal latch-off or a wrong connection?

• A: CS at ~4V should not be internal latch-off or wrong connection. But it might relate to the Vout or duty ratio clamp. Could you please provide the waveform of Vc and Vout?

• Is there any known sensitivity of TPS23753A during bench bring-up (with RTN–VSS shorted)?

• A: If there is no PSE connected and 24V is supplied from bench power supply, the RTN-VSS short should not have influence.

You mentioned no secondary output. Does that mean no voltage at all or the output is lower than 12V but has some value? Could you provide the Vout waveform?

It would be helpful if you can also share the schematic for the PoE PD part circuit.

Best regard,

Zhining

Hi Zhining,

Thank you for your follow-up. Please find more details below.

• Rcs = 0.39 Ω.

• VC waveform: ~10.7 V DC (no switching ripple observed).

• Vout = 0 V; I cannot observe any secondary voltage.

• No output from the auxiliary winding either.

About the schematic

• My original design was based on SLVA305C (TPS23753AEVM-004) reference design.

• Since the transformer HA3803-BLB used in that design was not available, I modified the circuit to follow PMP7355 by “air wiring” (uzay montaj).

• In this process I removed unnecessary parts and added the secondary Schottky diode according to PMP7355.

• R47 (RSN) and C30 (CSN) were updated to 330 Ω / 470 pF.

• Because I use transformer EP1322S-10 instead of HA3803-BLB, I disabled the “Drive” section present in HA3803-BLB design and used a Schottky diode connection like PMP7355.

Summary

So effectively my PD/front-end follows TPS23753AEVM-004 (SLVA305C), but the transformer/secondary section was adapted to PMP7355 due to transformer availability (EP1322S-10). Despite these changes, I still cannot get VC to sustain above UVLO, no AUX voltage, and no Vout.

Would you please advise if these modifications could explain why the converter never starts, or if I should focus on other aspects (for example transformer polarity/aux connection or startup bias path)?

I hope that has been explanatory.

Best regards,
Servan

Hi Servan,

Thanks for the updates.

According to the schematic you provided, you need to disconnect the aux winding and primary winding.

When they are connected in series, the 24-V will be connected to GND through primary and aux windings. According to the DCR of these two windings, the expected voltage on Vc is 10.7 V which aligns with your measurement and there will be no output voltage.

Best,

Zhining

Hello Zhining,
In order to explain my issue more clearly, I have updated the schematic design according to the transformer (EP1322S-10). While making this update, a mistake occurred in the schematic; however, this mistake does not exist on the PCB that I am currently testing. The PCB I am testing at the moment was designed based on the SLVA305C reference design. I implemented the required modifications through bodge wiring to convert it into the PMP7355 reference design.

I am not sure whether the reason I cannot obtain an output is due to the selected component values or if there is another underlying issue.I would be grateful if you could help me. Thank you.

Best regards,
Servan

Hi Servan, 

Thanks for the explanation.

As you described, you bodge wired the new transformer. And you have modified the secondary side circuit.

Could you also explain how did you bodge wire the new transformer? How are those pins connected to the previous schematic pins.

Also could you note on your previous schematic for the modification you did? 

Best,

Zhining

Hello Zhining,

You can find a PDF document here that explains the previous schematic design and how I made the connections.

Best regards,
Servan

Hi Servan,

Thanks for the update. I will get back to you tomorrow.

Best,

Zhining

Hi Servan,

Sorry for the delay. I will get back to you in 2-3 business days.

Thanks for your understanding.

Best,

Zhining

Hello Zhining,

Yes, I’ve verified that all connections are complete. I scraped the silkscreen above transformer pin 2 and tied it to the GNDD polygon directly underneath.

As you noted, since I wasn’t getting energy from the AUX winding, the VC voltage stayed below the threshold and there was no GATE drive. After changing the VC-related capacitor values, that issue was resolved. When I power the board from an adapter, I now get the desired output on the transformer secondary.

I’m now facing a different issue. As shown in the schematic, when I feed 36–52 V through J4 or J3, the design works correctly and I observe a stable 12 V at the output. However, when I power the board via J6 (RJ-45) from a PoE switch, the output toggles on and off at 1-second intervals—12 V comes up and then drops. Measuring between VDD_B and VSS_B, the voltage likewise rises to 52 V and then collapses to 0 V every second.

What could be causing this behavior?

Best regards,
Servan

Hi Servan,

Glad to hear that your previous issue has been resolved.

For the new issue you mentioned, do you mean that the 12V went on and off repetitively in each second? Do you have load at 12V port or not?

Do you have the waveform for VDD or VCC or Vout? Can you also provide the specs of the diode bridge D9/D10?

Thanks,

Zhining