TPS2378: TPS2378 serial connection of four PD modules can't power up issue

Sam Shyu36

Part Number: TPS2378

Dear Sir,

We design a PoE Anchors module. When we connected to the four PoE Anchors module, when the fourth PoE Anchor module was unable to supply power properly, we measured the voltage of the PD to be oscillating around 40V, such as below picture.

When just three modules are connected in series, the power supply can be normally performed. For example, the picture of the accessory, we measure the normal voltage of the PD voltage to 55.2V.

Such as below picture.

Can you let us know why? How to resolve the problem of connecting four modules in series? Thanks!

Best Regard,

Sam Shyu

over 5 years ago

0 Darwin Fernandez over 5 years ago

TI__Mastermind 40100 points

Hi Sam,

Do you have the schematic of your design using the TPS2378? How much power on each module is being outputted?

0 Sam Shyu36 over 5 years ago in reply to Darwin Fernandez

Intellectual 735 points

Hi Darwin,

Schematic as below. 4Watt per module.

Best Regard,

Sam Shyu

0 Thomas Amlee over 5 years ago in reply to Sam Shyu36

TI__Genius 9230 points

Hi Sam,

Could you describe how these individual PoE anchor modules connect together? Are the PoE Controllers be daisy chained?

Also, what are the signals you are measuring on these waveforms?

Is the 1st waveform of VDD-VSS with 3 modules & the 2nd waveform VDD-VSS with 4 modules? What are the operating conditions?

- Tom

0 Sam Shyu36 over 5 years ago in reply to Thomas Amlee

Intellectual 735 points

Hi Tom,

In and out is RJ45. Use RJ45 connect together each PoE anchor module. Like daisy chained. The waveform is measure C2096. The 1st waveform of C2096 with 3modules & the 2nd waveform of C2096 with 4 modules. Input is 30W (IEEE802.3AT) and each module is 4W.

Best Regards,

Sam Shyu

0 Thomas Amlee over 5 years ago in reply to Sam Shyu36

TI__Genius 9230 points

Hi Sam,

Could you send waveforms of a single PD powering on & only 2 modules in series? It will help us understand where those transients are coming from. Additionally, could you send a waveform of the voltage across the Vdd-VSS input at the same time as this Vdd-RTN output (C2096) with & without the 2nd daisy chained PD?

- Tom

0 Thomas Amlee over 5 years ago in reply to Thomas Amlee

TI__Genius 9230 points

Hi Sam,

Any updates?

- Tom

0 Sam Shyu36 over 5 years ago in reply to Thomas Amlee

Intellectual 735 points

Dear Tom,

Please refer below waveform.

a. 1 PoE Anchor-PowerON-TiME:

b. 1 PoE Anchor-PowerON-Voltage

c. 2 PoE Anchor-PowerON-Time:

d. 2 PoE Anchor-PowerOn-Voltage

0 Thomas Amlee over 5 years ago in reply to Sam Shyu36

TI__Genius 9230 points

Hi Sam,

Are you powering directly from a power supply or a PSE?

If power supply:
- How long of a cable are you using to connect to your board?
- Have you considered twisting the the positive & negative cables together? It may reduce that ringing I'm seeing.

Also, consider reducing your bulk cap (C2096).

0 Sam Shyu36 over 5 years ago in reply to Thomas Amlee

Intellectual 735 points

Hi Tom,

Are you powering directly from a power supply or a PSE?

PSE.

If power supply:
- How long of a cable are you using to connect to your board?

10M.

- Have you considered twisting the positive & negative cables together? It may reduce that ringing I'm seeing.

How to do it?

Also, consider reducing your bulk cap (C2096).

Yes, I'm already remove this one, but still fail.

Are you have any other idea for this issue?

Best regards,

Sam Shyu

0 Thomas Amlee over 5 years ago in reply to Sam Shyu36

TI__Genius 9230 points

Hi Sam,

The comment regarding twisting the cables together ONLY applies to power supply input, not PSE input.

Beyond that, I recommend ordering TPS2378EVM-105 & comparing.

- Tom

0 Sam Shyu36 over 5 years ago in reply to Thomas Amlee

Intellectual 735 points

Hi Tom,

Customer use PoE adapter to power supply input, just like some issue.

The PoE adapter only 48V power supply, not have PSE IC.

Best regards,

Sam Shyu

0 Thomas Amlee over 5 years ago in reply to Sam Shyu36

TI__Genius 9230 points

Hi Sam,

Can the customer see if VDD-VSS gets back to the intended 48V on a single board after inrush (82ms)?

I can see that the time scale is only 5ms/div on all the waveforms. I'm curious to see when the VDD-RTN signal of the 1st board goes to zero (meaning the RTN-VSS pass FET is fully on).

Regards,
Tom Amlee

0 Thomas Amlee over 5 years ago in reply to Thomas Amlee

TI__Genius 9230 points

Hi Sam,

Any updates?

Regards,
Tom A.

0 Sam Shyu36 over 5 years ago in reply to Thomas Amlee

Intellectual 735 points

Hi Tom,

After the experiment, using DC source to supply power, the required power for six sets of units requires about 13W, 48V/0.3A under normal load, and the normal starting voltage will not fall. Confirmed that inrush current caused PSE limiting current. We will try below solution to reduce inrush current:
1. Reduce DC/DC converter output capacitance after the TPS2378.
2. Add a load switch at DC/DC converter output.

Do you have other suggest solution for reduce inrush current? Thanks!

Best regards,

Sam Shyu

0 Thomas Amlee over 5 years ago in reply to Sam Shyu36

TI__Genius 9230 points

Sam,

No, that looks good to me. Let me know what happens.

I recommend putting load switches at the output of each PD so that each PD can finish inrush in sequence.

Regards,
Tom A.

0 Thomas Lewis over 5 years ago in reply to Thomas Amlee

TI__Expert 5615 points

Sam,

As the marketing manager, I am going back through some older threads where the customer has not confirmed our answers as "resolved" (answering their questions to satisfaction).

Is there anything else we could assist you with on this thread?

If not, thank you for your interest in PoE Products from Texas Instruments and it would be great if you could mark this as 'Resolved'.

Thanks,

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TPS2378: TPS2378 serial connection of four PD modules can't power up issue