TPS23754: TPS23754 EVM-420

Namith HL

Part Number: TPS23754

Hi Darwin,

As per your suggestion, we changed the Optocoupler to the recommended CTR. According to the design we are supposed to get 24VDC but we are getting around 21V with large voltage spikes. The image of the output voltage is attached for your reference. This is without connecting any load at the output of the PD circuit. Can you suggest us a solution for this issue. Also attached are the Gate pulse waveform

After connecting with the rest of the circuit which will take around 200mA max. The ouput was fluctuating, so we had a check on the supply from the PSE (we were using TPS23861EVM) and we found the 48VDC was not constant. The detection was happening and 48V is supplied but after around 800ms falls to 0V and again the cycle repeats. Attached is the waveform for your reference. Also attached is the waveform measured at VC in this scenario.

Can you please look into this and suggest us the solution.

over 7 years ago

0 Thomas Amlee over 7 years ago

TI__Genius 9230 points

Hi Namith,

My response will be delayed this week, due to short staffing. I will look into your issue & respond when when an answer is available.

Thank you,
Tom A.

0 Namith HL over 7 years ago in reply to Thomas Amlee

Intellectual 570 points

Hi,

Can i expect feedback by today? Please

Regards,

Namith

0 Thomas Amlee over 7 years ago in reply to Namith HL

TI__Genius 9230 points

Hi Namith,

I have prioritized your E2E request to be next on my queue. I will try to get to it today, but cannot guarantee.

Thank you,
Tom A.

0 Thomas Amlee over 7 years ago in reply to Thomas Amlee

TI__Genius 9230 points

Hello Namith,

Are the attached waveforms:
1. Vc
2. Vdd-Vss
3. Vout
4. Gate

This app note is a good resource for calculating flyback component values: www.ti.com/.../slva305c.pdf

I would walk through the whole document, especially section 2.8 & onward. I noticed you are missing several components in your feedback loop (R21, R16, C13 in figure 1) which may result in the instability you are seeing.

Regards,
Tom A.

0 Namith HL over 7 years ago in reply to Thomas Amlee

Intellectual 570 points

Hi,
If we are using TYPE 2 PSE(IEEE802.3AT) can we able test the POE ouput with TYPE 1 PSE .

Also i have doubt for fixing classification resistor,if we fix 1.27K ohm as per datasheet minimum power is 0.44W and maximum 13W. What does it meaning?

Regards,
Namith

0 Thomas Amlee over 7 years ago in reply to Namith HL

TI__Genius 9230 points

Hi Namith,

If you use a type 1 PSE with a type 2 PD with the 1.27k ohm class resistor, you will classify as class 0 & only receive up to 13W. The PSE will default to class 0 if it cannot recognize the class current as class 1, 2, or 3.

Regards,
Tom A.

0 Thomas Lewis over 6 years ago in reply to Thomas Amlee

TI__Expert 5615 points

Namith,

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 question?

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

Thanks,

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TPS23754: TPS23754 EVM-420