TPS23751: Question regarding choice of reference design

Hi Akshay,

Thanks for reaching out to us!

1. VFO reduces switching frequency to lower the switching losses. But VFO also has higher requirement on transformer / coupled inductor to avoid flux density saturation since frequency drops. If the efficiency difference is not significant at 500-800 mA, the VFO is selectable.

2. PMP11254 (https://www.ti.com/tool/PMP11254) is an example of diode rectifier. You could refer it and change the Vout to 5 V (refer the close-loop feedback circuit, transformer, FET selection of the two 5-V reference designs). 

3. Another concern about the idle mode is the maintain power signature (MPS): if the PD input current is less than 10 mA for a period (< 1s), the PSE will cut off this PD channel. To avoid it, the PD input power should large than 57 V x 10 mA = 570 mW and you may need the load always > 400 mW consider some efficiency. Chips like TPS23731 has autoMPS to avoid it but it does not have reduced switching frequency at light load condition. 

Please feel free to contact us if you have any further questions.

Best regards,

Diang

Hi Diang,

Thank you for your response. 

Based on your feedback, we plan to go with TPS23751 with diode rectifier instead of synchronous rectifier. Even in the idle state our PD will draw current of more than 400mA, so MPS should not be a concern for us. Our PD is currently implemented on a separate discrete board. We powered it up using the TPS23751EVM-104 and did not have any MPS issues.

We have designed the schematic for the TPS23751 using diode rectifier for 5V output, based on the reference design you provided for 12V. It is attached to this message. If you can give it a quick look and let me know if there any issues, that would be greatly appreciated!

Thanks again,

Akshay

PoE-DC-DC-diode.pdf

Hi Akshay,

Thanks for your follow-ups. Please find my comments below.

1. It is recommended to add a 0.1uF capacitor between VB-RTN to stabilize VB voltage.
2. Q5 direction looks incorrect in the gate driving circuit. Could be a symbol issue but please double check.
3. Could you share the transformer PA3885.002NL’s datasheet? I could not find details on line. Or please make sure the saturation current, turns ratio, rms current are suitable for your design.
4. D20 is 60-V 8-A rated. You could consider one with lower voltage rating and higher current rating for 5-V output case.
5. It is recommended to add a resistor in series with C45 for damping. Now the TLV431’s cathode and Vref only has a capacitor.
6. By the way, if you do not need the VFO function, other TI's products like TPS23731 or TPS23754/6 could be considered for Class-4 (<25.5 W) applications as well. While keeping with TPS23751 will also be a good choice.

Best regards,

Diang

Thank you for taking at look at the schematic, Diang.

1. Noted.

2. You are correct. The emitter and collector connection had been interchanged. Thank you for pointing this out.

3. The datasheet for the transformer is here: https://www.mouser.com/datasheet/2/447/P719-2903969.pdf . We chose this transformer based on the PMP20220 (5V, 3A ref design) and from the datasheet it looks like has suitable parameters. The reference designs for higher currents used the NA5730-AL. Would you recommend this one instead?

4. How about the PDS1040: https://www.diodes.com/assets/Datasheets/PDS1040.pdf ? It is rated for 40V 10A. 

5. Thanks, we will do this.

6. Will take a look at those controllers as well.

-Akshay

Hi Akshay,

Thanks for the follow ups.

#3 and #4 look good to me.

Besides, you could consider to add a snubber circuit at Q6. It is not necessary but we did see some customers have design with large stray inductance and high Vds turn-off overshoot at the primary side FET.

No other comments from my side.  

Best regards,

Diang