TPS23754EVM-420: POE Design for 24V

Hi Darwin,

Thanks for quick reply,

I have not worked on POE design before so please suggest me some information becoz iam runing out of time by understanding POE design.

 As you suggested after reviewing my schematics changes to be done are:

1) D10 backwards,D17 backwards ..is it i have to reverse the diode?

my specification:

• POE output voltage and secondary dc supply 21-30V shall be diode OR'd with POE supply as an alternative means of powering the unit
• According to loading on power amplifier  2 modes of POE operation  i.e 802.3af (POE) and 802.at(POE+) as Different speaker load to be connected .
• status LED on the PCB whether POE or  Secondary DC is in use.
• speaker configuration will be 20W load and 6W load.
• 6W Speaker use POE ( 802.af)
• 20W Speaker use POE+ (802.at)

my doubts in poe design:

1) changes as u have suggested to change 5V/25W Synchronous flyback design are sufficient for our requirement 24V/25W

2) I have changed Transformer that can support 24V/1.25A

3)Is secondary gate drive (GAT) is not required in 24V DESIGN?

4)TPS23754 -420EVM  has secondary gate drive ,and Q1,T3,Q3 and D17  whether all this not required?

5) my main doubt T2P usage what is the function of T2P?

6)AS my secondary i/p is 21-30V we are planning to convert 24v and then diode OR 'd with POE as alternative means of powering the power amplifier load.

7)where to give my secondary 24V output.

now i have attaching changes that u have told included in pdf ,kindly suugest me any other changes to be done,.

Hi Namith,

Yes it should be reversed. This is because the diode is on the low side. 

1,2. Please see attached to clarify the 24V schematic.HPA420C_24V_sch.pdf

3. Secondary gate drive is not required because it is a diode rectified flyback.

4. You can remove Q1,T3,Q3 and D17 

5. T2P will let you know if af PSE is connected or at PSE is connected. If af then T2P will be high, if at then T2P will be low. You should use this to know to change load output power between 6W and 20W.

6,7. Yes I agree it should be or'd with the 24V output of the converter (on the secondary). But in the schematic it was not referenced to secondary ground.

Unfortunately, I do not see an attachment for new schematic, can you attach for review? Thanks!

Hi Darwin,

I have attached schematics file ,kindly review it.

 I have connected my Auxiliary dc voltage and poe output diode (OR'd) with same ground ,i hope this is what you have suggested earlier

1) For T2P side is there any changes to be required?

2)how to provide status led which i/p is available poe or Auxiliary voltage?

3)If two i/p available at same time what is probality? any damage to PD

4)Please suggest any other modification to be required in schematics?

if nothing is can i proceed for further ,becoz already i wasted so much time in this  design

5POE design for ti review.pdf)Our requirement maximum 22 W can this design withstand for power?

Hi Darwin,

I would like to ask final queries before i will proceed further for pcb layout, i hope you will not mind.

• Resistor values for R (FRS) = 69.8 k, since 250Khz switching frequency.is it correct?
• what is the value for DT i.e R(DT) since we not using secondary gate drive (GAT 2) ?
• Datasheet says Connect DT to VB to set the dead time to 0 and turn GAT2 off.? what to do this RDT?
• What is the value for BLNK? RBLNK=80.6K? is it correct ?
• what is the efficiency of design?
• what is the maximum current for this design?1.02A right
• becoz i need minimum 1A current for my load
• can you share layout guidelines for pcb layout link for this design if have

Hi Darwin,

I have attached updated schematics as per your suggestion  , kindly have look at this before layout.

still i have doubt on T2P

If T2P is high then it is 802.3af(POE) and T2P low then it is 802.at(POE+)

our load is different speakers like 8ohm 20W  and 8ohm 6W,

1. Question:If POE I/P available T2P  on the chip will recognize whether it is POE or POE+  right?
2. becoz based on the input our load has to change i.e speaker configuration by seeing the T2P status?MAIN BOARD _ 05_POE.pdf

Hi Darwin,

I have question regarding Classification Resistor(CLS) pin ,in my design i have connected 63.4 ohm for  class 4 i.e min 13W and max 25.5W Power at PD.

This means minimum power that the power device can deliver 13W and maximum 25.5 W is it my understanding is correct?

If that is the case our customer was asking control to set POE mode from processor since they does not want 25.5W power at all the time 

only one instance they will run their load at that power ratings, they doesnt want power wastage all the time.

my idea to over come to switching circuit for CLS pin, i will use two resistor class 3  i.e 90.9 ohm and class 4 i.e 63.4ohm using optocoupler by giving control to select each CLS pin by processor.

Please suggest more about CLS pin.

I have attached  my plan in schematics ,please look at the once and tell me feasibility of that design .POE for TI review.pdf

Hi Darwin,

I mean to say we will fix class resistor  first either  class 3 or class4. Then we provide  hardware control to select class by connecting hardware control  circuit  for class pin to vss.

Ex:initially  will  fix our resistor to 63.4ohm then  it will run at class 4mode.

Then based on our requirement  we will control another resistor by pulldown the intial class.

Either way we will  fix our classification start up time only.

I have sending  this through phone  please ignore typos. Difficult  to attach schematic for your reference.

Regards,

Namith

Hi Darwin,

how are you,

I have attached the schematic for POE mode selection feasibity, can check and confirm possibilty.

Initial it is fixed to 90.9 ohm i.e MAX 12.5W 

if we needed we can select other mode using GPIO

Regards,

Namith

POE mode selection feasibility.pdf

Hello Darwin,

I have a question regarding TVS diode used in reference schematic P/N:SMAJ58A  is 58V reverse voltage rated.

Any reason for that.

I understand that POE maximum voltage rating is 57V ,so selecting 58V rated is suitable, is it correct?

Regards,

Namith