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LT4295 PoE++ 66W Froward topology

Category: Hardware
Product Number: LTC4295

Hi Eric,

The product which I am currently working on uses the LT4295 in a 71.3W PoE++ forward configuration, mostly inspired from the DC2584A EV Board.

I have used PoE injector which is is capable of supporting 802.3 af.

We have lowered the classification to type 3

My question is without PoE classification whether this circuit is expected to work. Because I am getting only 3.7V at the output of this circuit.

Attaching VPORT waveform and Primary Gate and Secondary Gate waveform. Switching frequency is matching

.PG_SG switching waveform

Are we missing anything here. Need your help to debug this issue and to proceed further.

  • Hello,

    By Type 3, do you means Class 3 i.e. 13W operation? This looks like the converter is operating near its minimum duty cycle. Have you changed any components related to the feedback path? Have you checked the SFST pin voltage, the ITHB pin, and the voltage at the Vref in of the ZR431 reference on the secondary?

    Best Regards,


  • Hi Eric,

    Hi Eric,

    I meant it is Class3/Type1/3 just by removing Rclass+ resisitor and populating Rclass pin with 52.3 ohm.

    Also we have received couple of review feedback from ADI to change the component from the feedback path. attached the review comment page of the PoE section .

    Also sharing the updated final PoE section of the schematic for your reference.


    Let me cross check and confirm the SFST pin voltage, the ITHB pin, and the voltage at the Vref in of the ZR431 reference on the secondary.



  • Also attaching VDS_FREE and T2P node waveform capture for your reference.

    Will revert back SFST pin voltage, the ITHB pin, and the voltage at the Vref in of the ZR431 reference on the secondary. 

  • Hi Eric,

    I also found one mistake on my PCB ZR431 footprint pin assignment got swapped.

    After doing the rework the PoE section is delivering 12V output. I have captured SFST, ITHB and VREF pin waveform with no load and full load condition.

    SFST Noload waveform:


    SFST with 47W load at 12V


    ITHB_No_Load Capture:

    ITHB_47W load:


    VREF with 47W load at 12V

    We were able to stress this circuit till 55W after that we heard some hissing sound from the PoE section and we stopped loading further.

    Also Noticed capacitor C509 is getting heated up 

    Disconnected the load the PoE section was powered up for sometime without any load. Noticed that PoE section is not working and the output voltage is back to 3.6V 

    Captures under failure case is attached:

    VPORT under failure case:

    12V output:

    PG_Under Failure Case:

    PG is not continuous:

    SG under Failure:

    SG is not continuous:

    After sometime PoE injector is displaying alarm and 55V is not present at the VPORT.

    Not sure which component went bad and we are looking forward your support.



  • Hi Eric,

    I will be continuing the conversation on this topic on behalf of my colleague Kalidoss who raised this issue in forum.

    I have powered up the POE circuit from external POE Injector. I have used Electronic Load to Increase the load in steps of 100mA till 4000mA which can load the circuit till 48Watts. The circuit worked and suddenly circuit trip happened. I found that 12V rail is still fluctuating in the same pattern as shown above.

    I also observe that:

    (1) Audio noise "Tick Tick" from Transformer area.

    (2)Primary MOSFET also went bad. I replaced the MOSFET , but still 12V pattern and audio noise is still same. 

    (3)Capacitor C509 is heating too much. I suspect it may not be expected. Looks like something went bad.

    Can you please suggest the areas to look into based on the above findings?


  • Hi Eric,

    Can you please help with suggestions to monitor any key signals to trace the issue?


  • Hi Eric,

    Do you have any suggestions for me? Please suggest.


  • Hi Srikanth,

    This is very strange. C509 should not be heating up. What is the IRMS rating for the component you are using? The intermittent switching indicates there is a short or overload present which is causing the sense voltage to exceed its current fault trip point. Or perhaps C509 is heating up the area around the LT4295 causing thermal shutdown but that is much less likely than overcurrent. Have you investigated the secondary to see if there are any components that failed?

    Have you tried reaching out to your local ADI support team? They may be able to provide direct support.

    Best Regards,


  • C509 has Ripple current rating of 130mA@100KHz.  LT4295 area is also hot. Secondary FET is not gone bad. I am not able judge if IC is gone bad or not? I need some more check points to verify.

    I tried reaching my local ADI support team. I didn't hear anything from them.