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TPS2375: Required changes to get a PoE device with 6 V output based on the given schematic for 5 V.

Mojtaba Mansour Abadi
Prodigy 110 points
Part Number: TPS2375
Other Parts Discussed in Thread: LM1086

I am not that familiar with switching power supply circuits. I have found the following schematic for a PoE device:

http://www.ti.com/lit/df/slvrb18/slvrb18.pdf

with the output of 5V @ 4.8 A. 

However, I need a voltage level of 6.5 V with lower current.

My questions are:

1- How can I change the circuit to get that voltage level at the output?

2- Since this PoE output is Mode 2, do I need to do extra things to make it work? (In the documentations a Data Link Layer (DLL) classification is also mentioned for Mode 2 PoE devices).

Cheers.

  • 0
    TI__Mastermind 40100 points
    Hi Mojtaba,

    1. 6.5V is a very unusual output voltage for PoE so it may be difficult to find an off the shelf transformer. What is the application? You might be able to get away with using the PMP5818 but the duty cycle will be slightly higher. You may also need to increase the inductance. I recommend reading the below app note that goes into detail regarding the active clamp forward topology.
    www.ti.com/.../slua535.pdf

    2. For Type 2 power levels, the PD system design should be able to do hardware classification (TPS23754 does this internally and outputs through the T2P signal) and data layer classification (or DLL). DLL means there needs to be a micro on the PD side that can negotiate with PSE (through the data lines) that it needs higher power. If the PSE can deliver this, it will communicate to the PD that it will deliver Type 2 power to the PD.

    An endspan PSE can choose to do either hardware or data layer classification, this is why the PD is required to do both.
    Note on our TI Designs and EVMs, it focuses on power and hardware. Since DLL class is a system design, it must be designed at a end product system level.

    Regards,
    Darwin
  • 0 in reply to Darwin Fernandez
    Prodigy 110 points

    Hi Darwin,

    Thank you for replying.

    I need the following voltages for optical transmitter and receiver boards:

    5.5 V @ 100 mA

    5.0 V @ 400 mA

    3.3 V @ 800 mA

    2.5 V @ 200 mA

    1 V @ 200 mA

    since the signal on the receiver board can be very small and sensitive to noise, I prefer not to use the direct output of the power supply and instead use the regulated voltage output.

    So here is my plan.

    The switching power supply gives me the output of 6.5 V with at least 2 A output current.

    Then by using two "LM1086" adjustable regulator from TI, I get stable 5.5 V and 5.0 V outputs.

    At the next steps, by using more regulators, I get other voltages from 5.5 V regulated voltage.

    Note that the DC supply needs to be as clean as possible.

    Please let me know if t this plan seems OK.

    Cheers.

  • 0 in reply to Mojtaba Mansour Abadi
    TI__Mastermind 40100 points
    Hi Mojtaba,

    Yes this plan makes sense. You can try modifying the PMP5818 for 6.5V by changing the feedback network. You can use the app note as a reference. Along with the considerations discussed in my initial post, the compensation might need to be changed as well.

    Thanks!
  • 0 in reply to Darwin Fernandez
    Prodigy 110 points
    Thank you Darwin for replying.
    I have gone through the document and it helped a lot.
    Now I have another problem which seems to be very odd. The transformer if PMP5818 is either "JA4270-AL " or "PA2328NL". But none of them are available in the market anymore.
    I wonder if there is an alternative I could use. I found this transformr "PA3856.002NL" from Pulse Electronics available on "Mouser". I wonder if I could use it as a substitute.
    products.pulseelex.com/.../

    It has two secondary outputs of 5 V and 7 A. It has a primary inductance of 100 uH the same as "JA4270-AL " and "PA2328NL". The only thing differences are PWM frequency and other primary input voltage. The frequency is 200 kHz instead of 250 kHz. The input voltage for "VC" section in primary side is "10 V, 50 mA" instead of "12 V, 10 mA". I wonder if I still can use this transformer in my circuit.

    Cheers.
  • 0 in reply to Mojtaba Mansour Abadi
    TI__Mastermind 40100 points
    Hi Mojtaba,

    Coilcraft has changed the name of the transformer as shown in the below link.
    www.coilcraft.com/Fct1xxm22sl.cfm

    It looks similar to use the PA3856.002NL so it should be okay to use.

    Also we've designed with the PA3856.005 in the past.