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Can a 13W PoE PD controller be used to deliver 18W power output?

John J Wu
Genius 3870 points
Other Parts Discussed in Thread: TPS23757, TPS23754, TPS23756, TPS23756EVM

We are looking at TPS23757, which is designed for 13W PoE. Can we use it in 18W output? What limits the application?

Thanks.

  • 0
    TI__Mastermind 42795 points

    You could use the dc/dc controller portion of the TPS23757 to control an 18W power supply (for an adapter powered dc/dc converter). For power applied over an ethernet cable (type 1 PD is limited to 13W at the RJ45 connector input), a compliant type 1 PD must limit it's input power to 13W.

    For higher power applications (up to 25W), a type 2 compliant PD is required. You can use the TPS23754 or TPS23756 for this case.

  • 0 in reply to Eric Wright
    Prodigy 15 points

    Glad the E2E site is back up. It went down while I was completing this post last night, near midnight PDT.

     

    We have the t.i. POE EVK. and have  been testing extensively.
    We have problems.
    POE : 
    • we can pull about 13 watts out @ 5vdc, which is good 
      • (target load on POE is 11-W max.)
    AC ADAPTER : 
    • we can pull around 17 watts @ 5vdc when the input is 48vdc
      • That's close (target is 18-W on AC-adapter)
    • we can only get 13 watts @ 5vdc when the input is 24vdc (the voltage of adapter that the customer is currently shopping for)
      • that's not nearly enough power
    QUESTIONS:
    • what is limiting the power?
      • the power-FET switching the transformer does not ever get warm
      • there's  a FET in the output that gets warm when the output voltage starts to drop at high load
    • is the transformer itself a limiting factor?
    • should we be using one with different turns-ratio if we want to optimize for 24-VDC input?
    PLEASE HELP!
    • we want 3.5+ AMPS OUT @ 5v with 24-vdc IN
    • and 2.2+ AMPS OUT @ 5v with POE input (that part is OK)
    customer wants to only target class 1 POE, as that's what's commonly deployed in offices (the target market)
    THANKS!
    --Brian

     

  • 0 in reply to Brian Empey
    TI__Mastermind 42795 points
    POE : 
    • we can pull about 13 watts out @ 5vdc, which is good 
      • (target load on POE is 11-W max.) Power limit at the RJ45 input for a compliant type 1 PD is 13W. An 85% efficient conversion will yield ~11W at the power supply output.
    AC ADAPTER : 
    • we can pull around 17 watts @ 5vdc when the input is 48vdc
      • That's close (target is 18-W on AC-adapter)
    • we can only get 13 watts @ 5vdc when the input is 24vdc (the voltage of adapter that the customer is currently shopping for)
      • that's not nearly enough power Which TI EVM are you using? Most likely the EVM is designed to deliver the rated output power (11W most likely) when operating from a certain adapter voltage range. If the EVM was designed to operate with 24 and 48V adapters, then the input range would be ~22V - 57V at max output power.
    QUESTIONS:
    • what is limiting the power? The design current limit.
      • the power-FET switching the transformer does not ever get warm
      • there's  a FET in the output that gets warm when the output voltage starts to drop at high load
    • is the transformer itself a limiting factor?
    • should we be using one with different turns-ratio if we want to optimize for 24-VDC input?

    There are many circuit factors in the design that are targeted at specific output power under certain conditions.

    PLEASE HELP! Take a look at TPS23756EVM. It is a type 2 PD controller that can operate from 12, 24, 48VDC adapters and deliver 25W at 5V. You can use the T2P signal to detect the PSE type and signal the load about how much power to consume (<13W or higher if/when a type 2 PSE is connected).
    • we want 3.5+ AMPS OUT @ 5v with 24-vdc IN
    • and 2.2+ AMPS OUT @ 5v with POE input (that part is OK)