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TPS23753A: 13W 5.0V @ 2.5A output design using SLVA305 as a guide and 12V aux input

Part Number: TPS23753A
Other Parts Discussed in Thread: , PMP7355, TPS23756

Hi,

I was wondering if there is a design guide for using TPS23753A delivering the full 13W available to a PD (802.3af) at 5.0V, 2.5A output.

I read document SLVA305 as a guide, however this explains an example delivering 5.0V at 1.4A, which is not quite enough for our applicaiton.

I have also read document SLVA306 for ORing an auxilliary wall power, and for a 12V input, we would need to use Option 2.

Our requirements are:

1. Using TPS23753A (since it is a handy small TTSOP16 part) - other recommendations in same family will be acceptable.

2. Using Coilcraft PoE13F-50 transformer - since it is the only one we have found with height 8.5mm low enough to fit in our chosen enclosure.

3. Simple design, like that shown in SLVA305. Other designs using more MOSFETs in leui of diodes looks too complicated, and will take more board area.

4. 48V PoE primary source, and auxiliary input of 12V / 24V industrial power supply. Also we would like to use this in battery / solar solutions, so a minimum operating input voltage of 10.5V is desired.

Can someone offer a starting guide?

PS: We are now focusing on PMP7355 which details a 5V @ 2A design using a POE13P-50 transformer, and inserting Dvdd between VDD and VDD1 pins, and injecting 12V into VDD1 via another diode as described in SLVU314F (TPS23753AEVM-004). However this suggests at 12V input, power is limited to 6W - why? Would reducing R18 below 0.39 ohms allow more current?

Kind Regards, Kyran McGlasson - SAGE Automation

  • Hi Kyran,

    I will look for a design that meets your requirements, however a diode rectifier will not be able to meet your 96% efficiency requirement (13W in, 12.5W out). We may need to go for the type 2 range to attain this power requirement while using only diode rectification.

    For your 10.5V wide Vin requirement, TPS23756 would be ideal due to its lowered Vc UVLO (9V) for converter startup.

    - Tom
  • Thanks Tom,

    I accept that we will only achieve 10W power due to the losses in the circuit. In the meantime we want to figure out how to achieve 10W on 12V input.

    We have gone ahead with the PMP7355 design, however modified using parts of TPS23753AEVM-004 schematic to allow 12V input, I assume this is the APD resistor divider, and possibly the diode between VDD and VDD1 pins.

    Regards, Kyran.

  • Hi Kyran,

    PMP7355 is an excellent choice for a reference design.

    The adapter configuration from TPS23753EVM-004 is PoE priority option 2 OR-ing detailed on page 5 of this app note:
    www.ti.com/.../slva306a.pdf

    12V input is limited to 6W is because the required input current for this power level is 4x higher than the input current for 48Vin.

    6W/12V = 0.5A
    6W/48V = 0.125A

    The current sense resistor is sized for operation at lower primary side current levels to prevent saturation of the transformer & prevent the FET from overheating.

    In order to get to 10W with 12Vin, you would need to pick a larger FET with a lower Rdson & a transformer designed for Wide Vin that would not saturate at the higher input current level required for 12Vin.

    Thanks,
    Tom

  • Thanks Tom,
    We have replaced the MOSFET with a FDS2582 (4.1A max) and changed Rcs = 0.33 ohms, which will allow more current (not sure how much more).
    Also the POE13F-50L flyback transformer datasheet states the maximum primary winding current is 2.1A, so at 75% duty cycle that should be 12V * 2.1A * 0.75 = 18.9W available power right?
    And if the Imax is 2.1A, Vcs is 0.55V, does this make Rcs = 0.26 ohms?

    Regards, Kyran
  • Hi Kyran,

    I'll perform the calculation & let you know what I get, shortly.

    Thanks,
    Tom
  • Kyran,

    While I am working on this, here is a more suitable transformer for your design:
    www.coilcraft.com/.../ha3585.pdf

    It is wide Vin (12V adapter) flyback transformer, but comes in at 8.51in for height clearance. You can probably work with Coilcraft on this height spec.

    Thanks,
    Tom
  • Hi Kyran,

    I worked SLVA305C from section 2.5 to 2.7 using the new wide Vin flyback transformer I found. Doing this validated that the transformer was correctly designed (~60% duty cycle at 10.5Vin), and determined which MOSFET current rating is appropriate (>2.33A) & what resistance Rcs should be (0.22ohm, >1W power rating).

    The transformer is the most important component because the inductance it is designed for will determine the peak primary current which is the key design spec for selecting your Rcs & MOSFET current rating.
  • First, I worked section 2.5 with the assumption that 60% would be a good max duty cycle. It is well below the 80% duty cycle the TPS23753A can handle.

    In section 2.6, I determined that a 60% duty cycle at 10Vin would give me a pri:sec turns ration of ~2.7. Without stepping through all the equations in 2.6, I arrived at a max peak primary current of 2.4A which requires a minimum of 20uH inductance. The transformer I found has 40uH, which satisfies this.

    The 4A mosfet satisifes this peak current approximate peak current (2.4A) found in 2.6.4 & actual peak current (2.33A) found it 2.6.8.

    In section 2.7.2, I used the peak current I found to determine that the right Rcs for your design (10vin min, 5V/2A) would be 0.236ohms. Since Vcsmax is 0.55V, the power dissipated across this resistor would be 1.28W, so you will need an Rcs resistor power rating that exceeds this.
  • I did not go into detail for each individual calculation on SLVA305C, but basically, determine your desired output power & Vin range first, select the correct transformer, then select the parts around it using the equations provided in the design guide.

    Cheers,
    Tom
  • Thanks Thomas,

    We appreciate your technical calculations on the transformer, especially the hint towards the main specification of  the Primary inductance and 60% duty cycle as a starting point.

    Regards,

    Kyran McGlasson (SAGE Automation)