LT3748 VS LT4295 for PoE

To whom may it concern

Hi, I'm HS.

I was looking for one of your DCDC solutions for PoE bt and found quite fascinating chip LT3748 which has no opto-coupler nor 3rd winding.

My design goals are V_IN = 37V ~ 60V, V_OUT=28V, Efficiency is 90% or higher, P_IN=71.3W( for PoE bt)

The datasheet of LT3748 has no reference design about it.

So my questions are..

1. I think LT3748 is really nice for PoE solution beacuse it has no opto-coupler nor 3rd winding but somehow none of your PoE bt Soultions has LT3748 but uses other chips that requires opto-coupler or 3rd winding.

Why didn't you guys use LT3748 or similar chips that requires no opto-coupler nor 3rd winding for PoE bt??

2. Can I use LT3748 for PoE bt solution? Or is there any better DCDC controller chips for PoE bt other than LT4295?

If I can, would you give me reference design for the goal I listed above??

3. Datasheet of LT3748 didn't specify the TR(transformer) for high power(more than 60W) but for kinda relatively low power(around 10~50W).

If you say i can use LT3748 for PoE bt(71.3W), Can I replace TR for high power?

4. I found out that LT3798 also doesn't need opto-coupler nor 3rd winding. What's the difference between LT3748 and LT3798?? Which one would be better for PoE bt if you have to choose?

5. What would be the advantages and disadvantages using flyback or active clamp forward??

Thank you for your hard work and I appreciate your help a lot.

Hope you have a nice day!

more questions
[edited by: feelyoursoul at 1:46 AM (GMT 0) on 14 Nov 2019]
  • +1
    •  Analog Employees 
    on Nov 15, 2019 2:46 AM


    We do offer several PoE PD solutions that can be configured as a non-opto synchronous flyback converter. LTC4269-1 uses a third winding for feedback and biasing, while LT4276 and LT4295 use the third winding for feedback only. LT3748 is a non-synchronous flyback, that means it uses a diode for the secondary-side rectifier. Non-synchronous non-opto flybacks are less efficient than synchronous flybacks and require a minimum load or Zener clamp at the output for light load regulation. Please note, the LT3748 can be combined with the LT8309 secondary-side synchronous rectifier driver for high current applications.

    LT3748 is indeed a nice part, and it can be combined with a PD interface controller such as LT4275, LT4293, or LT4294 so it can be used in a PoE PD application. Please note, the LT3748 datasheet recommends using a third winding to bias INTVCC. From the datasheet: For a typical 48Vin, 10W application, the third winding may improve efficiency by several percent at full load and as much as 30% at light loads.

    The output diode limits the output current for non-synchronous designs. Flyback transformers rated for more than 60W are more complicated to build and are usually limited to 100W.

    LT3798 is another great non-opto non-synchronous flyback part. It is designed for off-line, AC to DC designs with active power factor correction. It can be configured for use in a PoE system. This part does require a third winding for feedback and biasing INTVCC.

    Flyback designs are generally smaller and simpler. The output power is lower, and with synchronous rectification they can reach 92% efficiency. Flybacks can also be designed to support a wide input range. Forward converters tend to be larger and more complicated, requiring additional components. They are better for higher current and higher power designs and can achieve 94% efficiency. Forward converters also require an opto-coupler or other means to sense the output voltage.

    Best Regards,