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  1. #1
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    Alternative Flyback Transformer

    Hi there I just started working on a PoE design. I am using the mEZS84801A as a reference design as shown below. It uses the MP8007 PoE chip. I have modified and tested this PoE system successfully and my camera can be powered and it can run at 1 Gigabit.

    Now I am in the phase of selecting components for the schematics and layout. However, the T2 coil 750316206 Wurth Np:Ns:Na=22:10:6, Lp=47.4μH, Core=EE13 47.4uH, cannot be found anywhere. The flyback transformer seems very specific to the designs, and the topologies seem to vary depending on PoE controller chip. I am having a lot of trouble trying to find an alternative for the design. My system runs on 12V and consumes ~10W. Does anyone have a transformer that can be an alternative to this one? Ideally would like to order on Digikey/Mouser and test out.

    I have read that it is a step-up transformer with very large ratios but another thought is, I am also wondering if there is a way to use the transformer to output 5V to the system instead of 12V since most of my circuitry runs off 5V. I currently have a step down converter but seems like a waste of space. If not, any help with a replacement transformer would be so helpful. I have reached out to MPS but gotten no reponse. Any help or suggestions would be very much appreciated! Thank you in advance!

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  2. #2
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    Re: Alternative Flyback Transformer

    Is this PoE or PoE+ ?
    The newer version provides more current than the older.

    You need 10W, and the supply is about 50V, correct? Then you wish to draw smooth current at about 250 mA.
    Then convert this to 12V at 1A.

    There are ways to construct one buck converter, or interleaved buck converters.

    Or maybe capacitive charge-pumps on a narrow duty cycle.

    To divide by 4 seems like a useful strategy, to reduce 50V to 12V.



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  3. #3
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    Re: Alternative Flyback Transformer

    Example, buck converter with LC smoothing filter at supply input. Notice Ampere draw does not go higher than 240mA. This is compatible with usual amount provided by PoE (50V, 350mA).

    However, the buck converter internal waveforms have peaks over 1A. This is suitable considering the load requires 800mA at 12VDC.

    Click image for larger version. 

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    In a real-life circuit the analog switch's job is done by a transistor. Voltage regulation needs to be added. You can design it for 12V or 5V.

    The 2 ohm resistor is unnecessary. It's a trick in the simulation, in order to dampen LC resonant oscillations on startup.



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    Re: Alternative Flyback Transformer

    Hi BradtheRad, thank you for the response. This is for PoE. Thank you for the waveform for the buck converter concept. I am looking to use the MP8007 chip. I looked into designing for 5V last night, I found out MPS has a simulation tool I can change the values and see what the VOUT transient curves would give. I read the MP8007 datasheet and it seems I have follow the parameter of F_SW< 200kHz. The 749119450 by Wurth and changing R1 to 379K and R2 to 50K looks like it works, except the Vout fluctuates a lot in the simulation, I have attached the simulation waveform, Any ideas on how to make the curve more smooth?
    Click image for larger version. 

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    Re: Alternative Flyback Transformer

    The jaggy waveforms are akin to squegging (oscillations that occur intermittently or at certain times in a cycle with a longer period. The cause can be hard to track down and solve. (The faster red oscillations have a varying decaying amplitude which resembles LC ringing.)

    Your schematic appears to be a flyback with all the features (three winding transformer, snubber network, feedback loop providing regulation). At this point it's too early to jump immediately to a complicated project like that.

    It's wiser to start out by experimenting with a simpler circuit around the MP8007, or else with a type of converter made from components which are familiar.



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    Re: Alternative Flyback Transformer

    A post filter on the 5V would greatly reduce the voltage fluctuations, say 10uH, 100uF low esr. The sim seems to give results based on very small o/p cap or o/p cap with high ESR - otherwise it appears to be working as intended ...

    - - - Updated - - -

    you have no feedback from Vout back to the chip - so you cannot expect tight load regulation of Vout with load steps ...



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