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LM5022: Isolated Flyback - Additional Output on Primary Side

Jan Seidel1
Prodigy 20 points
Part Number: LM5022

Hi,

I am currently working on an isolated flyback converter design

Vin: typ. 24V, 20V - 30V min/max

Vout: 12V, 3A max

using the following WEBENCH design and design guide:

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=71FCBC144E2150BF

http://www.ti.com/lit/an/snva866/snva866.pdf

My question is: Can i use a second primary winding/ auxiliary winding as an output on my primary side? I would need 5V @ 150mA max.

In the design guide it is not really mentioned how they ensured the maximum current rating on the aux winding nor the expected voltage ripple on the aux winding.

Best regards,

Jan

  • 0
    TI__Genius 17540 points

    Hi Jan,

    In a flyback, you could use the auxiliary winding to externally supply power to VCC, which will reduce power dissipation.

    You can use another winding to generate another voltage as long as the turns ratio from the secondary-side output winding to the primary-side output matches what you need. However, I'm afraid that your regulation on that rail may not be so good since it is based on secondary-output. 

    The reason maximum current rating is not calculated on the aux winding is because it is a low current winding-so a diode in series with the winding, and a capacitor to smooth the output should be good enough. If you would like to have a general approximation of the peak current, you could take it as the sum of the reflected current from the secondary winding and your load current divided by 1-D, where D is the duty cycle. I_winding = I_sec_reflect + I_out/(1-D). 

    Voltage ripple can be mitigated through a capacitor, and a zener diode in parallel to the capacitor for clamping.

    Another method is to use a LDO and generate your desired voltage- a potentially quicker solution, but may require either higher power dissipation, or more components depending on your chosen solution. 

    Thanks,

    Richard  

  • 0 in reply to Richard Fu22
    Prodigy 20 points

    Hi Richard,

    thank you for your answer and explaination. I guess your suggestion to use a LDO is the most sensible solution for my application. Especially, after i realized that finding a "simple" 1:1 transformer for my isolated use case is hard enough, if i want to avoid going for a custom made solution. Do you maybe have a recommendation for a transformer I can check out? The problem with the ones suggested in the WEBENCH tool is the pin layout. The isolation of primary and secondary side is not really possible in my eyes. This is one of the suggested ones: https://www.we-online.de/katalog/datasheet/749196521.pdf

    Best regards,

    Jan

  • 0 in reply to Jan Seidel1
    TI__Genius 17540 points

    Hi Jan,

    Unfortunately, I do not have a recommended solution for a transformer. In order to utilize that specific transformer, you would need to connect multiple winding together to generate the appropriate turns ratio.

    However, what I can do for you is to pinpoint you to a reference design that you can utilize. 

    https://www.ti.com/tool/PMP20551

    This design also has some threads related to it below, which will link you to the transformer datasheet, and you can contact the vendor for more questions.

    Thanks,

    Richard