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LM5185: Using a flyback transformer with higher primary inductance

Arttu Rasanen
Prodigy 180 points

Part Number: LM5185

Hi,

I'm designing an isolated power supply with the LM5185 controller IC and would like to ask TI's opinion about my transformer selection.

  • PSU main parameters are:
  • input 16-28 VDC
  • output 36 VDC, 0.5 A
  • galvanic isolation between input & output

Here are screenshots from the LM5185 design calculator worksheet:

kuva.png

kuva.png

I used the LM5185-DESIGN-CALC.xlsx worksheet to simulate the following transformer:

kuva.png

I'm intending to use the transformer with its primary windings connected in series. The windings ratio would then be 2:6 = 1:3. With series-connected windings, the total inductance would rise up to 10µH*2^2=40µH, four times higher than the recommended inductance of 10.6µH.

My questions:

  1. Do you think this transformer would be oversized for this application?
  2. What are the disadvantages of using a transformer with higher than recommended primary inductance?

Thanks in advance!

  • 0
    TI__Genius 16735 points

    Hi Arttu,

    The device can function. 

    But the device performance may not be optimal as it design to function in 3 operating modes based on output load and input voltage. Please check the operating frequency wrt to output load, it looks the Fsw remains below 100kHz for max operating period.  

    Currently we don't have any test data with over sized transformer, our EVM transformers are designed for optimal performance. 

    Thanks.