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LM5023: How to Select Conductor Size in Flyback Transformer?

Raj Shahu
Prodigy 80 points
Part Number: LM5023

Hello, 

I am designing a Flyback Transformer similar as the data sheet provided by webench for LM5023 controller IC. 

In that datasheet I have a doubt about the AWG selection of the secondary winding of transformer.

The rating of the flyback converter is 19V, 3.43A at output side. That's why I think the secondary winding's wire have sufficient current carrying capacity to conduct that much of current.

But in the data sheet provided by webench they are using 2 strands of 25AWG wire. The current carrying capacity of 25AWG wire is 0.457A. Hence the total current carrying capacity will become 2 times 0.457A i.e. 0.914A which is very much lower than 3.43A.

Could you please help me to explain how this gauge selection (25AWG) is made? 

Attachments:

Data Sheet of Transformer:B66423G0000X187.pdf

AWG Chart: American Wire Gauge Conductor Size Table.pdf

Also, If I will select the AWG with the current rating equal to or more than 3.43A, then the winding will unable to fit inside the above mentioned transformer core.

  • 0
    TI__Guru**** 189399 points

    Happy New Year! One of our experts will be back in office after the holidays and answer your questions. Thank you! 

  • 0 in reply to Don Dapkus
    TI__Mastermind 21835 points

    Hi Raj,

    Thank you for the query on LM5023.

    For your application (19V/3.4A) there is  already a transformer available from one of our reference designs and the Wurth transformer number used is 750313417. You can check the datasheet from the manufacturer for exact details of the winding they have used there. I am not sure on the discrepancies in the Webench tool. I would recommend you to manually design using the formulas of the following app note below and verify your results with the actual transformer mentioned above

    https://www.ti.com/lit/ml/slup127/slup127.pdf

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    TI__Mastermind 29400 points

    Hello Raj, 

    I'd like to expand on what Harish is saying.  The Webench transformer design tool uses certain design criteria and formulas to arrive at its core and winding recommendations.  Unfortunately, we don't know what those criteria and formulas are.  However, they are based on power-transformer considerations. 

    The Max Current wire-rating table that you reference has notes at the bottom of the table which qualify the ratings per AWG as being extremely conservative, because they are all based on a 700cmil/A rating used in AC power transmission.  One of the main goals of power transmission is to minimize the voltage drop in the distribution over long distances.  If you note, the product of (Resistance/km x Max Current) = ~48.5V (ignoring effect of temperature rise on resistivity) and is constant for each AWG listed.  This voltage drop makes sense only in the context of power distribution using the large diameter wires. 

    For 0AWG, 150Arms x 0.3224R/km = 48.4Vrms/km of power line.  For a 120V distribution, this is obviously too much drop, so it applies to a higher-voltage line.  For a 1% drop, it would be a 4850V line, but still 1% drop per km would lose 10% in only 10 km.  for >100 km, at < 10% loss, the distribution would need to be > 49kV.   150A x 100kV = 15 MW, which is where these ratings make sense.    

    That's beside the point, though.  The main point is that the entire ratings table is based on the same criterion as for power distribution, yet no one would use a 25AWG wire to route power over long distances.  So this table is not applicable to low power flyback transformer design.  The current capacity of a wire used in a transformer depends on several factors, of which temperature rise is one of the main considerations. 

    Here is another table of ratings, which uses different criteria that are a little more clearly described. current_rating chart.pdf

    Even this one is not completely clear where the ratings come from, but the fine print at the top of the chart indicates temperature rise to some limit of some type of insulation.  The main point here is that these current ratings are much higher than those of the table you referenced.  Yet, even this is not conclusive.

    For a transformer, the design will need to account for the voltage drop of the length of windings at the current in those windings, with respect to its self-heating plus heating from adjacent windings and core loss, within the limits of the insulation system used in the transformer.  I'm sure that the Webench design tool does this, although I don't know what its algorithms are.  

    As Harish says, you can obtain the 750313417 datasheet from Wurth Electronik and compare the windings with Webench, or simply use the Wirth design. 
    And maybe use the SLUP127.pdf App-Note to calculate your own design and compare it to both of the others.   

    Regards,
    Ulrich