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WEBENCH® Tools: WEBENCH : ISOLATED TRANSFORMER CONNECTION

Arun Gopalakrishnan
Prodigy 150 points


Tool/software: WEBENCH® Design Tools

i design an isolated power supply using WEBENCH, in that there is a transformer at the output of the switcher IC.

The transformer connection in the workbench schematics is given below,

But the when i look into the datasheet of transformer i got like this,

Now, i am in a trouble... how i connect the transformer. In the BOM given transformer has 12 pins buts in the schematics it was only four(2 input and 2 output pins).

please explain the connection?

Transformer details,

Wurth Elektronik

Part No. : 749196521

Lp= 48.0 μH
Rp= 142.2 mOhm
Leakage_L= 480.0 nH
Ns1toNp= 2.0
Rs1= 284.4 mOhms

Full Schematics given below,

  • 0
    TI__Expert 8365 points
    Hi Arun,

    The connection pins of the transformer is used to configure the primary inductance of the transformer. For example, if 12 and 2 are connected then the total inductance is Lprim*2^2. Since Lprim = 12uH for 749196521 ,the total inductance calculated is 12*4=48uH.Please use the appropriate pins of the transformer based on your application. The remaining pins of the transformer can be floated.

    Thanks & Regards,
    Praveen B
  • 0 in reply to Praveen Badiger
    Prodigy 150 points

    Did you mean that, for primary side connection short 12 and 2 pins?

    Transformer design details as per webench is given as,

    Lp= 48.0 μH
    Rp= 142.2 mOhm
    Leakage_L= 480.0 nH
    Ns1toNp= 2.0
    Rs1= 284.4 mOhms

    Could you please elaborate the design.

    Also it is given as "Ns1toNp= 2.0"

    I would be thankful if you can explain input and output connections w.r.t. the designand transformer

  • 0 in reply to Arun Gopalakrishnan
    TI__Expert 8365 points

    Hi Arun,

    We know that for an inductance winding, L is directly proportional to N squared. So when you connect 2 windings in series on N turms the effective inductance becomes directly proportional to the 2N whole square which is 4*N*N. Hence the L turns out to be ~48uH

    The turns ratio calculation:

    As you can see FB node is at Vref. So Vo1 will be at Vref*(Rfbt+Rfbb)/rfbb

    Now you know Vo1 and Vout. Turns ratio will be Vout/Vo1 ideally. In the actual equation, we take care of diode drops as well.

    Hope this is clear.

    Thanks and Regards,

    Praveen B

  • 0 in reply to Praveen Badiger
    Prodigy 150 points

    Dear Praveen,

    Thanks for the detailed explanation,

    few more help needed,

    As per you reply i understood that at the primary side i need to connect two winding's in serial(to achieve primary L = 48uH approx).

    also in this design it is stated that "Ns1 to Np is 2.0", so please explain how i connect the secondary side winding's.

    Do i need to connect all other windings(4 number) in serial to achieve Ns to Np ratio to 2?

    I will be thank full if you explain the full connection, because i'm a litlle confused about the transformer connection.

  • 0 in reply to Arun Gopalakrishnan
    Genius 4910 points

    Hi Arun,

    Please note that in order to achieve turns ratio "Ns1 to Np as 2.0" you need to connect four secondary winding in series.

    In this case you need to connect 9 to 5, 8 to 6 ,7 to 3  and the output is measured between 4 and 10.

    Let us know if you have any further questions. Also please share the input/output conditions for the future queries.

    Thanks and Regards,

    Atul