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LM5155: How to set transformer Lp

Y.Ottey
Mastermind 6145 points
Part Number: LM5155

Dear all,

Our customer is considering a Flyback circuit using the LM5155.
The simulation with the following WEBENCH shows that the transformer(T1)'s Lp is 5.79 μH.
How is this Lp set?

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=D6E438A754C6BEC4

Also, when they calculate with the following formula, Lp will be the following value. Isn't this way wrong?

If Lp smaller than Lp set in WEBENCH is set, will there be an effect such as lower efficiency?
·conditions
Vin = 8-36V
Vout = 12V
Iout = 4.2A
Fsw = 100khz

* When Vin = 8V
Lp = Vin ^ 2/8 * Pout * fsw
= 8 * 8/8 * 12 * 4.2 * 100000
= 1.58μH

* When Vin = 36V
Lp = Vin ^ 2/8 * Pout * fsw
= 36 * 8/8 * 12 * 4.2 * 100000
= 6.32μH

Best Regards,

Y.Ottey

  • 0
    TI__Guru* 78060 points

    Hi Y. Ottey,

    Thank you for reaching out.  Transformer Lp is determined by setting the inductor ripple current.  Please refer to the following app note:

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

    The equation that you use assumes 50% duty and it should not be 50% for both Vin=36V and Vin=8V.  

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Guru 12155 points

    Hi Youhao,

    May I cut in? 

    I work with Ottey for this issue. And I tried to calculate that Lp value which is resulted by WEBECN with that equation on the App-note.

    But it does not match the calculation result of WEBENC.

    My calculation result was 83uH. (WEBENCH result is 6uH)

    Could you give me advice what is wrong with my calculation?

    (WEBENCH result)

    https://webench.ti.com/power-designer/switching-regulator/customize/875

    (condition of calculation)

    Vin=8~36V, Vo(Vload)=12V

    ILRR=0.6, Fsw=99.6kHz, Pout=50.4W, Np=1,Ns=0.42

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Guru* 78060 points

    Hi Hamamoto-san,

    Thank you for chiming in. I am afraid you used a wrong value for Ns.  It should be 1/0.42. 

    For flyback transformer selection, the min VIN is used in general, because the highest peak current occurs at the min VIN. I followed your conditions and also the equations in the App Note (yes I changed Ns to 1/0.42=2.38), and I got minimal Lm=3.2uH.  You should choose something not smaller than that. 

    Note that Webench is a good tool and it can give you a good start point, but t also has limitations because the Webench mode references to the EVM circuit and also its limited part library.  My recommendation is to use the Webench to start a design, then cross check with the calculations, and make some updates.  

    Best Regards,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Guru 12155 points

    Hi Youhao-san,

    I appreciate you support. 

    I recalculated the Lm value as you suggested Ns should be 1/0.42. But it still does not match to your calculation result (2.8uH?).

    I understand this is the start value for the design but is is very important to check that the calculation procedure is correct or incorrect.

    Could you check my calculation procedure ?

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Guru* 78060 points

    Hi Hamamoto-san,

    You need to use min VIN instead of max VIN for the calculation, because the flyback transformer is basically determined by the min VIN which has the max peak current.  When you replace it with 8V, you will get about 3.2uH.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Guru 12155 points

    Hi Youhao-san,

    I appreciate your support. I am still getting confused for this calculation.

    On the datasheet it is mentioned as bellow,

    As it's mentioned on above datasheet, Lm is calculated at max Vin condition because the max ripple current is at max Vin condition.

    So, actually which condition we should calculate Lm at?

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Guru* 78060 points

    Hi Hamamoto-san,

    Sorry for the confusion. Designing a power supply is always an iteration process with trade-offs. The app note assumes CCM throughout the input voltage range, and it is a good start point.  However, you need to check both conditions to get a design with the min BOM cost. When you compare the Lm selected by low Vin, you can decide which is a more suitable selection for your application.  Either one should work. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Guru* 78060 points

    Hi Hamamoto-san,

    Can I assume this case is closed?  You can always re-open this by adding a new post.

    Thank you for supporting our products.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Guru 12155 points

    Hi Youhao-san,

    I appreciate your support. I had some discussion with our customer about this calculation.

    They are going to follow that calculation way as it's mentioned at application note anyway. I would like to ask again if I got new question about this.

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Guru* 78060 points

    Thank you Hamamoto-san.   You are welcome to contact us if we can be of further assistance.  Let me close this thread here for now.  You may open a new thread.

    Thanks,

    Youhao