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UCC28750: Transformer turns ratio and magnetizing inductance

Rohan Kadam
Prodigy 30 points
Part Number: UCC28750
Other Parts Discussed in Thread: UCC28C42

Tool/software:

I have generated the offline flyback converter  design report from TI WEB Bench Power designer tool for following specifications .

  

As per the design report , turns ratio is Np/Ns= 64/7 =9.14 and magnetizing inductance is Lm= 378uH .

UCC28750 datasheet also has design procedure explained in detail . 

by referring equations 6,7,8 and 9 from design procedure 

Turns ration comes out to be 16.:1 and Lm =2mH for Vbulk_min =109V , DMax =0.65 , Vout=12V , Iout=5A  , n=80% , Kccm= 0.166

values of turns ratio and magnetizing inductance are not matching to web bench report .

Is it incorrect to use datasheet formulas to determine the aforementioned parameters?

please advise .

  • 0
    TI__Guru* 86266 points

    Hello,

     

    Your inquiry has been received and is under review.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    Thanks for notifying us.  I will look into this.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points

    any update on above issue ?

  • 0 in reply to Rohan Kadam
    TI__Guru* 86266 points

    Hello,

     

    Equations 6 through 9 in the data sheet are correct.

     

    I checked the product folder for the UCC28750x devices and do not see a Webench design tool.  There is an excel design tool that you can find at the following link.

    https://www.ti.com/tool/download/UCC28750-CALC

     

    Could you send me a link to the Webench tool that you used?

     

    Thanks,

  • 0 in reply to Mike O'
    Prodigy 30 points

     https://webench.ti.com/power-designer/switching-regulator?powerSupply=1 

    I used this link to generate the design for mentioned input /output ratings and then tool generated attached design report for UCC28750 .

    UCC28750 design report.pdf

    is there a another IC for offline flyback design  (fixed frequency operation) whose web bench tool i can use ?

  • 0 in reply to Rohan Kadam
    TI__Guru* 86266 points

    Hello,

     

    Thankyou for sharing the link.  In the past they had Webench designer setup in the design development tab for the device product folder.

    https://www.ti.com/product/UCC28750?keyMatch=UCC28750&tisearch=universal_search&usecase=GPN#design-development

     

    I did enter your design parameters in the link you sent and it gave me a UCC287506 design.  It did give me a turns ratio of 9.    I checked to see what the maximum duty cycle would be on this and it would be 98.7%.  So, this is in error.

     

    If you need to use Webex I was going to recommend using UCC28C42.  However, I checked that tool and it came up with the same calculations for the turn’s ratio.   So more than likely it uses the same calculations for the transformer.  I am going to report this to the Webench team so they can look into fixing it for you.

     

    If you want to design with the UCC28750x devices I would recommend using the excel design tool instead of Webex.  You can find it at the following link.  https://www.ti.com/tool/download/UCC28750-CALC

     

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

     

    Can someone on the Webench team look into the UCC28C42 and UCC287506 Webench calculators.  They are giving incorrect transformer turns ratio for flyback designs.  The details of the issue are contained in the thread.

     

    Regards and Thanks,

  • 0 in reply to Mike O'
    Prodigy 30 points

     thanks for detailed reply . I will refer  excel sheet to design using  UCC28750 and also looking forward to updates on TI webench tool. i wanted to ask  if you can recommend any general Kccm value to start off the design process ? at what percentage of full load power it show enter into  DCM mode ? 

  • 0 in reply to Rohan Kadam
    TI__Guru* 86266 points

    Hello,

     

    I have moved this to Webench tools forum for support.  So they can fix the tools for you.

     

    Can someone on the Webench team look into the UCC28C42 and UCC287506 Webench calculators.  They are giving incorrect transformer turns ratio for flyback designs.  The details of the issue are contained in the thread.

     

    Details in regards to the errors can be found in this link.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Expert 5810 points

    Hi Rohan,

    We are able to replicate the issue. We are looking into the root cause for this. We would be able to get back with more details by end of this week.

    Thanks and Regards,

    Umayal

  • 0 in reply to Umayal Ramanathan
    Prodigy 30 points

      looking forward to outcome as we want to know the webench suggested  magnetizing inductance , turns ration , core and bobbin part . 

    I wanted to ask  if you can recommend any general Kccm value to start off the design process ? at what percentage of full load power it show enter into  DCM mode ? any general  rule of thumb considering 60 watt of output power ?

  • 0 in reply to Rohan Kadam
    TI__Expert 5810 points

    Hi Rohan,

    We will keep you updated on the query pertaining to Webench suggested turns ratio and magnetizing inductance in device UCC287506.

    For further assistance on following query pertaining to design, we are moving the post to product experts.

    "I wanted to ask  if you can recommend any general Kccm value to start off the design process ? at what percentage of full load power it show enter into  DCM mode ? any general  rule of thumb considering 60 watt of output power ?"

    Thanks and Regards,

    Umayal

  • 0 in reply to Mike O'
    TI__Guru 50090 points

    HI Mike,

    I will work with the WEBENCH team to get this corrected.

  • 0 in reply to JCHK
    Prodigy 30 points

    any update on above issue ?

  • 0 in reply to Rohan Kadam
    TI__Guru 50090 points

    HI Rohan,

    I submitted the bug fix to the WEBENCH team. Normally this takes a few weeks to resolve.

  • +1 in reply to JCHK
    TI__Expert 5810 points

    Hi Rohan,

    We compared our Webench turns ratio and magnetizing inductance calculation with the Excel calculator equation.

    Following is the detailed calculation for Primary to Secondary Turns Ratio (Nps) as per Excel Calculator for the reported condition,


    Primary to Secondary Turns Ratio (Nps) = ((Vbulk,min) * DMax_Est) / ((Math.abs(Vout_Usr)+VF) * (1 - DMax_Est))
    Primary to Secondary Turns Ratio (Nps) = ((75.5)*0.65)/(((12+1) * (1 - 0.65))
    Primary to Secondary Turns Ratio (Nps) = 10.7857
    Excel calculator considers the secondary side rectifier voltage drop (VF) of 1V hence Primary to Secondary Turns Ratio (Nps) = 10.7857

    In Webench as per Webench standards secondary side rectifier voltage drop of 0.45V and sense resistor voltage drop is considered to compute the
    Primary to Secondary Turns Ratio hence we are observing 9.14 on Webench.

    Similarly for magnetizing inductance (Lm) following is the detailed calculation as per Excel calculator.

    Num=Math.pow(Vinmin_bulk,2)*Math.pow((DMax_Act),2)*(1/IC_fsw_Act)*0.85
    Den=2*(Vout_Usr *Iout_Usr)*0.65
    Lm=Num/Den;


    Num=75.5*75.5*0.65*0.65*(1/100000)*0.85
    Num=0.0204

    Den=2*(60)*0.65
    Den=78

    Lm=261.53uH

    In Webench for reported condition Lm is 378uH.
    As per Webench standards ripple voltage across bulk capacitor is considered to compute the Vinmin_bulk.
    Hence discrepancy in Vinmin_bulk is causing the mismatch in Lm.

    In Webench there is always some margin taken from the actual calculated value just to give some headroom so that to enable selection of different off the shelf transformers for the design, if it is possible for that operating condition.

    As per our observation Webench uses accurate model to compute the turns ratio and magnetizing inductance considering other parameters like sense resistor voltage drop and ripple voltage across bulk capacitor to estimate Primary to Secondary Turns Ratio and magnetizing inductance hence we are observing the discrepancy but we are in the ballpark range of excel calculator values.

    Kindly revert back to us for any further queries.

    Thanks and Regards,
    Umayal

  • +1 in reply to Umayal Ramanathan
    TI__Guru 50090 points

    HI Umayal,

    Thank you for clarifying why there is a discrepancy between the WEBENCH result and the datasheet calculations.