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WEBENCH® Tools/UCC28740: Flyback Converter Design

Christopher Dyck
Prodigy 70 points
Part Number: UCC28740

Tool/software: WEBENCH® Design Tools

Hello there,

I am designing a Flyback converter using the UCC28740 IC controller, and have used the WEBENCH application and Design Calculator (Rev. B) spreadsheet on the UCC28740 page. The WEBENCH application provides a complete design of the converter, however I have a few questions regarding this and for steps moving forward when assembly this converter. 

In order to validate my design, I am want to receive the same component values that are being outputted by WEBENCH and by the Design Calculator. Do these two tools use the same calculations to find the component values? I have tried with limited success to duplicate the WEBENCH results using the Design Calculator. WEBENCH does provide some calculations (such as determining Vout) but this calculated Vout on WEBENCH changes from the Vout I specify before WEBENCH builds my circuit. Are there any open source SPICE tools to validate this design in simulation?

Thank you for your time and help!

  • 0
    TI__Mastermind 21755 points

    Christopher,

    There might be some differences in the equations and  the parameters used amongst the excel design calculator vs WEBENCH. I have contacted appropriate team members who will try to find out the significant differences and let you know. Would it be possible for you to share your exact Design Calculator (Rev. B) spreadsheet version you are updating and the WEBENCH design?

    You can share the WEBENCH design by following the reference screenshot below:

    At the moment, there is only a SIMPLIS transient model available at http://www.ti.com/product/UCC28740/toolssoftware if you want to validate design via simulation.

    Regards,

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 70 points

    Hello Amod,

    Many thanks for your reply and assistance.

    I have attached the spreadsheet which I have done my best to use the WEBENCH design values for here: calculationsChris.xlsxWEBENCH Public Link: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=EF1F8ED6FF499A46

    I also have a question I would like your advice on. In your experience, how well does the design calculator performed with respect to "real-world" implementation?

    Best Regards,

    Chris

  • 0 in reply to Christopher Dyck
    Prodigy 70 points

    One particular calculation within our design that may be of concern is the size of the magnetizing inductance for the high-frequency transformer. It seems very large. Perhaps this may be a point of interest when looking at the differences between WEBENCH and the Design Calculator (Rev. B)

  • 0 in reply to Christopher Dyck
    TI__Intellectual 1710 points

    Hi Christopher,

    The equations used in Webench are different from what is in Design calculator. We're working with Applications engineering team to get more information on this matter. I'll keep you posted on the proceedings.

    Regards,

    Vishwanath

  • 0 in reply to Vishwanath Tigadi
    TI__Mastermind 26760 points

    Hello Christopher,

    I was asked to comment on your question of the performance of the design calculator with respect to the "real world".  The Excel Design Calculator tool for the UCC28740 controller closely follows the datasheet design procedure.  It is an augmenting tool to the datasheet.  The introduction box of the calculator recommends to "clear all user input cells before starting a new design."  Do not leave previous example values unchanged because they will influence the results.  Each user input should reflect what your design target is intended to be. If realistic values are entered into the User Inputs, then realistic recommendations for component values and design parameters will result from the calculator outputs. 

    However the user should strive to follow those recommendations closely.  Minor deviations (generally due to commercially available standard value increments) will not affect the final design significantly, but gross deviations from the recommended values will distort the design and one or more design targets may not be achievable.  Having followed the recommendations, your prototype should behave very much as you would expect, although some tweaking of parameters may be necessary to overcome some unanticipated situation or improper estimation.

    Regarding the extra-high inductance value: A relatively high inductance in the mH range is normal for a 5-W off-line design, however this value is influenced in part by the target switching frequency and the chosen current-sense resistor value.  In your calculator, fSW is set quite low and Rcs is chosen higher than recommended, both of which cause the Lm value to be higher than necessary.  Furthermore, the high Rcs value will prevent the design from delivering the full 5W target output power.  I recommend for you to clear all of the values and restart the Excel tool program with values based on your design targets.  The tool results are based on what you put into it.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 70 points

    Hi Ulrich,

    Thank you very much for your detailed comments and for sharing your expertise. We will work with the design calculator as you have advised.

    Best Regards,

    Chris

  • 0 in reply to Vishwanath Tigadi
    Prodigy 70 points

    Hi Vishwanath,

    Thank you for looking into this. Looking forward to future proceedings.

    Best Regards,

    Chris

  • 0 in reply to Vishwanath Tigadi
    Prodigy 70 points

    Hi again Vishwanath,

    I noticed what I think to be a bug in the Design Calculator spreadsheet. Sheet labeled "CALCULATIONS" does not update the opto-isolator user inputs based on what is input on the "START HERE" sheet. Instead, it says these are "User Input" but they do not change and are locked in the "CALCULATIONS" sheet.

    Best Regards,

    Chris

  • 0 in reply to Christopher Dyck
    TI__Intellectual 1710 points

    Hi Chris,

    There are minor differences in two design approaches mainly in transformer selection. We're working on matching Webench design with Design calculator as closely as possible. This may take us some time. I'll reply to this thread as soon as we implement the changes and complete the internal review. Thanks for your patience.

    Regards,

    Vishwanath

  • 0 in reply to Vishwanath Tigadi
    TI__Intellectual 1710 points

    Hi Chris,

    The changes are implemented in webench and internal review is in progress. We'll move these changes to ti.com soon.

    Regards,

    Vishwanath

  • 0 in reply to Vishwanath Tigadi
    TI__Intellectual 1710 points

    Hi Chris,

    We're making minor tweaks and still in the phase of internal review. I'll keep you updated on the proceedings as we progress.

    Regards,

    Vishwanath

  • 0 in reply to Vishwanath Tigadi
    TI__Intellectual 1710 points

    Hi Chris,

    The changes will reflect on ti.com by mid December release. We're working to make sure all changes get moved in time.

    Regards,
    Vishwanath

  • 0 in reply to Vishwanath Tigadi
    TI__Intellectual 1710 points

    Hi Chris,

    The changes will be moved to ti.com in a week's time.

    Regards,

    Vishwanath