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LM5021: Can customer utilize TINA TI to simulate how much phase margin would be there in specific schematic ?

Masaharu Takahashi
Expert 4170 points
Part Number: LM5021
Other Parts Discussed in Thread: TINA-TI, TL431

Hi, team.

i have been asked by customer as below, please tell us

Q1.Can customer utilize TINA TI to simulate how much phase margin would be there in specific schematic ?

  • 0
    TI__Mastermind 40580 points

    Hello Masaharu-san,

    Thank you for your interest in the LM5021 PWM controller.

    The short answer is "Yes", phase margin can be simulated by TINA-TI or most other simulation programs.

    However, accurate results depend on how accurately the loop components are modeled.  The transformer and output capacitor are fairly easy to model accurately. The secondary-side shunt regulator (typically at TL431 or similar device) usually has one or two poles to consider, whose break-point frequencies may or may not be well specified.  The most widely-varying component is probably the opto-coupler, whose dominant pole depends on the biasing resistance and whose CTR varies with photo-diode forward current and temperature.    

    If the opto-coupler is not modelled accurately, any phase-margin results will be meaningless.

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Expert 4170 points

    Hi. Ulrich-san

    thanks your quick answer.

    do you happen to know how we do that on TINA TI?

    (can you show us those steps how to simulate it ?)

    i'll emphasis on what you said , its simulation result would be meaningless without accurate other components models for customer.

  • 0 in reply to Masaharu Takahashi
    TI__Mastermind 40580 points

    Hi Masaharu-san,

    I have not used TINA-TI so I don't know exactly what functions are available, but I imagine that controlled current and voltage sources are available.  Hopefully the gains can be expressed as polynominals or exponentials to obtain curve fitting to the datasheet curves.  Be aware that the entire CTR curve does not need to be modeled from 0 to 100mA, only the limited IF range where the system will be operating.  So this can help simplify the polynominal terms. 

    I am attaching a few app-notes that I have accumulated over time concerning opto-coupler modeling.  There may be newer ones available from a web-search, too.  These notes should help with constructing a reasonable first-order model of a typical opto-coupler.  Note that even though the papers discuss a particular part number, these are guidelines that can be adapted and applied in general to most optocouplers from any vendor.

    If your customer uses an opto-coupler with a comprehensive datasheet, it may be modeled from datasheet parameters.  Otherwise, it may require some bench characterization.

    CEL_AN3017_Optocoupler_PSpice_Model.pdfTI_AN-1095_Measure_Optocoupler_Pole_SNVA005b.pdfOmi_App_Note_Optocoupler_Freq_Resp_V2_0.pdf

    Regards,
    Ulrich