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UCC28740: Calculation Tool Inductance Value

Keith Keller
Expert 7230 points
Part Number: UCC28740

Hello,

We are working with the UCC28740 design spreadsheet calculator and had a question.

  • In box C54 I see a calculated primary inductance of 493uH (which is expected) with a 0.9 ohm sense resistor (recommended 0.089).
  • Then if I change the "Actual Current Sense Resistor" from 0.9 ohm to 0.075 (which is lower than recommended 0.089) then the calculated inductance drops to 41uH.
  • I don't see why the sense resistor value would affect the inductance so much?

>>>>> Excel Calculation with 0.9 ohm sense resistor >>>>>

>>>>> Excel Calculation with 0.075 ohm sense resistor >>>>>

>>> Calculation Tool >>>

sluc487b_revised.xlsx

Thanks very much for your help.  Keith

  • 0
    TI__Mastermind 40120 points

    Hello Keith,

     

    Thank you for your interest in using the UCC28740 flyback controller, and for providing the calculator tool that you are using.

     

    The transformer inductance depends on several design factors that are established at earlier rows in the calculator tool.  Among them, you are targeting a 139-W max output level, 100kHz max frequency, and 80V minimum bulk voltage (at the valley of the bulk ripple voltage).

     

    To deliver the cycle-by-cycle energy needed under these conditions within the on-time portion of each 10-us switching period window (about ~5us), the algorithm determined (row 51) that an Rcs value of 0.089R is needed to get the peak primary current. In row 54, the inductance is calculated to be 41uH. 

     

    Consider that the high output power requires a high peak primary current of 0.773V/0.089R = ~8.7Apk. To get this peak within 5us at only 80V bulk voltage, the inductance must necessarily be around 40-ish uH.  The calculator does the exact math.  Mine are quick approximations, but indicate that 41uH is not an unreasonable value based on the design conditions.

     

    Entering 0.9R for Rcs gives you ~493uH which may have been expected based on previous lower-power designs, but 0.9R is an order of magnitude off from the required value for this design. Also, selecting an Rcs value much lower than the recommended value will result in early current limit, because it will allow excess Ipk level on the primary, which cannot be sustained within the Kcc limit (maximum magnetization duty cycle of 0.425) of the IC.

     

    I hope this answers your questions.

     

    Regards,
     Ulrich