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UCC28180: Calculation of Inductor Current

Naveen Bevara
Intellectual 1970 points
Part Number: UCC28180

Hi, 

I am going through the inductor value calculations in UCC28180 datasheet. Ripple current is calculated at minimum input mains. But inductor value is calculated at duty cycle of 0.5. As in boost converter, Vout = Vin/(1-D), Vin would be at Vout/2 for a 0.5 duty cycle, and at lower mains duty cycle would be greater than 0.5. If I calculate ripple current at Vin = Vout/2, the value would be less than below shown value. This increases the value of required minimum inductor. But the EVM has value of inductor as per below calculations. Please help on these calculations. 

  • 0
    TI__Mastermind 25570 points
    Hello Naveen
    The first thing to keep in mind is that the exact choice of PFC inductance is somewhat arbitrary. If the inductance is lower then it will have less parasitic resistance and may be lower cost but the peak to peak ripple current will be higher. With a lower inductance the system will drop into DCM mode at a higher load current and at lower input voltages. A higher PFC inductor will have a higher parasitic resistance and may be more expensive but the peak to peak ripple currents will be lower and the system will operate in CCM down to lower load currents and over a wider input voltage range. So, the choice of 40% peak to peak inductor current ripple at 50% duty cycle is a design choice made against the background of the tradeoffs above.

    The equations 16 through 25 are used to calculate the input capacitor needed at the input to the PFC stage. The capacitance is based on the allowed ripple voltage at the input to this stage calculated at the peak of the low line input voltage where the input current is greatest.

    The other calculations calculate the inductor based on the 40% peak to peak allowed ripple current - this will happen at 50% duty cycle as you have pointed out.

    The whole calculation is somewhat arbitrary and depends on some design assumptions that may not be exactly the same as another designer will make - but they will give a good first pass estimate for L_PFC and C_IN

    Regards
    Colin