The selection of the PFC inductor value is usually based on a number of different considerations. Cost, core size, EMI filter, and inductor ripple current are some of the factors that have an influence. In previous versions of this data sheet, the design method to choose the inductor targetted the peak to peak inductor ripple current (ΔI_L) at the minimum input voltage to have the same amplitude as the peak of AC-line current in each phase. The line current flows equally in the two phases so ΔI_L is half I_{in_pk} calculated in Equation 38. That method worked well for relatively low minimum input voltages, but was found to generate excessively low inductances for minimum inputs with peak voltages near ½ V_OUT.

A new method of calculating boost inductance is presented in this datasheet where low input-current distortion is the main design criterion. In recent years, low distortion at lighter loads and higher input voltages has become a major design requirement in many applications. In a CCM-Boost-PFC, the total harmonic distortion of the input current (THDi) increases greatly when the inductor current operates in DCM over a significant portion of the input AC-line cycle. To maintain low THDi at any given line and load point, it is necessary to maintain CCM in the boost inductors at that operating point. Since a PFC converter is intended to present an equivalent or emulated resistance Re to the AC line, it can be shown [5] that inductor current operates in CCM over the entire line cycle when:

By rearranging terms and substituting, the minimum boost inductance necessary to maintain CCM is calculated as:

where

• V_{rms_CCM(max)} is the highest rms input voltage at which CCM operation is to be maintained
• P_{O_CCM(min)} is the lowest output power level per inductor where CCM is to be maintained
• η is the expected conversion efficiency at P_{O_CCM(min)} and V_{rms_CCM(max)}

Lower values of boost inductance than that calculated by Equation 42 can be used for PFC, but THDi will increase as the amount of DCM increases in the line cycle. To match the previous data sheet inductor selection, it can be seen that for CCM operation at 100Vrms input, 150W per phase, 95 % efficiency, and 200kHz PWM switching frequency, L_B must be ≥ 158.333µH.
Select L₁ = L₂ = 160µH.

Given that inductance, ΔI_L at the peak of low-line can be calculated as follows:

Then, the peak current in each boost inductor is approximately:

The basic inductor specifications for this application example are:

• Inductance: 160µH
• Peak current: 4A