Active Boost PFC Inductor & Capacitor Calculator

Principles of Active Boost Power Factor Correction (PFC)

Standard bridge rectifiers with smoothing capacitors draw current only during brief voltage peaks of the AC sine wave, producing high current crest factors and severe harmonic distortion (THD $> 80\%$, Power Factor $< 0.60$). Under international standard IEC 61000-3-2 Class D, electrical loads drawing more than 75W must incorporate Active Power Factor Correction (PFC) to shape the input current into a clean sinusoid in phase with line voltage.

Boost Inductor Sizing in Continuous Conduction Mode (CCM)

In CCM, the inductor current never falls to zero during a switching period. The worst-case inductor ripple occurs at the peak of the minimum AC input voltage:

Where:

• $V_{in,pk,min} = \sqrt{2} \cdot V_{in,min}$: Peak input voltage at low line ($85\ \text{V}_{rms} \to 120.2\ \text{V}_{pk}$).
• $I_{in,pk} = \frac{\sqrt{2} \cdot P_{out}}{\eta \cdot V_{in,min} \cdot PF}$: Peak AC line current.
• $\Delta I_L = r \cdot I_{in,pk}$: Peak-to-peak inductor ripple current (typically $r = 0.20$ to $0.30$).

Bulk Capacitor Sizing for Mains Dropout Holdup

The output bulk capacitor must fulfill two independent criteria:

1. Holdup Time ($t_{hold}$): During a complete AC line outage (typically 1 mains cycle, $16.6\ \text{ms}$ or $20\ \text{ms}$), the capacitor must supply downstream DC-DC stages without voltage collapsing below $V_{out,min}$: \[ C_{bulk} \ge \frac{2 \cdot P_{out} \cdot t_{hold}}{V_o^2 - V_{out,min}^2} \]
2. Double-Frequency Voltage Ripple ($2 f_{line}$): Because instantaneous AC power pulsates at $2 f_{line}$ ($100\ \text{Hz}$ or $120\ \text{Hz}$), peak-to-peak ripple is: \[ \Delta V_{o,pk-pk} = \frac{P_{out}}{2\pi \cdot f_{line} \cdot C_{bulk} \cdot V_o} \]