Power Factor Correction

Inverters and DC-DC converters can be highly efficient, but most switch-mode power supplies begin with a rectifier and bulk capacitor that draw short, high-amplitude current pulses from the AC mains instead of a smooth sinusoidal current.

These pulsed currents reduce power factor, increase harmonic distortion, and inject harmonics into the power grid. Power Factor Correction (PFC) reshapes the input current so it more closely follows the sinusoidal mains voltage.

What Is Power Factor?

Power factor is the ratio of real power delivered to the load to apparent power drawn from the source. A power factor of 1 means all supplied power is converted into useful work. A low power factor means extra current is drawn for the same useful power.

PF = \frac{P_{real}}{S_{apparent}} = \frac{P}{V_{rms} \cdot I_{rms}}

Two Components of Power Factor

Power factor has two independent components. Both must be good for the overall PF to be high.

1. Displacement Power Factor

Caused by phase shift between voltage and current — typical in inductive loads like motors and transformers. If current lags voltage by angle φ:

PF_{disp} = \cos\varphi

2. Distortion Power Factor

Caused by harmonic distortion in the current waveform. Even if current is perfectly in phase with voltage, a non-sinusoidal current shape still lowers power factor. This is common in rectifiers, SMPS front ends, and diode-capacitor input filters.

PF_{dist} = \frac{1}{\sqrt{1 + THD_i^2}}

The overall power factor combines both:

PF = PF_{disp} \times PF_{dist} = \cos\varphi \cdot \frac{1}{\sqrt{1 + THD_i^2}}

Common misconception

Many beginners assume "current in phase with voltage" automatically means good power factor. But SMPS rectifiers often have nearly zero phase shift yet still poor PF — because the current is highly distorted (narrow pulses, not a sine wave). Displacement is only half the story; distortion matters just as much.

The Problem: Bridge Rectifier + Capacitor

A diode bridge with a large filter capacitor draws current only near the voltage peaks — the capacitor charges in a brief burst, then the diodes stop conducting until the next peak. The resulting narrow current pulses give a PF of roughly 0.5 to 0.65.

Figure 1. Sinusoidal mains voltage (blue) vs peaky input current (red) — the current pulses are in phase with voltage but highly distorted

Notice: the current pulses are centred on the voltage peaks — there is almost no phase shift. The poor power factor comes entirely from the distorted waveform shape, not from a lagging current.

Oversized wiring and fuses are needed because RMS current is much higher than necessary for the real power.
Transformers must be derated, and triplen harmonics overload the neutral conductor in three-phase systems.

Passive PFC

Adding a series inductor after the bridge spreads the current pulse wider, improving PF to roughly 0.7 to 0.8. The inductor is heavy and bulky, limiting passive PFC to low-power or cost-sensitive designs.

Figure 2. Passive PFC: bridge rectifier with series inductor, filter capacitor, and load

Active PFC (Boost Converter)

A boost converter stage between the bridge and bulk capacitor shapes the input current to follow the voltage sine. A control loop modulates the MOSFET duty cycle so the inductor current tracks a sinusoidal reference.

Figure 3. Active PFC: bridge rectifier feeding a boost converter (inductor, MOSFET, diode, output capacitor) and load

PF exceeds 0.99 with active PFC — the current waveform is nearly sinusoidal.
The boost topology is dominant because it naturally draws continuous current from the mains.

Approach	PF	THD	Weight	Cost
No PFC	0.5 -- 0.65	100 -- 150%	Lightest	Lowest
Passive PFC	0.7 -- 0.8	30 -- 50%	Heavy	Low
Active PFC	> 0.99	< 5%	Light	Higher