Power Factor Correction Basics

It is given as:

PF = Real Power (W)/Apparent Power (VA) 

The above equation shows that PF is a number that can vary between 0 and 1. PF is 1 when the real power and the apparent power are the same. This happens only when both the current and voltage waveforms are in phase and sinusoidal (Figure 1). However, if both are sinusoidal but out of phase, the apparent power is more than the real power, and PF is the Cosine of the phase angle θ between current and voltage waveforms. In other words, PF = Cosθ. In practice, PF = 1 is an ideal situation where the load is pure resistive and linear. In reality, off-line AC/DC power supplies found in electronic systems are switched-mode, presenting a nonlinear load.  

Figure 1: Power factor is 1 when both the input current and voltage waveforms are in phase and sinusoidal. (Source: Infineon)

Since power supplies today are primarily switched-mode, they draw a non-sinusoidal waveform, resulting in a phase angle θ between the input current and voltage waveforms. When the current waveform does not follow the voltage waveform (Figure 2), it results in a PF below 1 based on Cosθ. For instance, if θ = 45°, PF = Cos45 = 0.707. Besides power losses, < 1 PF causes harmonics that travel down the neutral line and disrupt other devices connected to the AC mains line. The lower the PF number, the higher are the harmonics content on the AC line, and vice versa.  

Figure 2: Power factor is below 1 when the current waveform does not follow the voltage waveform. (Source: Infineon)

Subsequently, there are stringent regulations to limit the harmonic distortion permitted on the AC mains line. A popular European regulation is EN61000-3-2, which was introduced to limit sending back reflected harmonics from electronic equipment into the mains. It is applicable to all Class D electronic systems, such as PCs (including notebooks and PC monitors), and radio and TV receivers consuming more than 75 W. Class D is one of the four classes (A, B, C, and D) categorized by the EN61000-3-2 standard, which imposes different harmonic-current limits on each class. It is now an international standard. 

To comply with the harmonic requirements of regulations like EN61000-3-2 and maintain high overall PF performance, it is necessary to incorporate power-factor correction (PFC) in the AC/DC front-end converter modules used in electronic systems consuming more than 75 W. Implementing PFC achieves a high PF number and ensures low harmonics. Today, there are a number of passive and active techniques available for numerous power-supply topologies employed in the AC front-ends.

Passive PFCs 

The simplest way to control the harmonic current is to use a passive filter comprising an inductor and a capacitor. By passing the current only at the line frequency (e.g., 50 or 60 Hz), this LC filter reduces harmonics, enabling the nonlinear device to look like a linear load; thus, helping the PF to approach near unity. However, the drawback is that the filter requires a large-value high-current inductor and a high-voltage capacitor, which is bulky and expensive.

Active PFCs 

Figure 3: The active PFC solution uses a semiconductor controller, which is placed between the input rectifier and the storage capacitor, followed by the DC/DC converter.  (Source: ON Semiconductor)

The active PFC solution uses a semiconductor controller chip, which is placed between the input rectifier and the storage capacitor, followed by the DC/DC converter, as shown in Figure 3. This circuitry shapes the input current to match the input voltage waveform and achieve a high value of PF, normally above 0.9. Fundamentally, there are three different types of active PFC controller ICs. These include critical-conduction mode (CrM), continuous-conduction mode (CCM), and discontinuous-conduction mode (DCM). There are several manufacturers offering a variety of these active PFC controller ICs, with each supplier offering its own versions along with the rationale for using them.