Introduction to Power Factor Correction Technology in AC/DC Front-End Converter Modules

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Introduction to Power Factor Correction Technology in AC/DC Front-End Converter Modules [Copy link]

By definition, the power factor (PF) of an AC power supply is defined as the ratio of the real power (watts) flowing into the load to the apparent power in the circuit, which is the product of the current and voltage. It is expressed as PF = real power (W) / apparent power (VA). The PF equation shows that it is a number between 0 and 1. Therefore, when the current and voltage are in phase and sinusoidal, the PF is 1. However, if both are sinusoidal but out of phase, the apparent power is greater than the real value. The power and PF are the cosine of the phase angle between the current and voltage waveforms. In practice, PF = 1 is the ideal case where the load is purely resistive and linear. In reality, offline AC/DC power supplies in electronic systems are switch-mode, presenting a nonlinear load.

Since most current power supplies are switch mode, they produce non-sinusoidal waveforms, which result in a phase angle between the input current and voltage. When the current waveform does not follow the voltage waveform, it results in a PF below 1. In addition to power loss, a <1 PF causes harmonics to propagate along the neutral line and damage other devices connected to the AC power line. The lower the PF number, the higher the harmonic content on the AC line, and vice versa. Therefore, there are strict regulations to limit the harmonic distortion allowed on the AC power line. For example, Europe's EN61000-3-2 [1] was introduced to limit the harmonics reflected by electronic devices sent back to the power supply. It applies to all Class D electronic systems, such as PCs (including laptops and PC monitors), and radio and TV receivers that consume more than 75 W. Class D is one of four classes (A, B, C, and D) classified by the EN61000-3-2 standard, which imposes different harmonic current limits for each class. This standard is now accepted internationally.

In order to comply with the harmonic requirements of regulations such as EN61000-3-2 and maintain a high overall PF performance, it is necessary to incorporate power factor correction (PFC) in AC/DC front-end converter modules used in electronic systems with power consumption exceeding 75 W. Implementing PFC achieves high PF values and ensures low harmonics. As we will see, there are many passive and active technologies currently available for the numerous power supply topologies adopted in the AC front end.

Passive PFC

As described in Chapter 1 of the Power Factor Correction Handbook published by ON Semiconductor[2], the simplest way to control harmonic currents is to use a passive filter that passes current only at the line frequency (e.g., 50 or 60 Hz). This filter reduces the harmonic currents, which means that the nonlinear device now looks like a linear load. Using filters with built-in capacitors and inductors, the power factor can be brought close to unity. However, the disadvantage is that the filter requires large value, high current inductors and high voltage capacitors, which are bulky and expensive.

Figure 1 shows the input harmonics of three different 250 W PC power supplies compared to the limits according to the EN/IEC61000-3-2 Class D equipment specification. The harmonic amplitude is proportional to the input power of these devices. As shown, the performance of the passive PFC barely meets the limit for the third harmonic. The unit with active PFC meets and exceeds the IEC61000-3-2 specification.

Figure 1: In comparison, the power supply with active PFC controller outperforms the passive PFC over the IEC61000-3-2 specification for power line harmonics. (Courtesy of ON Semiconductor.) Despite their simplicity of design and use, passive PFC circuits have several disadvantages. First, the size of the inductor limits its usability in many applications. Second, for global operation, a line voltage range switch is required. The incorporation of switches makes the appliance/system prone to operator error if the switch selection is not done correctly. Finally, the unregulated voltage rail results in cost and efficiency loss in the DC/DC converter following the PFC stage.

Active PFC

Beyond performance, the rising cost of copper and core materials, combined with the falling cost of semiconductors, tip the balance in favor of active PFC solutions, even in the most cost-sensitive consumer devices. In the scheme below (Figure 2), the active PFC circuit is placed between the input rectifier and the storage capacitor, followed by the DC/DC converter. The PFC IC with associated circuitry shapes the input current to match the input voltage waveform, achieving PFs of 0.9 and higher.

Figure 2: The active PFC controller circuit is placed between the input rectifier and the storage capacitor. (Courtesy of ON Semiconductor.) Fundamentally, there are three different types of active PFC controller chips. These include critical conduction mode (CrM), continuous conduction mode (CCM), and discontinuous conduction mode (DCM). There are several manufacturers that offer a variety of these active PFC ICs, but each vendor offers its own version and reasons for using them.

The CrM control scheme keeps the inductor current at the critical limit between continuous and discontinuous conduction. For this reason, some suppliers prefer to call it boundary conduction mode or BCM. Since the waveform is always known in this scheme, the relationship between the average current and the peak current is also known. ON Semiconductor offers a variety of voltage-mode CrM PFC ICs for medium-power applications up to 300 W. The latest product in this category is the MC34262/MC33262 controller.

Another CrM PFC controller supplier is Fairchild Semiconductor. Its FAN6920MR integrates a CrM PFC controller and a quasi-resonant PWM controller in a single package. For PFC, the FAN6920MR uses a controlled on-time technique to provide a regulated DC output voltage, as well as perform power factor correction.CCM control is widely used in many applications, ranging from medium to high power, due to lower peak current stress, reduced ripple current and easier filtering tasks. Some of the major vendors offering CCM-based PFC controllers include Fairchild Semiconductor, Infineon Technologies, International Rectifier, NXP Semiconductor, ON Semiconductor, Power Integrations and Texas Instruments. In the DCM space (also preferred for low- and medium-power applications), Cirrus Logic has used digital technology to create a discontinuous mode active PFC controller that eliminates the need for multiple passive components to achieve a low-cost PFC solution for PCs, notebook adapters and digital TV receivers. The CS1500 (Figure 3) uses a variable on-time and variable frequency algorithm to achieve near unity power factor and low EMI emissions, simplifying EMI filtering. Figure 3: Cirrus Logic’s digital PFC controller implements an adaptive digital algorithm to shape the AC mains input current waveform to be in phase with the input voltage waveform.