Power Factor Correction Effective December 2017 (PFC) application notes Technical Note 4040 Supersedes 2010 Power Factor Correction (PFC) application notes Overview Every year, millions and millions of notebook computers, LCD monitors and LCD televisions are produced. With such a fast growing number of these and other electronic devices using more and more power, actions must to be taken to ensure the functionality of the nationwide power grid. In 2001, the European Union put EN61000-3-2 into effect to set the harmonic regulation standard on any power grid supplied application with power consumption over 75 watts. This essentially requires power factor correction (PFC). Additionally, a standby power dissipation limit is set to conserve power when a load is OFF. 80 PLUS is an initiative funded by electric utilities to integrate more energy efficient Power Supply Units (PSUs) - especially for desktop computers and servers. 80 PLUS certifies to more than 80% energy efficiency at 20%, 50% and 100% of rated load. To meet the 80 PLUS certification, PSUs require a PFC of 0.9 or greater at 100%load. This means PSUs that waste 20% or less electric energy (as heat at the specified load levels) will lead to reduced electricity consumption and lower bills. Rebates are sometimes given to manufacturers who use 80 PLUS certified PSUs. Implementing power factor correction (PFC) into switch mode power supplies will maximize: Power handling capability of the power supply Current handling capacities of power distribution networks Input power factor (PF) is defined as: Real Power (watts) PF = Apparent Power (VA) PF is expressed as decimal number between zero and one (0 and 1). A non-corrected power supply with a typical PF equal to 0.65 will draw approximately 1.5 times greater input current than a PFC supply (PF = 0.99) for the same output loading. The non-corrected supply requires additional AC current to be generated which is not 2 consumed by the load, creating I R losses in the power distribution network. There are two types of PFCs: Active PassivePower Factor Correction Technical Note 4040 (PFC) application notes Effective December 2017 Passive PFC Boost inductor The simplest form of PFC is passive (Passive PFC). A passive PFC The boost-circuit based PFC topology is the most popular. It is an uses a filter at the AC input to correct poor power factor. The economical solution for complying with regulations (Figure 3). The passive PFC circuitry uses only passive components an inductor inductance value is selected based on the desired current ripple in and some capacitors (Figure. 1). the boost inductor. The inductance value is expressed as follows: Although pleasantly simple and robust, a passive PFC rarely achieves low Total Harmonic Distortion (THD). Also, because the pK V (min) * d(max) circuit operates at the low line power frequency of 50 Hz or 60 Hz, L = in the passive elements are normally bulky and heavy. fs * i where: PFC Inductor DC Bus pK V (min) is the peak minimum input voltage in + fs is the switching frequency AC i is the ripple current d(max) is the maximum duty cycle expressed as: - pK 1- V (min) in d(max) = where V is the output voltage o V o Figure 1. A passive PFC circuit requires only a few components to increase efficiency, but they are large due to operating at the line power frequency The rms boost inductor current is expressed as: I (pk) in IL (rms) = A Active PFC 2 Active PFC offers better THD and is significantly smaller and L2 lighter than a passive PFC circuit (Figure 2). To reduce the size and 3.3Vout F1 F2 cost of passive filter elements, an active PFC operates at a higher PFC L1 C2 + switching frequency than the 50 Hz/60 Hz line frequency. DC/DC B oost C1 A C C Converter Line out Active PFC functions include: C3 M odule Active wave shaping of the input current 5V Filtering of the high frequency switching out L3 F3 Feedback sensing of the source current for waveform + control DC/DC Converter Feedback control to regulate output voltage Buck, boost, flyback and other converter topologies are used in active PFC circuits. Figure 3. PFC Boost - Typical application circuit, 3.3 & 5 V, 60 W combined The DC-DC converter input capacitor also benefits from active output power. PFC. The capacitor can be sized to filter the high frequency ripple of the active PFC circuit instead of a much larger capacitor that Inductor selection would be required to smooth the 50-60 Hz input. The regulated input of the DC-DC converter also demands a lower range of duty Eaton s PFC inductors are available for use with a wide variety of cycle from the DC-DC converter. Other benefits of active PFC PFCs from 100 W to 250 W. They operate with controllers from include increased hold-over-time. Hold over (brownout several IC manufacturers to provide PFC supply solutions that protection) benefits from always starting at the maximum voltage utilize either passive or active PFC applications (Table 1). 2 and because energy in the capacitor is related to 1/2CV , the capacitor can be much smaller than a capacitor in a converter Eaton s PFC inductors range from 200 H to 1.2 mH. The without active PFC. standard input voltage range is 85 V to 385 V with different core materials such as ferrite, iron powder and Kool-Mu to provide significant low core loss. The E-core and toroidal geometries PFC allow using thicker wire to decrease DC resist-ance and yield DC Inductor higher current capacity. Many vertical or horizontal through-hole Bus mounting options are available with an operating temperature + range of 20 C to +105 C (Table 2). A C PFC Control - Figure 2. An active PFC circuit produces low THD and uses relatively small passive components. 2 EATON www.eaton.com/electronics