VISHAY GENERAL SEMICONDUCTOR www.vishay.com Rectifiers Application Note Power Factor Correction with Ultrafast Diodes More and more switched mode power supplies (SMPS) are being designed with an active power factor correction (PFC) input stage. This is mainly due to the introduction of regulations aimed at restricting the harmonic content of the load current drawn from power lines. However, both the user and the power company benefit from PFC, so it just makes good sense. Non-PFC power supplies use a capacitive input filter, when powered from the AC power line. This results in rectification of the AC line, which in turn causes high peak currents at the crests of the AC voltage, as in Fig. 1a. These peak currents lead to excessive voltage drops in the wiring and imbalance problems in the three-phase power delivery system. This means that the full energy potential of the AC line is not utilized. V IAV a. No Power Factor Correction L Bridge Rectifier Power Not Used I IL D Vin Power Used V Mathematically, Power Factor (PF) is equal to Real Power / Apparent Power. The basic concept behind PFC is to make the input look as much like a resistor as possible. Resistors have a power factor of 1 (unity). This is ideal, because it allows the power distribution system to operate at its maximum efficiency. Lets consider a continuous conduction mode (CCM) boost converter being used for active PFC. The boost topology was chosen because it is the least expensive (cheapest) solution, and cost is always a major consideration. Please refer to Fig. 2. I C Vout MOSFET Fig. 2 - Continuous Mode Boost Converter Circuit IAV b. Power Factor Corrected Input Fig. 1 - Non-PFC vs. PFC Waveforms (Current, Voltage) Revision: 25-Nov-15 Document Number: 88839 1 For technical questions within your region: [email protected], [email protected], [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE Power Factor Correction (PFC) can be defined as the reduction of the harmonic content, and / or the aligning of the phase angle of incoming current so that it is in phase with the line voltage. By making the current waveform look as sinusoidal and in phase with the voltage waveform as possible, as in Fig. 1b, the power drawn by the power supply from the line is maximized for real power. Real power is equal to VRMS × IRMS × cos , where is the phase difference between the voltage and current waveforms. Therefore, as approaches zero, cos approaches unity, which maximizes the real power (now just VRMS × IRMS). The input full-wave bridge rectifier converts the alternating current (AC) to direct current (DC). The MOSFET is used as an electronic switch, and is cycled “on” and “off” by an external source. While the MOSFET is “on”, the inductor (L) current increases. While the MOSFET is “off”, the inductor delivers current to the capacitor (C) through the forward biased output diode (D). The inductor current does not fall to zero during each switching cycle, which is why this is known as a “continuous conduction mode.” The MOSFET is pulse-width-modulated so that the input impedance of the circuit appears purely resistive, and the ratio of peak to average current is kept low. The most cost-effective way of reducing losses in the circuit is by choosing a suitable diode for the application. Diodes for use in PFC circuits typically have higher forward voltages than conventional ultrafast epitaxial diodes, but much shorter (faster) reverse recovery times. Vishay recommends the use of the UH-series for PFC applications. Application Note www.vishay.com Vishay General Semiconductor Power Factor Correction with Ultrafast Diodes TABLE 1 - PFC ULTRAFAST RECTIFIERS - MINI SELECTOR GUIDE VISHAY PART NUMBERS CASE OUTLINE DESCRIPTION IAV (A) VRRM (V) trr (ns) USB260 DO-214AA (SMB) Plastic SMD 2 600 30 MURS260 DO-214AA (SMB) Plastic SMD 2 600 50 31GF6 DO-201AD Plastic Axial 3 600 30 SUF30J P600 Plastic Axial 3 600 35 MURS360 DO-214AB (SMC) Plastic SMD 3 600 50 MUR460 DO-201AD Plastic Axial 4 600 50 UHF5JT ITO-220AC Isolated Power Pack 5 600 25 UH5JT TO-220AC Plastic Power Pack 5 600 25 UG5JT TO-220AC Plastic Power Pack 5 600 25 UGB5JT TO-263AB Power Pack SMD 5 600 25 UGF5JT ITO-220AC Isolated Power Pack 5 600 25 UH8JT TO-220AC Plastic Power Pack 8 600 25 UHF8JT ITO-220AC Isolated Power Pack 8 600 25 UG8JT TO-220AC Plastic Power Pack 8 600 25 UGB8JT TO-263AB Power Pack SMD 8 600 25 UGF8JT ITO-220AC Isolated Power Pack 8 600 25 UG12JT TO-220AC Plastic Power Pack 12 600 30 UGB12JT TO-263AB Power Pack SMD 12 600 30 UGF12JT ITO-220AC Isolated Power Pack 12 600 30 UG15JT TO-220AC Plastic Power Pack 15 600 35 UGB15JT TO-263AB Power Pack SMD 15 600 35 UGF15JT ITO-220AC Isolated Power Pack 15 600 35 APPLICATION NOTE Revision: 25-Nov-15 Document Number: 88839 2 For technical questions within your region: [email protected], [email protected], [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000