SWITCHMODEâ¢ Power Supply Reference Manual

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SMPSRMA Power Factor Correction (PFC) circuit is a switchingpower converter, essentially a boost converter with a verywide input range, that precisely controls its input currenton an instantaneous basis to match the waveshape andphase of the input voltage. This represents a zero degreesor 100 percent power factor and mimics a purely resistiveload. The amplitude of the input current waveform isvaried over longer time frames to maintain a constantvoltage at the converter’s output filter capacitor. Thismimics a resistor which slowly changes value to absorbthe correct amount of power to meet the demand of theload. Short term energy excesses and deficits caused bysudden changes in the load are supplemented by a ”bulkenergy storage capacitor”, the boost converter’s outputfilter device. The PFC input filter capacitor is reduced toa few microfarads, thus placing a half–wave haversinewaveshape into the PFC converter.The PFC boost converter can operate down to about30 V before there is insufficient voltage to draw any moresignificant power from its input. The converter then canbegin again when the input haversine reaches 30 V on thenext half–wave haversine. This greatly increases theconduction angle of the input rectifiers. The drop–outregion of the PFC converter is then filtered (smoothed)by the input EMI filter.A PFC circuit not only ensures that no power isreflected back to the source, it also eliminates thehigh current pulses associated with conventionalrectifier–filter input circuits. Because heat lost in thetransmission line and adjacent circuits is proportional tothe square of the current in the line, short strong currentpulses generate more heat than a purely resistive load ofthe same power. The active power factor correctioncircuit is placed just following the AC rectifier bridge. Anexample can be seen in Figure 34.Depending upon how much power is drawn by the unit,there is a choice of three different common controlmodes. All of the schematics for the power sections arethe same, but the value of the PFC inductor and thecontrol method are different. For input currents of lessthan 150 watts, a discontinuous–mode control scheme istypically used, in which the PFC core is completelyemptied prior to the next power switch conduction cycle.For powers between 150 and 250 watts, the criticalconduction mode is recommended. This is a method ofcontrol where the control IC senses just when the PFCcore is emptied of its energy and the next power switchconduction cycle is immediately begun; this eliminatesany dead time exhibited in the discontinuous–mode ofcontrol. For an input power greater than 250 watts, thecontinuous–mode of control is recommended. Here thepeak currents can be lowered by the use of a largerinductor, but a troublesome reverse recoverycharacteristic of the output rectifier is encountered,which can add an additional 20–40 percent in losses to thePFC circuit.Many countries cooperate in the coordination of theirpower factor requirements. The most appropriatedocument is IEC61000–3–2, which encompasses theperformance of generalized electronic products. Thereare more detailed specifications for particular productsmade for special markets.Switch CurrentInput VoltageConduction AngleC smallIControlV senseC largeV out+To PowerSupply–Figure 34. Power Factor Correction CircuitVoltageCurrentI AVGFigure 35. Waveform of Corrected Inputhttp://onsemi.com33
SMPSRMBibliography1. Ben–Yaakov Sam, Gregory Ivensky, “Passive Lossless Snubbers for High Frequency PWM Converters,”Seminar 12, APEC 99.2. Brown, Marty, Power Supply Cookbook, Butterworth–Heinemann, 1994, 2001.3. Brown, Marty, “Laying Out PC Boards for Embedded Switching Supplies,” Electronic Design, Dec. 1999.4. Martin, Robert F., “Harmonic Currents,” Compliance Engineering – 1999 Annual Resources Guide, CannonCommunications, LLC, pp. 103–107.5. ON Semiconductor, Rectifier Applications Handbook, HB214/D, Rev. 2, Nov. 2001.http://onsemi.com34
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