Active Power Factor Correction
Active power factor correction can achieve very high power factors—0.98 and above—with reasonably sized components, though the energy efficiency may be slightly lower than with passive techniques (94 percent compared to 96 percent, for example) due to the addition of switching components.
In one of the simplest architectures, an inductor, a metal oxide semiconductor field-effect transistor (MOSFET) or insulated gate bipolar transistor (IGBT) switch, and a diode are added between the rectifier bridge and the bulk capacitor, in a boost switch-mode power supply configuration. The figure below shows an improved rectifier with active power factor correction (PFC), which draws current from the AC mains exactly in-phase with the mains voltage, for a high power factor.
Improved Rectifier with Active Power Factor Correction
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This is how the AC-to-DC boost converter works: The intermediate DC bus voltage is chosen to be higher than the peak voltage of the rectifier bridge, so the switch-mode controller will be working in boost mode. The controller driving the switch (Q) will adjust the duty factor of the switch control signal so that the desired current and voltage targets are maintained. The switching frequency is chosen to be much higher than the AC mains frequency (60 Hz compared to 20 KHz, for example). The small current ripple at the switching frequency and its harmonics can be filtered using a passive filter on the AC mains input, similar to passive PFC, but much easier because of the lower amplitude and higher frequency of the ripple current.
During phase one the switch is closed, shorting one end of the inductor (L) to ground. Current is drawn through the inductor via the rectifier bridge, energizing the inductor's magnetic field. During phase two, the switch is open. Since the current on the inductor cannot change instantaneously, the voltage on the inductor increases almost instantaneously until it is above the DC bus voltage by enough to turn on the diode (D). The current going through the inductor charges the bulk capacitor (C) as the magnetic field collapses slightly and simultaneously raising the DC bus voltage slightly. All the while, the motor inverter is drawing current from the DC bus in order to power the motor, using up the charge stored on the bulk capacitor and thus reducing the DC bus voltage.
