With the increase of power line current harmonics have become
a significant problem. Typical problems include increased
magnitudes of neutral currents in three-phase systems,
overheating in transformers or induction motors, as well as
degradation of system voltage waveforms among others.
Several international standards have been issued to regulate
and limit the harmonics of equipments connected to the
electricity distribution network. High frequency switching
converter topologies include a PFC circuitry section or
Preconverters where the heart is the PFC inductor operating
in either Continuous or Discontinuous Mode Current (DCIMor CCIM).
Passive PFC methods use additional passive components in conjunction with the diode bridge rectifier from Fig. 1.
One of the simplest methods is to add an inductor at the AC-side of the diode bridge, in series with the line voltage as
shown in Fig.2a, and to create circuit conditions such that the line current is zero during the zero-crossings of the line
voltage. The maximum power factor that can be obtained is 0.76, with the theoretical assumption of constant DC output
voltage. The DC output voltage of the PFC circuit has a ripple at twice the line-frequency. The ripple is also dependent
on the load current. Simulated results for the rectifier with AC-side inductor are presented in Fig. 2b, where the inductance
DL has been chosen so as to maximize the power factor.
Figure 2a) Rectifier circuit PFC with inductor on AC side.
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Figure 2b) Waveform example for PFC correction with inductor on AC side.
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The inductor can be also placed at the DC-side, as shown in Fig. 3a. The inductor current is continuous for a large
enough inductance L. In the theoretical case of near infinite inductance, the inductor current is constant, so the input
current of the rectifier has a square shape and the power factor is 0.9. However, operation close to this condition would
require a very large and impractical inductor, as illustrated by the simulated line current waveform forDH L
(without Ca ), shown in Fig. 3b.
For lower inductanceDL , the inductor current becomes discontinuous.
The operating mode being identical to the case of the AC-side inductor previously showed. An improvement of the power
factor can be obtained by adding the capacitor Ca as shown in Fig. 3a, which compensates for the displacement factor
cos p. A design for maximum purity factor p K and unity displacement factor cos is possible, leading to a maximum
obtainable power factor 0.905. This is exemplified by the simulated line current for D275mH L and a 4.8 µC which is
shown in Fig. 3b.
Figure 3a) Rectifier circuit with PFC inductor on DC side.
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Figure 3b) Waveform example for PFC correction with inductor on DC side.
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