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Issue #214 May 2008
Problem 1—The following figure shows the essential features of a modern power-factor controller, which is a specialized form of boost regulator.
What sort of current waveform would you expect to see in the inductor? How does this relate to the input line current?
Answer 1—The primary purpose of the PFC circuit is to produce a line current waveform that precisely tracks the rectified line voltage waveform, in order to simulate a purely resistive load.
Therefore, working backwards, the line current waveform should also be a full-wave rectified sine wave shape.
The line current is the average of the current in the coil, so although the basic coil waveform is a discontinuous sawtooth/triangle wave, its overall envelope follows the rectified sine shape as well. The average value of a sawtooth or triangle wave is half of its peak value. If the coil current is discontinuous, then the average value is reduced by the duty cycle as well.
Obviously, we don’t want this high-frequency inductor current to appear on the input line, so a smallish capacitor, C1, is used to decouple it. This capacitor has a low impedance at the switching frequency (10 s or 100 s of kHz), but a fairly high impedance at the rectified line frequency (100 or 120 Hz). Sometimes a more elaborate filter (e.g., pi network) is used in this position to improve the performance.
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