Power Electronics
A simple bridge rectifier + large capacitor draws current from the mains only in short, sharp pulses at the voltage peaks. The result: the voltage is a clean sine, but the current is just spiky pulses. This lowers the power factor (PF) (typically 0.5–0.6); the mains sees far more apparent current (VA) than the real power used, transmission losses rise, and the grid gets polluted. PFC solves this by shaping the current to resemble the voltage.
The most common solution is boost-PFC: a boost stage is placed between the rectifier and the load, and the controller forces the inductor current to follow the shape of the input voltage sine. The current then becomes sinusoidal and in phase with the voltage → PF ≈ 0.99. The output is typically boosted to a 400 V DC bus, and an isolated DC-DC stage feeds the rest.
The power factor is actually the product of two things: PF = cosφ × 1/√(1+THD²). The first term penalizes phase shift, the second the harmonic distortion (THD) in the current. Rectifier pulses are full of 3rd, 5th, and 7th harmonics; standards like IEC 61000-3-2 limit them. A good PFC pulls both cosφ toward 1 and THD below 5%.
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