PFC Power Factor Corrector Circuit

Power factor in power network is defined as the ratio of real power delivered to the load to apparent power. Strict mathematical definition of power factor

(cos(φ)) is

pfc-definition

In here, P is active power, S is apparent power and φ is electric angle. Power factor can’t be higher then unity. If power factor is low, waste of energy is high and vice versa. This is because overall power delivered to the load is equal to average value of integral of voltage and current product. If power factor is low it means that the phase difference between voltage and current is large, so average value of integral of voltage and current product i.e. power delivered to the load is small, as it can be seen in figure bellow.

active-reactive-apparent-power

If power factor is close to unity it means that the phase difference between voltage and current is small, so average value of integral of voltage and current product i.e. power delivered to the load is close to apparent power. Phase shift can be corrected with passive compensator (capacitor to compensate inductive load and vice versa). If DC-DC converter is used to do power factor correction, we are talking about active power factor corrector (PFC).
Linear rectifiers,

diode-bridge-simulation-pfc

distort the current drawn from the system, since they draw current from the network only in short time intervals when rectified input voltage is higher then input capacitor voltage.

pfc-waveform01

In order to correct this, DC-DC converters known as power factor correctors (PFC) are used to widespread input current and so to correct power factor. Power corrector circuits must draw current from the network even when rectified voltage is few volts only (when input sine is close to zero). For this reason boost and flyback topologies are used as power factor corrector circuits. Usual topology for this PFC purpose is boost DC-DC converter. Advantage of the boost converter is that input inductor is with one terminal permanently connected to input voltage source. Inductor is the only electrical element capable to transfer energy from power source on lower electrical potential to power source on higher electrical potential if and only if is previously charged with energy. During “ON” time of the main switch, second terminal of input inductor is connected to the ground. For this period of time, current flows from power network to inductor, even if input voltage is few volts only.

pfc_boost01

When the main switch is “OFF”, inductor current decreases linearly, but still charging output capacitor.

pfc_boost02

As the final result, output voltage is higher then input voltage, input current is in phase with the voltage (near unity power factor).

pfc_waveform02

Due to energy conservation law, power delivery is zero when input voltage is zero, so that output voltage must have small sine-like waveform ripple on frequency twice as mains.

External links:

Power Factor on Wikipedia
Power Factor on Hyperphysics
Power Factor Corrector Circuit on radio-electronics
Power Factor Corrector Circuit on powerfactor

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