What is power factor ?

The power factor of an AC power system is defined as the ratio of energy which is capable to be absorbed by the load to the total amount of energy it draws from the input power source. It is an key indicator to show whether the system functions efficiently or not.

In a power system, there are two factors may cause low power factor: displacement and distortion. Some passive components such as inductor and capacitor make the current and voltage waves fall out of phase. The phase difference between voltage and current is defined as displacement. And the distortion is caused by the presense of harmonics in the currnet waveform, which are commonly caused by the non-linear load.

Why power factor correction?

Either dispalcement or distortion may make the current pumped back into the power grid, which will result in significant heat, and sometimes cause irretrievable damage. To avoid this situation, several components are being put together to improve power factor, which is called power factor correction(PFC).

Passive PFC, for example, uses combination of passive components, capacitor and inductor, to remove unwanted frequencies of signal. The power factor of the passive PFV is 70%~80%, not good enough for the daily use today, and the size and weight of the passive components become a problem when applied to the higher power levels. However, active PFC not only has the benefits passive PFC has, but also use a switch (MOSFET、IGBT, etc) modify current to match the phase of the input voltage, which significantly increase the power factor over 95%.

The swtich here plays an inportant role. The higher the switching frequency is ,the smaller the passive components will be. The typical switching frequency for silicon mosfet is below 100kHz. Owing to the smaller gate charge of GaN FET, GaN FET can be operated to the higher switching frequency, which can reduce the volume of the passive components, and thus increase the energy density of the power supply.