Power Factor Correction (PFC)



As technology advances, electric power usage is moving from simple, non-electronic loads (tungsten lamps, motors, relays, resistive heaters, etc.) to more complex ones (fluorescent lamps with energy-efficient ballasts, motors with solid-state drivers, personal computers and home appliances) with more detailed electronic components in them. The electric current drawn by these new devices is typically different from that of the predecessors, and causes problems in overall capacity of the electric utilities. Modern power supplies are among the devices that have a characteristic of distorted input current. Quite different from resistive heaters, toasters and tungsten light bulbs, typical switched-mode power supplies such as those in personal computers draw input current in short pulses rather than in smooth sine waves. In order to deliver the same amount of power in short pulses, the current peaks are much higher. This puts more stress on the wiring in the home or office, circuit breakers and even the generation and distribution equipment provided by the electric utilities. Figure 1 shows a typical distorted input current of an electronic load (a switched-mode power supply in this example).

To minimize these stresses and maximize the power handling capabilities of a switched-mode power supply, circuitry can be added to improve the shape of the input current. Ideally, the input current should have a sine wave shape, and be in phase with the input voltage. In this case, the maximum amount of power can be drawn from the AC line within the limits of power available from the source.

Power Factor Correction (PFC) allows power distribution to operate at its maximum efficiency. There are two types of PFC, Active PFC and Passive PFC.

Active PFC

The preferable type of PFC is Active Power Factor Correction (Active PFC) since it provides more efficient power frequency. Active PFC also markedly diminishes total harmonics, automatically corrects for AC input voltage and is capable of a full range of input voltage. Since Active PFC is the more complex method of Power Factor Correction, it is more expensive to produce an Active PFC power supply.

Passive PFC

The most common type of PFC is Passive Power Factor Correction (Passive PFC). Passive PFC uses a capacitive filter at the AC input to correct poor power factor.