Why we need PFC in PSU

About PFC
To understand the meaning of PFC (Power Factor Correction), it is important to figure out the meaning of Power Factor. The power factor can be performed with the following formula, then the difference between efficiency and power factor will be introduced, followed by the categoies of PFC. Having indicated both of them, the passive and active PFC power supply will be addressed respectiively.
Power Factor = Real Power / Apparent Power
This ratio can indicate the usage of power.

The difference between efficiency and power factor

Take our daily life as an example; the current we use everyday is alternating current while most of the household products are belonging to resistive load (light bulb, heater, etc.) and inductive load (engine, air condition, etc.). When alternating current connects to inductive load, it will result current waveform’s phase angle behind voltage waveform, which is named as non-efficient power. If the waveforms of current and voltages can have the similar wave shapes, then the power factor value will close to 1, which can be also seen as 100 percent transform. This condition indicates that there is no loss during the electronic power transferring process and the power is used with efficiency.

The difference between efficiency and power factor efficiency indicate the usage degree of power results from household electronics; it can be seen as the electronic fee we spend. When the power supply performs with low efficiency it can produce extra heat which increases its temperature. With this result, cooling performance will need to increase to satisfy environmental requirement.

Power factor reveals the electronic usage ratio which the household electronics consume, mainly focus on the degree of usage and waste. Although power factor’s influence to electronic fee is not as large as efficiency, the requirement of large power is increasing with the computer device improvement and this results increasing expenditure of power. Although the low performance of power factor of power supply is not the main reason that affects our electronic fees but can be seen as a huge loss for the limited power in the world.

While environmental protection is gradually becoming an important issue, the European Union has practiced the PFC safety standard for power supply over 70 Watts selling within Europe, which mandates the inclusion of PFC function in order to reduce the requirement of power voltage. Also in 2006, the Europe Union also introduced RoHs standard.  For limited global resource, and for environmental protection, SilverStone’s power supplies are all designed to meet the RoHs standard, high PH value, and high efficiency.

To increase the efficiency of power usage, the input of household applications are added a loop which is named as power factor correction. With this design, the inputting time and waveform of alternative current can be controlled to increase the high power factor. This kind of design is very important to the electronic device which needs a huge requirement of power. Without this design, the consumption of power from electronic devices may become over spec. and may interfere with other device. Normally, the power factor value of electronic device without power factor correction is approximately 0.5.

The categories of PFC

PFC can be divided into active and passive, this is classified by controlling PFC component characteristics. The main difference is that the active PFC power supply is without manual switch voltage and support all voltage range.




PF value



None PFC

With input voltage, switch or fixed input voltage





Passive PFC

With input voltage, switch or fixed input voltage





Active PFC

Without input voltage switch






Introduction of passive PFC power supply

assive PFC power supply is consisted of passive components, its PF value is between 70%~80% approximately. The more voltage of power supply, the requirement size of PFC components becomes greater. The input power voltage is required to switch via manual operation. Passive PFC power supply is using a low pass filter to filter the distortion’s high harmonic wave, and only leave the 60 Hz (or 50Hz) basic wave in order to increase the power factor. Passive PFC power supply can only decrease the current wave within the standard and the power factor can not be corrected to 1. This kind of design has the following advantages, simple structure, easy design, low cost, and only require a filter to correct, but will have the following disadvantages, large dimension of filter, heavy and low efficiency resulted from thermal and voltage decrease while current pass through the filter.

Introduction of active PFC power supply
The PF value of Active PFC power supply is over 90 percent. It is even possible to reach 99.99 percent and include the full range special feature. Active PFC power supply can detect the input voltage automatically, supports 110V to 240V alternative current, its dimension and weight is smaller than passive PFC power supply which goes against the traditional view that heavier power supply is better.

To distinguish active PFC power supply, the input resistant should be pure resistance, in other words, active PFC power supply can enforce the current tracking the voltage waveform in order to have the same wave as voltage. This usually can correct the active power factor by current inverter and digital I.C. which can reduce the harmonic wave and can stabilize the voltage. The benefits of active PFC power include light weight, small dimension and the power factor value of over 0.95. The disadvantage of active PFC is that the layout design is complex, and high cost (needs PFC control IC, high voltage MOSFET, high voltage U-fast, choke and other circuit).

Active PFC power supply can achieve 99 percent power factor; therefore, it can save more energy and its power factor performance is more stable to meet higher environment standards.
Active PFC Circuit

Active PFC Circuit on Board