PFC Circuit in Controller Design

1. What is PFC

PFC (Power Factor Correction) is power factor correction, which is mainly used to indicate the efficiency of electronic products in utilizing electrical energy.A higher power factor indicates better efficiency in utilizing electrical energy. All computer power supplies that pass the CCC certification must include PFC circuits. These circuits are located after the second layer of filtering and before the full-bridge rectifier circuit.There are two types of PFC, one is passive PFC, and the other is active PFC.

2. Explanation of Basic Concepts of Power Factor

(1)Power Factor (PF):PF represents the ability of a power supply to output real power. Power is a measure of the rate of energy transfer. In DC circuits, power is the product of voltage (V) and current (A). In AC systems, it’s more complex due to the presence of reactive currents or harmonic currents, which cause the apparent power (voltage multiplied by current) to be greater than the real power. In AC systems, part of the AC current circulates in the load without transmitting electrical energy. This is called reactive current or harmonic current, which makes the apparent power (voltage multiplied by current) greater than the actual power. The inequality between apparent power and actual power leads to the power factor, which is equal to the ratio of actual power to apparent power.
(2) Apparent power: the product of the effective value of the AC voltage and the effective value of the AC current, expressed by the formula: S = UI. In this formula, S is the apparent power in VA (Volt-Amperes), U is the effective voltage in V (e.g., 220V, 380V), and I is the effective current in A.
(3)Apparent power consists of two parts: active power (P) and reactive power (Q). Active power refers to the part of power that does real work, such as lighting a lamp, rotating a motor, or powering an electronic circuit. This power eventually becomes heat and can be directly felt. Some may mistakenly think that active power is the same as apparent power, but active power is actually only a part of the apparent power, which can be expressed by the formula: P=SCosΦ=UICosΦ＝UIF.In the above formula, P is the active power, and the main units are watt (W), kilowatt (kW), and megawatt (MW).F=CosΦ is called the power factor, and Φ is the phase difference when the voltage and current are out of phase when there is a non-linear load. Reactive power is the power stored in the circuit but not directly doing work, expressed by the formula: Q = SSinΦ = UISinΦ. In the above formula, Q is reactive power, with units of Var, kVar.

3. Types and characteristics of PFC

(1)Passive PFC
Passive PFC refers to a correction circuit that does not use active devices such as transistors. It typically consists of passive components like diodes, resistors, capacitors, and inductors. Add an inductor between the rectifier bridge and the filter capacitor (select the inductance appropriately), and use the characteristic that the current on the inductor cannot change suddenly to smooth the fluctuation of the capacitor charging strong pulse, improve the distortion of the current waveform of the power supply line, and use the characteristic that the voltage on the inductor leads the current to compensate for the characteristic that the current of the filter capacitor leads the voltage, so as to improve the power factor, electromagnetic compatibility and electromagnetic interference, as shown in the following figure:

This method cannot be considered a true passive PFC circuit but is rather a simple compensation measure. It can be applied to the equipment without PFC function designed in the early stage. Simply add a suitable inductor (appropriately select the values of L and C) to achieve the purpose of suppressing the instantaneous mutation of current. However, this simple, low-cost remedy has a large output ripple, a low DC voltage across the filter capacitor, poor current distortion correction and power factor compensation capabilities, and poor quality control of the L winding and core, which can cause serious interference to the image and sound. It can only be a temporary measure to enable early non-PFC devices to enter the market.
(2)Active PFC
The active PFC circuit is highly effective. It can essentially eliminate current waveform distortion and maintain the phase consistency between voltage and current. It basically completely solves the problems of power factor, electromagnetic compatibility, and electromagnetic interference, but the circuit is very complicated. The basic idea is to remove the filter capacitor after the 220V rectifier bridge stack (to eliminate the current waveform distortion and phase change caused by capacitor charging), and use a “chopper” circuit to convert the pulsating DC into high-frequency (about 100kHz) AC. After rectification and filtering, the DC voltage is then used to supply power to a conventional PWM switching power supply. The process is AC→DC→AC→DC.
The basic principle of the active PFC circuit is to add a DC-DC chopper circuit between the rectifier circuit and the filter capacitor of the switching power supply, as shown in the figure (the chopper circuit is equivalent to adding a switching power supply). For the power supply line, the output of the rectifier circuit is not directly connected to the filter capacitor, so it presents a purely resistive load to the power supply line, and its voltage and current waveforms are in phase and have the same phase. The operation of the chopper circuit is also similar to that of a switching power supply, so the active PFC switching power supply is a switching power supply circuit of a dual switching power supply, which is composed of a chopper and a voltage-stabilized switching power supply.
A chopper circuit (also called a DC chopper circuit) is a circuit that “chops off” part of a sine wave for some reason in the application of electricity. (For example, when the voltage is 50V, use electronic components to cut off the 50~0V part at the back, and the output voltage is 0.) Later, it was borrowed to DC-DC switching power supplies. Mainly, during the voltage regulation process of the switching power supply, the original straight line of power was “chopped” into pieces of pulses by the line. Converts direct current into another fixed voltage or adjustable voltage direct current. Also called a DC/DC converter.
(3)The difference between passive PFC and active PFC
In power supply design and power system debugging, there are two common PFC application methods: passive PFC and active PFC. Power supplies rated at 250W to 300W mostly use passive PFC, while active PFC is commonly used in power supplies rated at 400W and above.
The difference between the two is that passive PFC is generally divided into “inductor compensation ” and “valley fill circuit “. The “inductance compensation” is to reduce the phase difference between the fundamental current and voltage of the AC input to improve the power factor. Passive PFC includes silent passive PFC and non-silent passive PFC. The power factor of passive PFC can only reach 0.7~0.8, which is generally near the high-voltage filter capacitor.
The “valley filling circuit” is a new type of passive power factor correction circuit. Its characteristic is that it uses the valley filling circuit behind the rectifier bridge to greatly increase the conduction angle of the rectifier tube. By filling the valley point, the input current is changed from a peak pulse to a waveform close to a sine wave, which increases the power factor to about 0.9 and significantly reduces the total harmonic distortion. Compared with the traditional inductive passive power factor correction circuit, it has the advantages of simple circuit, significant power factor compensation effect, and no need to use a large inductor with large volume and heavy weight in the input circuit.
Active PFC is composed of inductors, capacitors and electronic components. It is small in size and uses a dedicated IC to adjust the current waveform and compensate for the phase difference between current and voltage. Active PFC can achieve a higher power factor – usually over 98%, but the cost is also relatively high. In addition, active PFC can also be used as an auxiliary power supply. Therefore, when using an active PFC circuit, a standby transformer is often not required, and the ripple of the active PFC output DC voltage is very small. This power supply does not need to use a large-capacity filter capacitor.

The three pictures are the effect of power factor correction. Red is the input voltage, blue is the input current. The first picture is the waveform without power factor correction. The second picture is the waveform with passive power factor correction. And the third picture is the waveform with active power factor correction.