How do pfc work

Rectification is the process of converting a signal from AC to DC, and is done using a rectifier. The negative voltage in the AC wave can be either cut off using a half-wave rectifier, or inverted using a full-wave rectifier. A full-wave rectifier is made up of four diodes connected in an arrangement called a Graetz bridge. This wave has a large voltage variation, called ripple voltage, so a reservoir capacitor is connected in parallel to the diode bridge to help smooth the output voltage ripple.

This creates a series of short current spikes in the capacitor, which look nothing like a sinusoid see Figure 2. This is a very big problem, not only for the power supply, but for the entire power grid. To understand the magnitude of this problem, we must first have a handle on the concept of harmonics. Most of the electric waveforms you have seen up to now are sine waves.

However, waves often stop being purely sinusoidal, especially when reactive components capacitors, inductors or nonlinear components transistors, diodes are present. These waves are defined by different, often complex mathematical functions. This could make the analysis of this wave much more difficult, because the mathematics behind the analysis become considerably harder see Figure 3.

Fortunately, in the 19th century, French mathematician Jean-Baptiste Joseph Fourier came up with a method of decomposing any arbitrary periodic waveform into a series of sine and cosine waves with different frequencies, called harmonics see Figure 4.

The first of these waves, which is referred to as the fundamental, is the wave with the lowest frequency. Then, several other waves are combined with the fundamental and given specific amplitudes and frequency. Harmonic frequencies must be integer multiples of the fundamental frequency. One of the most important parameters of harmonics is their amplitude, which is a measure of their effect on the fundamental frequency.

The amplitude of these harmonics can be graphed, showing what role each harmonic plays in the creation of the arbitrary waveform. The problem with the current in the capacitor is that it looks very similar to a delta function. This type of wave is, ideally, an infinitely short, infinitely powerful pulse. A wave of this shape is understandably complicated to decompose into sine waves, and results in a large number of powerful harmonics, spanning practically all frequencies see Figure 5.

However, if there are too many high-power devices with low power factor connected to the grid, it could be problematic, and even cause a blackout!

There are three types of AC power. The first is called active power, and is usually referred to as real power, or P. This represents the net energy that is transferred to a load. If the load is purely resistive, all power in the line is active power, and the voltage and current oscillate in phase with each other. Second, if the load is purely reactive, like an inductor or a capacitor, the power will be purely reactive, often expressed as Q. This power is used to generate and maintain the magnetic and electric fields in reactive components.

This means that the overall power generated by these purely reactive loads is zero, because the positive reactive power is canceled out by the negative reactive power. In practical applications, loads are never purely resistive or reactive, but are a combination of both. The third AC power type is the addition of both active and reactive power, and is called apparent power, or S. This addition is quadratic, and the relationship between active, reactive, and apparent power is often expressed in the form of a triangle.

Power factor is the relation between the active power and the apparent power, and is useful for measuring the efficiency of power transmission in the circuit see Figure 7. A low power factor is a combination of two factors: displacement and distortion.

The first, in linear loads, is due to the presence of reactive components that make the current and voltage waves fall out of phase. The effect that the phase difference between the voltage and current has on the total power factor is defined by the displacement factor, which is calculated as the cosine of the angle between waves using Equation 1 :. However, if we return to the matter at hand, the problem faced by power supply designers is not only that the current and the voltage waves may be out of phase, but also the fact that the current waveform has become a pulse train, which is a nonlinear function.

This means that the multiplication of the voltage and current, also known as power, is also nonlinear and highly inefficient. This occurs when a circuit has nonlinear loads, such as fluorescent lights, electronic devices, and full-bridge rectifiers.

The active PFC shown above is in the form of a boost regulator and as a result the voltage appearing across the load R1 must be greater than the highest value of the peak voltage appearing at the input. Normally, the DC voltage is set to 10 to 20V higher than the expected maximum peak input voltage. Below is the basic block diagram of a PFC boost regulator.

Unlike a standard power supply input there is no holdup capacitor directly across the bridge rectifier so that there is no large inrush current or transient currents as the input voltage rises above the voltage on the capacitor. The PFC works by inducing a current in the inductor L1, See Figure 1 above and causing the current to track the input voltage. The control circuit senses both the input voltage and the current flowing through the circuit. By controlling the on time in the switch Q1 that places L1 across the output of the rectifier, the current in the coil increases as the input voltage increases.

The switch is turned off periodically and the voltage at the drain end rises until the current in the inductor achieves the charge level. In general, interleaved circuits and CCM control are often used for circuits with high output power. The following graph is an example of comparison of these methods. By continuing to browse this website without changing your web-browser cookie settings, you are agreeing to our use of cookies.

Application Examples that Exploit Feature. What is PFC? Single PFC and Interleaved PFC In basic PFC operation, a sawtooth-wave current is passed through an inductor, and by executing control such that the current average value is sinusoidal, the shift in the voltage and current phases is corrected. This website uses cookies. Privacy Policy.