The important role of power factor corrector (PFC) in power supply applications

Conventional off-line switch-mode power converters generate non-sinusoidal input currents with high harmonic content. This can stress power lines, circuit breakers, and power utilities. In addition, harmonics can affect other electronic devices connected to the same power line. Active power factor correctors (PFCs) that shape the input current before applying it to the switch-mode power supply can solve this problem.

PFC has become more important since the European Union established the EN61000-3-2 standard and the A14 amendment for electronic equipment. The standard stipulates that ac line current harmonics are allowed. The regulations vary depending on the input power, product type, and specific harmonics. The original equipment classification and the A14 amendment classification are listed in the table below.

The Class D regulations are of most interest because they cover PCs, computer monitors, and TV receivers. All other equipment only has to meet Class A regulations. To understand how PFC works, let's first look at the basic concept of power factor. Power has two components: real power (W) and apparent power (volt-amperes or VA, vars = reactive power, not total VA). When a pure sine wave is applied to a resistive load and a reactive load, the vector relationship for the power factor is:

Where cosθ = the cosine of the phase angle between voltage and current;

Vin = RMS input voltage

Iin = RMS input current

Reactive loads can be inductive or capacitive to produce current with a phase angle that lags (positive) or leads (negative) the voltage, respectively. If the apparent power is very high relative to the real power, the power factor approaches zero. But if the apparent power equals the real power, the phase angle is zero and the power factor is 1. Therefore, one of the goals of PFC is to make the power factor as close to 1 as possible so that the load behaves as closely as possible to a purely resistive load.

Original Equipment Classification and A14 Amendment Classification List

Equation (1) is only valid for pure voltage and current sine waves. For non-sinusoidal input currents, i.e. when the power supply has a rectified input, the situation is different. To find out why, see Figure 1, which shows a typical power supply rectified input and the resulting input current and input voltage waveforms.

Figure 1: This diagram shows a typical power supply rectified input and the resulting input current and input voltage waveforms.

Here, the rectifier and input capacitor cause the power supply to produce input current in short pulses (rather than a pure sine wave). The capacitor charges only when the input voltage is close to its peak, at which point it produces high peak current, high RMS value, and a power factor of about 0.6.

Fourier analysis of a typical rectifier input shows that odd harmonics dominate the input current (Figure 2). There are also some even harmonics, but their amplitudes are relatively low. In the case of a non-sinusoidal input current with harmonics, the power factor consists of a displacement factor related to the phase angle and a distortion factor related to the waveform.

Figure 2: Fourier analysis of a typical rectifier input shows that odd harmonics dominate the input current.

This would produce the following relationship:

in,

PF = Power Factor

Irms(1) = fundamental harmonic component of current

Irms = total RMS value of the current

Kd = Distortion Factor

Kθ = displacement factor

Therefore, for the non-sinusoidal current waveform produced by the switch-mode power supply, the PFC must minimize the input current distortion and make the input current in phase with the input voltage.

Boost Converter PFC

To create a PFC, boost converters are widely used. ICs from several manufacturers simplify the implementation of boost converters specifically for PFC applications. In its most basic form, a switch controls a boost circuit (Figure 3). Closing the switch allows current to flow into the inductor. Opening the switch allows current to flow out of the diode. As the capacitor charges with the inductor current, multiple switching cycles bring the capacitor to the output capacitor voltage. The resulting output voltage is higher than the input voltage.

Figure 3: In its most basic form, a switch controls a boost circuit.

In a more specific circuit (Figure 4), the PFC IC provides internal control circuitry, and an external power MOSFET replaces the mechanical switch in Figure 3. This circuit uses a large energy storage capacitor at the output of the boost converter (rather than after the diode rectifier). When averaged over each high-frequency switching cycle of the boost converter, the inductor current (which charges the capacitor) is controlled to be proportional to the low-frequency input voltage wave.

Figure 4: In a more detailed circuit, the PFC IC provides internal control circuitry, and an external power MOSFET replaces the mechanical switch in Figure 3.

The input voltage range of the boost converter is between zero and the peak of the ac input. To operate properly, the boost converter must simultaneously meet:

The boost converter output voltage must be higher than the peak value of the power supply voltage. Typically 385V DC is used, allowing connection to a high power line of 270V ac rms;

In any case the current drawn from the power line must be proportional to the voltage.

The boost converter voltage is higher than the input voltage, allowing the converter to draw current in the same phase as the ac supply voltage, minimizing harmonics.

This boost converter configuration forms the front end of a power factor corrected SMPS or switch-mode power supply (Figure 4). Since it only provides the PFC function, the boost converter is considered a stand-alone PFC circuit.

Critical Conduction Mode

Critical conduction mode PFC ICs operate in between continuous and discontinuous modes. To understand critical conduction mode, compare the difference between discontinuous mode and continuous mode in switch-mode designs such as flyback converters.

In discontinuous mode, the magnetizing inductance of the transformer starts charging from zero current when the switch turns on. It then discharges to zero after the switch turns off. It then remains at zero current for the off time before the switch turns back on. In continuous mode, the magnetizing inductance is not fully discharged, so it starts charging from some positive current value each time the switch turns on.

In critical conduction mode, the off time is zero and the switch is turned on only when the inductor current reaches zero. The average value of the AC line current is a continuous waveform, and the peak switch current is twice the average input current. In this mode, the operating frequency varies with a constant on time.

Average Current Mode

Operation in continuous average current mode involves a PFC controller IC with a gain regulator that has two inputs and one output. The other input at the end of the gain regulator comes from the voltage error amplifier. The error amplifier compares a stable reference voltage with a portion of the output voltage after the boost diode. The error amplifier has a low bandwidth so that it is not affected by sudden changes in the output or ripple voltage. The gain regulator then doubles the reference current and the error amplifier output.

The gain regulator sends its output current (IGM) to the current amplifier, which then applies its output to the comparator that drives the RS flip-flop. As a result, the pulse width modulation (PWM) circuit controls the switching of the power MOSFET.

The key blocks in this standalone PFC controller include the current control loop, voltage control loop, PWM control, and gain regulator blocks. The current control loop forces the inductor current waveform to change with the input voltage waveform. The output voltage of the continuous inductor current boost regulator must be set to exceed the maximum peak of the input voltage for the PFC to work properly. The output should be 1.414 times the maximum RMS input voltage. In addition, the internal current amplifier must have sufficient bandwidth to turn off the switch immediately when the desired current threshold is reached.

The gain regulator block and the voltage control loop work together to sample the input voltage and output voltage respectively. The output voltage is compared with the internal reference to generate an error signal, which is then multiplied by the input voltage to set the threshold of the current control loop. This threshold is compared with the input (switch) current to determine the PWM duty cycle. PWM control uses trailing edge modulation.

Input current shaping

Input current shaping is another control method for continuous current mode PFC, which is different from the traditional/typical average current mode PFC controller. This PFC configuration does not require input voltage information and multipliers (gain regulators). It changes the slope of the internal ramp according to the error amplifier output voltage, and the current sensing information and the ramp signal are used to determine the turn-on time. As shown in Figure 5, the PFC switch turns on when the current sensing voltage reaches the internal ramp signal. An internal clock signal turns off the switch.

Figure 5: The PFC switch turns on when the current sense voltage reaches the inner ramp signal.

To control the output voltage, the PFC IC adjusts the slope of the internal ramp signal. If the slope increases, the average current increases; if the slope decreases, the average current decreases. With the continuous mode characteristic, the inductor current is proportional to the sine wave at turn-on. Therefore, the minimum inductor current in one switching cycle varies with the sinusoidal current reference. Of course, the peak inductor current in one switching cycle is not controlled according to the sinusoidal reference. Therefore, the average inductor current may not be a sine wave. In order to make the average inductor current close to the sinusoidal reference, there must be a high enough inductance to reduce the current ripple.