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

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Traditional off-line switch-mode power converters produce 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 for electronic equipment and the A14 amendment. 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 regulation is of most interest because it covers PCs, computer monitors, and TV receivers. Other equipment only needs to meet the 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 = cosine of the phase angle between the voltage and current; Vin = RMS input voltage Iin = RMS input current         Reactive loads can be inductive or capacitive to produce currents with phase angles that lag (positive) or lead (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 unity. Therefore, one of the goals of PFC is to get the power factor as close to unity 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 contains 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 yields the following relationship: Where, PF = Power FactorIrms (1) = Fundamental harmonic component of currentIrms = Total RMS value of currentKd = Distortion factorKθ  = Displacement factorThus , for the non-sinusoidal current waveforms produced by switch-mode power supplies, PFC must minimize input current distortion and keep the input current in phase with the input voltage.Boost Converter PFCTo create 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 causes current to flow into the inductor. Opening the switch causes current to flow through the diode to the output. As the capacitor is charged by the inductor current, multiple switching cycles cause the capacitor to reach 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, which allows the converter to draw current in phase with the ac mains 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 PFC functionality, the boost converter is considered a standalone PFC circuit.

Critical Conduction Mode

        Critical conduction mode PFC ICs operate 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 does not fully discharge, so it starts charging from a positive current value each time the switch turns on.

        In critical conduction mode, the off time is zero, and the switch turns 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

        Operating 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 value 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. The 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 ramp signal are used to determine the turn-on time. As shown in Figure 5, the PFC switch is turned 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.

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