Why the current decreases after power factor correction

How the Boost PFC Converter Circuit improves power quality

How the Boost PFC Converter Circuit improves power quality

Linear power supplies, even those with passive filtering, have a low power factor and high harmonic currents. Learn more about the PFC converter (Boost Power Factor Correction), a circuit that can improve the voltage quality of power supplies.

Related information

  • Calculation of the power factor
  • Practical power factor correction
  • Power factor, THD, and why linear power supplies fail to meet performance standards

All electronic devices require power supplies to convert the AC voltage from the mains into DC voltage for the electronics. Linear power supplies, even those with passive filtering, have a low power factor and introduce harmonic currents into the system.

The overall effect of a single PSU is not great, but considering that millions of such PSUs are in use, the combined effect on the voltage quality of those PSUs can be significant. We can improve this situation by using power supplies that contain a power factor correction circuit that increases the power factor and reduces harmonic currents. The Boost Power Factor Correction Converter is a circuit that can be added to power supplies to significantly improve their performance quality.

Power factor and power factor correction

One type of power factor correction (PFC) involves passive correction in which the reactive power of a system is compensated for by adding a component that uses an equal but opposite amount of reactive power. For example, if a load is inductive with a reactive power of 1.754 kVAR, the system would require a capacitive load with a reactive power of 1.754 kVAR to counteract the inductance.

One way to implement this type of power factor correction is to have a large bank of capacitors that can be switched into the circuit if necessary. This type of power factor correction works well with large-scale linear loads where the cost of the power factor correction system can be offset by the size and cost of the overall system.

With much smaller scales - for example with individual power supplies - the power factor is also important. It doesn't matter because a single power supply has a huge impact on the system, but because there are so many power supplies. Even more difficult is the fact that these power supplies are non-linear loads, so the power factor cannot be corrected by simply adding reactive components (i.e. capacitors or inductors).

To ensure that electronic devices do not have a significant cumulative impact on the power factor of the network, international standards such as EN61000-3-2 and Energy Star 80 Plus set limits on the power factor degradation and harmonic distortion from power supplies.

A previous article showed that simple passive filters are not enough to adequately improve power factor or harmonic distortion. Instead, we need to use an active power factor circuit that forces the AC current to track the AC voltage.

The boost power factor correction converter

One of the most common active PFC circuits is called a boost PFC converter, which is a relatively simple and inexpensive circuit. The only additional components needed beyond those used in a linear AC-DC converter are a switch (usually an FET), a diode, and an inductor.

Figure 1 shows a boost PFC converter. You can see that it is essentially a linear power supply with a boost converter between the rectifier and the filter capacitor.

Illustration 1. Boost PFC converter circuit

The general goal of the Boost PFC converter is to turn the switch (S1) off and on quickly and with a varying duty cycle in order to increase the input current ( iac ) sinusoidal and in phase with the input voltage ( vac ) close. .

Boost PFC converter operation

The Boost PFC circuit quickly switches between two states. The first condition occurs when S 1 is closed as shown in FIG. In this state, if the inductor is excited by the AC side of the circuit via the rectifier, the inductor current will thus increase. At the same time, the diode D pfc biased in the reverse direction (because its anode via S 1 connected to ground), and the load is powered by the capacitor.

Figure 2. Boost PFC converter with the switch (p 1 ) closed

3 shows the second condition which occurs when S. 1 is open. In this state, the inductor drops (the current decreases) as it adds energy to the load and recharges the capacitor.

Figure 3. Boost PFC converter with open switch (p 1 )

(Note that both Figure 2 and Figure 3 show only the positive half of the input voltage cycle. The negative half would be the same except that the current would flow through the other two diodes of the rectifier.)

The change between the two states takes place at a high frequency, which is at least on the order of a few tens of kHz, but is often an order of magnitude (or even more) higher than this. The cycling forwards and backwards between states is done quickly and in a manner that both maintains a constant output voltage and controls the average inductor current (and subsequently the average AC current).

Since the inductor current increases in state 1 and decreases in state 2, the duty cycle determines the length of time during which the inductor current increases and the length of time during which the inductor current decreases. Thus, the average inductor current can be adjusted by varying the duty cycle. By having this average current follow the expected current, you can get a significant improvement in power factor and overall harmonic distortion (THD).

For an ideal system, the expected inductor current would be a rectified sine wave and the expected AC input current would be a sine wave. Because of the switching characteristics of the system and the difficulty of getting a perfect tracking of the expected current, the AC input current (I. ac ) not an ideal sine wave and the inductor current (I. (L) ) won't be ideal rectified sine wave, but instead looks like this:

Figure 4. AC current and inductance current of a boost PFC converter

These currents are the general shape they should be (sinusoid / rectified sinusoid) but one thing that stands out is that the lines of the signals look thick. This thickness occurs because the current increases during a cycle and then decreases as the average current is controlled to track the reference sine wave voltage.

Zooming in on the inductor current shows the inductor currents repeatedly increasing and decreasing as the system alternates between the two states.

Figure 5. Enlarged view of the inductance current in a boost PFC converter

Boost PFC control system

A closed loop control is required to ensure that the output voltage is maintained and the alternating current is sinusoidal and in phase with the alternating voltage. It is beyond the scope of this article to describe how the control system is designed, but Figure 6 gives you an overview of the overall system; it shows a boost PFC circuit with a controller block that accepts four inputs and generates a pulse width modulated (PWM) output that is applied to the gate of S1.

Figure 6. Boost PFC converter circuit with control system

The control system in Figure 6 requires three things:

  • Measurement of the output voltage (V dc ) to ensure that they are at the reference level (V ref ) is held
  • Measure the AC voltage to provide a reference for the inductor current
  • Measure the average inductor current to ensure it follows the rectified AC voltage

The control system would typically be a PI or PID control system which ensures that the difference between the reference signals and the required signals is as small as possible.

The results of a successful design are improved power factor and THD, as well as a regulated output voltage. The AC voltage and current for a boost PFC converter are shown in FIG.

Figure 7. AC voltage and current

You can see that the current and voltage are close to phase and that the current is a general sinusoid with minimal distortion.

An analysis of this system shows that the power factor is just below 0.99 and the THD is around 10%. These numbers indicate that the power quality is reasonably good and would be sufficient to meet the harmonic current requirements of IEC 61000-3-2 and the power factor requirements of Energy Star 80 Plus.

Conclusion

Linear power supplies have a negative effect on the voltage quality of an electrical system. Adding passive filters to the power supply can improve power quality, but it is not enough to meet power quality specifications such as IEC 61000-3-2 and Energy Star 80 Plus. Active power factor correction is necessary to meet these specifications, and one of the cheapest and most common ways to implement active power factor correction is to use a boost PFC converter.

The Boost PFC converter uses a switching element to cause the AC input current to be sinusoidal and in phase with the input voltage. The example used in this article showed that a significant improvement in the voltage quality of a power supply can be achieved by using a Boost PFC converter.