Power factor correct (PFC) basics and design considerations

Hello, this is Jason from Industrial System team, and we all focused on appliance application. My base in Shanghai, China. Today I will bring you a topic about PFC. I hope you can get a profile of PFC after my introduction. What is the PFC? What is the relevant standard of PFC? What is the common used PFC topologies? How to design a cost competitive and the high efficiency PFC circuit? What should we pay attention in PFC design?

And this is the agenda about the PFC topic. First I want to give you a brief introduction of PFC, then I will tell you the classification of PFC and what is the commonly used control law in PFC circuit. In the third part, I would take four different topologies used in active PFC, and give the comparison of them. In fourth part, I want to introduce how to design a cost competitive and effective PFC, and what is the consideration we should notice in the design. Finally, I will give an example of PFC design to show you what is the key performance and the character we should notice.

OK, so first we should know what is the PFC. PFC means Power Factor Correction. When we mentioned the PFC, the key point we should know about is the PF and the THD. Definition of the PF is real power divided apparent power. From this figure, you can see that the horizontal axis is real power, and the vertical axis is reactive power. The other side of the triangle is apparent power. If we want to get real power as 100 kilowatts and the power factor is only 60%, then input apparent power should be 167 kilowatts. But if we can improve the power factor to 95%, then we only need a input power as 105 kilowatt. It means that we can save the 62 kilowatts.

Another key point about PFC we should know is the THD. THD means the Total Harmonic Distortion. It can be calculated by this equation, in which the l 1 means the first harmonic, and l n means the n-th order of the harmonic. One thing we should know is the THD is used to measure the power quality of the electrical grid. Maybe you would have a question, is why we need a PFC circuit. The reason is in the real system. Usually we use a large capacitor on the DC link to maintain the output voltage stable just like this.

As you see, this is the input voltage, and this is the voltage on the capacitor. When the input voltage is lower than the voltage of the capacitor, the load only powered by the capacitor. At this moment, input apparent is 0. Only when input voltage is higher than the voltage capacitor the load can draw the power from input AC source. And only at this moment [INAUDIBLE] has the current flow into the input just like this curve. But input current is no more a sine wave as the input voltage. That means the THD is very poor.

The targets we add did it. The PFC circuit is to shape the input current to be the same phase with the input voltage, and the maximum real power drawn from the mains. If we add the PFC circuit, the benefits we can get-- the first one is, can reduce the RMS input current. As just mentioned at page one, the apparent power is RMS input voltage, multiple RMS input current. If we can improve the power factor, that means we can reduce the RMS input current. The second benefit is, it can help improve the power supply holdup. The active PFC circuit control the DC bus voltage independent of the input voltage. So the energy store in the back capacitor does not decrease with the input voltage decrease. This allows we can use smaller and cheaper back capacitor.

The third one is, can improve the efficiency of downstream converters. It means PFC reduce the dynamic voltage range applied to downstream inverters or converters, reducing the voltage rating of the [INAUDIBLE] that results in lower forward drops. The final benefit is, can increase the efficiency of the power distribution system because a lower RMS current can reduce the distribution wiring losses. Now in order to improve power quality of the electrical grid, a lot of standards have been published. Just like the 80 Plus Energy Star EN61000-3-2, EN60555, and so on, in which the EN61000-3-2 is focused on the line-current harmonic limited, while the Energy Star requires the power factor of power supplies must more than 0.9 at the full load with 115 voltage input.

But one thing we should note is that if the power supply is universal input, it should meets the requirements of both standards. The standard of the EN61000-3-2 classified different equipment into four categories. Household appliance belong to Class A, and most of the power supply belong to Class D. In this stage we should notice that if equipment not specified in one of the three other classes, and it should be considered as Class A equipment.

And this table shows that the maximum permission harmonic current for every odd harmonic. So if we want to design a [INAUDIBLE] PFC circuit, we not only must increase the PF to meet the requirements of the Energy Star, and also need to attenuate the harmonic to meet the requirement of the EN61000-3-2. Now let's recap what we discussed so far. First, we know what is the PFC, what is the PF, what is the THD. And then, we know what is the benefits if we add a PFC circuit. Then, we know the relevant standards of the PFC, just like the EN61000-3-2, Energy Star, and so on.