功率因数校正 (PFC) 基础知识和拓扑分类

Welcome to the power factor correction topic. My name is Brent McDonald. And I'm a system engineer at Texas Instruments. I'm in charge of defining and architecting TI's high voltage controllers.

This seminar is an introduction to power factor correction. As such, we are going to spend some time defining what PFC is, why it's needed, and how it's measured. Once we have our definitions of PFC established, we'll spend some time going over the more popular realizations of PFC controllers and circuits, how those circuits work, and some of the associated advantages and disadvantages of each.

We'll start out with critical conduction mode and go through it in a fair amount of detail. From then, we'll move on to cover continuous conduction mode and interleaved and bridgeless PFC controllers. At the conclusion of this seminar, I want everyone to leave with an understanding of the foundational elements that need to be considered when designing a PFC solution.

This is part one of six of the power factor correction circuit basics seminar. The single biggest reason power quality needs to be addressed are regional requirements stipulated by different government entities worldwide. This map displays some of the more significant requirements and the associated geographies that require them. While the different requirements have a lot in common, it's important to keep in mind that there are often significant differences that can make a huge difference.

Please make sure you carefully study the requirements you need to meet in order to understand how they will impact your overall design. In my experience, the specifications can be very nuanced and are best understood in concert with someone experienced with the document. Most organizations have a local expert that can help with this.

As an example, in some regions, capacitor banks are installed at substations or locations with high usage to counteract the inductive power factor caused by loads. Examples include US and Australia. Other regions place this entire burden on the equipment itself. An example of this is Europe.

What is power factor and why should I care? The bottom line is poor power factor creates a burden on power generation and distribution. As an example, consider a 60 watt laptop computer with a power factor of 0.4 versus 1.0. The system with the power factor of 0.4 has a peak current demand of just under two amps. While the high power factor system has a peak current demand of just over 1/2 an amp. This means that the low power factor system will need a thicker cable to handle the larger current.

Now while a 60 watt adapter might not be that big of a deal, think about what it would take to scale that up to a country the size of the United States. The US has something like 3.2 terawatts of power. A power factor of 0.4 would really tax the distribution of this power. And hopefully this example makes it a little clearer why this is so important.

Now let's talk about exactly what PFC means to these distribution systems. There are two fundamental things that influence power factor, displacement and distortion. The plots shown here illustrate the displacement element of this. This is the phase shift that you see between the voltage and current.

In fact, I think this is what most people think of when they hear the words power factor. Basically, this means that power is only delivered when the voltage and current are in phase. Any out of phase component results in a current flow but no actual power delivered to the end load itself.

The idea of displacement and power factor is just the tip of the iceberg. In most cases, the distortion element is the most important. And in fact, many power quality standards actually specify the allowable distortion at a given frequency. The plot in the middle here shows what a typical power supply without a PFC will do to the line current.

Essentially, these supplies place a large capacitor across the input diode bridge to convert the sinusoidal input voltage to a DC input voltage. This capacitor tends to get replenished only at the peaks of the line resulting in narrow slugs of current getting pulled from the line. These current slugs have enormous distortion.

Also shown on this slide are three common ways that power factor quality is measured. Power factor itself is defined as the cosine of the displacement factor divided by the square root of one plus the total harmonic distortion, where the total harmonic distortion is the square root of the sum of the squares of all of the harmonics in the system other than the fundamental. The square root of that entity divided by the fundamental itself.

Some regions, in many end use cases, require careful adherence to power factor and specific distortion limits like those shown on the right here. These are the so-called IEC 61,000 3-2 class d limits. And you can see for each individual harmonic, there is a maximum milliamp per watt value specified. In order to give you a feel for what this means, I've calculated and displayed the distortion itself for the wave form shown with the power factor of 0.69.

So given those definitions, how is the so-called power factor correction done? In other words, what do we put in the PFC box to magically go from the nasty current shown on the left to the nice smooth sinusoidal wave form shown on the right? There are a huge number of ways to solve this problem.

But the most popular and effective techniques all use some form of boost converter. However, please be aware that this is not the only way to solve the problem. And depending on your need, you may find yourself using one of the other approaches shown on this slide.

Lastly, I think it's very important to point out that the PFC has benefits that extend beyond just power quality. They also enable a regulated output that can simply downstream DC to DC converters such as an LLC or a phase shifted full bridge. PFC systems are also characterized by a large output capacitor.

I'll discuss more about why this is the case later. However, this capacitor can provide a substantial amount of energy hold up for brownout events. Since the output of the PFC is regulated, it provides a natural way to create a power supply with a universal input.