In the bad old days, most of the field applications engineers in power management had served several successful years as power supply designers prior to going into field applications. They had an intrinsic knowledge of how stuff works, from whiteboard down to the pcb layout, EMI signatures, transformer design and component attributes. Times have changed. Due to lots of restructuring, downturns and basic evolution at IC companies, what used to be clear concise information from those that knew it is now, by and large, grey and cloudy hearsay. As an active power supply designer and also a Field Applications Engineer, I’d like to make some attempt at clarification. To accomplish this, I’d like to go through the reasons for using active PFC, the claims made in favor of active PFC, and lastly the topologies and design careabouts.
I once heard one of the newer FAE’s tell a room full of designers “PFC of course stands for power FREQUENCY correction” the words that followed were more shameful. In his defense, he was a software engineer prior to this engagement. For better or worse, if this are the new sales tactics, we really need some old school knowledge!
Let’s start out with the basics. Active power factor correction is simply a means to bring the distortion of the line current down to a low value with active power electronics circuitry. In switchmode power supplies, current and voltage tend to be in phase; however, the harmonics in the line current waveform are excessive, as shown in Figures 1 and 2, left side. (Note: due to the high impedance of my isolation transformer, the clipped top of the voltage in Figure 1 caused by the narrow conduction angle of the diodes is exacerbated.)
A good engineer would ask the question: Why do we need PFC? To answer this question requires delving into some of the fundamental concepts of power distribution and transmission.
If we investigate power distribution in the U.S., there are two different categories. The first is residential, and the second is commercial/industrial. There may be subcategories, too. I’d imagine that a steel mill with 128 kV linesets coming into the premises is on a different scale than the 480 V drop at the local Kinko’s. We won’t worry about that here.
In the commercial and industrial arena, a surcharge is assessed when PF drops below 0.85. PF is monitored in these applications. It’s easy to see why PFC would be advantageous for appliances in this market—it’s direct. Low PF = higher bill. Correct that and save money.
In most residential applications, we see a slightly different story. The energy consumed by most households today is measured by a rotating disc watt-hour meter as shown in Figure 3. This type of power meter measures kW*H. This meter is set up like a shaded pole induction motor with the mains voltage driving the potential winding.
In this winding the voltage is fairly constant and the inductance is very high. The resultant excitation current in this winding lags the mains voltage by 90 degrees due to the high inductance. The current legs then drive shading windings. The phase difference between the potential and current windings is then 90 degrees for a resistive load, thereby setting up a maximal resultant torque on the disc and spinning the disc at a speed proportional to the power flowing through the meter. There are some damping and counter-torque mechanisms on the disc that we won’t discuss here. The rotating disc then drives gears and a display that is calibrated to kW*H.
The flux in the current winding is a measurement of MMF or magnetomotive force. Now for the odd part, the stuff that Mr. (or perhaps it was Dr.) Power Frequency Correction didn’t tell us: What do we pay for? If we assume that the potential winding voltage is fairly constant, the MMF caused by the line current drawn then determines the speed of rotation of the disc. This mechanism measures real power. As justification, consider a purely reactive load. All current is at the fundamental, and 90 degrees out of phase. There is no displacement torque on the disk in this case.
MMF is a time-averaged function. For an AC half cycle, any combination of harmonics in the line current that results in the same time-averaged value will make the disk in the meter spin at the same rate (assuming the fundamental is at the same phase angle with respect to the potential winding). There are practical limits to this. For example, if we were to draw heavy current in the 601st harmonic, the steel laminations would have a lot of eddy current loss, as would the copper. The rotating disk would never see that energy. The outcome of this time-averaged function is odd for residential households in the greater U.S. at present. Power factor doesn’t matter, at least in terms of the billable kW*H that we pay for. The question still stands unanswered: Why do we care about power factor at the residential level?