SSZTBR1 January   2016

 

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    2.     Power Factor Is like a Glass of Beer.
    3.     Additional Resources

Mark Ng

I’d always thought that power factor (PF) was an advanced and complex topic until a colleague explained the relationship between PF and beer.

In principle, PF (expressed as a value between 0 and 1) determines how effectively an electronic device uses electricity.  Designers will always strive to achieve ideal unity PF (a value of 1), meaning that no phase angle exists between voltage and current. In other words, we are maximizing energy transfer. Anything less than unity PF means that the electric company must supply more current than necessary to run the electronic device. This is inconvenient for utility companies because they have to build more infrastructures to support inefficient power demand, and a nuisance to consumers because they will pay more for electricity than they actually need to.

Take for example a large-screen high-definition television (HDTV). A 200W HDTV running at a power factor of 0.5 means that the TV will need 400VA (200W/0.5) to operate – but the TV really only needs half that.  You would therefore be paying double the amount of energy needed to watch TV!

GUID-8C9F90E9-920A-4434-9AC0-3D4E6C290E49-low.png Figure 1 A Standard Size Large-screen HDTV

It is true that the average power consumption of TVs has gone down over time with the advent of LED technology. But talking about TVs gives me a chance to return to my initial comment about beer, since TV and beer (sometimes) go hand in hand.

Power Factor Is like a Glass of Beer.

After ordering a beer from the bar, you may receive a glass with quite a bit of foam at the top. The foam isn’t drinkable, and it is more of a nuisance, but you’re actually paying for it – it is, in effect, a waste. Since you actually paid for the entire glass, it would be much better if the beer went all the way up to the rim. In this example, the beer itself represents the “real power” that an electronic device actually requires. The foam at the top represents “reactive power,” and the entire glass of beer plus the foam represents the “apparent power” that the utility company supplies.

(I’m not exactly sure who came up with the PF and beer correlation, but apparently it’s quite popular – just search for “power factor correction and beer” on the Internet)

GUID-5414BE88-A895-4F20-A139-447652F885F7-low.png Figure 2 Think of PFC as a Glass of Beer or Soda

The goal of active PF correction (PFC), therefore, is to maximize the use of all available energy rather than to use some but not all of the available energy. Regulatory standards in all corners of the world are mandating improved PF. The IEC61000-3-2 standard governs Europe; China and Japan have also adopted similar standards. The U.S. follows Energy Star guidelines from the U.S. Department of Energy. As a general rule, all equipment rated 75W or greater is required to incorporate active PFC. The easiest way to comply is to integrate active PFC in your design. The regulations do not expressly demand the use of active PFC, but they do require PF to be over a certain percentage (Energy Star does), with harmonics lower than those stated in EN61000-3-2.

The cost of energy is increasingly important, and we will all have to reduce our overall dependence on energy resources. It is only a matter of time before all electrical equipment (not just those >75W) will require PFC. Much of this requirement will be driven by the desire to reduce utility costs, as well as by the new and increasingly stringent government regulations.If you wish to get ahead of these regulations, or simply comply today, with your next AC-powered design, I suggest you follow these instructions:

First, grab a beer.

Second, check out TI’s portfolio of analog, digital and combination PFC controllers which achieve unity PF, or try some light reading on how to design for PFC:

Additional Resources