By Michael O’Loughlin, Texas Instruments
A majority of electronic product consumers are unaware of what power factor correction (PFC) is or how it can conserve energy by reducing power transmission line losses. But, environmental agencies are aware of the benefits of PFC in offline power converters. It started with the European Union (EU) EN61000-3-2 input current harmonic content specifications for power converters.
Michael Oââ'¬â”¢Loughlin, Texas Instruments
By Michael O’Loughlin, Texas Instruments
A majority of electronic product consumers are unaware of what power factor correction (PFC) is or how it can conserve energy by reducing power transmission line losses. But, environmental agencies are aware of the benefits of PFC in offline power converters. It started with the European Union (EU) EN61000-3-2 input current harmonic content specifications for power converters.
While this specification was not a power factor (PF) spec per se, power supply designers found it easier to meet this requirement with PFC pre-regulators. This specification became a driving force for offline power converters to have PFC. To help meet the world’s growing demand for energy and reduce power distribution losses, power companies prefer electronic devices with PFC pre-regulators. Several power utility companies funded the 80 PLUS incentive program to integrate more energy efficient power supplies with PFC.
In 2008, to receive incentives offered by the 80 PLUS program requires having a full load PF of 0.9. However, PFC pre-regulators do not come for free as it makes offline power converters less efficient. Even though these power converters are slightly less efficient, they can save energy by reducing transmission line losses. This energy savings would save the power supply company money as well as make more electricity available to the end consumer. If the power company had more available energy to distribute, they could possibly reduce their rates, saving the end consumer money as well.
Before discussing how PFC pre-regulators will save electronic consumers’ money, lets review the definition of PF. Power factor is defined as the cosine of the phase angle between real power and apparent power. For offline ac-to-dc power converters, PF is the total dc output power (VOUT×IOUT) divided by the apparent ac input power multiplied by efficiency.
For several years now the United States has been experiencing somewhat of an energy crisis. During peak usage periods such as during summer heat waves, energy companies struggle to meet consumer demands for energy. In California, some cities experienced brownouts or even blackouts. With consumers switching to plug-in electric cars, this demand for energy can only get worse.
Of major concern is the loading from consumer electronics. These devices may include power converters that are less than 80 percent efficient with a PF of less than 0.45. These power supplies generally consist of an EMI filter, bridge rectifier on the front end, and simple switching power converter. Unfortunately, these offline converters only draw power for a portion of the line cycle, which makes these power supplies draw 120 percent more apparent power than the load demands. Even though this additional apparent power is not dissipated, it causes distribution problems for the power company. To reduce line loading and better utilize line power, utility companies have added a PF of 0.9 to the 80 PLUS incentive program. Power factor correction makes the ac-to-dc power converter resemble a resistive load, forcing the power converter to draw an amount of apparent power that is almost equal to the converter’s output power plus the converter’s losses.
A tradeoff of PFC is that it drops the power converter’s overall efficiency. You can see this by studying the efficiency of an offline power converter with PFC. The diagram below shows a functional block diagram of an offline power converter with PFC.
Converters like this typically have a PFC boost pre-regulator (Stage 1), followed by a step down converter (Stage 2). The PFC pre-regulator’s efficiency at 115V is quite high, typically greater than 95 percent. The step down converter’s efficiency is typically 85 percent. The total efficiency of this power converter is the product of the efficiency of Stage 1 and Stage 2. This power system’s total efficiency is 80 percent – roughly five percent less efficient than highly efficient power converters designed without PFC. Note that if all offline power converters had PFC, power companies could still distribute more power, save money and energy, and reduce costs for power consumers, even though power converters with PFC are less efficient.
If more power converters had PFC, transmission line losses would be greatly reduced.
A power converter that meets 80 PLUS specifications would draw up to 47 percent less RMS current compared to an offline that was 85% efficient with a power factor correction, that had a PF of 0.45. The 47 percent reduction in RMS load current by using PFC could result in up to a 72 percent reduction in power line conduction losses, saving the power supply company money. If more power converters had PFC, the power companies could distribute more power and save money because of the reduced conduction losses. The power companies could possibly reduce their rates due to having more distributable power. While the 80 PLUS specification has been adopted by computing power, it is voluntary and not required by electronic devices that use line power. Even though power converters with PFC are less efficient than converters without PFC, it should be mandatory that all offline ac-to-dc power converters have a full load PF of 0.9 as 80 PLUS requirements recommend.
I have heard that some power supply companies are achieving 80 PLUS Silver. To meet this specification, a power supply must have greater than 85 percent efficiency when the converter is loaded from 20 – 100 percent of the rated output power. Meeting this standard also requires a PF of 0.9 when the converter is loaded with 50 percent or more of the converter’s rated output power. Even though 80 PLUS Silver is not mandatory, power supply manufacturers are aware that consumers are energy conscience, and believe that consumers will be more likely to buy power supplies meeting these higher efficiency specifications to save some money.
If we can make power supplies that are 85 percent efficient, environmental protection agencies should make it mandatory that all future offline ac-to-dc power converters have PFC. In addition to better utilizing line power and reducing power line conduction losses, PFC pre-regulators also recover losses to the power system, saving consumers more money. These more efficient power converters may even give scientists a little more time to develop environmentally safe energy alternatives.
For more information on power factor correction, you can visit: ti.com/pfc
About the Author:
Michael O'Loughlin is an Applications Engineer with the Power Supply Control Products group at Texas Instruments. He specializes in offline and isolated power supply design and has authored numerous articles on power factor correction and power supply design related topics. Michael received his Bachelor of Science degree from the University of Massachusetts. Mike can be reached at .
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