[Technology Report]
New Techniques Enhance Efficiency Across All Loads
Governments around the world are moving from voluntary to mandatory power-supply efficiency efforts. Challenges may lie ahead, but designers now have methods to cope with them.
Engineers who design products that plug into the ac mains face upcoming efficiency mandates that will make power-supply design tougher—and, one hopes, more lucrative. To some extent, the same is true for designers of portable equipment, as consumers get used to ever more powerdraining features while simultaneously demanding equal or longer battery life.
Back in the 1990s, the Environmental Protection Agency (EPA) created a voluntary labeling program called Energy Star, which only addressed sleep mode. It was a first step, but since it was voluntary and ignored operational efficiency, it had modest impact.
Focusing on operational efficiency in personal computers, Ecos Consulting partnered with a group of electric utilities to create another voluntary program called 80 Plus. The name of the program signifies a requirement for 80% or better efficiency at 20%, 50%, and 100% of rated load, plus a power factor greater than 0.9 at rated load.
Manufacturers of desktop computers and desktop-derived servers earn $5 and $10 rebates, respectively, for every unit with a certified power supply sold in participating utilities’ territories. Also, even though 80 Plus is voluntary, U.S. government
agencies will pay a premium for 80 Plus-qualified computers. In the past, efficiency curves tended to show a pronounced hump at slightly less than maximum design load, with a severe droop—even down as far as 40% or 50%—at low current demands. Yet many applications spend days just idling and far less time drawing peak power. The 80 Plus initiative aims at reducing total wasted watt-hours, not just optimizing efficiency at one point on the curve. Figure 1 demonstrates the kind of success that is possible.
Last July, the EPA incorporated 80 Plus into a revised Energy Star computer specification. The EPA also revised the Energy Star specification for laptop adapters, mobile phones, printers, scanners, digital cameras, and other appliances. Similar programs that harmonized with Energy Star exist in other countries, including Japan, China, and the nations of the European Union.
Although these programs are still voluntary, the U.S. and other countries are considering mandatory standards for power-supply efficiency. For example, the Energy Policy and Conservation Act (EPCA) directed the U.S. Department of Energy to determine by August 8, 2008 whether energy conservation standards shall be developed for battery chargers and external power supplies.
Meanwhile, the California Energy Commission’s mandatory Appliance Efficiency Regulations (CEC-400) include new Energy Star requirements for external power supplies. And that’s where the bite comes. Nobody would consider building a new electronic product they couldn’t sell (or even warehouse) in California, yet that’s what’s exacted by CEC-400.
Paralleling global efficiency requirements are mandated limits on power factor—effectively the harmonics of switching- regulator frequencies placed on utility lines. “Crossing the threshold of 75 W input has significant consequences. In effect, 75 W is the power threshold beyond which EU regulation (IEC1000-3-2) for the reduction of harmonic currents applies to class D electrical equipments,” explains the reference design notes from a few years ago for an ON Semiconductor notebook acdc adapter.
The notes also state that “notebook adapters are classified under class D. This regulation stipulates the maximum level of harmonic currents that class D equipment can inject on the mains ac line. The IEC1000-3-2 regulation is currently mandatory in Europe and Japan. In a sense, the mobile/global nature of the notebook adapters make them the first mass-market power supply to fall under the IEC1000-3-2 target.”
THE ENGINEERING PROBLEM Turning f rom regulation to design approaches that meet those regulations, consider the drags on efficiency in a basic step-down (buck) voltage regulator. Allowing for circuit differences, there are parallels in all switching-regulator topologies.
In a basic, non-synchronous buck-regulator circuit, the forward-voltage drop across the low-side rectifier diode is in series with the output voltage, so its losses seriously impact efficiency (Fig. 2). “Even at 3.3 V, rectifier loss is significant,” points out Maxim Appnote AN652. “For step-down regulators with a 3.3-V output and a 12-V battery input, the 0.4-V forward voltage of a Schottky diode represents a typical efficiency penalty of about 12%, aside from other loss mechanisms,” the note says. The situation only gets worse at the lower regulator output voltages required by the latest processors and FPGAs.
Synchronous rectification—essentially replacing the diode with a switch, usually another MOSFET—improves powerconversion efficiency. Appnote AN652 goes on to say that “For an input voltage of 7.2 V and an output of 3.3 V, a synchronous rectifier improves on the Schottky diode rectifier’s efficiency by around 4%.”
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