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%.”
Continue to page 2