IEEE Power Electronics Magazine - September 2015 - 64

White Hot

by Robert V. White

The Amazing Server
Power Supply

ttending conferences and reading transactions leaves the
impression that not much is
happening in the industry-because
the vast majority of papers are from
academia. Why that is the case is a
topic for another time. However, there
is a lot happening in the power electronics sector of the industry. We do
not hear about it because it is often
proprietary. Even if an industrial development is not proprietary, engineers in
industry do not have the time or support to write papers.
I have spent a good part of my career working in the commercial power
supply world, mostly in power supplies, dc-dc converters, and power
systems for computing, networking,
and telecommunications equipment.
Today's power supplies for these
systems are simply astounding. The
typical power supply for a server
operates from ac power anywhere
in the world with essentially unity
power factor. The output is typically
+12 V, rated for 1-2 kW. The standard
efficiency for these power supplies
at 50% of rated load is on the order
of 92-94% with an input voltage of
120 Vac and 94-95% with an input voltage of 240 Vac. Efficiencies of 96-97%
are readily available for an additional
cost. While these efficiencies are incredible, server power supplies typically maintain efficiencies greater
Digital Object Identifier 10.1109/MPEL.2015.2449273
Date of publication: 4 September 2015

IEEE POWER ELECTRONICS MAGAZINE

than 90% with output loads from 10 to
100% of the rated maximum.
The power conversion density is
typically in the range of 30-40 W/in³
(1,800-2,400 W/L). A typical 2-kW power
supply is 1-U tall, 4-in wide, and 8-in
long (4.5-cm tall # 10-cm wide # 20-cm
long). Conversion densities of 50 W/in³
(3,000 W/L) are coming soon. I consider that level of performance for a
commercial power supply to be astounding. Even more astonishing is
that this performance, including a full
microcontroller-based serial interface,
comes for only US$0.07-0.08/W.
There are a number of techniques
used to achieve this kind of performance. Advanced semiconductors
are important, with superjunction
MOSFETs playing a major role. Silicon carbide diodes are also important
for reducing reverse recovery loss.
The typical topology is a bridgeless
active rectifier on the input (often
called a power-factor-correcting converter). There are several bridgeless
rectifier topologies, but the totempole
topology seems to be the most common. This topology has one pair of
high-speed (60-65-kHz) switches that
shape the input current into a haversine wave shape and one pair of slowspeed (line frequency) switches that
toggle the polarity.
The most popular topology for the
dc-dc converter stage is the LLC converter. The complexity of the design
and control slowed the adoption of the
LLC, but its high efficiency makes it

 September 2015

the first choice. The key advantage of
the LLC converter is the ability to essentially eliminate switching losses.
At this point, efficiency is gained by
the very careful design of the resonant
inductor and the transformer. It is also
possible to drive down the conduction
losses by reducing the on-resistance of
the switching devices, both in the primary and secondary, by using larger
devices or adding devices in parallel.
It helps a lot that the output voltage
stress on the output rectifying devices
is only twice the output voltage. For a
12-V output, designers, depending on
how much risk a designer is willing
to take, can use output devices with a
voltage rating of 40 V, although more
cautious designers might choose devices rated for 60 V.
Another key to the adoption of the
LLC was the introduction of controller ICs specifically for this topology.
Standard controller ICs make the design easier, but the LLC design is still
much more challenging than the typical PWM converter.
Also enabling the high efficiency-and high efficiency over a wide
range of load current-of the commercial server power supply is digital
power management. It is typical to
use two or three active rectifier circuits operating in parallel. These rectifier circuits are turned on and off as
the load varies to minimize the power

(continued on p. 62)

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2015