By moving digital feedback and PMBus control
upstream from point-of-load (POL) dc-dc converters
in intermediate-bus-architecture (IBA)
power-distribution schemes, Ericsson Power
Modules may have sidestepped a patent problem that
has all but dried up new IBA developments. The brains are
now in the formerly “dumb” bricks that step down 48 V dc
to whatever the POLs need. In addition to jumpstarting
a stalled digital IBA, Ericsson’s engineers have improved
system efficiency across a range of loads.
IBA THE OLD WAY
IBA and approaches like it are used in datacenters, telephone
switching offices, and other places that employ
multiple racks or cabinets of electronics gear. Essentially,
incoming ac from the utility is stepped down and rectified
to a nominal 48 V for distribution. It’s “nominal,” because
it can in practice cover a 36- to 70-V range. That comes
in part from old telephone standards for central-office battery
backup systems. It also represents a compromise
between conduction losses and personnel safety.
Inside the cabinet, the 48 V is stepped down to 12 V or
less for distribution on the circuit boards within. The dc-dc
switching converters that handle this function are often
termed “bricks” because that is what the form factor of the
original converters reminded people of. (Later generations
of bricks are smaller, so now these converters can range
down to “1/16-brick” size.)
At each microprocessor or FPGA on the system board,
the roughly regulated dc from the brick is separately
stepped down for the core, memory, and I/O dc voltages
required by the semiconductors. These voltages demand
tight regulation and power-up/down sequencing, as well
as voltage ramping control.
POL-CONTROL IP COMPLICATIONS
Microcontroller and FPGA fussiness about the voltages
applied to them opened up a market for “smart” POL
converters that could be controlled and/or programmed
remotely. Until last fall, there were three approaches to
designing those POLs: one approach employs analog
feedback for the regulation function with digital control of
voltage output, and two approaches use digital feedback
and control. One is open-source, the other proprietary.
The open-source version used PMBus, which is an
extension of SMBus. The proprietary approach was Power-
One’s Z-bus. Both have merits. PMBus has all the economic
advantages of open sourcing, and a lot of smart
people from different companies are working on it. It does,
however, place a higher coding and computational load on
the system controller.
Z-bus uses a single controller that can handle many
comparatively simple POLs. Most of the POL variables
can be accomplished simply by using SMBus to
address the controller through a very elegant
user interface.
Last autumn, the race between Z-Bus and PMBus entered
a new phase when a jury agreed with Power-One that any use
of PMBus to control POLs infringed Power-One’s patents.
Abruptly, the number of new product announcements about
digital POLs for IBA applications dried up.
CUTTING THE GORDIAN KNOT
Meanwhile, back in Sweden, Ericsson wasn’t looking so
much at POLs as it was looking at the whole issue of energy
efficiency in the datacenter and in similar facilities. The company
then focused on those bricks, which the industry has
treated somewhat cavalierly—to date, most bricks use analog
control loops, and their regulation is fairly loose.
The Ericsson engineers then asked themselves what would
happen if the tight regulation took place in the brick and they
used sophisticated digital techniques inside the bricks to flatten
the efficiency curve across all levels. They also asked what
would happen if they gave the system more control at the
point where the voltage steps down from 48 V. They asked
a lot of server and switch
makers these questions
as well and began a lot of
testing on real hardware.
The results, which
avoid the Z-Bus/PMBus
impasse, have been
embodied in new products.
Specifically, Ericsson’s
industry-standard quarter-brick (2.28 by 1.45 by 0.46
in.) BR453 digitally controlled dc-dc converters work with
an input range of 36 to 75 V and output up to 33 A at 12 V.
Output voltage can be set via PMBus between 8.5 and 13.5
V, ±2% (Fig. 1). That’s desirable, because in cases where
POL output is on the order of 1 V, using a lower input voltage
improves POL efficiency. Previously, different output voltages
meant using different bricks.
That 400-W rating and the voltage precision are higher than
any other quarter-brick. Traditional quarter-brick dc-dc converters
achieve up to 300 W with an output accuracy of ±2%,
while intermediate bus converters (IBC) can achieve up to 377
W with an output accuracy of +4/–9%.
Thanks to the digital control loop, typical efficiency is 96%
at half-load, and that’s nearly flat out to full load (Fig. 2). The
only thing distinguishing the Ericsson bricks physically from
conventional quarter bricks is an additional header for the
communication bus at the opposite end from the standard
input/output pins. That bus includes pins that allow two
BR453s to load-share while automatically phase-interleaving
their switching signals to minimize conducted interference.
The BMR453 (with communication interface) will cost
$55.50 in OEM quantities.