Electronic Design

Smart As A Brick—A New Approach Rejuvenates IBA

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.

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