Electronic Design

DC-DC Converters Deliver Better Performance For Distributed Power

With their enhanced efficiency and features, modular components deliever high current and low voltages in small packages.

Falling supply voltages and rising supply currents are driving the adoption of distributed power architectures (DPAs) in a growing number of applications. In systems that were previously well served by centralized power schemes, evolving supply requirements are forcing a change. At the lower supply voltages and the higher current levels encountered in new designs, voltage drops across the power bus often become unacceptable. Even when conductors can be sized appropriately, parasitics may make it impossible to meet demands for a faster transient response from the supply. More and more, these factors necessitate the use of DPAs.

For designers seeking to implement DPAs, there are various modular power components that simplify power-supply usage at the system level. Packaged ac-dc front ends and dc-dc converters come in standard and nonstandard formats for operation at popular input and output voltage levels. These devices take into account the increasingly high standards being set by current and emerging applications. Vendors who manufacture the modular components face constant pressure from customers who demand smaller, more efficient power solutions at lower cost.

The performance and packaging of board-mounted dc-dc converters significantly impact DPA-powered systems. These factors become even more important as the system's complexity increases. OEMs are looking to pack circuit boards more densely in telecom and datacom equipment. Therefore, they're pushing power-supply vendors to lower the overall package height on board-mounted dc-dc converters.

One way to cut package height is to dispose of the heatsink. Many of the power converters now arriving on the scene have eliminated the extra metal. In many cases, this was done by switching from Schottky diode rectification to the more efficient synchronous rectification with MOSFETs.

They've improved efficiency even further by taking advantage of better MOSFET performance. Each new generation of MOSFETs offers enhanced on-resistance and gate charge characteristics, among other specifications. Some of these transistors have even been optimized for use in specific dc-dc converter applications. Smarter switching controllers and better power-management schemes have helped too.

Smaller ICs and discrete components have also contributed to the shrinking of converter packaging. Still, in many of the newer converter designs, thermal management has been the key to downsizing the package and losing the bulky heatsink.

After reading the latest converter specifications (see the table part 1, 2 and 3), designers may wonder whether the numbers reflect real-world performance or if power-supply developers are figuratively blowing smoke. In some cases, they're doing so literally, using wind-tunnel tests to optimize their physical designs and make the most of forced-air cooling.

Mounting Also A Factor
Power-supply vendors are battling it out to produce power converters with higher levels of output power in small form factors. Package size is not the sole focus, however. Suppliers are also migrating to surface-mountable packaging to lower manufacturing costs for their OEM customers. OEMs want to eliminate the extra processing steps required to assemble a through-hole power converter onto an otherwise all-surface-mount board.

Meanwhile, manufacturers of dc-dc converters are also addressing demands for greater reliability and the need to guarantee full output power over the application's temperature range. Besides boosting single-unit performance, suppliers are also recognizing the necessity for paralleling multiple units. Doing so enables higher output powers and fault tolerance. As a result, features such as current sharing, power-on sequencing, and N+1 redundancy are being incorporated into many models. Such functions make for a more seamless connection of units, simplifying control of operations and preventing operating glitches.

Some of the newly developed products offer faster transient response—a feature sought most aggressively by the microprocessor world. Moreover, work done for the motherboard in creating voltage-regulator modules (VRMs) has influenced the development of dc-dc converters in other ways. Consequently, converters with digitally programmable outputs are competing with the traditional fixed-output types. Of course, DPAs aren't just about dc-dc converters. Innovations in front-end ac-dc converters and front-end filters are also making the task of building DPA systems easier.

The issue of power efficiency is inextricably linked with available output power, package size and weight, mounting and air-flow requirements, reliability, and cost. Raising power-conversion efficiency means more available power for a given-size package and greater reliability due to reduced component heating. In addition, it minimizes the need for forced-air cooling and allows the front-end ac-dc converter to be downsized. Furthermore, if the reduction in power dissipation is sufficient, enhanced efficiency can eliminate the need for an external heatsink altogether. At the same time, construction of the dc-dc converter is greatly simplified.

SynQor's PowerQor Tera product series is an example of this. Replacing Schottkys with synchronous rectifiers raises efficiency levels to the point where the heatsink can be eliminated and an open-frame construction can be used (see the graph). As a result, the converter's potting material, casing, and metal baseplate are no longer necessary. The recently introduced half-brick version of PowerQor delivers 165 W at 3.3 V (or 60 A at voltages of 2.5 V and below), and yet stands only 0.4 in. high.

Low-profile packaging is a major concern for developers of space-critical, card-based systems. A potted converter with heatsink is likely to be the tallest component on the board. So it will dictate the minimum spacing between cards—and the number of cards that can be packed onto a backplane. According to Marty Schlecht, CEO of SynQor, some customers in the telecom/datacom markets want the total height of the converter with a heatsink to be no more than 0.7 in.

As indicated by some of the converters listed in the table, manufacturers are responding to the need for low package heights. A product introduced last fall, Lucent's JAHW/JAHC series of 50- to 100-W half bricks, offers a 0.4-in. package height. (For more information, visit www.lucent.com/networks/power/100wjhcjhw.html.) Due out later this year, PowerCube's QED series of 0.5-in. tall half-brick converters promises to deliver 150 W with air cooling.

In the quarter-brick format, Broadband TelCom Power is releasing 0.34-in. high 100-W converters that operate without an external heatsink. Each of these converters uses an air-cooled, open-frame construction built on a high-efficiency synchronous-rectification design.

Given the size and weight of the potted dc-dc converter models, it's not surprising to find that they've been mostly through-hole components. But the packaging situation is changing now that the need to streamline production flows with all surface-mount parts has become more apparent.

Until recently, surface mounting has only been an option at lower power levels. According to Barry Papermaster, director of marketing for Lucent Power Systems, most of the industry is only offering the surface-mount option for dc-dc converters up to 25 W. Be that as it may, innovators such as Lucent are raising the bar for surface-mount packaging. The company's JAHW/JAHC series generates 50 to 100 W in a ball-grid surface-mount package. According to Papermaster, Lucent plans to use the ball-grid package to transfer all of its open-frame converters to surface-mount technology.

Surface-mount packaging seems to be a natural extension of the low-profile, open-frame construction. SynQor's PowerQor Tera line was initially offered in through-hole packaging, but is scheduled to be brought out in surface-mount form later this year. Astec also plans to offer a surface-mount option for its BK60C series of 132-W half-bricks.

One vendor, though, is taking a different approach to make surface mounting an option for high-power converters. Vicor will produce a family of surface-mount header assemblies that will mate to the company's second-generation dc-dc converters.

With the SurfMate solution, a compromise is made between existing surface-mount and through-hole solutions. The socket assembles to the board as part of a standard surface-mount reflow process, which requires the converter to be plugged into the socket as a final assembly step. SurfMate's most outstanding trait is that it extends the surface-mount approach to converters rated up to 600 W.

Advanced Intel-style microprocessors provide some of the most demanding applications for distributed power. Since the Pentium Pro was introduced, processor performance has depended on having a point-of-use, well-regulated dc supply tuned to a specific output voltage, which varies from chip to chip. As a result, the output of the VRM or dc-dc converter must be programmed on-board to the value specified by the individual processor. In addition, the processor's current demands can shift rapidly from milliamperes to tens of amperes. For this reason, the dc-dc converter or VRM used to power the processor must exhibit a very fast transient response.

The processor-dedicated VRMs have typically operated from either a 5- or 12-V bus. Now, however, power-supply vendors are promoting more advanced dc-dc converters with VRM capabilities. These devices can operate from the higher, standard input voltages (24 and 48 V) encountered in telecom and datacom applications and come with the extensive feature set that normally complements these designs.

An early example of this trend is Lucent's Onami series, which generates a programmable 1.3 to 3.5 V at 30 to 40 A, while powered from a 48-V input. Transient response for this product is 50 A/µs. Onami devices can be paralleled for true current sharing during load transients, when performance is critical.

Other vendors have developed similar components. Power Trends modules from Texas Instruments offer a programmable 1.3 to 3.5 V at 30 A with operation from either 48- (PT4480) or 24-V (PT4470) supplies. Brian Narveson, product development manager for TI's plug-in power solutions, points out a primary benefit of this device. Rather than using conventional primary-side regulation, it relies on secondary-side regulation to achieve faster transient response and tighter voltage regulation. Transient response is 5 A/µs, while line and load regulation are each specified at 0.1% typical (1% maximum).

TI's secondary-side regulation is the basis for a series of dc-dc converters. One of its chief advantages is that it permits units to be paralleled in controller-slave configurations, thereby boosting output current. For example, the company's 24-V-input PT4472 voltage-programmable converter can be paralleled with its related PT4495 booster modules for outputs up to 60 A. Another way to raise output current is with an N+1 redundant configuration, which affords a higher level of fault tolerance than the controller-slave arrangement. N+1 redundant operation is also among the features offered in the PT4470/4480 series.

Cherokee International is yet another developer of voltage-programmable converters. The company's Advanced D2D converter boasts packaging in a card-edge pluggable style. A similar type of device from Celestica Inc. is the AD2D30A. Operating from a 43 to 53-V input, this vertical-mount, plug-in unit generates a programmable 1.8- to 3.3-V, 30-A output. It delivers the full 100 W of power at 25°C without heatsinking, but requires a forced airflow of 300 LFM.

Choices are also available to designers seeking fast transient response without the adjustable output voltage. For example, Lucent has been offering a little-publicized "-t" option with its board-mounted dc-dc converters. Geared toward applications that call for high current, these units can be tuned to differing load requirements.

Progress in DPA design is not limited to the dc-dc converter realm. Vendors are also turning their attention to matters on the front end. One company providing both the ac-dc and dc-dc converters for DPAs is Artesyn. Its family of front ends contains the AFE-2000, which is a 2000-W, 48-V output front end. Also included is the soon-to-be-released AFE-1200—a 1200-W version. Designed to operate with air cooling, these silver-box supplies provide a high level of reliability, while minimizing the in-system requirement for airflow.

Getting clean power to the dc-dc converter is another concern for designers. Functions such as transient protection, EMI filtering, and inrush current limiting may be addressed with discrete designs. On the down side, this puts an added burden on system designers. Not only must they provide enough filtering to make the design work properly, but their efforts have to satisfy Bellcore, FCC, ETSI, and European regulations.

Vicor intends to remove this burden by introducing a modular solution, its filter/input attenuator module (FIAM). The half-brick FIAM accepts 36- to 76-V inputs and handles either 10 or 20 A, depending on the model. This device complements the company's second-generation 48-V-input dc-dc converters.

For both front-end and dc-dc converter products, the goal of the power-module vendor is to simplify system design. This is done through improved power performance and the integration of features that make for a seamless interface between DPA components. Power requirements become more challenging with each new generation of silicon technology. As this trend persists, vendors will continue to shrink packaging, fine-tune component efficiency, and enhance control options to lighten the load on system designers.

See associated figure

Manufacturers Providing Information For This Article
Artesyn Technologies
(508) 424-2867
www.artesyn.com

Astec Power
(760) 930-4787
www.astec.com

Broadband TelCom
Power Inc.

(714) 850-7320

Burr-Brown Europe
+44 1506 472309
www.burr-brown.com

Calex
(925) 687-4411
www.calex.com

Celestica, Inc.
(905) 707-4568

Cherokee International
(714) 554-6665

Datel Inc.
(508) 339-3000
ext. 207
www.datel.com

ETRI Inc.
(704) 289-5423
www.etrinet.com

International Power
Sources

(508) 429-4440
www.intlpower.com/pages/
ibt.html
e-mail: [email protected]

Lucent Technologies
(800) 526-7819

Newport Components Inc.
(919) 571-9405
www.newport-comps.com/frames/
pdf-files.html
e-mail: [email protected]

Powercube, A Natel
Company

(818) 734-6500

RO Associates, Inc.
(800) 443-1450
www.roassoc.com

Switch Power
(408) 871-2400
www.spinet.com

SynQor
(978) 567-9596
www.synqor.com

Texas Instruments/Power
Trends Products

(800) 531-5782
www.ti.com

Vicor Corp.
(978) 749-3396
www.vicr.com/
products/home.htm

Wall Industries
(888) 597-WALL
www.wallindustries.com
[email protected]

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