Electronic Design

Power: DC-DC Converters

Distributed Power Enters New Development Stage

Distributed power architectures, like those used in telecom and networking applications, have traditionally relied on isolated dc-dc converters (bricks) to generate the various supply voltages required on pc boards. But over time, the number of supply voltage levels necessary on a typical pc board has jumped.

So, designers depend more on point-of-load (POL) converters to generate the required array of voltages. In some cases, the brick is used simply to convert a 48-V bus down to some intermediate voltage, such as 12 V, which then powers all of the POLs. This trend toward two-stage dc-dc conversion has pushed power-supply manufacturers to step up development of POLs to handle emerging intermediate voltage bus architectures. Con-sequently, POL options multiply.

Meanwhile, semiconductor vendors continue to offer chip-based alternatives to the POL modules. These chips provide highly integrated solutions for implementing embedded dc-dc converters. Steady improvement in both POL modules and embedded dc-dc converter solutions comes from offering higher-current density and ease of use. Thus, the lines separating these two approaches continues to blur as the cost of the modules falls.

>ISOLATED, BRICK-STYLE DC-DC CONVERTERS developed for telecom will continue their performance gains in the coming year. In the quarter-brick format, top-of-the-line performance is now at 50-A output at 3.3 V and 60-A output at 2.5 V and below. Expect much of the industry to catch up to these levels next year, while market leaders push the quarter- brick outputs up to 70 A at the lower voltage levels.

>MORE EIGHTH BRICKS will emerge with the ability to deliver 30 A—currently an industry best—while at least one vendor introduces a 40-A eighth brick. In half-brick converters, the industry-high 100-A output will become more common, while one or more new converters reach 120 A.

>INTERMEDIATE VOLTAGE BUSES in distributed power architectures will increase in response to the greater number of supply voltages on pc boards. As a result, companies will continue to roll out isolated bus converters and nonisolated point-of-load (POL) converters. The bus converters represent an emerging category of isolated dc-dc converters specifically designed for intermediate voltage bus applications. Although system designers may elect to use standard telecom-style bricks to generate the intermediate voltage bus, bus converters will offer potential advantages. For example, bus converters can be designed with unregulated outputs, since POLs ensure regulation at the load. In some applications, where the full 36- to 72-V input range is not required, bus converters can also be designed for narrow input voltage ranges. These modifications to the standard bricks allow power-supply designers to raise the efficiency of the isolated dc-dc converter into the mid- to high-90 percentages. That boost in efficiency will help offset the losses incurred by the added stage of dc-dc conversion in intermediate voltage bus architectures. The higher efficiency will also help limit the size of the bus converter. Expect to see these types of bus converters introduced as well as a few variations. Brick-style packaging should provide a standard format for the new bus converters. On the other hand, in the POL area, standard formats haven't been firmly established. Nevertheless, two de facto standards have emerged—a 2-in. long SIP and a 1.3-in long surface-mount package. Expect more POLs in these packages.

>POL VENDORS will offer products with lower output voltages and programmable output voltages. Meanwhile, more POLs with 12-V inputs will arrive, as this voltage becomes an important intermediate bus voltage. There should also be new POLs designed for operation somewhere between 5 and 12 V. System designers will attempt to find a new intermediate voltage that obtains the best efficiency from the isolated bus converter and the POLs. At up to 20 A of output, these POLs will tend to be single-phase synchronous buck converters, but at higher-current levels, multiphase designs will prevail. In surface-mount versions, POLs are now limited to about 10 to 15 A. But these ratings could increase as demand for surface-mount packaging grows.

>EIGHTH-BRICK DC-DC CONVERTERS only arrived last year, but some vendors may make the next leap to sixteenth-brick converters. Finding a format that everyone can accept might be a sticking point though. With eighth bricks, power-supply vendors could shrink the converter yet maintain backward-compatibility with existing quarter bricks. But maintaining that same compatibility in a sixteenth brick won't be feasible.

>WITH MICROPROCESSOR CURRENT REQUIREMENTS headed to 100 A and beyond, digital techniques for voltage regulation will help attain high performance in multiphase power designs (see sidebar, "Digital Power Architecture Arrives," p. 100). For instance, digital control techniques will be used to perform phase and gain compensation. This type of approach enables the implementation of complex operating modes that are difficult to employ in analog. Benefits include layout flexibility, fast transient response, and the ability to change the number of phases in real time for improved efficiency.

>ONLINE DESIGN SIMULATION will foster easier simulation of embedded dc-dc converter designs. Semiconductor vendors are developing these tools to simulate common synchronous buck converters. But in general, these tools only work with the vendors' components.

>MULTIPHASE POWER CONTROL should improve the performance of POL converters. This means better transient response, lowered input and output noise, and a higher effective switching frequency that reduces the size of input and output capacitors. For instance, Bel Power Products plans to introduce a POL converter family with which users can synchronize the phase between modules. As a result, they can obtain the benefits of a multiphase converter rather than simply paralleling modules for boosting the current output. The Bel Power POL will have an output capability of 20 A per phase.

>INTEGRATING LOW-RDS(on) MOSFETs onto the same silicon with the dc-dc controller has produced an approach to POL power that falls between the traditional power modules and the multichip solutions in terms of ease of use, flexibility, and cost. Texas Instruments' SWIFT family of buck converter ICs has pushed the monolithic approach to its present 9-A limit in a 28-pin TSSOP-style device that steps down a 5- or 3.3-V input. Don't expect the 9-A rating to rise in the coming year, but do look for new additions to this family, such as a 12-V input model.

>WITH POWER-SUPPLY MANUFACTURERS competing for business in a slow economy, pricing on dc-dc converters—especially POLs—should continue to fall. But don't expect as dramatic a drop as the 50% reduction in pricing that occurred over the past two years. Current pricing is around $1 per amp for 10,000-unit quantities on POLs in the 10-A range. Pricing may fall another 5% to 10% this year as POL pricing continues to approach that of embedded dc-dc converters based on chip-set solutions.

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