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.