Low Voltages Challenge DC-DC Supplies
Falling supply voltages and rising currents have greatly enhanced the value of stepdown dc-dc converters. As voltages migrate to 1 V and possibly lower, I*R losses make it impractical to bus these voltages around the system. Point-of-use dc-dc converters let designers keep bus voltages at more acceptable levels, like 5 or 12 V, and generate the lower values on-board where long interconnects can be avoided.
But the challenge for dc-dc converter developers—whether they're power-supply vendors offering fully functional converter modules or IC suppliers developing buck converter chips—is to minimize the impact of the converter on pc-board space, spacing between boards, and heat dissipation. Consequently, the coming advances in dc-dc converter design will be judged in terms of their ability to raise the converter's efficiency as well as their density in terms of power or current per cubic inch. Increasingly, it's the latter spec that counts most, and over time we'll see more of an emphasis on amps than watts. That concern is re-flected in many of the Top 10 developments awaiting dc-dc converters in 2002.
Smaller units will satisfy the requirements of more dc-dc converter applications. The lion's share of the converter market is currently in half bricks. With power performance rising, though, the market is transitioning to quarter bricks.
The drive to pack all the building blocks (namely power MOSFETs, Schottky diodes, a PWM IC, and passives) in one package, like a multichip module (MCM) or some other compact format, is already under way. Designers are now moving toward complete multiphase solutions in these packages. To minimize stray inductances and interconnect delays, it won't be too long before faster CPUs and complete dc-dc converters are combined in one package. As the pressure to miniaturize and cut time-to-market continues, more and more integrated dc-dc converters will bring power MOSFETs on-board to simplify design and cut external component count. Six-amp capability has been demonstrated, and the trend is toward much higher current at sub-2.0-V outputs.
Three factors are pushing dc-dc converters to higher power density:
- Greater use of synchronous rectification
- Better MOSFETs with lower RDS(ON) and gate charge
- Surface-mount pc-board construction.
This year, expect to see brick-style converters exceeding 130 W/in.3, or more than the 600-W output now available in a full-brick format.
Increasing use of a distributed-power architecture is driving the growth of integrated regulators and dc-dc converters. These converters step-down the voltage from a bus to the desired voltage at the point of load. While 48 V has been the norm in this architecture, the latest trend is toward 12-V buses. In fact, 48 V will be downconverted to 12 V. So, converters with a 12-V input supply will begin to gain momentum. As boards are getting denser and denser, these devices are moving into smaller and smaller packages.
Package heights will decrease. With height requirements as low as 8.5 mm (0.33 in.) for applications in Japan, converter makers should continue to be pressed to reduce the converter's height off the pc board. Another ongoing trend is the movement to surface-mount designs. Yet it's likely the higher-power units, 50 W or more, will continue to be leaded units.
The emergence of multigigahertz microprocessors is forcing designers to adopt multiphase techniques to tackle the very fast transient responses demanded by these CPUs. These next-generation processors also need large current supplies (greater than 50 A) at 2.0 V and below. Single-phase PWM techniques are running out of gas to handle these challenges. By partitioning the power converter into several phases, multiphase PWM techniques will solve these problems. However, the number of phases required will depend upon the output specifications.
Voltage regulator modules (VRMs): While the overall trend in processor power is greater current at lower voltage, supply requirements for the processors coming out this year may not vary greatly from those of the present generation. So while Intel's next generation of Pentium 4 processors may reach speeds of 2.5 GHz, the use of finer transistor geometries will tend to offset the rise in power consumption dictated by the faster operation. VRMs should see peak current requirements up to about 65 A at voltages ranging from 1.1 to 1.85 V. Package heights for VRMs will become critical, particularly in 1U applications. By employing multiphase controllers with four- or five-phase operation at about 1.2 MHz per phase, VRM manufacturers will be able to employ smaller capacitors and inductors. Delta Electronics (www.deltaww.com) plans a height reduction this year that will reduce the current 2.4- or 2.5-in. profiles to 1.2 or 1.3 in. In general, VRMs should predominantly feature 12-V inputs, which serve the needs of desktop PCs and midrange servers. But there should also be an increase in the number of 48-V input VRMs to satisfy high-end servers.
While switching at higher frequencies has been the trend for some time, the interest now is to go beyond 1 MHz. Hence, many recent dc-dc converter introductions have offered switching frequencies up to 1.4 MHz. The intent now is to go beyond 2 MHz to drastically cut the size of capacitors and magnetics. Some developers are pondering support for frequencies as high as 4 MHz. But as the frequencies get closer to 2 MHz and up, the switching losses are going to get pronounced, prompting some tradeoff between power density and conversion efficiency. Although the MOSFETs' on-resistance and gate-charge specifications continue to improve significantly, the 90% and better conversion efficiency required by new equipment will make designers think twice about very high switching frequencies.
The ability to complete a dc-dc design online and then build a virtual prototype with proper layout was introduced a few years ago. That capability is on the rise as more and more players join the fray. Besides extending the complexity of the design, newer versions are coming online to enable users to add heatsinks and other thermal management techniques to the desired board and view the results instantly. This is motivating digital designers to confidently design power-supply circuits.
The Semiconductor Industry Association roadmap predicts chip supply voltages will fall to less than 1 V by 2004. Some power-supply manufacturers are already offering modular dc-dc converters with 1 V either as a nominal output or as a value within the output trim range. With the SIA forecast in mind, power-supply manufacturers will step up introductions of synchronous, rectified, dc-dc converters with outputs of a volt or less. Another trend among these high-efficiency dc-dc converters will be the continued rollout of dual-output devices.