As per usual, the power-supply industry is feeling the heat from the semiconductor industry to meet upgraded requirements for voltage, current, power, and switching speed. That pressure is no more apparent than with point-of-load (POL) converters, which power most low-voltage loads.
On the bright side, this challenge has spun out several new trends:
- Power-supply companies are developing their own POL ICs.
- New POL architectures promise better performance.
- Digital control is elevating POL converters to a new level.
- Improved power MOSFET packages boost POL efficiency.
- Monolithic IC regulators with higher switching frequency allow for smaller POL capacitors and inductors.
Ultimately, system designers and integrators must investigate whether to design and build POLs or buy OEM POLs. A company with limited technical resources would probably buy tested and qualified POLs, whereas a company with adequate resources may design and build its own POLs. Regardless of the approach used, designers have many different ways to achieve POL power conversion.
To encourage in-house designs, semiconductor companies now provide more "hand-holding." For example, Carl Smith, marketing manager for International Rectifier (IR), points out that his company has developed simulation tools and "demo" boards to assist circuit and system designers who may not have the necessary design resources in-house. IR provides support for power MOSFETs as well as its power-oriented ICs that drive and control the MOSFETs. Thus, IR's support is at the POL system-design level.
WHAT'S A POL?
Why exactly are these trends taking place? To get that answer, designers first must understand POLs. These non-isolated converters accept power input from an isolated dc-dc converter and provide dc power to close-proximity loads. This configuration costs less than an isolated dc-dc converter approach, because the non-isolated POL converters don't have a transformer for I/O isolation. (For a look at some groups that have formed in this area, see "POLA And The Power-Management Bus," p. 50.)
POL dc-dc converters are usually found in small, nonbrick packages, such as a single inline package (SIP) or surface-mount-technology (SMT) module. POL converter modules must be located as close as possible to the circuits they power, which aids voltage regulation. This has become increasingly important as processors operate at lower voltages and higher currents. Also, higher clock frequencies mean that POLs must possess fast dynamic response.
Today, most POL converters are part of a distributed power architecture (DPA). Many but not all distributed power systems employ an intermediate bus architecture (IBA) that inserts a bus converter in front of the POL (Fig. 1).
Randy Malik, a member of IBM's senior technical staff, is responsible for computer power-system design. Therefore, he's involved with power-system design of desktop and industrial computer systems. He points out that the company's POL goals include at least 90% efficiency (458C to 558C) with minimum space requirements. The company purchases POLs based on its specifications and occasionally designs and builds some of its own.
These POL converters operate from a 12-V-input bus derived from an ac-dc front-end power supply (Fig. 2). Because the 12-V supply is required for fans and hard drives, it's also used directly for the POLs that provide output voltages in the 1- to 2-V region. No intermediate bus converters are necessary. Virtually all circuits use synchronous rectification and multiphase conversion. Most multiphase circuits employ four phases, while a handful use six or eight phases. A typical multiphase POL for a microprocessor might handle over 100 A at about 1 V.
At the system level, the major problems Malik has encountered are those related to thermal management. Problem heat sources emerge primarily from inductors and multilayer pc boards.