Editor's Viewpoint: Digital Control Treads a Complex Path

Feb. 1, 2005
When switch-mode power supply technology entered the mainstream, it was initially adopted in higher-power applications. Over time, as the technology got

When switch-mode power supply technology entered the mainstream, it was initially adopted in higher-power applications. Over time, as the technology got better and cheaper, switch-mode designs were able to migrate to lower power levels, supplanting linear power supplies in countless applications. Today, the more-efficient switchers are cost competitive with linears at even the lowest power levels.

Could a similar evolution come to pass with digital power control? But instead of a migration of new technology across different power levels, perhaps we'll see digital control techniques migrate across different levels of application complexity.

Over the past year or two, representatives of the power semiconductor and power supply industries have been debating the merits of digital control. The early discussions focused mostly on why digital control would be valuable in many power supply applications and how it could be implemented to varying degrees. Some have described four levels of digital control ranging from the use of a simple microcontroller to add simple functions like soft-start to a power converter (Level 1) to complete digital control of power conversion using a DSP (Level 4).

But in recent months, IC and module vendors have gone from advocating digital approaches to implementing them. One example is the ongoing effort to develop a digital communications protocol known as PMBus. Many semiconductor and power module suppliers have voiced their support for this effort. PMBus has even garnered support among competing groups like POLA and DOSA.

As I write, a special interest group is being formed to develop the PMBus specification. Open to all interested parties, the special interest group will allow participants to view the preliminary specification and comment on it. The group plans to unveil rev. 1.0 of the PMBus specification to the general public at the upcoming Applied Power Electronics Conference (APEC) in Austin.

The degree of interest in digital power control is also reflected in the rapid pace of product development now unfolding. In this issue, we describe a microcontroller from Microchip Technology that includes power-optimized peripherals. This chip is meant to support Level 3 digital control, which still uses an analog power control loop, but could employ digital techniques to manage operations such as current sharing in an adaptable multiphase converter.

Another recently introduced product is C&D Technologies' QBC bus converter, a Level 2 digital control implementation in which a microcontroller supplements the PWM to enable programmability of various fault-protection parameters. For instance, the microcontroller can adjust the converter's output overcurrent limit.

APEC attendees will hear many presentations on digital power control this year. But they'll also witness actual products like those from Texas Instruments, which plans to unveil a number of power conversion ICs with digital control. Others like Astec Power and Artesyn Technologies plan to introduce power modules with digital control later this year. The modules to be unveiled by Artesyn in mid-2005 will be nonisolated, point-of-load converters with fully digital (Level 4) control and a PMBus-compatible interface.

Digital control promises to significantly reduce the component count on the pc-board. One IC vendor described how its customer cut the parts count from 1700 to 1100 in a UPS design. Similarly dramatic reductions have been described for board-level power systems and isolated power converters.

However, the value of digital implementation still rests largely on the functional complexity of a power design. If all you need is a simple buck converter, it's tough to beat an analog controller for simplicity and lowest cost. But that's today's conventional wisdom. Digital control techniques could take root in applications with the most complex power management needs only to find their way into what we now view as the simplest of power supply applications. With the pervasiveness of digital design techniques in so many areas, it's impossible to predict where the future boundaries for digital power control might lie.

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