EnergyStar or no, power supplies deliver

Jul 1, 2010 12:00 PM,

Vincent Biancomano contributing editor

Though the Energy Star standard for power supplies is going dark, the broader issue of energy conservation continues to steer high-efficiency designs.

Digital developments

It took time to knock down the proponents of so-called “digital power” technology for medium and high-power applications. The point of contention pertained to “digital” being “faster” and “more accurate” when placed directly in the feedback loop of a power conversion stage. Those rather vague claims have receded, but digital power techniques still have a following, particularly as one easy way to improve the transient response, albeit indirectly, in multiphase architectures.

“The talk on digital control loops in dc/dcs has largely evaporated,” says Linear Technology's Don Paulus. “The argument on digital power is meaningful in power system telemetry to measure and monitor current and voltage levels, power loss, temperature and so on to optimize system level power efficiency and usage.” The view is fairly universal. “Digital is a buzzword; people have to be careful how to use the technology,” says Vicor's Steve Oliver. “Digital technology is excellent when it comes to communications---setting voltage, reading back system voltage, and control. But when it comes to power converters, it still comes back to the analog system. Anything's possible in time, but if you want the fastest, most accurate feedback loop, use an optosensor and an analog op amp. It's the most elegant way.”

“Digital feedback is much slower, much less precise than an analog feedback loop,” says Oliver. The ideal solution is to keep the analog loop the way it is, but add power management features----digital interface, gathering temperature, voltage, and current info, and feed that information back through a digital bus. Now the loop doesn't stay stable (with load variations) if you don't re-optimize the compensation. Doing that in analog is very difficult. Doing it with digital, you just change some internal coefficients. Internally, you have a look-up table that relates how many phases to the loop coefficients.”

And it's in the power management mode that digital power increasingly comes into play for multiphase applications. “The controller has little impact on efficiency,” says International Rectifier's Deepak Savatti. “But a digital controller gives you one advantage at light load: phase shedding. Phase shedding is independent of the power stage. You can do it automatically without decision-making (the controller is smart). If my output current is 50% of load, going from four to two phases improves efficiency by almost 5%. On the other hand, digital power has no impact on peak efficiency and full-load efficiency. But (indirectly), the transient response in a multiphase system is going to be better.”