Electronic Design

Portables Prepare For New Power Challenges

Hungrier laptops and busier handhelds keep power-supply designers on their toes.

Big changes loom on the power horizon. For instance, the new PMBus power-management bus standard is going mainstream in 2006. Also, alternative power sources like direct-methanol fuel cells are on the way, though they may take a bit longer. In November, the International Civil Aviation Organization decided that air passengers would be able to carry on two spare methanol containers for their microcell-powered laptops starting in 2007.

Meanwhile, laptop supplies need a boost. Handheld power management is about to get more complicated, too. Also, keep an eye on the seemingly omnipresent LCD-screen backlighting, appearing in everything from personal media devices to big-screen TVs.

Virtually across the board, Intel's next generation of laptop chips will demand more power—enough to push ac power converters into a whole new set of design requirements.

Operating at 1 to 1.3 V, chip sets will demand 30 to 50 A of peak current. The CPU alone will need 40 to 70 W of peak power. Add peripherals and memory, and full-function notebooks will require 150 W and up. On the board, CPU voltage-regulator modules (VRMs) will multiply, using interleaved phases.

With laptop ac-dc converters rated for more than 75 W, they will have to incorporate power-factor-correction (PFC) circuitry. A few monolithic PFC controller chips already populate the market, with more expected.

Those laptop ac-dc converters will also likely move from diodes to synchronous rectification. This may seem as if it would provide only small gains, given the converters' relatively high output voltage. But advances in reducing FET on-resistance and in packages like International Rectifier's directFET make synchronous rectification more attractive.

That greater-than-150-W number represents peak power. Intel's thermal-design-current (TDC) specifications do not call for average operating power at such levels. So, maybe those chips won't have too big an impact on battery run time.

However, they still represent a challenge for power-supply designers. The trick will be to build a supply that can handle the thermal requirements at peak power levels while maintaining efficiency during periods of low to medium power demand.

Also, laptops may get an intermediate bus like server and comm boards. Today's laptop wall adapters range from 19 to 24 V in output voltage, which is what gets stepped down at the VRM. On the other hand, the front-end ac-dc converters in telco and server systems offer a 48-V level output. The higher voltage makes the two-stage step-down of the intermediate bus architecture (IBA) attractive in those systems. Compared to 48 V, 12 V to the point-of-load offers an efficiency advantage for buck regulators because they help keep the ratio of input voltage to output voltage low.

See Associated Figure

For more on the battery "fuel gauge" and CCFL trends, go to www.elecdesign.com, Drill Deeper 11844.

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