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Understanding adaptive power control strategies

Posted: 17 Feb 2012     Print Version  Bookmark and Share

Keywords:intermediate-bus architecture  digital power converters  point-of-load regulators 

The intermediate-bus architecture (IBA) that the telecoms industry has refined over decades to become cost-competitive with traditional offline-only sources is an evermore attractive proposition for system architects needing high availability in a wide range of industries. Some of its features are the ability to swap seamlessly between utility ac line and local battery backup power. Continual development in areas such as intelligent power management strengthens the arguments in favour of the IBA approach and while such strategies have been possible for many years, the recent commercialisation of digital power converters vastly simplifies their implementation. Contemporary economic and environmental sensibilities make potential energy savings impossible to ignore—and this particularly applies to demand-driven systems that experience significant load-level swings.

Evolution or continuous revolution?
Re-examining long-established practices and actively exploring new ones has never been more appropriate for a power-system designer. However, revolution is also a realistic description of the step-change in capability that recently available power converters, which exploit digital inner-loop control, deliver over analogue counterparts that have decades of evolution behind them, making significant functionality or performance gains unlikely for such a well-refined platform.

To compete, the digital approach has to be better from initial product launches onwards, which is one reason it's taken so long to successfully commercialise. Commodity mixed-signal processes that allow silicon architects to pack a measurement and control sub-system and communications interface alongside the digital PWM controller core at negligible additional cost now enable products that are electrically superior to analogue designs. For instance, at 396 W with ±2% regulation Ericsson's BMR453 almost doubles the power density delivered by tightly regulated analogue quarter-brick intermediate-bus converters.

Figure 1: A digitally controlled buck converter with power management and PMBus link.

Moreover, the digital platform enables a raft of programmable functions that range from setting constants such as output voltage, sequencing delays and slew rates, and fault-condition thresholds in a one-time programming step to dynamically optimising key parameters in a running system. With its SMBus hardware basis and a standard power-control command language, PMBus—an industry success story in its own right—makes it easy to explore and implement a level of control that's unprecedented in analogue converters and in turn enables compelling opportunities for system design improvements. Figure 1 shows the key functions within a representative digitally controlled buck converter.

Figure 2: Adaptive digital inner-loop control minimises losses over wide-ranging conditions.

Able to adapt to line and load conditions in real time, digital inner-loop control mitigates losses using techniques that include adaptive dead-time control—that is, to vary the period between the power switches conducting to avoid shoot-through. Using a buck-converter example, the objective is to minimise the conduction period of the relatively lossy body diode in the lower sync FET. The efficiency improvement that results from optimising this conventionally fixed parameter becomes greater with increasing down-conversion ratios and higher switching speeds, and can be several per cent higher. Relative to analogue DC-DC converters that are typically most efficient between about 50 – 70% of full load, the result that figure 2 shows for the BMR453 is to widen the efficiency curve, which is almost flat from about 10% of full load upwards and achieves 96% or better in typical operation while also being relatively insensitive to input voltage levels. Significantly for power-converter designers, other family products of widely-varying power levels that employ common core designs reflect virtually identical efficiency characteristics that promise far greater scalability than delivered by analogue circuitry.

Classic 48 VDC suits high-power systems
Reliable power is increasingly crucial as society's reliance upon communications-system availability rises, and that data-centric infrastructures structurally consign any meaningful division between telecoms and datacomms to history. The generic model for assuring continuously available power in such systems combines an AC-DC front-end that sources utility-line power alongside a battery backup system and standby generator to supply a DC power-distribution bus; and it's a well-proven arrangement that's attractive to multiple industries. Keeping in mind that its roots lie in lead-acid accumulator technology that pre-dates AC-line supplies, the 48 VDC level that telecom-spec ETSI EN 300 132-2 defines as a service voltage of 40.5 – 57.0 VDC remains an excellent choice for a DC power-distribution bus for systems that may require kW+ power levels, either initially or as they evolve to meet increases in demand. Relative to say a 12 VDC distribution level that will struggle to service high-power applications, 48 VDC eases Ohmic-loss issues and reduces wiring and connector bulk.

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