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Digital power control for high-current DC-DC design

04 Feb 2014  | Ben Dowlat

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Digital power control has, in the past, been one of those technologies that seems initially attractive, but is unable to gain a toehold in the market because of a single, deal-breaking drawback. In the case of digital power, this drawback has been latency – the time between the acquisition of the feedback signal and the generation of the subsequent modulated output.

This latency has resulted in slower transient response than equivalent analogue controllers. In high-performance systems that require an instantaneous response to a change in load, the use of a slow digital power controller is out of the question.

Now two changes are serving to swing the balance decisively in favour of digital power control in high-performance computing and networking applications such as data centres and server farms. Here, the demand for improved energy efficiency and energy cost reduction is leading to new and innovative uses of granular and real-time power consumption data – information that is easily available from digitally-controlled power architecture.

At the same time, incremental improvements in the implementation of digital power control designs have recently led to reductions in latency in the digital control loop, resulting in transient response times even better in some cases than equivalent analogue controllers can offer.

Telemetry in large server installations
One of the great advantages of a digital power controller is that intelligence is an inherent feature of the device's mode of operation – it comes, as it were, for free, in the same IC that implements power control. This intelligence supports the provision of attractive features that digital system designers can readily appreciate. At the simplest level, these will typically include output voltage margining, and telemetry of key parameters such as voltage, current and temperature.

The digital architecture also enables the easy configuration of these and many other operational parameters in a software design environment normally hosted on a PC. The straightforward selection of the required parameters in a software tool provides for an easier and quicker design process than the equivalent analogue circuit design.

In the context of high power-consuming installations such as data centres, however, the great value of a digital power controller lies in telemetry: the ability to capture operating data at the level of the individual powered device. When equipped with an I2C or PMBus interface, the controller can stream these data in real time to a host processor. This ability to track power usage in real time can be used to achieve substantial savings in power consumption, as well as computing performance improvements.

For instance, the processors in data centre or telecom central office servers reach peak efficiency at full load. They are also designed to draw a very small current when in stand-by (no load). They are least efficient when between these two states – in active operation, but at something less than full load.

Telemetry enabled by a digital power controller enables the implementation of dynamic load-sharing applications in arrays of telecom and networking equipment, either taking computing workload away from a partly-used device so that it can be put into Sleep mode, or diverting workload from other partly-used devices until it is fully loaded.

The provision of granular operational data over an I2C or PMBus interface also allows for the analysis of historical operating data. Such analysis might reveal what triggers over-voltage or over-current events, or uncover patterns in the transitions of devices into and out of Sleep mode. The insights gained from such analysis can enable data centre operators to refine the configuration of server arrays in order to reduce downtime and to use power more efficiently.

The transient problem
For all the attractions of the telemetry supported by a digital power controller, however, it is the main control function that determines whether it is valid for any given application. In computing and networking applications, accurate, fast and stable voltage-mode regulation is essential, enabling the server to respond instantaneously to demands for throughput. Previously, the slow response time available from a digital control loop put these applications out of reach of digital power controllers.

Network and data centre operators remain hungry for device-level power telemetry, however. And now a new generation of digital power controllers is emerging with the aim of solving the transient problem. Typical of the breed is the MIC21000 from Micrel. This device is a voltage-mode digital DC-DC controller compatible with industry-standard DrMOS devices, which means that it can support a wide range of output voltages. It can be configured to operate at any one of 12 switching frequencies from 177kHz to 1MHz.

Figure 1: Simplified block diagram of the digital compensation system implemented in the MIC21000.

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