Making digital power control viable for high-current DC-DC designs
Digital power control has, in the past, been one of those technologies that appears superficially attractive, but that fails 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 analog 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 favor of digital power control in high-performance computing and networking applications such as data centers 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 a 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 analog 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 analog circuit design.
In the context of high power-consuming installations such as data centers, 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 center 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 center 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 center operators remain hungry for device-level power telemetry, however. And now a new generation of digital power controllers is emerging that finally looks to have solved 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 177 kHz to 1 MHz.
In the grand tradition of electronic component technology, the device has achieved a new high level of performance not through a single, radical breakthrough but by combining a number of smaller improvements to the conventional design of digital power controllers.
Figure 1: Simplified block diagram of the digital compensation system implemented in the MIC21000
For instance, the device’s digital control loop is based on a familiar PID (proportional-integral-derivative) compensator structure. But in this case, it is implemented as an adaptive PID structure (see Figure 1). The controller analyses the voltage feedback before it is fed to the compensator, and decides whether it represents steady-state operation or a transient. It then applies the more appropriate compensation mode, with the transient mode offering fast response and settling times. The coefficients for the two modes can be derived from the device’s digital design software tool (see Figure 2).
Figure 2: The Micrel Digital Designer GUI greatly simplifies the programming of the MIC21000 power solution
In addition to this adaptive PID structure, three other techniques are applied to improve the feedback mechanism, the response to the feedback and the handling of large transients.
‘Ultra Fast Sampling Technology’ implements high-frequency sampling of the voltage feedback in the digital control loop. By continuously acquiring accurate voltage signals, the compensator can provide a quicker response to changes in demand for output power. The effect of Ultra Fast Sampling Technology is to reduce the phase lag caused by sampling delays, to cut noise sensitivity and to improve transient performance.
Another technique, Ultra-Fast Transient Response (UFTR), is a method for driving the digital pulse width modulator (PWM) asynchronously during load transients (for non-linear control). This has two beneficial effects: it limits the extent of the deviation of the output voltage from its target value; this means that the output capacitors can be recharged more quickly (see Figure 3a).
Third, the device applies a non-linear gain adjustment during large load transients. This momentarily boosts the gain applied by the control loop, and serves to reduce the voltage settling time (see Figure 3b).
Figure 3a: The implementation of Ultra-Fast Transient Response (UFTR) in the MIC21000
Figure. 3b: The use in the MIC21000 of techniques such as non-linear gain adjustment provides for rapid settling after load transients
The result of the implementation of these new techniques is to provide outstanding transient response from an entirely digital control loop. In fact, transient response is in some cases even better than an equivalent analog controller can offer (see Figure 4).
At the same time, the device offers all the protection, monitoring and telemetry features that are valued by operators of computing and network equipment, including real-time voltage data acquisition via I2C and PMBus™ interfaces. Sophisticated power sequencing can easily be configured using the Micrel Digital Designer tool, and in volume production configuration settings can be saved in an embedded one-time programmable non-volatile memory.
In addition, the digital architecture offers the designer great flexibility to configure appropriate settings for the various protection features, such as over-voltage, over-current, under-voltage and overloaded start-up.
Applicable across a broad range of high-current, non-isolated DC-DC power supply designs using DrMOS devices, these technology advances promise to enable useful energy savings and to provide operational data that improve capacity and energy utilization in environments such as data centers and telecom central offices. This suggests that the time for mass adoption of digital power control technology has come a step closer.
Figure 4: New digital control techniques implemented by Micrel enable transient response even better than that of equivalent analog controllers
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