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Server Efficiency could be improved with Advanced Control Electronics

Power-management methods and three-phase BLDCs to cool down the reduce width=”752″ height=”508″ />The electronic devices allow inexpensive control over server components with minimal contributions to thermal signature, power draw, or physical size. Some, like the Allegro MicroSystems A4942 three-phase sensorless BLDC fan- motor driver chip, are sufficiently small to suit onto the hub PCB of mini ducted fans. The hub PCB is a small ring-shaped board with less than 5-mm effective width, to accommodate the rotor shaft (figure 1). Monitor ICs, like the ACS761, provide current and power monitoring and control, enabling hot-swap management in the individual server blade level.

Energy reduction strategies

The latest generations of servers provide several new methods to energy management, which allow rapid recovery of conversion costs, often within a year. For example, microprocessors happen to be designed for higher throughput in smaller packages, requiring less power and generating less heat.

Studies of the baby thermal sources, principally power supplies and microprocessors as well as their enclosures, have led to optimized heatsink geometries and component layouts, with channeled shrouds to direct laminar airflow across these key areas. This complements the greater recent high-efficiency ducted miniature (less than 40-mm) tandem fan-motor assemblies, arranged in series or parallel arrays within these flow paths.

To increase airflow efficiency and reduce footprint, the integrated fans assemble in tandem pairs that share the same ducting. The 2 fans are, however, completely independent when it comes to mounting shaft and drive electronics. Although this could gain a benefit from modular control, actually it may introduce problems effecting reliable sensorless motor startup: left privately, among the motors will begin first, causing airflow within the other fan and dragging the motor and disturbing the open-loop startup sequence.

A similar problem can occur when one fan has not yet stopped turning when the motors restart. In the past, this phenomenon made it essential to allow both fans arrive at a complete stop before restarting. The new motor-driver ICs contain an adaptive startup algorithm that may interpret when the motor is being driven by airflow within the fan blades in the tandem fan or once the motor and fan are already moving from the previous power cycle. The advanced IC can modify the power-on sequence to adjust with this and permit both fans to function synchronously at maximum efficiency through the power cycles.

Optimization of airflow is bound, however, and improvements in PID control systems have to optimize fan usage in terms of speed and idle time. Many servers are employed only a small percentage of times. Energy during the low demand periods could be saved by low-power modes or perhaps power-down modes with automatic startup.

This can be accomplished by monitoring current consumption because the components operate, using current sensing ICs that may mount around the PCBs in the servers for lower-current onboard applications, or on supply lines for high-side current sensing. These compact ICs measure current magnetically, while using Hall effect, eliminating the need for sense resistors, which dissipate heat. For example, an integrated conductor, such as in the Allegro ACS758, presents only 100 μΩ resistance, which is an order of magnitude lower than typical sense resistors, to cause significant power savings.

This technology offers isolated current sensing in a compact package, providing a low voltage output signal for closedloop feedback. Applied with advanced PWM motor drivers, these units can control supply current surges and be sure direct closed loop fan speed control to carry the flow of air rate consistent as well as in proportion towards the actual cooling requirement.

This also results in material savings because motors don't have to be overdesigned to pay for big motor-to-motor torque and speed variations. Individual motors often have electrical characteristics that vary more than 10% between units. In addition, the neighborhood environments where the motors mount vary substantially and inconsistently in terms of electrical power and cargo, as well as thermal loading from coolant flow and adjacent heat sources.

Advanced PWM motor drivers and hot-swapping current monitoring ICs can suppress current surges because the motors turn on. New device types apply soft-start PWM current-ramping techniques that permit the designer to optimize tradeoffs between surge current and power cycle times (figure 2).

Figure 2. Effect of soppy start in reducing surge current

Additional efficiency is gained by the test device-in this case, the A4942-which has advanced features that start to energize the motor phase windings prior to the timing based on the rotor position.

This phase advance technique helps to ensure that the phase windings have reached the required current level at the point where the resulting forward torque around the rotor is going to be best, thereby improving motor efficiency. Observe that the beginning and stop conditions are the same however with soft start, the utmost current is greatly reduced. The longer power-up may not be significant in start-stop fan applications, and can be programmed to tradeoff with power surge.

Integrating Hot-Swap Management

Existing server-blade technology seeks to minimize these variances through the modular approach, placing power supplies and cooling fans off-board from the memory storage and processor elements. However, this incurs significant risks in hot swapping. Current sensor ICs with integrated hot swapping controls manage power surges that occur because of the makes and breaks of electromechanical connections. The soft oncoming of an external FET controls the hot-swap power surge and offers current limiting (figure 3). By controlling the FET turn-on time when power is connected, the hot-swap current-sensor IC, within this example the ACS761, reduces the inrush current from 32 A to 12 A.

Hot-swap management affects the style of another components in the server. This cuts down on the requirement for components to become rated for top inrush current levels. Additionally, by integrating current and power limiting, the hot-swap IC not just minimizes the board area that must definitely be isolated in the operator for compliance with UL 60950, but also provides short-circuit protection.

Three-phase motor advantages

Figure 3. Hot-swapping current-surge suppression simulation

Although single-phase BLDC motors are less expensive than three phase motors, increasing energy costs make the larger efficiency of the three-phase motor a fiscal offset. Typical efficiency improvements from single-phase BLDC motors to three-phase BLDCs are approximately 25%.

Designs achieve further cost reductions using techniques such as motor soft start to reduce the current surges in the power at startup. This decrease in surge current also allows smaller FETs and reduces costs for power supplies.

Along with optimized motor drivers, power-regulation techniques can optimize the whole process of various components and systems within the server. QFN-sized DC-to-DC regulators provide point-of-supply management integrated with advanced features, for example synchronous rectification for high efficiency, short minimum controllable on-times, and optimized high- and low-side FET

RDS(on) ratios for VIN / VOUT ratios commonly present in servers. These provide robust fault-tolerant power management to resist variable operating conditions, and detect and report a multitude of fault conditions.

Three-phase BLDC motors combined with advanced integrated circuit control and monitoring are selling significant efficiency gains now, and supply a path to future improvements. Since these technologies can apply at the subsystem level, they can scale to both DG (distributed generation) and CHP (combined heat and power) systems. Using the improved electronic evaluation techniques, these devices enhance server-system microgrid integration with Smart Grid systems.