Digital POLs Take Pressure Off Power Supply Design
Power conversion and distribution is under pressure in data centers, telecom offices, cellular base stations, and countless industrial computing applications. Modern life is reliant on massive computing power to handle tasks ranging from managing huge numbers of small IoT data packets, to streaming high-definition video on demand, or high-speed medical image processing. Demand for processing capability and storage in The Cloud is increasing, but server board dimensions remain constrained by industry-standard rack dimensions. And as successive generations of processors, FPGAs and ASICs continue to pack extra capability into smaller component-package sizes, equipment designers are eager to cram more of the devices on board to achieve the performance advances they need.
Power conversion and distribution is under pressure in data centers, telecom offices, cellular base stations, and countless industrial computing applications. Modern life is reliant on massive computing power to handle tasks ranging from managing huge numbers of small IoT data packets, to streaming high-definition video on demand, or high-speed medical image processing. Demand for processing capability and storage in The Cloud is increasing, but server board dimensions remain constrained by industry-standard rack dimensions. And as successive generations of processors, FPGAs and ASICs continue to pack extra capability into smaller component-package sizes, equipment designers are eager to cram more of the devices on board to achieve the performance advances they need.
Into this shrinking board area must squeeze the power conversion and distribution components, including bus converters, point of load modules, and any external components such as decoupling capacitors. Moreover, although available space is decreasing, power supply design is becoming more critical as system performance continues to increase. The transient performance of the power supply is assuming greater importance, as the low operating voltages of the latest ICs manufactured on advanced process technologies can tolerate fluctuations of only a few millivolts before unpredictable effects such as spurious resets can occur.
Digital power modules such as bus converters and Point-of-Load (POL) converters have evolved to address these challenges. In combination with advanced thermal engineering as seen in devices such as Ericsson Power Modules’ latest BMR466 60A POLs, digital technology enables increased power density and greater energy efficiency.
Digital Power, Always Optimized
Digital converters can adapt continuously as load conditions change, to minimize energy losses at any operating point from no-load to full-load. This overcomes the major limitation of analog converters, which are designed according to fixed parameters and therefore deliver optimum performance within a strictly limited range. Compared with analog converters, digital POLs display much more consistent energy efficiency, particularly at light loads (Figure 1). In addition, bus voltages can be adjusted on the fly to optimize system efficiency.
Figure 1. Digital power delivers more consistent efficiency from light load to full load.
Figure 2 illustrates the main functional blocks of a digital converter. The output voltage is sensed in the same way as in an analog design, but is digitized by an A/D converter instead of passing through an error amplifier. These digitized values drive a control algorithm hosted on the microcontroller. The microcontroller can switch between various control strategies stored in memory, to adjust the converter characteristics dynamically and maintain optimum performance as system demands change. Monitoring of the input and output voltage and current, internal temperature, duty cycle, and switching frequency not only facilitates adjustment of converter settings, but also enables the supervising host system to identify any problems with the power supply or the board and determine maintenance requirements.
Figure 2. Digital control allows dynamic adjustment for optimum performance across the load range.
For an application with a specific input voltage, output voltage, and capacitive load, the control loop of a digital converter can be optimized to ensure stability and faster load-transient response. Optimizing the control loop in this way reduces the number of output decoupling capacitors needed to meet the required transient response, which helps reduce bill of materials costs as well as saving board space. Space savings can be extremely valuable in complex systems such as data-center servers, which may contain 10 or 15 POLs – or sometimes even more – to supply on-board processors, FPGAs or ASICs that each require multiple power rails. Dynamic Loop Compensation (DLC), which is featured in Ericsson’s BMR466 digital POLs, automatically calculates the required loop-compensation coefficients, which saves power designers having to work out how to stabilize the converter and reduces the number of filtering capacitors needed.
Advanced Thermal Design
In the BMR466, Ericsson’s thermally efficient LGA package platform measuring just 0.98" x 0.55" x 0.276" high ensures excellent power density. These compact dimensions also allow the module to be positioned physically close to the load for optimum transient performance. The internal layout has been optimized to promote dissipation from the integrated FETs, which have low on-resistance and RDS(ON) x Qg figure of merit for optimum energy efficiency. The BMR466 can deliver its maximum rated current of 60A at 70°C ambient air temperature with natural convection cooling alone. Minimal derating at 85°C allows up to 48A with natural convection, or 55A with airflow of 1.0m/s (Figure 3). The efficient thermal design and low internal component count also allow very high reliability of 50 million hours MTBF according to the approved Telcordia test method.
Figure 3. Thermal efficiency helps minimise temperature derating.
For loads that require greater than 60A, up to eight BMR466 digital POLs can be connected in parallel to supply up to 480A. In such an array, the converters can be synchronized with an external clock to enable phase spreading, which greatly reduces input ripple current and the associated capacitance requirements and efficiency losses.
Digital Design Flow
Digital power technology also benefits from a more streamlined approach to product development and manufacturing. Engineers can use software-based design tools such as Ericsson Power Designer to build systems offline and subsequently streamline hardware development using an evaluation board. Real-time status monitoring of key parameters such as temperature, current, and voltage helps identify and fix any faults.
Simple systems or multi-module/multi-phase systems can be configured, and modules can be setup and initialized quickly by accessing the converter’s control loop through the Power Designer GUI to optimize load-transient response and stability. The tool helps optimize the input and output filters using the minimum number of capacitors, and also aids setting up more advanced features such as current sharing, sequencing and tracking, synchronization, and phase spreading. Once the design is complete, users can easily generate configuration files that can be applied directly to units on the production line.
Conclusion
Digital power technology is unlocking significant improvements in power supply efficiency, stability, flexibility, ease of use and time to market. The latest digital POLs in thermally efficient packages deliver extremely high power density and extended reliability.
