Using digital power modules to power FPGAs
The proliferation of voltage input rails for delivering point-of-load (POL) power to FPGAs is making power supply designs ever more challenging. As a result, encapsulated power modules are seeing increased use in telecom, cloud computing and industrial equipment because they operate as self-contained power management systems. They are easier to use than discrete solutions and speed time-to-market for both experienced and novice power-supply designers. Modules include all of the major components -- PWM controller, FETs, inductor and compensation circuitry -- with only the input capacitor and output capacitor needed to create an entire power supply.
This article discusses a FPGA reference design generator and walks you through the steps for selecting an FPGA, required power rails, backplane and digital power modules for POL. We will highlight a graphical user interface (GUI) that configures, validates and monitors the FPGA’s power supply architecture, and we will explain the GUI’s sequencing feature to power up the voltage rails, and select the power sequence order and rise and fall times.
Power Supply Software Tools
FPGA manufacturers provide various tools that help estimate the power requirements during the power supply planning stage. These tools take into account device selection, architecture evaluation and thermal modelling to arrive at an estimated solution. For example, power supply designers can use the Xilinx Power Estimator (XPE) tool at the pre-design and pre-implementation phase. Power management vendors then take the results from XPE and use the information to provide the necessary guidance for component selection of the power supply.
Since the programmable FPGA is a variable at the planning stage, rule of thumbs can be established for the device families that will vary based on FPGA utilization. Low, middle and high utilization estimates can help determine the power demand under these conditions. Table 1 breaks up the power requirements with a low, mid, and high current estimation for a Virtex 7 FPGA.
Table 1: Power supply requirements for a Virtex 7 FPGA.
Using the Table 1 chart as our guide, we can select various options such as analog discrete or module solutions and digital discrete or power module solutions. Tools such as the FPGA Reference Design Generator make it easy for you to select a solution for your targeted FPGA hardware. Simply select the FPGA vendor, FPGA family, current requirements, desired backplane, and solution of interest. The tool then provides all of the necessary design collateral associated with the desired solution, including design schematics, layout, BOM and a high-level block diagram.
Figure 1: FPGA Reference Design Generator finds the right Intersil power device for your design.
For high performance applications, you probably want to minimize the time spent on your power supply, and instead focus your attention developing the application on the FPGA. In high performance systems, the FPGA code is not set and the solutions code for the FPGA will often vary. With an analog-based power supply solution, most of your time will be spent redesigning the inductor along with recalculating the compensation network in order to maintain the power supply’s performance. The calculations of course take time, and under some circumstances, it could mean redesigning an inductor. In addition, if the package size changes, you also might have to spend extra time on a PCB redesign. With some digital solutions, such as the ISL827xM, you will not have to redesign the inductor or recalculate the compensation network; the device automatically handles it for you.
The leading power IC suppliers provide both analog and digital solutions, including power modules, which gives you many options to evaluate the trade-offs. For example, you could select the ISL85003 3A switching buck regulator for the fixed rails of the Virtex solution and then go with a digital power module to support the various Virtex device options. In some cases, the power rails not only power the FPGA but also provide power to other devices in the application. In our example, we will use a digital solution for all three voltage rails. We’ll use the ISL8270M 25A digital power module, and the ZL2102 6A digital integrated buck regulator for the remaining rails.
Depending on your board bring-up and test strategy. Intersil’s PowerNavigator GUI software will help accelerate the bring-up, testing and finalization of your hardware. Both the ISL8270M and the ZL2102 offer evaluation boards that can be connected together and then set-up using the USB-to-PMBus interface for your specific application requirements. Figure 2 shows the PowerNavigator tool in offline mode ready for us to plug in our hardware, and make the configuration setting selections. We will use these settings during the bring-up stage to test and verify the design in sections.
Figure 2: PowerNavigator GUI simplifies design of densely populated power systems.
Continue reading the next page on Embedded's sister site, EDN: "How to power FPGAs with digital power modules."