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Powering FPGAs with digital power modules

Posted: 29 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:FPGAs? power supply? GUI? inductor? PCB?

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 componentsPWM controller, FETs, inductor and compensation circuitrywith 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 utilisation. Low, middle and high utilisation estimates can help determine the power demand under these conditions. The table breaks up the power requirements with a low, mid, and high current estimation for a Virtex 7 FPGA.

Table: Power supply requirements for a Virtex 7 FPGA.

Using the table chart as our guide, we can select various options such as analogue 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 minimise 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 analogue-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.

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