Reduction of Switched Mode Power Supply Electro‐Magnetic Emissions in Automotive Cockpit Applications by Using of a Field Programmable Gate Array to Implement Pseudo Random Spread Spectrum Modulation

Introduction At the heart of automotive design is the requirement to pass Electro-Magnetic Compatibility (EMC) limits, by minimising conducted and radiated emissions and susceptibility. The use of Switched Mode Power Supplies (SMPS) is now common-place, yet a prime source of electro-magnetic emissions. Minimising these emissions is a key goal of SMPS design. Use of spread spectrum techniques is an increasingly common means of achieving this goal, and more complex techniques, such as pseudo-random spread spectrum modulation, can yield the greatest emissions improvements. While some SMPS Integrated Circuits (IC) have an integrated spread spectrum function, many don’t. Implementation of spread spectrum techniques, can be achieved using Field Programmable Gate Arrays (FPGA), with a minimal burden on the FPGA’s resources. Many automotive audio and visual systems utilise an FPGA as a key part of the system. At the outset of deciding upon the overall system strategy the power supply design is often not of primary concern. However with some advance consideration of the power supply design and FPGA system resources, the FPGA can be used to help minimise power supply electro-magnetic emissions, thereby reducing counter measure costs, and time spent attempting to pass EMC limits. This approach requires a single FPGA I/O pin and a compatible SMPS IC with a synchronisation (SYNC) pin. Switched Mode Power Supply A “Buck” or “step-down” SMPS converts a higher voltage to a lower voltage, the ratio of the voltage at the input to that of the output termed the duty cycle, D. The aim is to regulate the output voltage at a fixed value; as the input voltage varies the duty cycle varies in proportion. The dc transfer condition (1) holds so long as the current through the inductor is continuous (continuous conduction mode).