Estimating the radiated emissions from cables attached to a switching power supply in a MIL-STD 461 test

Common-mode currents on cables attached to a switching power supply generate radiated emissions which may interfere with near-by components. A relatively simple equivalent circuit model is developed to predict the radiated emissions measured in a MIL-STD-461 or RTCA/DO-160 test. The intent of this model is to provide an estimate of emissions that allows the designer to better understand the mechanisms behind emissions issues and to rapidly predict the impact of changes to the system, like adding filtering, changing components, or modifying cable connections. The model represents cables connected to the power supply as transmission lines, represents coupling from the cables to the antenna using lumped capacitors, and represents the balun in the antenna using a transformer. The simulated results match the measured results well. This simple SPICE model allows EMI issues to be investigated early in the design of switched mode power supplies.