Modelling interference properties of SMPS DC power distribution buses

The potential for a switched-mode power supply (SMPS) DC distribution bus to create electromagnetic interference (EMI) is examined using the SPICE circuit analysis program. Attaching a SMPS to a long length of transmission line essentially places a time changing load across that line. This generates reflections on the line that result in short periods over which the line voltage can change by a significant percentage of the nominal voltage (as much as 100% of the nominal DC bus voltage). Although these spikes do not substantially change the DC voltage of the bus, their rapid rise/fall times can generate high-frequency signals on the bus (as on long antenna) that can radiate and cause interference with other electronic subsystems. The frequency content of these voltage spikes can extend to frequencies well above the basic switch rate of the individual SMPS units that are connected to the bus. The potential EMI of these spikes is strongly related to the nominal DC operating voltage of the DC bus. The higher the bus voltage, the larger the amplitudes of these spikes