EMC Considerations in the Design of Switch Mode Power Supplies
Almost every electronic product requires converting efficient conversion of some primary AC or DC power source to some DC voltages for powering its electronic circuits. Switch Mode Power Supplies are the current state of the art in high efficiency power supplies.
Conventional series-regulated linear power supplies maintain a constant voltage by varying their resistance to cope with input voltage changes or load current demand changes. The linear regulator can, therefore, tend to be very inefficient. The switch mode power supply, however, uses a high frequency switch (in practice a transistor) with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are a source of significant electromagnetic interference (EMI) filtered out by an LC filter.
In recent years, switch mode power supplies have become particularly popular when multiple and diverse DC voltages (e.g., 5V, 3.3V, 2.5V…) are required in any single printed circuit board with maximum power conversion efficiency. Increase in switching frequency is desired for achieving goals of size, conflicting with production of harmonics and subsequently... noise, or EMI.
For the adequate control of electromagnetic interference (EMI) emanating or entering the product via their power ports, strict international commercial and military standards have been developed. These standards require the suppression of electromagnetic emissions from circuits and systems, and their increased immunity to externally induced interference. Indeed, switch mode power supplies are major sources of EMI and if EMC
is not embedded in the design, high level emissions, and increased immunity will be the result, with possible non compliance with the very stringent commercial and military standards.
Embedding EMC considerations in the design of switch mode power supplies is a cost effective approach for the control of EMI on the power port in particular and in the equipment and system level, in general.