Design of Power Converters under EMC Constraints

Choosing a conversion structure for power electronics remains a difficult exercise, which has to meet a wide variety of increasingly demanding criteria. Among the most common are compactness, cost, efficiency and compliance with electromagnetic compatibility (EMC) standards. EMC in power electronics is a particularly critical aspect given the switching mode of operation, and is involved in the sizing of the converter over frequencies ranging from the switching frequency to several tens of megahertz [D 3 290] . Compliance with all these criteria requires a sound knowledge of all converter components, combined with the expertise of the designer.

Once the conversion structure has been chosen, the sizing phase aims to determine the values of the components required to ensure converter operation: values of inductors and capacitors to respect the ripples of input and output quantities, but also all the technological choices linked to the components (current and voltage rating of switches, maximum RMS current, choice of magnetic material, number of turns, etc.). It is also necessary to dimension the coolers needed to evacuate semiconductor losses, as well as the filters required to comply with electromagnetic compatibility standards.

It is only at the end of these stages that the candidate structure can be compared with others in terms of the performance criteria selected (weight, cost, etc.).

It's easy to see how difficult the task is for the designer, if he doesn't call on his expertise to investigate all possible solutions within ever-shorter deadlines. What's more, expertise can be called into question with every technological leap or appearance of new constraints. The example of wide-gap semiconductors is highly instructive: the potential for much higher voltage withstand and switching frequencies could make hitherto abandoned solutions competitive, provided that all other converter components are up to standard. On the other hand, increased switching speed means that more efficient EMC filtering is needed, and it's not certain that the whole converter will end up being more compact.

This article will be structured in three parts: the first will illustrate the large number of possible converter structures based on two classes of representative applications, and analyze them from the dual perspective of loss and EMC. The second part will propose a methodology for converter pre-sizing, enabling us to identify a global optimum for...