Simple unified analysis of single inductor DC-DC converter topologies

​​​​​​​
Kurt Beneder, Siemens
Timm Ostermann, Johannes Kepler Universität

Poster-Abstract

Discontinuous voltage nodes and branches with discontinuous currents are the primary contributors for EMC problems in DC-DC converters. To identify these nodes and branches one must analyze a DC-DC converter topology either per simulation or by circuit analysis. Traditionally this is done separately for each topology although unified analysis approaches are also well known.

Our research presents a simple unified approach for related topologies. If the analysis is completed for one topology, the results are immediately applicable to a whole family of converters.
It is well-established that a DC-DC converter like a Buck converter function as an ideal transformer with the duty cycle as the transmission ratio. Here, we extend this concept to an autotransformer equivalent circuit. From this equivalent circuit, one can derive the voltage transfer ratio vs duty cycle for the Buck, the Boost and Buck-Boost converters trough a unified analysis. The wave forms of discontinuous voltage nodes and branches with discontinuous currents are also similar up to the sign, due to a different direction of power flow. Consequently, also the associated part stress due to these waveforms is the same. We validate our unified analysis results by comparing them with simulations.

Further insight will be got, if one analyses not only switch 1 and switch 2 in opposite state, but also the states where both switches are on (short circuit), or both switches are off (high impedance) state. These conditions reveal further EMC-relevant behaviors which has to be considered.