Analysis of PCB parasitic influences in uninterruptible power supplies

The paper presents a method of analyzing the influence of the parasitic elements of a PCB on a push-pull DC-DC converter. The push-pull converter is a critical stage in an uninterruptible power supply (UPS). It must handle all the available back-up power in case of a line voltage drop or failure. For the correct push-pull converter functionality, it is essential to assure perfect circuit symmetry. Also, due to the higher power levels involved in this case study, the parasitic elements must be kept at the lowest possible values. Due to the lack of space on the PCB, a perfect symmetrical geometry design was not possible. Our goal was to analyze and simulate the influences of the PCB layout on the converter behaviour. To compute the resistances of the PCB copper shapes and traces, we used a method based on conformal mappings. For the parasitics estimation of the PCB traces and copper zones, we used an electromagnetic full wave planar simulator. Based on the lumped model of the passive interconnection structure, a SPICE transient analysis was realized. The analysis results permit us to observe the voltage and current wave shapes at the switching elements and to estimate the circuit functionality. This method of analysis is not time effective but permits better prediction of the influences of PCB parasitics on real circuit behavior in comparison to other dedicated signal integrity software we have tried