Multisecondary Transformer: A Modeling Technique for Simulation

Pulse step modulation (PSM)-based high-voltage power supplies have played a significant role in fusion research. In many cases, multisecondary transformers are used for generation of tens of kilovolt range outputs with fast (microsecond-order) transient response. In this scheme, large numbers of isolated voltage sources are connected in series to generate the output. Isolated voltage sources can be achieved by a large number of separate transformers or by a single unit of multisecondary transformer. Naturally, a transformer having numbers of secondary windings (40) on a single core is the preferred solution for PSM topology due to its space and cost consideration. For design and simulation analysis of such a power supply, the model of a multisecondary transformer poses special problem to any circuit analysis software. While many simulation software packages provide transformer models with limited number (3-6) of secondary windings, they still fail to predict the actual observed results from a manufactured unit. This paper discusses the difficulties in modeling, step by step with representative schemes of multisecondary transformer, and advantages/disadvantages of each scheme with simulation results. Finally, a model using flux-coupled cores as basic building blocks is proposed; this model is able to simulate actual transformers very close to its observed parameters in test and actual usage.