Modeling of cross-regulation in converters containing coupled inductors

A general model of the multiple-winding coupled inductor is described, in which all parameters can be directly measured. The magnetics and the resulting cross-regulation models are employed in a tutorial explanation of the mechanisms by which leakage inductances and effective turns ratios affect cross regulation and discontinuous conduction mode boundaries in a multiple-output converter with arbitrary number of outputs. Analytical results are found for the discontinuous mode boundaries and for the steady-state characteristics when one of the outputs operates in discontinuous conduction mode. Three basic approaches to coupled-inductor design are compared: near-ideal coupling, practical moderate coupling, and the zero-ripple approach. It is shown that the best cross regulation can be obtained via the zero-ripple approach with relatively loose coupling in applications where there is at least one output whose load current variations are relatively small so that all windings can always operate in continuous conduction mode. The conclusions are supported by experimental results.