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. This model is 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. Three basic approaches to coupled-inductor design are compared: near-ideal coupling, practical moderate coupling, and the zero-ripple approach. Near-ideal coupling results in good cross-regulation even when some outputs become discontinuous; however, realization of near-ideal coupling may be difficult in practice, and the resulting circulating currents can also lead to increased output voltage ripples and reduced efficiency. 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 the continuous conduction mode. The conclusions are supported by experimental results.