Transformer and Inductor Design for Optimum Circuit Performance

The energy absorbed and released by transformer leakage inductances during each switching period usually ends up as loss, thus impairing switching power supply efficiency. Even when much of this energy is recycled and recovered through the use of resonant or active clamping techniques, leakage inductance remains as the main factor in cross-regulation errors among multiple outputs, a problem that cannot be eliminated or reduced with a single control loop. This topic discusses how proper thinking about the causes and effects of leakage inductance can result in optimized transformer design with minimum parasitic inductances and ac winding losses, as well as establishing the winding hierarchy to reduce cross-regulation errors. Also discussed are ways to use reluctance modeling and duality principles to develop the transformer equivalent electrical circuit model which provides provide the best visibility of the effects of parasitic inductances. Examples are presented of forward and flyback transformer applications with multiple outputs.