Magnetising Inductance of Multiple-Output Flyback DC-DC Convertor for Discontinuous-Conduction Mode

This paper presents the following: (a) detailed derivation of the expressions for the maximum value of the magnetising inductance of the ideal and lossy two-output flyback dc-dc convertor operating in discontinuous-conduction mode (DCM), (b) a method to appropriately select the duty cycle for the MOSFET based on the rated output dc voltages of the two stages, and (c) a design approach for a three-winding transformer with a gapped core used in the two-output flyback convertor. The expressions derived and the proposed design technique can be extended to flyback convertors with more than two output stages with equal or unequal load voltages. A universal power supply (ac line adapter) employing a flyback dc-dc convertor with output voltages 15 V and 32 V, supplying a rated output current of 0.563 A and 0.533 A, and operating at a switching frequency of 85 kHz is designed using the proposed methodology. Simulation and experimental results are presented to validate the theoretical predictions.