Optimized coupling factor of multi-winding coupled inductor for minimum inductor current ripple in rapid traction battery charger system

This paper investigates the design of 3-winding coupled inductor for minimum inductor current ripple in rapid traction battery charger systems. Based on the general circuit model of 3-winding coupled inductor together with the operating principles of dc-dc converter, the relationship between the ripple size of inductor current and the coupling factor for a 3-winding coupled inductor is derived under the different duty ratio. The optimal coupling factor which corresponds to a minimum inductor ripple current becomes -0.5, i.e. a complete inverse coupling without leakage inductance, as the steady-state duty ratio operating point approaches 1/3 or 2/3. In an opposite manner, the optimal coupling factor value of zero, i.e. zero mutual inductance, is required when the steady-state duty ratio operating point approaches either zero or one. Coupled inductors having optimal coupling factor can minimize the ripple current of inductor and battery ripple current resulting in a reliable and efficient operation of battery chargers.