Problems of paralleling windings for planar transformers and solutions

In today´s power converters, paralleling windings in planar transformers is common, especially for low voltage and high current applications. Although this practice is usually believed to be an effective way to increase the current handling capacity of the windings, the effect might not be so significant due to the unequal current distribution within parallel layers if the layers are not properly arranged for high frequencies. In this work, factors affecting the current distribution for parallel layers are identified. General rules for reducing the unbalance of current distribution among parallel layers are presented, which is very useful for practical applications.