Prevention of Iron Core Saturation in Multi-Winding Transformers for DC-DC Converters

This paper deals with prevention of saturation in the iron core of a multi-winding transformer. It is a substantial part of dc-dc converters used in resistance spot welding systems. The discussed resistance spot welding system consists of a semiconductor input converter, a single-phase welding transformer with one primary coil and two secondary coils, and a full-wave output rectifier connected to the transformer's secondary coils. The paper shows that the interaction among magnetically nonlinear behavior of the iron core combined with unbalanced parameters of the circuits with the two transformer's secondary coils can cause iron core saturation even when elements connected to coils are passive elements. The first part of the paper focuses on analysis of saturation phenomena in multi-winding transformers. It is performed on specially designed laboratory transformer composed of the iron core in the form of two C-shaped segments and modular coils used to form the single-coil primary winding and two-coil secondary winding. Knowledge acquired on the laboratory transformer is applied to develop two different solutions for active prevention of the iron core saturation in multi-winding welding transformers. Both solutions are presented in the second part of the paper.