Design and control of an active reset circuit for pulse transformers

In pulse modulators applying pulse transformers reset circuits are used to achieve optimal utilization of the core material, which results in lower costs, downsized pulse transformer/system volume and therefore in an improved pulse behavior due to the smaller parasitics. Because of its simplicity the most common method to reset the core is a DC reset circuit, where a DC current is used to premagnetize the core. However, the DC reset circuit - even with an optimal design-leads to significant losses in the freewheeling path. By applying an active reset method the losses due to the passive reset circuit can be reduced significantly. So far, only the theoretical behavior of the active reset circuit has been examined. Therefore, the detailed design and measurement results are presented in this paper. Furthermore, a new control method for achieving symmetrical flux swings in the core is presented.