Zeroing transformer´s DC current in resonant converters with no series capacitors

DC current unbalance in the windings of a transformer may initiate a runaway process and ultimately drive the transformer into saturation. Such a situation could arise in a transformer coupled resonant converter that does not include a series capacitor. This issue was explored in this study theoretically, by simulation and experimentally. A method is proposed for automatically adjusting the transformer´s DC current to zero by correcting the asymmetry of the drive. This balancing control is facilitated by a signal obtained from a DC/AC current sensor which senses the transformer´s primary and secondary currents such that they cancel each other for the no DC case. The proposed method was tested on a self oscillating, parallel loaded DC-DC resonant converter of 12V input voltage 500V output voltage and 700W power level. The proposed approach allows the design of resonant converters without DC decoupling capacitors which could lead to a significant cost and size reduction.