Planar transformers with no common mode noise generation for flyback and forward converters

Flyback and Forward converters are two commonly used topologies for isolated low power applications. In order to enhance the performance of these converters, planar transformers (PT) can be used that feature lower height, considerably lower leakage inductance, excellent thermal characteristics, repeatability, and manufacturing simplicity. Selecting a proper winding arrangement for PT is a significant challenge, in particular given the large capacitances involved in flat structures. While interleaved structures significantly reduce the AC resistance and leakage inductance of PTs, they also lead to very large inter-winding capacitance, which produces significant levels of undesired common-mode (CM) noise that causes EMI problems. This paper focuses on the creation of interleaved winding structures that minimize CM noise and EMI commonly present in PT and at the same time achieves low AC resistance and leakage inductance. The experimental results show that the proposed PTs not only have very low AC resistance and leakage inductance, but also generate extremely low levels of CM noise. Considering that the proposed PT has inter-winding capacitance equal to 700pF, it is very interesting to see that it generates significantly less CM noise than does a traditional wire-wound transformer that has only 10pF parasitic capacitance. Therefore, the proposed method gives designers the opportunity to minimize AC resistance and leakage inductance by using highly inter-leaved structures, without having to worry about CM noise caused by large inter-winding capacitance.