Minimising conducted common mode EMI by charge balancing in a nonisolated DC-DC converter

Common mode current is a major concern in switch mode power supplies. As the magnitude of current increases so too does the size of supply side EMI filtering elements. An antiphase winding implementing a charge balancing technique can produce good results, but requires the use of external capacitors and a well defined ground. A new implementation of the charge balancing technique is presented here that does not require any external components and makes use of the inherent parasitic capacitance of the magnetic component. The technique is particularly successful with a planar magnetic component. A description of how charge balancing is achieved is presented as well as limiting conditions. Experimental results are shown to verify the theory.