Radiated emissions of multilayered coreless printed circuit board step-down power transformers in switch mode power supplies

This paper addresses radiated EMI issues in Coreless PCB step-down transformers used for power transfer applications. In case of SMPS circuits, since the waveforms are not sinusoidal in nature, most of the radiated emissions are not only from fundamental frequency component but also from harmonic contents. According to antenna theory, radiated EMI for three different power transformers of different radii were estimated for fundamental frequency of 2MHz to 300MHz. The computations were made for both sinusoidal and square wave excitations and at a load power of 20W. The calculated radiated power obtained for simulated waveforms and for practical measured current waveforms are in good agreement with each other up to certain bandwidth. These computational results confirm that radiated power can be reduced in three layered 2:1 transformer compared to two layered 2:1 transformer. Also the radiated emissions are negligible in case of sinusoidal excitations compared to square wave excitations.