EMI reduction using symmetrical switching and capacitor control

An EMI reduction technique using two MOSFETs instead of a single MOSFET in a step-down converter is investigated in this article. A circuit that implements this technique together with external capacitor control was designed and measurement results were compared against simulations. The switching element in the proposed circuit is an IRF7309 that consists of a p-channel and an n-channel MOSFET in the same package. The entire circuit also consists of an input circuit for the control pulses and a controller circuit responsible for optimizing the turn-on and turn-off of the p-channel MOSFET and n-channel MOSFET. The effect of difference in the threshold voltage between the two MOSFETs is controlled by external capacitances in a configuration referred to as capacitor control. The analyzes of simulations and measurement results show that the symmetrical switching (or double MOSFET switching) technique can successfully be applied to reduce the RF emission in the low frequency and medium frequency range when compared to the single MOSFET switching.