Reduction of EMI in Switched Mode Converters by Shaped Pulse Transitions

An idea of reducing the amplitude of higher order harmonic frequencies from PWM switching pattern is presented and analyzed in this paper. For experimental studies a flexible curve shaping circuit was developed and implemented for achieving and controlling smooth transitions in a DC/DC PWM controlled converter for a 12 V system. A comparison between trapezoidal and sinusoidal transitions is made and it is found that a sinusoidal transition gives better EMI performance without increasing the switching losses. In order to further reduce the EMI, a strategy of controlling the output voltage transitions in a MOSFET by ”pre-charging” its gate is derived and investigated. The developed practical circuit is tested with sinusoidal reference for a purely resistive load and an R-L load. Frequency responses obtained from simulations and measurements are compared and discussed.