MOSFET Modeling Aimed at Minimizing EMI in Switched DC/DC Converters Using Active Gate Control

Licentiatavhandling, 2009

This thesis deals with electromagnetic interference that can arise from switched DC/DC-converters intended for low-power applications, e.g. within the telecom or automotive industry. It analyzes measures and methods that can be applied when a reduction of EMI directly at the source without using any additional means such as shielding and filtering is desired. By investigating the physical properties of the two most important ingoing components, the diode and the MOSFET, an improved MOSFET model and a new gate voltage control method is proposed. This method is referred to as active gate control with a operating principle where a controller circuit shapes the desired output to a sinusoidal trajectory during the entire switching event. By doing this, it is shown that the harmonic content in the output signal can be reduced. The proposed MOSFET model is used as a base for extracting suitable controller parameters with the help of a linearized state-space system. Two different outputs were selected and investigated, either the drain-source voltage or the drain current. The general conclusion from simulations and measurements are that active gate control where the drain current is selected as the controlled quantity has good potential for reducing the harmonics and the emitted electromagnetic disturbance in a switched DC/DC converter even though it sets high demands on the controller circuit. The analysis shows that a controller with static parameters derived on basis of the proposed MOSFET model is not sufficient due to the complexity of the system which includes many nonlinearities and varying parameters. In order to obtain better transitions that are valid over a wider range of operating points, adaptive control needs to be implemented. Simulations shows that by carefully selecting the controller parameters based on the proposed MOSFET model and adapting the parameters to the current operating point an improvement in performance and robustness can be achieved.