Choosing a nonlinear controller for a series resonant DC-DC converter

The control goal in the case of resonant converters is to regulate the output voltage in the presence of large load and supply perturbations by varying the switching frequency. The performance of the controller in terms of reduced oscillations and lowest possible response time is a matter of great concern especially when it is related to high-end medical equipments. The fast dynamics of the high frequency resonant tank circuit makes compensator design difficult especially when there is a dynamic change in the input or output loading conditions. This leads to large oscillations in the tank current and the output capacitor voltage, which renders the usual control techniques inefficient. This can be understood from the fact that the resonant converters are linear piecewise systems whose global behavior is highly non linear. This nonlinear behavior creates difficulties when computing control laws and for practical implementation. The development of nonlinear controllers for resonant converters has been found to give better performance compared to fixed linear compensators especially during transient conditions. This paper attempts to make a comprehensive review of the different nonlinear controllers put-forth in the literature for resonant type DC-DC converters. This analysis presented in the paper, which based on literature review would then be used to make an initial choice of a suitable controller for the control of a series resonant DC-DC converter.