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A diode bridge rectifier followed by a boost-chopper circuit is a common topology of the generator-side converter for the direct-drive surface-permanent-magnet-synchronous-generator-based wind energy conversion system. Owing to its nonlinearity, it is difficult for the system to maintain good performance within normal operating range under the ordinary proportional-integral control. In this paper, a piecewise nonlinear mathematical model for the whole system, including both generator and converter, is proposed based on the commutation points of the diode bridge rectifier for more accurate controller design. The input-output feedback-linearization-based nonlinear transform for the mathematical model of the system is piecewise made. Then, a speed controller is designed according to the converted linear model, considering the integral of time multiplied by the absolute error. The proposed strategy has the advantages of relatively simple transform of state variables for linearization and developed parameter tuning method. The parameters of the linearized controller for different model intervals are the same. Finally, simulation results indicate that the proposed nonlinear controller is able to reject parameter perturbation to some extent, and experimental results are presented with a 3-kVA prototype, demonstrating that the dynamic performance of the system is improved effectively.