In this article, the method of ‘extended linearization’ is used for designing stabilizing non-linear proportional-integral (P-I) controllers which regulate to a constant value either the average output voltage or the average input current of PWM controlled DC-to-DC power converters such as the boost, the buck-boost and the Cuk converters, The design is carried out on the basis of the Ziegler-Nichols P-I controller design method applied to a family of linearized transfer function models of converters parametrized by constant operating equilibrium points of the average PWM controlled circuit.