An efficient routine for the calculation of internally controlled switching instants is presented in this paper. It has been developed for numerical time-domain analysis of switched networks, where internally controlled switching instants must be calculated as zeroes of strongly non-monotonic non-linear functions, corresponding to a current or voltage within a given time interval. The routine is based on the Chebyshev series expansion to integrate the state-space equations and on an Interval Analysis-method for the root-finding of non-monotonic polynomials. Adoption of a Chebyshev expansion minimizes the order of the polynomial approximation of the switching functions, and together with a number of basic properties of the expansion results in an efficient integration method.