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Series resonant converters based on a transistor inverter, a transformer, a series capacitor and a rectifier with buffer capacitor find widespread application today. They provide soft switching and are thus of interest when low EMI and reduced switching losses are required. Due to the high quality factor of the corresponding resonant circuit, formed by the leakage inductance of the transformer and the series capacitor, the operation point of a converter strongly depends on the ratio of the control frequency to the resonant frequency. This may cause problems for the control if the resonant frequency changes. The problem can be solved by a fast control algorithm presented in this paper. It estimates the unknown or variable resonant frequency of a series resonant converter by successive measurement of the converter current at certain time instants within a conduction period. As a result the converter frequency can continuously follow the resonant frequency of the system, which leads to a perfect matching (sinusoidal current and ZCS). The algorithm has been implemented in a DSP (TMS320LF2407) and successfully proved experimentally. Furthermore, it is shown how the estimation principle can be adapted for a low speed DSP and for a simple 8-b μC.