CLL resonant converters with output short-circuit protection

The use of harmonic excitation of resonant converters as a means of limiting excessive electrical stresses on power switches, and tank components, in the event of over-current or output short-circuit fault conditions is investigated. It is demonstrated that the excitation of the primary tank resonance, by odd harmonics of the square-wave input voltage, facilitates reduced power dissipation for systems that are subject to long periods of standby operation. Due to the inherent limitations of traditional fundamental mode approximation techniques to predict sub-harmonic resonant peaks, an alternative, steady-state, cyclic-averaging methodology, is proposed, that is shown to provide accurate and rapid analysis of the resonances when the converter is operated under short-circuit conditions, thereby allowing the effects of high electrical components stresses to be mitigated, and eliminates the necessity for time-consuming integration-based simulations. Measurements from a prototype converter, along with simulation results from a derived state-variable model and a component-based simulation package, are used to demonstrate the accuracy of the proposed cyclic predictions, and the reduction in losses imparted by the use of harmonic excitation when compared to other previously published techniques for over-current/short-circuit protection.