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A novel fast-diagnostic method for open-switch faults in inverters without sensors is proposed to improve the reliability of power electronic system in this paper. The presented method is achieved by analysis of switching function model of the inverter under both healthy and faulty conditions. Due to the different voltages endured by each power switch under healthy condition from open-switch faults in some cases, open-circuit faults can be detected by sensing the collector-emitter voltages of the lower power switches in each leg. The diagnostic scheme employs the simple hardware circuit to obtain indirectly the voltages of lower power switches and to get rid of high cost and complexity as a result of sensors. Also, this method minimizes the detection time and is available for the inverter faults in the systems with open-loop or closed-loop current control strategies. To ensure the effectiveness and reliability of the proposed diagnostic method, the rising-edge delays for switching signals are used to avoid misdiagnostic results because of the ON-OFF processes of power switches, and the delay durations are determined considering various factors. Simulation and experimental results validate the proposed scheme for detecting switch and leg open-circuit faults.