A fast open-circuit fault diagnosis method based on residual analysis is proposed for a dual active bridge (DAB) DC-DC converter in this paper. The proposed method uses the magnetic component voltage and the primary-side current for fault diagnosis. A state estimator of the primary voltage and current is established by the switching function, and then the residual characteristics are extracted by comparing the estimated values with the measured values. Furthermore, after the specific switch signals are injected, the change trend of the current residual is extracted to locate the faulty switch. The magnetic component voltage can be obtained by the auxiliary winding in the magnetic core, and the measurement of current needs to add a sensor. The proposed method can accomplish the accurate location of the faulty switch within 3 switching periods by increasing a small number of hardware devices. The hardware-in-the-loop (HIL) test results have verified the validity and feasibility of the proposed diagnostic method.