To combine the advantages of both model-driven and data-driven methods, this article proposes a model-data-hybrid-driven method to diagnose open-switch faults in power converters. This idea is based on the explicit analytical model of converters and the learning capability of the artificial neural network (ANN). The process of the method is divided into two parts: offline model analysis and learning, and online fault diagnosis. For both parts, model-driven and data-driven are combined. With the model information and data-based learning capability, a fast diagnosis for various operating conditions can be achieved without a high computation burden, tricky threshold selection, and complex rulemaking. This can greatly contribute to the practical application. The open-switch fault diagnosis in a two-level three-phase converter is studied for the method validation. For this converter, an ANN is trained with two input elements, seven output elements, and two neurons in the hidden layer. Experimental results are given to demonstrate good performance.