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This paper presents a fixed frequency operating sliding mode (SM) current control method with fast response and improved stability. Different from the conventional SM control with variable switching frequency, the fixed-frequency SM controller is proposed to control the modulation index and shoot-through duty ratio of the voltage-fed quasi-Z-source inverter (qZSI), which will not increase the passive components and filter design difficulty. A large-signal dynamic model of the system has been established, which can be used for the system stability control in a wide operating range. By using linear approximation, the system small-signal model is also obtained to analyze the control system stability and transient response. Compared with the conventional current mode controller, the proposed SM controller can achieve faster response, lower current ripple and better stability for qZSI when the supply and load variation is large. Experimental results are presented to demonstrate the validity of the theoretical design and the effectiveness of the proposed controller.