Based on the linear-time-periodic (LTP) modeling technique, this paper presents an approach for single-input and single-output (SISO) impedance modelling of a single-phase voltage-source-converter (VSC). The derived SISO model is utilized for analysis and comparison of the stability properties when the average or the peak value of the dc voltage is controlled. The validity of the derived SISO impedances for these two control methods is verified by the experimental frequency scanning. Since the peak dc voltage control is intended for single-phase VSCs with reduced size of the dc capacitor, it is necessary to evaluate its performance with respect to grid interactive stability. By evaluation under various system conditions and different control parameters, it is shown that the peak dc voltage control exhibits a lower stability margin than average dc voltage control. Therefore, carefully attention should be directed towards the tuning of the control system when a single-phase VSC with peak dc voltage control is connected to a weak ac grid.