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In recent years, the multilevel inverter synthesising the output voltage with a step pulse, has been widely used in high power and high voltage applications. To acquire the conducting angles of switches, the conventional method needs to solve simultaneous equations, corresponding to the fundamental and harmonic components, by the use of a Fourier series. Since they are calculated by an iterative method, the process is requires with a large amount of calculation and time. Moreover, they are calculated by means of an off-line operation. To overcome these drawbacks, this paper proposes a simple method of obtaining the conducting angles. This method reduces the amount of calculation needed to obtain the conducting angles and determines them through an on-line operation. It makes use of the voltage-second areas of the divided reference voltage according to the output voltage levels of the inverter. It does not solve the set of nonlinear transcendental equations, but calculates several trigonometric functions. The validity of the proposed method is demonstrated according to each modulation index through simulation and experiment results. The results indicate that the harmonic components obtained by the proposed method are similar to those from the conventional one in the high modulation index. However, in the low modulation index, the proposed method includes more harmonics than the conventional one.