I. Introduction

 Multilevel inverters represent a novel type of inverter technology. Conventional inverters employ a single switching device per bridge arm. In contrast, multilevel inverters incorporate multiple series-connected switching devices within each bridge arm, enabling precise control of the output voltage. Differentiating the sinusoidal waveform produced by the inverter yields a waveform that approaches a perfect sine wave as the number of differentiations increases. Common configurations include three-, five-, and seven-level inverters. Their power switching elements operate at lower frequencies, reducing switching losses, minimizing electromagnetic interference, and enhancing inverter efficiency. The drawback is the requirement for a greater number of power switching elements, along with higher technical demands for drive modulation and testing/validation.

II. Working Principle of Multilevel Inverters

Bridge circuits are commonly found in standard two-level inverter configurations. Composed of upper and lower arms, they are categorized as single-phase or three-phase depending on application scenarios. MOSFETs Q1 and Q2 are positioned between the voltage source and ground. By controlling the conduction of Q1 and Q2, the desired voltage is output from the midpoint. (See Figure 1)

The operating waveform of a two-level inverter is shown in the figure. The output voltage has two levels: when Q1 is on and Q2 is off, the voltage is U (voltage source voltage); when Q1 is off and Q2 is on, the voltage is 0 (ground voltage). (See Figure 2)

A two-level inverter has only one switching device in each bridge arm, whereas a multilevel inverter has multiple switching devices connected in series within each bridge arm. (See Figure 3)

The operating cycles are as follows: Q1 and Q2 conduct, Q2 and Q3 conduct, Q3 and Q4 conduct. The output voltage has three levels: U when Q1 and Q2 conduct, U/2 when Q2 and Q3 conduct, and 0 when Q3 and Q4 conduct. (See Figure 4)

If both inverters above have a voltage source of U, the theoretical output voltage amplitude of the two-level inverter is U, since the output voltage is either U or 0, and the voltage applied to each device is also U. In contrast, the output voltage amplitude of a three-level inverter is U/2. Since the output voltage levels are U, U/2, and 0, the voltage applied to each device is also U/2. Therefore, for a three-level inverter, the voltage applied to each switching device is half that of a two-level inverter, allowing the use of devices with half the rated voltage.

III. Selection Recommendations

Weibei Semiconductor's low-voltage MOSFET series covers a broader range of application scenarios. Featuring extremely low on-resistance, low power dissipation, high avalanche withstand capability, and high efficiency, these products deliver stable performance and reliability, meeting the demands of harsh operating environments. With diverse packaging options, internal resistance values, and voltage ratings, the comprehensive product parameter series fulfills the requirements of multilevel inverters operating at different voltages and power levels.