###### Abstract  
Switch-sharing-based multilevel inverters offer great advantages in terms of efficiency improvement and output quality enhancement for low-power photovoltaic (PV) applications. However, the capacitor voltage balancing issue may critically deteriorate the output voltages, and thus could nullify the aforementioned benefits. Hence, this paper proposes a capacitor voltage balancing solution based on a buck-boost converter with performance-based tuning controllers to address the issue. The proposed controllers are designed based on the proportional-integral (PI) configuration equipped with an online tuning mechanism. The main purpose of the online tuning mechanism is to fine tune the proportional gain according to the DC input voltage, based on the measured output current total harmonic distortion (THD) performance of the inverter, while at the same time preventing the controller from reaching a state of saturation. By using the actual measurement of current THD performance, online tuning accuracy can be improved since no ideal condition is assumed, and thus, the associated error can be minimized. Simulation and experimental results reveal that the capacitor voltages can be balanced at high modulation indexes with improved current harmonic performance able to be be acquired at the inverter's output. buck-boost converter; capacitor voltage balancing; online tuning; switch-sharing-based multilevel inverter 29 March 202125 April 202113 May 20213 May 20213  
a classical multilevel inverter typically increases as the number of levels increases. This contributes to reduction in the reliability as well as complicated control of the inverter. As a result, many new multilevel inverter topologies have been proposed [12; 13] to overcome the problems encountered in the classical multilevel inverters, and one of them is the switch-sharing-based multilevel inverter. Figure 1 presents the circuit topology of the three-phase five-level switch-sharing-based multilevel inverter [14]. The switch-sharing-based multilevel inverter is modified from the full-bridge configuration with a switch-sharing capability. The inverter offers advantages for low power solar PV applications including lower harmonic contents and enhanced output quality [15]. However, it has a shortcoming that is quite similar to that of the DCMI. The main problem is the unbalanced capacitor voltages at the DC-link, for voltage levels more than three [16]. This problem deteriorates as the number of levels increases. In a five-level switch-sharing-based multilevel inverter, four DC-link capacitors may experience unbalanced voltage across each of them when they are not charged and discharged accordingly. This voltage-drift phenomenon deteriorates the AC-side voltage waveforms and consequently results in an unsatisfactory performance. Therefore, finding a proper solution is paramount since this affects the operation and performance of the inverter. As for DCMI, there are two possible solutions that can be adopted to overcome the unbalanced voltage problem. One solution is via the modification of the pulsewidth modulation (PWM) switching pattern [17; 18; 19; 20; 21]. The switching pattern is normally selected from the redundant switching states. It is observed that many researchers prefer this method, so as to avoid extra cost since there is no additional circuit needed. However, the limitation of this method lies in the control complexity and the range of the modulation index allowed. Another solution is via the implementation of a voltage balancing circuit on the DC side of the inverter [22]. The use of the voltage balancing circuit based on a buck-boost converter is most commonly employed to balance the capacitor voltages [23; 24]. A single-pulse control, which is regarded as one of the easiest methods, has been adopted. Multi-pulse control would also be another option when single-pulse control may seem inappropriate. Another auxiliary circuit is the three-level flying-capacitor-based chopper for a three-phase DCMI [25].