###### Abstract
This circuit is well-recognized in reducing voltage stress across chopper switches and in improving reliability. Despite the advantages offered, the control philosophy of the chopper could become complicated and tedious owing to the high number of switches required. Meanwhile, [26] proposes a resonant switched-capacitor  
Figure 1: Five-level switch-sharing-based multilevel inverter. converter (RSCC) for a new voltage balancing circuit with new phase shift control, in order to enhance the performance of voltage balancing. As compared to the buck-boost converter, RSCC can reduce the inductor to one tenth in volume. However, the increased number of switches used can be an issue. This would consequently turn the control system to a more complex capacitor voltage balancing scheme. In [27], a parallel switch-based chopper is proposed with the application of hysteresis control scheme. However, the main problem is the fact that additional components are needed, which would result in an increase in cost and circuit complexity. Various control approaches have been proposed for the capacitor voltage balancing purpose. Among these approaches is the application of a fuzzy controller, model predictive controller and proportional-resonant controller. Fuzzy controller application with a self-tuning capability for capacitor voltage balancing has been revealed to be used with a PI controller to control DC bus voltage and capacitor unbalanced voltage in the three-level boost converter [28]. It is reported that the output capacitor balance voltage is guaranteed with a low quadratic error. In an input-series output-series (ISOS) modular DC-DC converter, a modified model predictive control (MPC) known as the duty cycle-based MPC (DC-MPC) has been proposed to address the capacitor voltage imbalance issue [29]. The difference between the proposed DC-MPC and the conventional MPC is the fact that the proposed DC-MPC optimizes duty cycles with a fixed switching frequency instead of employing optimized switching states with variable switching frequency. By predicting the effects on input voltage sharing (IVS) and output voltage sharing (OVS) of the constituent modules, the optimized duty cycles can be determined. With the optimized duty cycles, the proposed DC-MPC can quickly attempt to achieve IVS and OVS when imbalances exist among the input capacitor and output capacitor voltages of the ISOS converter. In another paper [30], the use of the quasi-proportional-resonant controller has been presented. The controller has been applied in an inter-arm balancing method that would guarantee an even distribution of capacitor voltages without affecting the input/output power quality. As previously mentioned, the switch-sharing-based multilevel inverter would suffer from the capacitor voltage imbalance problem when no proper voltage balancing solution is put in place. Considering the fact that no such solutions have been properly presented for this type of multilevel inverter, to the best knowledge of the authors, it is the purpose of this paper to present a study on the capacitor voltage balancing solution based on an online tuning-controlled buck-boost converter for a five-level configuration of this type of multilevel inverter, taking into account the inverter's output performance. The contributions of this paper can be summarized as follows:  
* We propose a capacitor voltage balancing solution based on a buck-boost converter with performance-based-tuning controllers for a five-level switch-sharing-based inverter. * The proposed controllers are based on the PI configuration that is integrated with an online tuning module to fine tune the proportional gain according to the DC input voltage. * The online tuning mechanism uses the actual performance of the inverter based on the measured value of the output current THD to execute the tuning process. * 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 tuning error can be minimized.