**1. Introduction**

Silicon-based devices have primarily been used and are still dominant in developing power inverters [1,2]. However, ultra-wide bandgap (UWBG) semiconductors have gained a significant amount of attention in recent years [3]. As a result, Ga2O<sup>3</sup> being a strong candidate for UWBG devices have the potential to be profoundly applied in the various applications in the field of power electronics ranging from Photovoltaic (PV) inverters and UPS systems to inverters for traction and space applications, among others [4–6]. In these power inverters, UWBG semiconductors can contribute to high efficiency, inverter size

**Citation:** Meraj, S.T.; Yahaya, N.Z.; Hossain Lipu, M.S.; Islam, J.; Haw, L.K.; Hasan, K.; Miah, M.S.; Ansari, S.; Hussain, A. A Hybrid Active Neutral Point Clamped Inverter Utilizing Si and Ga2O<sup>3</sup> Semiconductors: Modelling and Performance Analysis. *Micromachines* **2021**, *12*, 1466. https:// doi.org/10.3390/mi12121466

Academic Editor: Francisco J. Perez-Pinal

Received: 8 November 2021 Accepted: 25 November 2021 Published: 27 November 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

reduction, and high-temperature environment operation, which are unlikely to be achieved otherwise [7]. These features of Ga2O<sup>3</sup> devices are due to the specific properties of the UWBG material. Gallium trioxide (Ga2O3) devices are capable of achieving these because unlike their conventional counterpart, the blocking voltage that is rated for these devices is nearly a hundred times higher for the same width of the drift region [8]. In addition, the high thermal conductivity, along with the fast-switching speed are two main factors that are offered by Ga2O<sup>3</sup> devices to gain this advantage [3]. Though the UWBG devices can be applied in medium power applications, the ongoing research has suggested that these devices have great potential to be applied in high power applications with modular multilevel inverters (MLIs) [9].

For medium-voltage-range Photovoltaics (PV), several DC-link voltages are proposed in recent years [10,11] considering different interests. However, when efficiency and reliability are the main concern, the 1.5kV DC-link-based PV generation system has gained significant attention along with systems [12,13]. In addition to that, for higher efficiency in PV systems, transformer-less configurations have shown better performance compared to other transformer-based configurations [6]. Though many inverter topologies had been proposed previously considering high voltage applications, a three-level neutral point clamped (NPC) inverter is one of the most optimal inverter choices for high voltage applications [14]. Since the clamped common-mode voltage (CMV) is enabled in this type of topology, it minimizes leakage-current-related issues [15]. That is why, for a transformerless system, it is a better choice than other systems which are incorporated by leakage current. Despite this, due to the unequal loss distribution among the switches, the NPC inverter has issues related to neutral-point voltage imbalance, as well as a shoot-through fault in the switching devices [16].

Various types of control strategies along with modified inverter topologies have recently been proposed to overcome the inconveniences in the NPC inverter. Since NPC inverters are prone to the shoot-through problem, a split inductor configuration can be used to solve this issue [17]. It should also be noted that in addition to successfully protecting the shoot-through fault, reduced leakage current and the eradication of CMV transitions that are high frequency in nature, can be achieved through this inverter. However, even though this configuration removes most of the inconveniences, it operates in a unity power factor region. Therefore, this configuration is hardly suitable for high voltage applications that are designed specifically for supplying reactive power to the grid. In addition to that, the previously mentioned non-uniform loss distribution problem in the switches of the NPC inverters still exists in these topologies.

In [18], the non-uniform current distribution was addressed, and it proposed an active neutral point clamped (ANPC) inverter. As per the switching states of ANPC inverters, additional redundant zero states can be gained in the ANPC inverter topology. Therefore, unequal switching loss distribution can be mitigated if different zero states can be appropriately exploited in the switching states of the ANPC inverter. Considering these additional states, some notable PWM-based control techniques are employed previously in the ANPC inverter topology [19,20]. The use of current and voltage sensors for the selection of the redundant zero states that are available in the ANPC topology focused on power factors. So, how the states of the inverters will be chosen is largely related to the feedback signals of those current and voltage sensors. This solution is optimized to achieve high efficiency in ANPC, which provides the states for the hybrid Si/Ga2O3-devices-based ANPC topology. The high efficiency and the low cost relative to all Ga2O<sup>3</sup> inverters can be obtained according to researchers [21]. Recent literature has also shown promising results using the aforementioned approach where the switching devices of the ANPC were mostly built using wide bandgap (WBG) materials or silicon carbide (SiC) [22]. However, one fact about their research is that they only considered low voltage applications, and the entire ANPC inverter was built using devices from the same bandgap materials. Furthermore, one fact about their research is that they considered the converters suitable for only low-power applications having low voltages. Because in the case of MV applications, unlike silicon

devices, the body diodes of Ga2O<sup>3</sup> MOSFETs are the cause of further switching losses along with overshoots that are significant in switching transient, the design criteria would be different [23]. In addition to that, as the dead-band time is declined in high-frequency devices, the severity of the shoot-through fault rises remarkably. It should also be noted that as high-frequency switching devices are employed at the output side, an increase has been seen in the voltage amplitude in the electromagnetic interference (EMI) frequency range, which ultimately contributes to the increased size and complexity in EMI filters [24].

Considering the issues stated above, this study proposes a hybrid ANPC inverter that utilizes both conventional Si and Ga2O<sup>3</sup> devices. As a result of this hybridization, the switching losses of the inverter are reduced significantly. The hybridization also made the implementation of a split-output structure achievable. Thus, the proposed circuit can also handle the switching transient overshoots. In this structure, since the UWBG switch is decoupled externally by the parallel diode, both overshoot issues in the switching transient, as well as switching losses, are declined significantly. These reduced overshoots ultimately also lead to decreased voltage and current stresses on the UWBG devices. As this converter topology is capable of supplying reactive power to loads with a wide range of power factors, it can be used for grid-tied PV systems. The key contributions of the paper can be listed as follows:


The rest of the paper is arranged as follows. The modeling of the proposed inverter topology is outlined in Section 2. Following this section, a characteristic and comparative analysis of the proposed inverter and conventional ANPC is presented in Section 3, including an analysis on fault currents, core losses, switching losses, efficiencies, EMI, and power factors. Section 4 discusses the summary and conclusion of the manuscript.
