## 4 DC-DC Converter Topologies for LEV Charging
### Off-Board Charging
#### 4.2.2 Non-Isolated Converter  
Considering the low charging power requirement for LEVs, non-isolated converter topologies remain popular among researchers. In the case of DC charging, reported non-isolated converters are mostly based upon conventional DC-DC converter topologies such as buck-boost, Cuk, and SEPIC, as seen in the case of on-board non-isolated chargers. One significant trend observed in recent implementations of non-isolated LEV chargers is the use of solar PV power supply as one of the inputs, or as the sole input. Design of such a system is described in [67], which takes into consideration the solar irradiation in a particular location. One prominent example of implementation of such a charger is described in [68]. In this work, a SEPIC converter is described by employing a solar PV input, thus resulting in a self-reliant off-board charging system. The converter operates in continuous mode to minimize voltage and current stresses on circuit elements, while also reducing current ripples. To further reduce charging current ripples, a small inductance is placed near the battery. Furthermore, this SEPIC converter, which normally controls battery charging process, also controls the MPPT of the solar PV input, thus addressing the issue of multiple conversion stages in standalone PV-based charging systems. This system is tested under variable solar irradiation and temperature variations, and it shows satisfactory performance in terms of MPPT tracking and charging efficiency. However, there is no clarity in the charging efficiency data reported in this work. Furthermore, standalone PV systems require an auxiliary battery storage to maintain power continuity in times of low solar irradiation. This aspect is not explored in this work. Authors of [69; 70] have also presented non-isolated charger topologies integrated with standalone solar PV power supply. The work in [69] presented a comprehensive design and analysis of an EV charger with standalone PV power supply. For the charging process, a first quadrant DC-DC buck converter is used. The auxiliary battery charging/discharging is controlled by the two-quadrant bidirectional DC-DC converter. This work also considers the sizing of PV power supply and auxiliary battery energy storage, based on typical variations of solar irradiation. This system is then tested under dynamic conditions--variable solar irradiation and a disconnection of PV supply--where it shows satisfactory performance. However, the charging efficiency data are missing in this work. Furthermore, an extra DC-DC boost stage is used in this work for PV MPPT control which may have an impact on the charging efficiency. Similarly, in [70], the authors presented a standalone PV-based non-isolated EV charger system, which also employed an auxiliary battery energy storage system. The power flow between the PV supply, EV battery and the auxiliary battery is coordinated by the use of three auxiliary switches. Operation of these switches depends on solar irradiation level. A schematic of this charging configuration is shown in Figure 18. As can be seen from this figure, the standalone PV supply uses the SEPIC converter to charge the EV battery and the auxiliary battery is charged/discharged through the interleaved bidirectional converter. Furthermore, to improve efficiency, the interleaved bidirectional converter is operated in discontinuous mode. The system is then tested under dynamic conditions--namely three different cases of varying solar irradiation. In all these cases, the charging efficiency is observed to be approximately 96%, which is satisfactory. Despite these promising results, the given topology has a relatively high switch count on account of the use of auxiliary switches, which may lead to increased control complexity. Another development in non-isolated topologies comes in the form of transformerless LLC converters. In this regard, the authors of [71] presented a transformer-less multilevel LLC DC-DC converter.