## 4 DC-DC Converter Topologies for LEV Charging
### On-Board Charging
#### 4.1.2 Non-Isolated Converters  
Owing to the comparatively low charging power required for LEV batteries, researchers have found it feasible to implement non-isolated converter-based charging topologies. One of the key advantages of non-isolated converter-based chargers is that due to the absence of isolation transformer, there is a big reduction in charger module volume. Furthermore, there is an improvement in efficiency and power density [49]. However, a general drawback of non-isolated converters is their low voltage gain range. This issue may be addressed by using switched capacitor or coupled inductor topologies [48; 50; 51]. Another drawback of non-isolated converters is the lack of galvanic isolation between the battery circuit and supply, which is disadvantageous from a safety standpoint. In this regard, the authors of [25] have comprehensively reviewed on-board unidirectional chargers, albeit for both short- and long-distance EVs. The authors segregated these chargers into fundamental converter topologies (buck, boost, buck-boost, Cuk, Zeta, SEPIC [52], Luo) and modified converter topologies, in which the aspects of fundamental converters are tweaked to enhance one or more features. Modification techniques for fundamental converters often include interleaving, bridgeless topologies, or both. The consequent effects of these modifications may include, but may not be limited to, reduced ripples in input and output current and improved efficiency. In non-isolated converters for LEV charging, recent publications have utilized switched inductors to enhance the voltage gain. For example, [53] describes one of the more recent implementations of non-isolated bridgeless LEV chargers. In this work, the authors have presented a single stage bridgeless on-board SEPIC converter-based charger, which uses a switched inductor structure. This charger configuration is shown in Figure 13. As evidenced in the circuit diagram, the identical switched inductors are charged and discharged in series and parallel, respectively--thus helping increase the overall gain of the charger. This converter achieves compact design and reduced component count, owing to absence of transformers. These switches also operate in discontinuous mode, which reduces the switched inductor size. Since a SEPIC topology is used in this work, there is an added advantage of simplified sensing of output voltage since the SEPIC converter delivers a non-inverted output voltage. The authors of [54] present another charger based on bridgeless non-isolated DC-DC converter with switched inductors. In this work, the authors present a bridgeless Cuk DC-DC converter-based single stage charger, and as in [46], utilize a switch inductor configuration. The only difference is that the switched inductor and diode's placements are interchanged (much like the fundamental SEPIC and Cuk converter). The converter's switched inductors are also charged and discharged in series and parallel, respectively. Figure 13: Bridgeless switched inductor SEPIC converter-based topology [53]. Discontinuous conduction is observed in switches, thus reducing the output inductor size. The component count is also identical to that of [53]. The proposed converters demonstrated satisfactory conversion efficiencies--92.5% for [54] and 93.3% for [53]. This is partly owed to the fact that these two topologies are single stage, thus greatly reducing conduction losses. Furthermore, due to the presence of switched inductors, the single-stage converter helps achieve wider gain range. Additionally, these implementations exhibit minimal impact on the supply current THD, as per IEEE-519 and IEC 61000-3-2 standards. However, there might still be sufficient conduction losses due to a maximum of three diodes providing a conducting path in each positive or negative cycle. Furthermore, due to the use of switched inductors in this circuit, there is also a potential for voltage spikes across switches [55]. This may necessitate the use of voltage clamp circuits alongside the switches. [PERSON] et al.