## 4 DC-DC Converter Topologies for LEV Charging
### On-Board Charging
#### 4.1.1 Isolated DC-DC Converter Topologies  
In LEV chargers, isolation can be achieved either by use of a transformer at the supply end or within the converter itself. The supply side transformer option is not feasible, as it requires the installation of a line frequency transformer, which has a bulky design. Hence, in subsequent topologies, researchers have employed high frequency transformers in charging converter design owing to their compact size. The provision of isolation in converters is advantageous in the following ways:  
* There is the obvious advantage of galvanic isolation between the battery circuit and supply, thus ensuring user safety. * The turns ratio of the high frequency transformer may be leveraged to achieve a wider voltage gain range, thus making the converter suitable for usage with a wider load range. The conventional on-board LEV charger topology with front-end diode bridge rectifier (DBR) remains popular among researchers. For example, [PERSON] et al. [35] presented a topology which used an interleaved canonical switching cell (CSC) at the front-end in conjunction with diode bridge rectifier for AC-DC conversion and power factor correction. This is further cascaded with an isolated Cuk converter at the back-end. This configuration exhibits simple control and is capable of charging batteries with voltages ranging from 24 V to 72 V. The circuit diagram for this circuit is shown in Figure 5. The CSC-based PFC ensures high power factor. The CSC is interleaved to reduce the size of filters and current ripples, and operates in discontinuous mode, which reduces the volume of magnetic components in the circuit, thus making it more compact. The back end Cuk converter operates in continuous mode, which reduces charging current ripples. The Cuk converter also allowsfor wide voltage gain variation and demonstrates robust performance in the event of supply voltage disturbances and varying battery voltages. However, the efficiency data in this work are missing. In yet another example, [36] presented an LEV charger which used a simple diode bridge rectifier in the front-end and an isolated SEPIC converter in the back-end. The charger schematic for this design is shown in Figure 6. The SEPIC converter has multiple benefits over the previously described Cuk converter, such as less susceptibility to EMI and inherent current limiting capability at the time of overload/startup. To reduce the size of magnetic components, this charger operates in discontinuous mode. Furthermore, the converter demonstrated satisfactory dynamic performance and posed minimum impact on supply power quality even during varying supply voltages, in compliance with guidelines set in the IEC 61000-3-2 standard. A charging current of approximately 20 A is observed and converter efficiency is approximately 80%. Furthermore, the input inductor of the SEPIC stage aided in power factor correction. The authors of [37] proposed a two-stage converter which uses a combination of Zeta-SEPIC converter as the PFC stage, followed by a flyback converter stage, which controls the battery charging process. Furthermore, the Zeta and SEPIC converter combination controls the voltage of input DC link voltage of the flyback converter. The circuit diagram of this converter is shown in Figure 7. The Zeta and SEPIC converters operate independently in the positive and negative half cycle of the input AC voltage, and both operate in discontinuous conduction mode. The Zeta and SEPIC converters are chosen for this topology due to their comparatively better ripple current suppression capability. Furthermore, there is also a reduced number of active components, which helps in the reduction in conduction losses. However, the use of DC link capacitor with a voltage of 300 V may make the circuit  
Figure 5: Isolated Cuk converter with canonical switching cell PFC [35]. Figure 6: Isolated SEPIC converter with DBR back end [36]. bulkier. Additionally, the leakage inductance of the transformer may necessitate the use of an appropriate clamping circuit across the switch.