## 4 DC-DC Converter Topologies for LEV Charging
### On-Board Charging
#### 4.1.1 Isolated DC-DC Converter Topologies
This charger ensures a wide output voltage range through variable DC link voltage control. The Cuk converter is chosen for the PFC stage due to its inherent input ripple current rejection capability. The circuit diagram for this configuration is shown in Figure 10. While the use of a half-bridge LLC converter helps in reducing the switching losses through zero voltage switching (ZVS), there is a drawback of increased component count in this circuit. Another recent implementation of a soft switched LEV charger is described in [22]. In this work, an LLC resonant full bridge converter is used, as shown in Figure 11. This configuration uses the resonant inductance of the transformer as a resonance circuit element, and the switching frequency is kept close to the resonant frequency, thus ensuring ZVS of the semiconductor devices. While a  
Figure 6.0 Bridgeless isolated Zeta converter with PV-fed Cuk converter [19]. satisfactory efficiency of 96.4% is achieved, the component count of this configuration is on the higher side. A similar efficiency of 96% is achieved in [23], which uses a two-stage converter; the first stage is an interleaved buck-boost stage and the second is an isolated half-bridge LLC stage with synchronous rectification. The resonant capacitor size is reduced by using a split capacitor structure. The interleaved stage achieves natural PFC through operation in discontinuous mode, and the LLC stage helps achieve soft switching. The LLC stage is unregulated, thus reducing control complexity. Another popular technique of modifying basic DC-DC converter-based LEV chargers comes in the form of interleaving. Interleaving--or multi-phasing--is a technique where multiple converter circuits are connected in parallel and turning on/off their respective switches after a phase delay as shown in (1) [44]. Figure 6.1  
Here, \(\phi\) denotes the phase delay (in degrees) and \(n\) is the number of phases connected in parallel. This process also tends to reduce the output current ripples. Another advantage of interleaving is that it increases the power handling capacity of the converter, thus lending itself to high current applications. Authors in [45] demonstrate an interleaved isolated Luo converter, which is also capable of bidirectional power transfer. The front-end comprises a voltage source converter, which interfaces with the AC grid via an LCL filter. The back end comprises the proposed interleaved isolated Luo DC-DC converter, which operates with high efficiency at light load. The schematic for this charger is shown in Figure 12a. The system control design is performed so that there is a provision for fast charging, slow charging, and discharging (bidirectional) operations. A 40 A/20 A charging current is ensured for fast/slow charging of the LEV. Discharging mode is enabled depending on utility requirement (normally during idle hours). While this charger has minimum impact on grid power quality (minimum THD), exhibits unity power factor operation and seamless bidirectional operation, this  
Figure 6.2 Full bridge LLC converter [22]. Figure 6.3 Two-stage charger with Cuk converter front-end and half-bridge LLC back-end [21]. paper lacks data on charging efficiency and charging time. Furthermore, this topology has a comparatively higher switch count. In [46], the authors present a topology identical to [45], only replacing the interleaved isolated Luo converter with interleaved isolated SEPIC converter, as shown in Figure 12b. The charger front-end comprises a full bridge voltage-source converter and the proposed isolated interleaved SEPIC converter makes up the charger back-end. As can be observed, inductors on the input and output side help in suppressing current ripples, especially since this converter operates in discontinuous mode. Just like [PERSON] et al. [45], this converter employs multistep charging for fast and slow charging. Depending on the supply side condition, this charger has vehicle-to-grid (V2G) capability.