## 4 DC-DC Converter Topologies for LEV Charging
### On-Board Charging
#### 4.1.1 Isolated DC-DC Converter Topologies
A similar topology was presented in [38], which used a bridgeless modified Landsman converter instead of the Zeta-SEPIC stage. Yet another variant of [38]--described in [39]--used an interleaved Landsman converter, which resulted in a lower input current ripple. To reduce the number of semiconductor devices in charger topologies, researchers have looked to bridgeless converter topologies [17; 18]. For example, the authors in [18] presented a single stage bridgeless isolated positive output Luo converter for LEV charging. This charger works for an AC input range of 85-265 V and ensures front-end high-power factor through use of Luo converter-based AC-DC converter topology. Eliminating the front-end diode bridge rectifier stage helps reduce component count and simplify design. Furthermore, the Luo converter-based topology achieves positive output polarity, which is contrary to prior implementations, which retained the negative output polarity. The significance of this feature is that this helps simplify output sensor design. Furthermore, the circuit operates in discontinuous mode to reduce magnetic component volume. This also reduces the switching/conduction losses of the switches in the circuit. The converter topology is shown in Figure 8. The authors of [19] use a bridgeless isolated Zeta converter shown in Figure 9, which also operates as a PFC converter, to charge LEVs. This on-board charger is further supplemented by a Cuk converter, which feeds solar photovoltaic (PV) power. This Cuk converter is controlled using the PV's maximum power point algorithm. The high frequency transformer ensures a high voltage conversion ratio. Furthermore, the isolation design choice in this case is desirable to reduce ripples. However, while this design uses a smaller number of switches, its drawback is that the Cuk converter control does not depend on the battery status. Furthermore, the behavior of switches during switching operation is not elaborated in this work. Figure 8: Bridgeless positive output Luo converter [18]. Figure 7: Flyback converter with Zeta-SEPIC PFC stage [37]. All the bridgeless converter topologies described above [17; 18; 19; 20; 37] also ensure improved power quality by reducing the THD of the supply current. This is done by the use of input inductance, which helps smooth the input current profile. Likewise, an output inductance is also incorporated in both circuits to reduce output current ripples. It is found through experimental results that the input current THD in these demonstrations is compliant with the IEEE-519 specified threshold of 5%. Furthermore, these bridgeless topologies have the advantage of reducing conductions losses (due to reduction in the number of semiconductor devices in the current path). Bridgeless converter topologies also exhibit poor performance related to EMI, as they have the tendency to introduce common mode noise on account of the presence of high frequency switch mode voltage [40]. This can result in degradation of supply current due to high ripples. Furthermore, because of the reduced switch count, the remaining switches of the bridgeless stage are subjected to high current stresses. For these reasons, research on two-stage converter topologies continues, owing to their improved EMI performance and manageable stresses on devices. However, lack of soft-switching techniques in the topologies described above is a limitation. This may limit its operation at higher frequencies due to higher switching losses. To overcome this limitation, resonant converter topologies have been incorporated by researchers into on-board charges [21; 22; 23; 24; 41; 42; 43]. This technique has been employed for numerous on-board chargers in conventional EVs [42]. For example, authors in [21; 43] presented a two-stage e-bike charger; this charger uses a Cuk converter as the PFC stage, which is followed by a half bridge LLC (inductor-inductor-capacitor) series resonant converter, which controls the battery charging process.