## 4 DC-DC Converter Topologies for LEV Charging
### Off-Board Charging
#### 4.2.1 Isolated Converter  
Off-board chargers with isolated configuration have been presented in recent literature, owing to their superior voltage gain range and potential for providing soft-switching capability. Authors of [16] presented an LEV charger which derives input power from a DC microgrid. This gives credence to the concurrent research on EV charging and DC microgrid. This LEV charger employs an LLC structure for soft switching of semiconductor  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & & & \multicolumn{6}{c}{**Non-Isolated Topologies**} \\ \hline \multirow{2}{*}{**Ref. **} & \multirow{2}{*}{**Topology**} & \multicolumn{6}{c}{**Component Count 1**} & \multirow{2}{*}{**Salient Features**} & \multirow{2}{*}{**Areas of Improvement**} \\ \cline{3-4} \cline{6-11}  & & & \multicolumn{1}{c}{L} & \multicolumn{1}{c}{C} & \multicolumn{1}{c}{S} & \multicolumn{1}{c}{D} & \multicolumn{1}{c}{**Tr. **} & \\ \hline \multirow{4}{*}{[53]} & Bridgeless switched inductor & & & & & & & Single power conversion stage & & & Increased conduction losses due to more diodes in conduction path \\  & SEPIC converter & & & 2 & 2 & 4 & NA & \(\bullet\) & Near-unity power & & & Charging current ripples may be optimized furtherdevices. This implementation also ensures wide output voltage capability through the use of variable winding sets on the secondary side of the transformer, as shown in Figure 15. For larger loads, an auxiliary switch operation ensures a greater turns ratio. Furthermore, a cascade switch structure is used on the primary side to reduce voltage stresses, thus allowing usage of switches with lower rating. Furthermore, two capacitors are used on the primary side to provide resonant condition for soft switching, and to provide split input voltage balance. Zero voltage turn-on and zero current switching turn-off is ensured for active switches and fast recovery diodes, respectively. This is due to the variable frequency switching scheme--pulse frequency modulation (PFM)--employed, in contrast to the conventional pulse width modulation (PWM). While this converter exhibits satisfactory performance for variable load, there are no data on efficiency reported. Furthermore, there is still scope for reduction in switch count, and the use of an additional switch for wide load operation may increase control complexity. Optimization of efficiency for the LLC-based charger is also a research avenue that can be taken up in subsequent studies [61]. Another suitable candidate for isolated LEV charging is the dual active bridge converter [62; 63; 64; 65]. In [63], the authors presented a multi-module dual active bridge converter with input-series output-parallel topology starting from the transformer secondary end, as shown in Figure 16. On the primary side, a single full bridge converter is used. The modularity of the system ensures scalability according to the load connected at any given time and ensures redundancy. Furthermore, the converter also maintains output current sharing and input voltage sharing through appropriate control scheme. Through experimental results, this converter topology achieves a high overall charging current of 94 A, which is fed in positive and negative pulses. While this ensures fast charging, this work lacks data on charging efficiency and charging time. Furthermore, the given topology has high component count and there may be issues related to high frequency transformer leakage inductance, such as switching transients and duty loss. Figure 15: LLC converter for LEV charging [16]. Moreover, one limitation related to dual active bridge converters is the EMI issue that may result from large deviations in input or output voltage, or light loads [66]. To increase compactness, the dv/dt limit may be pushed, and issues such as current ringing may occur. One more potential candidate for implementing isolated LEV chargers is the flyback converter. The use of flyback converter for LEV charging is exemplified by works [4], and [60].