## 5 Summary, Gaps, and Improvement Measures in Reviewed Literature  
Figure 20 shows the distribution of collected articles--by type and year--for this review study. One key takeaway from this plot is the increase in published research articles over the five-year timeframe considered. Furthermore, the research output has increased considerably since around 2020.  
From the reviewed literature, it is evident that on-board charging continues to remain a highly researched topic. This is a testament to the level of maturity this technology has gained over recent years. It is also seen that within on-board charging, isolated topologies are widely preferred. This is because of the safety aspect stemming from galvanic isolation between supply and vehicle, and the inherent capability for soft switching. However, non-isolated topologies are a viable solution as it helps increase power density. This makes the charger circuit more compact and cost-effective. One gap that needs to be addressed in on-board chargers is to find an optimal balance between component count and converter performance. This is evident in flyback converter-based topologies. In these, transformer leakage inductance causes voltage spikes which requires the use of snubber circuits, thus increasing component count.  
On the other hand, research on off-board charging--while not as mature as off-board technology--has been gradually gathering pace in recent years. Like on-board, the aim must be to strike a balance between component count and converter performance. LLC converters, for example, can achieve soft switching but at the cost of high switch/component count. Furthermore, the issue of control complexity also needs to be addressed, since there is a reduction in power conversion stages.  
One important takeaway from reviewing non-isolated off-board technology is the integration of solar PV power supply in the charging system. However, application of off-board chargers with standalone PV supply and DC distribution systems is an encouraging trend. To fully unlock the potential of this method, future works can focus on topologies with simple design and controls--managing both PV maximum power point tracking and charging control. Furthermore, the sizing of backup battery bank is another significant research area, which has implications on the viability of a standalone PV-based charging station.  
Furthermore, efforts must be made to unlock the vehicle to grid (V2G) potential of LEV chargers. One way to do this is by eliminating the DBR stage and using a bidirectional converter. Apart from the obvious advantage of V2G potential, they are also characterised by  
Figure 20: Distribution of reviewed papers by type and year.  
low component count and increased power density, as compared to multistage converters which use a DBR stage. Bidirectional functionality is especially beneficial for non-isolated topologies, as evident in works such as [72; 73] which also demonstrate high step-down gain. As an example, [PERSON] et al. [73] presented an extendable quadratic converter, which was suitable for both grid-to-vehicle (G2V) and V2G capability. This converter was characterised with simple operation, low component count, and also a common ground between supply and battery.  
Based on the articles reviewed, a qualitative comparison of on-board and off-board DC-DC converter topologies are presented in Figure 21. This comparison includes characteristics such as impact on supply THD, control and circuit complexity, technology maturity and suitability for PV integration.