On-Board Chargers for Electric Vehicles: A Comprehensive Performance and Efficiency Review

Abstract

The transportation industry is experiencing a switch towards electrification. Availability of electric vehicle (EV) charging infrastructure is very critical for broader acceptance of EVs. The increasing use of OBCs, due to their cost-effectiveness and ease of installation, necessitates addressing key challenges. These include achieving high efficiency and power density to overcome space limitations and reduce charging times. Additionally, the growing interest in bidirectional power flow, allowing EVs to supply power back to the grid, highlights the importance of innovative OBC solutions. This review article provides a thorough analysis of the current advancements, challenges, and prospects in EV on-board charger technology. It aims to offer a comprehensive review of OBC architectures, components, technologies, and emerging trends, guiding future research and development. Addressing these challenges is essential to enhance the efficiency, reliability, and integration of OBCs within the broader EV ecosystem.

1. Introduction

The global transition towards a sustainable and carbon-neutral future highlights the critical role of electric vehicles (EVs) in mitigating climate change and reducing greenhouse gas emissions [1]. Policy initiatives such as the European Green Deal, which aims for net-zero carbon emissions by 2050 [2,3], and India’s commitment at the 26th climate change conference to achieving net-zero emissions by 2070 highlight the urgency of this transition [4]. This shift is driven by environmental regulations, advancements in battery technology, and innovative propulsion systems, leading to an increasing preference for EVs over traditional internal combustion engine (ICE) vehicles. This statement is supported by the reports in [5,6], according to which 14% of all car sales in 2022 were electric vehicles, almost 9% more than sales in 2021.

We can broadly classify EVs as into the following categories: Battery electric vehicles (BEVs) consist of a battery powered electric motor. For charging purpose, BEVs need to be plugged into a wall outlet. Plug-in Hybrid Electric Vehicles (PHEVs) consist of an electric motor and an ICE powered by a battery and some fuel respectively. PHEVs are either charged from an outlet or make use of regenerative braking. Hybrid Electric Vehicles (HEVs) make use of an ICE along with one or more electric motors. The ICE and regenerative braking are used for charging purposes. Fuel Cell Electric Vehicles (FCEVs) use fuel cell technology to produce the electricity. They employ a propulsion system which converts hydrogen energy into electrical power. The primary subject of this study is battery electric vehicles (BEVs).

Even though conventional ICE vehicles have been dominant in transportation, they still pose significant environmental threats and face many technological problems. They have a major impact on greenhouse gas emissions and air pollution. Additionally, only 20–30% of the fuel energy in ICEs is converted into productive work. This low efficiency results in higher fuel consumption and increases operational costs. The mechanical complexity of ICE vehicles, with numerous moving parts, also leads to higher maintenance requirements and costs. Components such as exhaust systems, fuel injectors, and oil filters need regular servicing and replacement, adding to the overall cost of ownership. The reliance on fossil fuels further exacerbates the problems associated with ICE vehicles. Fluctuations in fuel prices, geopolitical tensions, and the finite nature of fossil fuel reserves create economic and energy security concerns. The extraction, transportation, and refining of fossil fuels also have significant environmental footprints, contributing to habitat destruction, oil spills, and water contamination.

In contrast, EVs offer a promising solution to these challenges. They produce zero tailpipe emissions, drastically reducing air pollution and greenhouse gas emissions [7]. In Figure 1, a comparison of emissions from the use of different fuel sources is shown. From the manufacturing to the time an EV starts to run, the process is environmentally friendly. Electric drivetrains are significantly more efficient than internal combustion engines [8,9], converting a higher percentage of stored energy into motion, which translates to lower operational costs [10]. Figure 1 gives us an idea about cost savings while using different fuels for same vehicle. From Figure 1, it can be inferred that for same vehicle among various fuels (gasoline, ethanol-85, diesel, LPG, and electricity), the maximum savings are experienced by using electricity. For the interpretation of the graph in Figure 1, we consider gasoline as fuel that results in maximum cost per kilometer [7]. The cost-saving percentages mentioned for all other fuels are calculated in comparison to gasoline. The simpler mechanical design of EVs, with fewer moving parts, reduces maintenance needs and enhances reliability [11]. The natural wear and tear due to vibrations and gasoline corrosion is absent in electric vehicles. As a result, there are fewer cases of breakdowns. Furthermore, the use of renewable energy sources for electricity generation can further minimize the environmental impact of EVs [7,12].

This interpretation is based on an approximation because the cost of various fuels can vary over time, as taxes and subsidies come into play. Therefore, for any cost-related comparison, we need to be extremely careful. The graph here gives us an idea about more effective technology in terms of cost only.

Despite these advantages, the widespread adoption of EVs faces several challenges [13]. There are certain areas in the case of EVs that need improvement. The limited range of EVs, determined by the energy density of current battery technologies, remains a significant concern for consumers. A worldwide statistics report [14] showed that the range of EVs has seen growth in the last 5–6 years. Although advancements have been made, such as the Tesla Model S achieving a range of up to 630 km [15], anxiety persists. The development of fast and accessible charging infrastructure is another critical area, as long charging times compared to the quick refueling of ICE vehicles hinder convenience. Range anxiety among drivers forces many to select a vehicle with higher battery capacity (in terms of kWh). Additionally, the high cost of lithium-ion batteries, driven by the need for large capacity to ensure adequate range, poses an economic barrier. Battery weight and thermal management are also critical issues [10,11]. The substantial weight of batteries impacts vehicle efficiency and performance, while effective thermal management systems are essential to maintain battery health and safety, particularly under extreme conditions. Addressing these technical and infrastructural challenges is imperative for the broader acceptance of EVs [16,17].

One of the most critical components in EVs that requires significant attention is the on-board charger (OBC). The OBC is responsible for converting alternating current (AC) from the grid into direct current (DC) to charge the vehicle’s battery. The efficiency and performance of OBCs are vital as they directly impact the charging time, energy efficiency, and overall user experience. The increasing demand for high-efficiency and high-power-density OBCs presents several technical challenges. These include managing heat dissipation, reducing size and weight, and ensuring reliable operation under various environmental conditions. The limitations of current OBC technology can result in longer charging times, reduced energy efficiency, and higher costs, which in turn affect consumer acceptance and the practicality of EVs [18,19,20]. The complexity of OBC design also poses significant challenges. High-power OBCs must handle substantial amounts of energy conversion while maintaining efficiency and safety. This requires advanced power electronics, thermal management solutions, and sophisticated control algorithms. Any failure in the OBC can lead to catastrophic consequences, including potential damage to an EV’s battery and overall vehicle malfunction. Therefore, improving OBC technology is crucial for the safe, efficient, and reliable operation of EVs.

Research and development efforts are focused on overcoming these barriers by advancing battery technology, improving energy efficiency, and developing intelligent thermal management systems. Innovations in high-power converters and control strategies aim to reduce charging times and enhance the reliability of EV charging systems. The integration of renewable energy sources with EV charging infrastructure is also being explored to maximize environmental benefits. Addressing the challenges of OBCs is particularly crucial, as advancements in this area will directly influence the overall performance, cost, and consumer acceptance of EVs.

In summary, while EVs present significant advantages over conventional ICE vehicles, addressing existing challenges, particularly in OBC technology, is crucial for their widespread adoption. Considering these requirements, this research focuses on innovations that are essential to realizing the full potential of EVs in achieving a sustainable and carbon-neutral transportation future. The article emphasizes current technologies and trends being researched, with a particular focus on improving OBCs, which will play a pivotal role in ensuring efficient energy conversion, reducing charging times, and enhancing the overall reliability and safety of EVs.

The major contributions of this literature are as follows:

• Analysis of existing EV charging methods, including various EV models, charging voltages, and charging modes.
• Comprehensive review of on-board charger (OBC) topologies, highlighting their advantages and drawbacks.
• Identification and review of topologies adapted for each stage, along with the significance of each adaptation.
• Overview of existing related standards and their formulating bodies.
• Identification of various research opportunities, challenges, and trends available to the academic and industrial communities in OBC technology.

The subsequent sections of the paper are organized as follows: Section 2 provides an overview of charging technologies of electric vehicles, which includes the different charging levels and charging methods available to us. Section 3 presents an analysis of all the relevant EV standards which comprise the safety, grid integration, and EV charging standards. In Section 4, the design considerations for OBCs and the targeted performance parameters are discussed. In Section 5, a summary of the present-day EV market is provided, with a detailed overview of the state-of-the-art OBCs. In Section 6, various trends, obstacles, and potential areas for further investigation are listed. In the end, Section 7 presents important conclusions that were drawn from this work.

2. Charging Infrastructure

This Section provides an overview of charging technologies of electric vehicles, which includes different charging levels and charging methods available to us [21]. The power infrastructure provides an electric circuit or system for power flow between EVs and the grid [22]. It may be categorized based on the types of power utilized, the charging circuit’s accommodation, the need for physical contact, and the direction of power flow [23]. There are three types of charging levels (Level 1, Level 2, and Level 3) for electric vehicles, based on the voltage and power handling capabilities according to established standards [24]. Figure 2 illustrates the three charging levels of an EV.

2.1. Charging Levels

• Level 1 Charger (AC Charging): The majority of electric vehicle charging occurs overnight at home, typically in a garage, where the EV can be easily connected to a nearby outlet [13,25]. For this purpose, a Level 1 charger is used, which deals with low power levels up to 1.92 kW [26,27]. A standard 120 V outlet is used here; therefore, it is the slowest charging process among the three [28].
• Level 2 Charger (AC Charging): This type of charging is the primary mode for the purpose of charging at private or public facility. It handles higher currents than Level 1 [28] and uses a 240 V outlet, providing power of up to 19.2 kW and hence faster charging compared to the Level 1 [29]. In other words, Level 2 is known as semi-fast charging.
• Level 3 Charger (DC Fast Charging): Level 3, or DC fast charging, makes use of AC and DC power for delivering high voltage DC power to the battery of an EV [20]. These chargers have the ability to manage power ranging from 20 kW to almost 350 kW, therefore supplying a DC voltage between 300 V to 800 V. These charging stations are mostly found in commercial places such as government buildings, airports, and refueling stations.

Table 1. Power level categories of EV chargers [20,26,30].

Specification		Level 1	Level 2	Level 3
Charging power		1.4–1.92 kW	3.1–19.2 kW	20–350 kW
Charging Type		Slow Charging (On-Board)	Semi Fast Charging (On-Board)	Fast Charging (Off-Board)
Charge Location		Residential	Private and Commercial	Commercial
Power Supply	Voltage	120–130 VAC	208–240 VAC	208–240 VAC 300–800 VDC
	Current	12–16 A	12–80 A	250–500 A

summarizes the range of voltage and power handling capacities for the three charging levels, along with the information about the type of the charging that is possible at different levels. From this information, it is clear that on-board chargers for electric vehicles can only support Level 1 and Level 2 charging, and similarly off-board chargers are a good choice if fast charging is required.

2.2. Charging Methods

There are different techniques for charging an EV available to us, and the type of charging facility that a customer chooses depends upon infrastructure availability, vehicle compatibility, charging speed, convenience, and cost considerations. A detailed classification of the EV charging methods is shown in Figure 3.

Charging methods can be classified as conductive charging, battery swapping, and wireless charging [20,31,32]. Since the focus of this paper is towards the on-board chargers, in this Section we talk about conductive charging and its types. Most of the commercial EVs use the conductive charging method [33]. There is a physical contact between the charger and the battery of an EV [32,34,35]. This technique involves connecting a battery to a power grid using a cord. From Figure 3, conductive chargers can be further classified into following two types:

• Off-Board Chargers: For off-board chargers, the charging framework is installed at the charging station. This type of charger is used in cases of higher power requirements. With an off-board charger, handling the charger’s weight and size is easier since there are no space and volume constraints set aside for these types of chargers [20]. These chargers bypass the OBC and connect directly to the battery [5,13]. Off-board chargers must communicate with the vehicle so that the correct voltage and current supply reach the battery of an EV. Particularly common among non-dedicated chargers in public charging stations, which are meant for supplying vehicles with varying voltages and different battery compositions [36].
• On-Board Chargers (OBCs): These are the electronic devices that are integrated into the powertrain of an electric vehicle. We do not need to buy one externally (they are inbuilt). These are supplied from power sources at home, charging stations, public places (such as parking areas), etc. Figure 4 shows the OBC integrated to an electric vehicle [37]. They convert the AC power from these sources into DC power to charge the battery pack of the electric vehicle. The OBC helps in determining the proper amount of current/power and the correct charging standard to be applied. OBCs result in better battery life compared to off-board chargers [12].

On-board chargers for electric vehicle are classified as follows:

(a)

Unidirectional OBC and Bidirectional OBC [38]: This classification is based on their design and purpose. Figure 5 represents power flow topology of both unidirectional and bidirectional chargers. Unidirectional on-board chargers are only meant for conversion of AC into DC while charging the battery of an EV, which is known as grid-to-vehicle (G2V) mode of operation. In most of the cases, a unidirectional charger consists of a diode bridge, a filter, and a DC–DC converter [13,39]. Since it minimizes hardware demands, streamlines challenges during interconnection, and typically lessens battery wear, unidirectional charging is logically the first step [39,40]. Most utility goals may be met by unidirectional chargers while avoiding performance, cost, and safety issues [41,42]. The unidirectional type is cheaper than the bidirectional type.

On the contrary, bidirectional on-board chargers enable power flow in the reverse direction, i.e., they serve the purpose of charging the battery of an electric vehicle (AC-to-DC) and can also discharge energy back to the grid (DC-to-AC) at a time of need [41,43,44,45,46]. This mode of operation of a bidirectional charger is known as the vehicle-to-grid (V2G) mode of operation [45,47]. Bidirectional chargers can either be an isolated circuit or use a non-isolated circuit configuration [13]. These chargers provide higher power density and faster control [48], but due to an increase in the components, there is an increase in the cost.

(b)

Single-Stage OBC and Two-Stage OBC: In case of a single-stage OBC, the process of conversion of AC–DC to charge the battery is a one stage operation [49]. Features like fewer number of components, longer lifetime, and high power density make this charger economical and also decrease the overall volume [28,50]. A single-stage OBC can either be isolated or non-isolated. Figure 6a shows the circuit diagram for a single-stage on-board charger.

In case of a two-stage OBC, the power conversion happens in two steps using a cascaded arrangement of two power electronic converters [50]. Rectification and correction of power factor take place in the first stage [49]. In this stage, a converter for getting a high power factor is used. The second stage, being a DC–DC converter, is meant for regulating the charging algorithm of the EV battery [28,51]. It works on the output from the first stage and then provides proper current and voltage to the battery. Figure 6b shows the circuit diagram for a single-stage on-board charger.

2.3. Control Algorithms for G2V and V2G Operating Modes

Grid-to-vehicle (G2V) and vehicle-to-grid (V2G) operating modes differ not only in their design features but also in the control algorithms used to manage power converters, which are essential for the effective operation of these systems.

2.3.1. G2V Control Algorithms

In the G2V mode, the control algorithm’s primary function is to regulate the flow of power from the grid to the EV battery during the charging process. The algorithm is designed to manage the charging current and voltage to maximize efficiency while ensuring the safety of the battery. Typically, this involves implementing strategies such as maximum power point tracking (MPPT), which adjusts the power flow to match the battery’s optimal charging conditions. The control algorithm must also integrate grid demand response mechanisms, where the charging power can be modulated in real time based on the grid’s current load and frequency conditions. This helps prevent overloading the grid during peak demand periods while maintaining a stable and efficient charging process.

2.3.2. V2G Control Algorithms

The V2G mode requires a more complex control algorithm due to the bidirectional nature of power flow, where energy is not only drawn from the grid but can also be sent back to it from the EV battery. The control algorithm in this mode must continuously monitor the grid’s condition, particularly the frequency and voltage, to determine the appropriate times and power levels for discharging the battery back into the grid. This real-time assessment helps support grid stability by providing ancillary services, such as frequency regulation. Additionally, the algorithm must manage the state of charge (SoC) of the EV battery, ensuring that the vehicle retains enough energy for the driver’s needs while contributing to the grid. This requires predictive modelling and optimization techniques that balance the grid’s needs with the preservation of battery health and performance.

2.3.3. Summary of Algorithmic Features:

G2V Mode:

• Charging Optimization: Utilizes maximum power point tracking (MPPT) to optimize charging efficiency by continuously adjusting voltage and current.
• Safety Mechanisms: Integrates protective features to prevent overcharging, thermal runaway, and other battery-related issues.
• Grid Coordination: Implements demand response strategies to modulate charging rates in real time based on grid conditions, preventing peak load stress.

V2G Mode:

• Bidirectional Power Management: Controls the flow of energy from the EV battery back to the grid, requiring complex bidirectional power flow management.
• Real-Time Grid Interaction: Continuously monitors grid parameters such as voltage and frequency to determine optimal discharging times.
• SoC and Battery Health Optimization: Balances the state of charge (SoC) and battery health by optimizing discharge cycles, ensuring sufficient charge for vehicle operation while maximizing grid support benefits.

These algorithmic features are pivotal for the effective differentiation and operation of G2V and V2G modes, ensuring efficient integration of EVs with the power grid.

3. Relevant Standards: Electric Vehicles

The introduction of EVs has resulted in creation of many new dimensions in the fields of transportation and power industries. Certain operational and safety standards have been set because of the increase in demand for EVs. This Section provides a brief idea about the formulating bodies and types of standards that are currently in force [52,53,54]. In the upcoming years, for effective deployment of EVs, the introduction and implementation of these norms and standards are necessary [13,23]. Various concerns pertaining to EVs are covered by these safety norms and standards [50,55]. This Section gives us an idea about different standards being used by the EV manufacturers. There are many international bodies that set the standards and norms. The EV charging standards that are taken into consideration can be divided into three categories; EV charging component standards, EV grid integration standards, and safety standards [55].

3.1. EV Charging Standards

EV producers in the U.S.A. make use of the Society for Automobile Engineers (SAE) and IEEE standards [56]. In European countries, the International Electrotechnical Commission (IEC) standards are used. Japan uses its own EV charging standards, namely Japan Electric Vehicle Association (JEVA) and CHAdeMO Association [23,57]. The International Organization for Standardization (ISO) is an EV charging component standardization organization. It focuses on EVs, while other bodies focus on specification at the component level [13,23,58]. The Standardization Administration of China (SAC) publishes national standards of China, known as Guobiao Standards (GB) [59].

3.2. EV Grid Integration Standards

Different standards have been designed to ensure that the process of integration of an EV to the grid is completely safe and dependable [60]. The grid integration standards deal with charging and discharging processes when an EV is connected to the grid [58,61]. The Institute of Electrical and Electronics Engineers (IEEE) and Underwriters’ Laboratories (UL) are the two major organizations in grid integration standards.

3.3. EV Safety Standards

The safety precaution is a need for grid connectivity and EV charging. Even though most standardizing bodies like ISO, IEEE, and UL define most of the safety standards, bodies like the National Fire Protection Association (NFPA) and the National Electric Code (NEC) are meant precisely for safety measures [23].

The standards are fundamental blocks to building an EV ecosystem that is secure, efficient, and adaptable. We can summarize the discussion about the EV relevant standards from the above sections in the following way:

• For EVs and their charging infrastructure to be safe, dependable, and compatible, relevant standards are essential.
• We need standards for preventing accidents, facilitating seamless communication between various EVs and chargers, and allowing the efficient integration of EVs with the electrical grid.
• These standards also provide guidelines for lowering the impact on the environment, avoiding electrical hazards, and boosting EV adoption on a larger scale.

There are numerous organizations, technical bodies, and groups worldwide, so there is a lot of duplication [13]. As a result of the fact that EVs can supply energy back to the grid, i.e., EVs can be treated as energy sources, there is a need for a stricter and more clearly defined set of standards [62]. In this paper, a tabular approach has been used to summarize the standards and codes that have been designed by the abovementioned organizations. The information about the associated bodies and their most significant standards is given in

Table 2. Standards associated with electric vehicles [13,23,63,64].

Organization	Standards	Detail
IEC [13,23]	IEC-61851 [13]	Standards dealing with conductive charging.
	IEC-61980 [23]	Guidelines for Wireless Power Transfer (WPT) for EVs and levels of AC and DC supply voltage.
	IEC-62196 [63]	Instructions for charger plugs, sockets, connectors, and vehicle inlets in case of conductive charging. Conductive charging modes are defined based on the method of power transfer to the vehicle.
SAE [52,56]	SAEJ1772 [52]	Instructions for voltage and current ratings of an equipment for AC and DC charging processes.
	SAEJ2293 [57]	Charging equipment for on-board and off-board chargers.
	SAEJ1773 [56]	Minimum requirements for contactless charging.
	SAEJ2847 [52] and SAEJ2836 [52,56]	Communication requirements between an EV and power grid.
	SAEJ2931 [58]	Guidelines for digital communication between EVs and grid.
	SAEJ2954 [56]	Standards for wireless charging of EVs.
GB [59]	GB/T 18487.1 [59]	Specifications of safety issues like emergency stops, overload and short circuit protection, and electric shock safety.
	GB/T 20234 [59]	GB/T 20234.2 provides specifications of a male, Type-2 AC connector. GB/T 20234.3 is compatible with a charging power of up to 250 kW.
	GB/T 27930 [59]	Communication between Electric Vehicle Supply Equipment (EVSEs) and battery management system during charging process.
JEVA and CHAdeMO [65,66]	JEVS-C601 [65]	Sockets and plug used to charge an EV.
	JEVS-D701 [65,66]	Battery testing procedure guidelines.
	JEVS-G101-109 [66]	Standards for EV fast charging.
IEEE [65,67]	IEEE P2690 [65,66]	Management of charging systems of electric vehicles.
	IEEE P1547 [67]	Guidelines for connection of grid to Distributed Energy Resources (DERs).
UL [68]	UL 1741 [69,70]	Specifications of power system’s converter, charge and output controllers.
	UL 62109 [70]	Safety arrangements during the designing process of grid connected PV converters.
	UL 1741 SA [71,72]	Safety standards for inverters to maintain grid stability
NFPA [73]	NFPA 70 [73]	Safety guidelines for DER grid integration.
	NFPA 70B [23,57]	Maintenance of Electrical Equipment.
	NFPA 70E [52,73]	Safety standards of the workplace.

[23,52,57].

3.4. Applications of Electric Vehicle Standards

In this Section we will get a brief idea about the applications of the EV Standards. There are different standards being used in different countries. In all the cases the design of connectors and ports differentiates one from the other. To tackle the standard dispute, there is a need for a common charging connector [74]. Connectors available to us vary due to their shapes, and automakers aim at harmonizing the standards. The connectors being made use of in the U.S.A. (SAE J1772) can be used with AC and DC. A connector with the same specifications has also been designed by Tesla. Tesla has designed an adapter for different vehicle models, which is meant for converting SAE J1772 connectors into Tesla connectors to use them at Tesla supercharging stations. These standards are vital for grid integration. Various standardization organizations deal with their respective fields. We must adhere to the norms for running the entire infrastructure, comprising of the grid, EVs, charging stations, and equipment [75].

4. Design Considerations; Targeted Performance Parameters

In case of any design-based process, the researchers set aside certain indicators to determine performance of the outcome. Electric vehicle chargers are also designed according to goals, demands, and limitations. Certain parameters need to be defined prior to starting the design process. Some of the most set targets that are taken into consideration in the process of developing an on-board charger for an electric vehicle are listed below:

• Efficiency: High efficiency makes sure that there is minimal loss of energy, making charging cheaper and enhancing the thermal performance. Engineers always aim at achieving high efficiency (usually above 90%). Designing an efficient OBC for an EV helps in saving anergy, reduces the generated heat, and increases the range of the vehicle.
• Power Density: High power density is used to abide by the packaging restrictions set by the automotive industry. To achieve this goal, researchers are trying to replace the traditional semiconductor material and introduce new materials with enhanced properties. Enhanced power density makes an OBC lighter and capable of delivering greater power in small packages.
• Thermal Management: It is very crucial to manage the dissipated heat and maintain the temperature of all the components within a safe value. Proper thermal management is crucial to ensure the longevity of an OBC, hence making it safe and reliable. It also results in minimizing the heat losses, therefore maintaining the efficiency of the charger.
• Safety: We need to consider the safety protocols that have been already set aside in the field of electric vehicles. Among these are protection against overvoltage, overcurrent, and overtemperature.
• Flexibility: We aim at achieving a design that lets our chargers operate in varying operating conditions. It should be able to adjust according to the upcoming advancements in the technology and also be able to adapt to various EV. A flexible OBC is scalable, which means that it can be scaled up and down to meet different power requirements. Additionally, flexibility improves user experience by offering more convenience.
• Sustainability: Prioritizing sustainability has positive effects on the environment as well as on the economy. Selecting the materials with consciousness results in minimal carbon footprints. Through decreased waste management, cheaper material prices, and increased energy efficiency, sustainable practices can result in cost savings.
• Cost Density: Cost is considered as an effecting parameter in case we make industrial products. We aim at optimizing the cost of our product by extensive examination of parameters like efficiency, power, thermal management, switching losses, and other significant characteristics, to make our product economically feasible.

5. Existing EV-Related Survey

On-board chargers can either be unidirectional and bidirectional, as discussed earlier. Unidirectional chargers only support the G2V flow of energy, but with bidirectional chargers G2V and V2G techniques are possible. The block diagram of an EV OBC is given in Figure 7. The input from the grid is initially fed to an AC/DC converter to receive a rectified voltage. An EMI filter is used for filtering out the noises and provides protection against surges [76]. The use of a filter capacitor to receive a ripple free voltage results in distorted input current. The distortion can have a very high value of up to 90% [77]. To eliminate the distortion, a power factor correction (PFC) circuit is used in EV chargers. PFC circuits which serve the purpose of improving the capacitor ripple are available in various topologies such as bridge topology, bridgeless topology, interleaved topology, totem pole, etc. The output of a PFC circuit acts as an input to a DC-link bus. Finally, a DC/DC converter (e.g., full bridge, flyback etc.) is used for getting a controlled DC voltage output. Hence, we get an efficient and safe charging [78,79,80]. A transformer is used for galvanic isolation in the DC/DC conversion stage.

Several research reviews have been carried out regarding charging infrastructure of an EV. In [20], a detailed review of infrastructure for charging of electric vehicles is present. These publications do not focus on or go into great depth about the specialized power electrical architecture of bidirectional OBCs. The study in [52] includes very few selected bidirectional OBCs, consisting of a report on the power density of four industrial OBCs. In [62], a detailed study of high power OBCs has been conducted. Many cutting-edge chargers were excluded in this study since only OBCs with ratings of ≥7.6 kW were taken into consideration; e.g., 6.6 kW chargers, which are very much in-demand in industrial areas as well as in academia, were not included. Most of the topologies that have been reviewed are unidirectional. This demonstrates a shift towards the use of high-power OBCs by summarizing international charging standards and modern trends in EV technology. The research in [81] assesses the V2G technique for its advantages in case of unidirectional and bidirectional chargers. In addition to the benefits, they examine the drawbacks, like battery degradation and very high costs. The authors of [82] review the potential impacts of various EV charging techniques on power distribution networks. They also mention and discuss the financial benefits of implementing the V2G technique. As a result, there is not a thorough evaluation that summarizes the SOA of OBCs as of now. To fill this research gap, this article reviews all the pertinent literature. The process of evaluating different OBCs with respect to their efficiency, design, and performance has been conducted using several approaches which include:

• Design and purpose of the OBC.
• Technical specifications like power rating, efficiency, and topology.
• Evaluation of electrical performance using power factor.
• Cost assessment and lifetime of OBCs.

The sales of electric vehicles have seen an increase globally. The reports suggest that there will be approximately around 230 million electric vehicles by the end of 2030, and in 2040 it is estimated that almost 58% of all the vehicle types will be electric [83]. A study by the International Energy Agency (IEA) in 2022 gave a detailed information about countries leading EV car deployment. According to this report, China is at the top, followed by Europe and the U.S.A. [84]. Different researchers have worked and summarized many EV-related topics such as charging infrastructure [85,86,87], smart grid integration [81,88,89], and effects of vehicle-to-grid (V2G) technique [82,85,90] in their previous works. However, due to high-speed progress in the field of EVs in recent years, there is a need for new and updated research. The previous ten years have seen notable advancements in several areas pertaining to the manufacturing process of electric vehicles, the use of modern technology, and their sales. There has also been an increase in the research efforts in this field of engineering.

In [91], the effects of OBCs when integrated with the EV on the powertrain of EV have been studied, while [92] focuses on 800 V powertrains, their advantages, obstacles, and associated trends. In [93], the authors discussed the history of development of EVs from the time they first came into existence to the present day. Furthermore, a categorization of the vehicles based on their powertrain parameters is performed. This research examines how the electric grid is affected when EVs are charged. The impacts that electric vehicles (EVs) can have on the necessary levels of potential of the grid, productivity, and efficiency have been investigated in [94]. The research also examines how electric cars affect the environment and the economy. In [95], a general idea about a new economic model present in electric vehicles is discussed. The study was conducted while taking the unidirectional and bidirectional flow of energy into consideration. To achieve this target, an analysis of various facilities of charging available for electric vehicles was conducted. The researchers also investigated if using these cars to store the energy from renewable sources is a practical option. In [96], a variety of practical solutions to issues pertaining to PHEV and BEV charging were discussed. Furthermore, they evaluate several charging methods in various locations, including residential garages, building apartments, and retail malls. Emphasizing fleet operators, ref. [97] offers an updated categorization of techniques for the smart charging of EVs mainly focusing on aspects like design, charging, and the protocols for communication.

5.1. State of the Art Unidirectional On-Board Chargers

The basic topology of a single-stage unidirectional OBC is shown in Figure 5. Enhanced power factor, price, reliability, and efficiency are the primary performance indices and the basis for getting different topologies. A challenging problem for the researchers nowadays is to minimize the time taken to charge an EV by increasing the efficiency of OBCs. Unidirectional single-stage OBC can either be an isolated type or a non-isolated type, but the isolated topology is more popular. The reason for this is the limitation in the ratio of conversion of non-isolated converters. Hence, they are not applicable in a wide output voltage range [98]. The research papers reviewed in this Section were chosen by taking three parameters (lifetime, efficiency, cost) into consideration [28].

The electrolytic capacitors are the ones with lowest lifetime. To increase the lifespan of a charger converter, the researchers in [99] proposed a converter making use of film capacitor in place of an electrolytic one. They propose a single-stage unidirectional OBC that uses a series resonant converter with diodes as clamping (DC-SRC) devices. However, the suggested architecture in [99] makes use of some additional components (current boosting circuit), therefore conduction losses increase and efficiency decreases (90.60%). Consequently, the overall price of the converter also increases.

Table 3. Comparison of technical specifications of unidirectional OBCs.

Ref.	Topology	Power Rating	Power Factor	Efficiency	Year
[99]	A diode-clamped series resonant converter (DC-SRC)	1.7 kW	0.995	90.6%	2013
[100]	Integrated Boost Converter and Full Bridge	1.5 kW	0.98	92%	2013
[101]	Isolated CUK	1.6 kW	1	91%	2015
[102]	DC-SRC	1.7 kW	0.992	92.1%	2015
[103]	Active-clamp and a series resonant circuit	2 kW	0.99	96.2%	2016
[104]	Current Fed Full Bridge Type	3 kW	0.995	94%	2017
[105]	Resonant converter and a non-isolated Buck converter	20 kW	-	>97%	2019
[106]	Non-isolated DC/DC converter with NLCC	120 W	0.99	97.2%	2020

summarizes the discussed topologies of unidirectional on-board chargers according to their rated power, efficiency, and power factor.

A full bridge converter (DC–AC) consisting of two control circuits has been proposed in [100]. This topology contributes towards decreasing the cost. A combination of an active full bridge and a boost converter has been used. Hence, this technique makes use of the desirable qualities of single- and two-stage strategies to reduce the overall cost. The suggested architecture finds its application when power ratings are greater than 1 kW. At a full load of 1.5 kW, the proposed charger recorded an enhanced efficiency of 92%.

In [101], the design for an Isolated CUK converter-based PFC has been put forward. The converter has a power rating of 1.6 kW. This topology makes use of the least number of components, due to which it becomes suitable for low-price OBCs. This topology is characterized by a short lifespan because of the higher value of capacitance and has an efficiency of 91%.

In [102], for improving the topology presented in [99], a passive power factor correction (PFC) circuit to minimize the switching losses is introduced. The switching phenomenon becomes soft and the efficiency of the converter increases and becomes (92.1%). This topology increases the lifespan by allowing the use of smaller capacitors, which permits us to replace the electrolytic capacitor with a thin-film type.

In [103], a circuit with enhanced lifetime, better efficiency, and minimal cost is proposed. The resultant power factor value is 0.99 and the overall efficiency increases and becomes 96.2% (at a full load of 2 kW). Hence, this topology has the highest efficiency among all the other reviewed strategies. It does so by replacing electrolytic capacitor converter by a combination of active clamp circuit and acceptor circuit.

The research in [104] suggests a modified version of a full bridge by installing a new path for the current to flow parallel to the full bridge. This main goal of this study is to enhance the efficiency. This study resulted in a zero-current setting, and hence an enhanced converter efficiency of almost 94% with no need for a DC-link capacitor. But the capacitor at the output in this architecture is electrolytic and very large in size. The authors of [105] suggest that a single-stage charger is employed when reduced cost and compact size are required. This research was able to achieve an efficiency of >97%.

The researchers in [106] have studied and proposed the use of nonlinear carrier control (NLCC) strategies. The suggested converter is a DC–DC converter with a single-stage configuration capable of achieving all the three modes of operation, plug-in charging, propulsion (PR), and regenerative braking (RB) for on-board applications. This results in eliminating the need for very costly and bulky circuit elements like DC capacitors and inductors. It also results in improvement of the power density and efficiency (97.2%) of the EV powertrain. On the grid side, it results in enhanced power factor.

To ensure high quality of input power for buck and boost mode, a non-isolated OBC has been proposed in [53]. The purpose of a two-mode average current control is to provide better input power quality while operating in a broad output voltage range.

5.2. State of the Art Bidirectional On-Board Chargers

The basic topology of a bidirectional on-board charger for an EV is already shown in Figure 5. The state-of-the-art bidirectional OBCs contribute towards development in academic field as well as in the industries. In this Section, we discuss the configurations and novel aspects of selected SOTA bidirectional OBCs for electric vehicles.

Table 4. Comparison of technical specifications of bidirectional OBCs.

Ref.	Topology		Power Rating	Power Density	Efficiency	Year
[107]	Interleaved Boost PFC	H-Bridge LLC	22 kW	-	94.5%	2014
[108]	TP PFC	CLLC	6.6 kW	3.2 kW/L	97.0%	2016
[109]	TP PFC	2P CLLC	6.6 kW	2.3 kW/L	96.2%	2018
[110]	CLLC with 3P Secondary		12.5 kW	9.5 kW/L	97.3%	2018
[111]	CLLC with 2P Secondary		6.6 kW	8.0 kW/L	97.8%	2019
[112]	TP PFC	Bidirectional LLC	6.6 kW	2.7 kW/L	95.6%	2019
[113]	TP PFC	Bidirectional LLC	6.6 kW	3.4 kW/L	96.0%	2020
[114]	Single-Stage 3P IMDAB		8.0 kW	7.3 kW/L	97.1%	2020
[115]	TP PFC	CLLC	6.6 kW	3.8 kW/L	-	2021
[116]	TP PFC	3P CLLC	11 kW	3.2 kW/L	96.0%	2022
[117]	Single-Stage TP PFC and DAB		3.7 kW	7.3 kW/L	97.1%	2022

summarizes the discussed topologies of bidirectional on-board chargers according to their rated power, efficiency, and power factor.

In [107], an OBC compatible with EV and PHEV rated at 22 kW has been put forward. In this paper, a modular circuit arrangement is seen, i.e., the 22 kW circuits are formed by the combination of three identical (7.4 kW) modules, which in turn consist of two identical (3.7 kW) units. The complete circuit works with an efficiency of more than 94%.

Another topology of a two-stage OBC rated at 6.6 kW has been proposed in [108] by Wolfspeed Inc. For the PFC stage, it makes use of a totem pole (67 kHz) in the first stage and a CLLC converter (148–300 kHz) for the second stage. The proposed topology is not interleaved and we get a power density of 3.2 kW/L and efficiency of 97%.

In [109], a two-stage bidirectional OBC configuration was presented by Virginia Tech., Centre for Power Electronics Systems (CPES), consisting of an interleaved two-channel totem pole (TP) PFC in the first stage and a CLLC converter in the next stage. The two converters operate at 300 kHz and 500 kHz respectively. The charger in this study has a rated power of 6.6 kW. The power density of the proposed configuration is 2.3 kW/L and can perform the job at a maximum efficiency of 96.2%. In this paper, a unique yet very basic PI control technique has been implemented.

In [110], the authors worked on the introduction of certain changes in the secondary of a three-phase CLLC converter, which resulted in enhanced power density of 9.5 kW/L, but the peak efficiency decreased by 0.5%.

A design identical to [109] has been proposed in [111] with an efficiency of 97.8% along with a power density of 8 kW/L. To reduce magnetic components and lower the size of a power converter, a unique magnetic structure based on PCB winding is developed in this study.

A two-stage OBC prototypes based on bidirectional LLC converter were proposed in [112,113]. The two topologies eliminated the below unity voltage gain problem in LLC Converters. The topology in [112] managed to achieve a power density of 2.7 kW/L with an efficiency of 95.6%. The prototype in [113], being an improved version of [112], achieved a better power density which was 26% greater than the previous value.

ETH Zurich in [114] proposed a single-stage OBC which has a power rating of 8 kW. The configuration used by the converter is an isolated matrix dual active bridge type three-phase rectifier (3P-IMDAB). A 400 V battery is being charged with a peak efficiency of 99%. Efficiency when only 10% of input voltage is applied decreases and becomes 98.7%. Its circuit layout resembles the dual active bridge (DAB) converter, but in this proposed design case, the primary winding of high frequency transformer is directly connected to the AC grid. The efficiency of the proposed converter increases because of the combined control techniques that are used, and the power density has a value of 4 kW/L.

Texas Instruments [115] proposed an interleaved configuration of a two-stage OBC of the same kind as in [109], but with an increased value of power density at 3.8 kW/L. The two stages in the proposed topology consist of a PFC circuit and a CLLC converter. The necessary voltage regulation is offered by a wide frequency range of 200–800 kHz.

A topology for a two-stage OBC rated at 11 kW is proposed in [116] by Virginia Tech (CPES). To charge an 800 V battery, it gets supplied from a single-phase AC grid. The two stages consist of a four-channel 350 kHz interleaved totem pole PFC and a CLLC 500 kHz resonant three-phase converter. Windings and inductors have been integrated for decreasing the size, therefore the topology enhances the power density. The two stages function at an efficiency of ≥98%, thereby yielding a peak efficiency of 96%.

Seoul Tech in [117] present a single-stage OBC converter which is extremely compact and uses an interleaved dual active bridge (DAB) type of configuration. The presented topology has a power density of 7.3 kW/L and a reduced efficiency value of 97.1%. In this paper the researchers did not talk about cooling arrangements or the EMI filter.

6. Current Challenges and Emerging Research Directions in On-Board Charger Technology for Electric Vehicles

6.1. Current Limitations of OBC Technology

One of the primary limitations lies in the efficiency and power density of current OBC designs. Traditional silicon-based power semiconductors are prone to significant switching losses, especially at higher operating frequencies. These losses, particularly above 100 kHz, limit the efficiency of the system and necessitate larger passive components, such as inductors and capacitors, which occupy substantial space within the OBC. The thermal management of these systems is another critical issue; as power densities increase, so do thermal stresses on components, which can lead to reduced reliability and lifespan. Conventional cooling systems, such as liquid cooling, add complexity and cost to the OBC, but may still struggle to manage localized hotspots generated by densely packed components.

The complexity of integrating multi-functional capabilities into OBCs also poses substantial challenges. Bidirectional operation, which is essential for vehicle-to-grid (V2G) and vehicle-to-home (V2H) applications, requires sophisticated control algorithms and additional components like bidirectional switches [1]. These additions increase the overall complexity and cost of the system, while also raising concerns about meeting electrical isolation requirements for safety. High-frequency transformers, necessary for galvanic isolation, introduce additional losses and bulk, further complicating efforts to reduce the system’s size and increase efficiency [11]. Furthermore, as OBCs aim to meet stringent electromagnetic interference (EMI) and electromagnetic compatibility (EMC) standards, the high dv/dt and di/dt rates in fast-switching devices exacerbate conducted and radiated emissions, necessitating complex filtering solutions that contribute to system size and cost.

The cost implications of advancing OBC technology cannot be overlooked. The adoption of wide bandgap (WBG) semiconductors like silicon carbide (SiC) and gallium nitride (GaN) offers superior performance but at a significant cost premium. SiC MOSFETs, for instance, can cost up to five times more than their silicon counterparts, which impacts the overall economic feasibility of OBC systems. Additionally, the precision manufacturing techniques required to achieve compact, high-efficiency designs increase production costs. Advanced cooling solutions and sophisticated PCB layouts for thermal management demand high manufacturing precision, adding to the complexity and cost of producing these next-generation OBCs.

6.2. Research Directions in OBC Technology

To address these limitations, several promising research directions are being explored, aiming to enhance the performance, efficiency, and integration of OBCs within EV systems.

One of the most promising areas of research involves the development of advanced converter topologies, particularly resonant and soft-switching converters. These topologies, such as LLC and CLLC resonant converters, enable zero-voltage switching (ZVS) and zero-current switching (ZCS) conditions, which significantly reduce switching losses and allow for operation at higher frequencies, up to 500 kHz [110]. This advancement can lead to substantial improvements in power density and efficiency. Additionally, the exploration of multilevel converter architectures, such as Neutral Point Clamped (NPC) and Flying Capacitor Multilevel Converters, offers improved voltage handling capabilities and reduced output harmonics, which can enhance overall system performance. Interleaved and modular designs are also gaining traction, as they allow for distributed thermal and electrical stresses, improving both reliability and efficiency.

Another critical area of research focuses on the integration of OBCs with vehicle powertrain components. Unified power electronics systems that combine OBC functionality with traction inverters and DC/DC converters can reduce the overall component count and improve system efficiency by sharing components and thermal management systems. For instance, the dual-use of motor windings in the charging process, as seen in Integrated Charger Inverter (ICI) systems, eliminates the need for separate inductors, reducing weight and improving efficiency [19,118]. This type of integration represents a significant step forward in creating compact, efficient powertrain systems.

The adoption of wide bandgap (WBG) semiconductors such as SiC and GaN is another promising research direction. These materials allow for higher efficiency and power density by operating at higher junction temperatures (above 150 °C) and switching frequencies (greater than 200 kHz), which can lead to a reduction in passive component sizes and an overall enhancement of system efficiency to levels exceeding 98%. However, further research is needed to optimize their application and address the challenges related to the long-term reliability and robustness of WBG devices, particularly under various operating conditions. Advanced packaging techniques, such as silver sintering and embedded die technologies, are being investigated to improve thermal management and reduce parasitic inductances, thereby enhancing device performance and lifespan.

Advanced control and optimization strategies are also being developed to maximize the performance of OBCs. Model Predictive Control (MPC) techniques are being implemented for real-time optimization of converter operations under varying load and grid conditions. MPC can anticipate future system states and adjust control variables accordingly, minimizing losses and improving dynamic response during grid disturbances or load changes. Furthermore, the application of Artificial Intelligence (AI) and Machine Learning (ML) in predictive maintenance and adaptive control strategies presents new opportunities. These technologies can analyze operational data patterns to predict component failures and optimize control strategies, thereby improving system reliability and reducing maintenance costs. Enhanced power quality and grid interaction are also critical research areas, with control methods being developed to improve power factor correction (PFC) and reduce total harmonic distortion (THD), ensuring compliance with grid standards like IEEE 519.

Bidirectional charging and V2G technologies represent another key research area. Developing robust communication protocols and standardized interfaces between EVs and the grid is essential for facilitating reliable V2G interactions. Protocols such as ISO 15118 [57] and Open Charge Point Protocol (OCPP) are being refined to handle complex energy markets and grid services. Moreover, energy management and grid support functions are being developed to allow EVs to provide ancillary services like frequency regulation and demand response. Real-time energy management systems are being designed to optimize charging and discharging schedules based on grid demands and electricity pricing, contributing to grid stability and offering economic benefits to EV owners. Standardization and interoperability across different EV models and charging infrastructures are also crucial for the widespread adoption of bidirectional charging capabilities.

Innovative thermal management solutions are critical for managing the increased thermal loads in high-power-density OBCs. Advanced cooling techniques, such as phase-change materials (PCM) and jet impingement cooling, are being explored to efficiently manage these thermal loads. For example, microchannel liquid cooling systems integrated directly into semiconductor packages can significantly enhance heat dissipation, allowing for higher power densities without compromising reliability. Optimization of Thermal Interface Materials (TIMs) is also a significant area of focus. High-performance TIMs with superior thermal conductivity and mechanical stability, such as graphene-enhanced composites, are being developed to improve thermal pathways between heat-generating components and heat sinks, reducing thermal resistance and operating temperatures.

Finally, modular and scalable design approaches are gaining importance in OBC development. Modular designs that allow for plug-and-play functionality enable flexibility in power scaling and simplify maintenance. For instance, modular interleaved converter designs can operate in parallel, allowing for easy adaptation to different power requirements and vehicle platforms. Standardized interfaces and components are also being emphasized to streamline manufacturing processes and facilitate easy integration across various EV models. This approach reduces design complexity and promotes interoperability among different manufacturers and suppliers, leading to more cost-effective and reliable OBC systems.

7. Future Trends and Challenges

Following the analysis of the SOA of EV OBCs, we talk about modern trends, barriers, and possible fields for additional development in the future. The current work that is being carried out in the design of OBCs for EVS is experiencing a growth in terms of popularity as well as technologically. This is being highlighted by the modern trends and challenges. Different research opportunities, challenges, and trends have been discussed in this Section.

7.1. Compact Converters with High Efficiency

In the last decade we have witnessed in increase in the efficiency of OBCs for EVs which is evident from the research that were carried out during this period. This increase in efficiency was helped by the introduction of modern semiconductor materials, advanced control techniques, and topologies exhibiting soft switching. It is already clear to us that single-stage OBC converter is more efficient compared to a two-stage OBC converter. The efficiency of single-stage converter OBCs is greater than the efficiency of two-stage OBCs. As discussed in the previous Section, in [114], a converter architecture with single-stage operation having an efficiency of almost 99% has been demonstrated. There is still a scope for improvement in the efficiency of the complete charging cycle of the EVs [1].

The use of compact OBCs creates room for several other components. Future OBCs keep getting lighter and smaller, making it simpler to install them in our vehicles without gaining a lot of weight. The use of compact OBCs requires addressing some challenges which include cost-effectiveness and improved heat dissipation methods.

7.2. Integration of the Powertrain

There has been an increase in efforts for creating and integrating different elements of an EV powertrain in a single compact package to achieve smaller and light weighted OBCs possessing very high power density. The use of motor windings instead of individual magnetic components, integration of cooling circuits, and combination of different Low Voltage Direct Current (LVDC) converters are potential areas of integration. The combination of circuit elements such as inverters, OBCs, and DC–DC converters into one unit results in saving of space, decreased weight, reduced overall cost, and enhanced thermal management. The process of merging an electric motor with its associated power electronic circuits elements results in increased efficiency and simplified vehicle design. The components being integrated should be compatible with each other. This technique makes use of advantages of different constituent elements to obtain best possible solution

7.3. Vehicle-to-Grid (V2G)

This comes under the heading of applications of bidirectional chargers. Various modes of bidirectional charging are collectively known as vehicle-to-X (V2X), which include vehicle-to-load (V2L), vehicle-to-vehicle (V2V), and vehicle-to-grid (V2G) [119]. This classification is determined on the basis of the load connected to EV [120]. If necessary, EVs can help the grid during periods of high load demand, as a result of which the stability of the grid is maintained. The V2G feature enables EVs to act as a source of energy and send energy (electricity) back into the grid, avoiding or at least delaying the load shedding [121]. V2G solutions are set for market launch and will improve grid support [20]. For the implementation of V2G [122], a dependable battery management system, and a precise state-of-charge predictions, and real-time grid data collection are very important [123]. In case of bidirectional charger topology, there is need for additional elements and control techniques to allow power flow in reverse direction. Although this sector is still in its infancy, the overall number of EVs on the road and the energy that they store in their batteries will substantially affect the services of the power grid.

7.4. Introduction of Wide Bandgap (WBG) Devices

WBG devices for OBCs has been widely established in the current scenario. Companies are working towards advanced substrates, interconnects, and sintering technologies to improve the voltage, current, and thermal capabilities of WBG devices. The application of WBG devices has resulted in enhanced power density and efficiency. At the same time, it also lowers the size and overall price [124].

Table 5. SiC and GaN material characteristics compared with silicon.

Properties	Silicon (Si)	Silicon Carbide (SiC)	Gallium Nitride (GaN)
Thermal Conductivity (Watts/cm2 K)	1.5	5	1.3
Critical Breakdown Voltage (104 V/cm)	0.3	3	3.5
Bandgap (eV)	1.1	3.2	3.4
Electron Mobility (cm2/V-sec)	1450	900	1500
Electron Saturation Velocity (104 cm/sec)	10	22	25

shows a comparison of properties between WBG devices and Si-based devices. It is evident from Table 5 that WBG devices are superior to traditional silicon devices [125]. The most widely used WBG materials include silicon carbide (SiC) and gallium nitride (GaN). The use of these WBG devices has increased, and this trend is expected to continue due to very strict packaging rules put forward by the automakers. By using WBG devices, there is no need for low-voltage series MOSFETs and fast-recovering parallel diodes [126]. Different substrates are being used in wide bandgap devices to further enhance their properties. These include SiC substrates (4H-SiC, 6H-SiC), GaN substrates (GaN-on-Si, GaN-on-SiC, GaN-on-GaN), diamond substrates, aluminum nitride (AlN) substrates, etc. The choice of the substrate is determined on the basis of requirement. In cases of Power MOSFETs and Schottky diodes, we can use a 4H-SiC substrate. For high frequency and high-power applications, as in Radio Frequency (RF) amplifiers, either diamond or GaN-on-SiC substrate can be used.

7.5. Modularity in Design

The concepts of modularity and scalability are very crucial during the OBC design for an EV. Module standardization is a key aspect of modular design, which means developing of OBCs that can work without any hindrance at various power levels. Modular converters are being adapted in the EV charger design process because of their versatility. A novel modular integrated OBC for EV is presented in [118] making use of a full bridge bidirectional converter on the AC side and an isolated bidirectional SEPIC converter on the DC side. The design should be able to be updated in case any upgrade is required with technological advancements. Modular design enables cost savings by using components that are common to several models. It also improves the platform’s adaptability to design, making it easier for automakers to manufacture vehicles with varying sizes and features.

7.6. Two-Layer Optimization Model for Improved Flexibility and Stability

In the context of optimizing OBCs, the flexibility and stability of the optimization network are critical, particularly as they relate to grid decision-making models. One of the emerging challenges in this domain is the integration of advanced optimization techniques that can handle the growing complexity and variability of EV charging demands.

A promising approach to address this challenge is the implementation of a two-layer optimization model. This model can enhance the decision-making process by separating the optimization problem into two distinct layers: one focused on the operational level (such as real-time charging management and power allocation) and the other on the strategic level (such as long-term planning and resource allocation).

The operational layer can address immediate needs, such as managing the dynamic interactions between the grid and a fleet of EVs, ensuring stability during peak loads, and optimizing charging schedules. The strategic layer, on the other hand, can take into account broader considerations, such as infrastructure investments, grid capacity planning, and the integration of renewable energy sources.

By adopting a two-layer optimization model, the EV charging network can become more adaptive to fluctuating conditions, ultimately improving both flexibility and stability. This approach also opens new avenues for research, particularly in developing algorithms that can efficiently manage the multi-objective nature of this optimization while maintaining computational efficiency.

8. Conclusions

To provide EV users with highly effective, affordable, and dependable charging options, research in the field of power converters and charging strategies is required. This also supports the widespread use of EVs. Modern charging infrastructure and intelligent control techniques are also fundamental aspects of achieving effective adoption of EVs.

Different types of EVs, their benefits over ICE vehicles, and the technologies employed have been reviewed in this article, along with various modes of charging and upcoming innovations. The article contains an in-depth discussion on EV charging technology and different EV standards. Three types of power levels exist in the charging process of EVs: Level 1, Level 2, and Level 3. According to the developing technology, conductive charging is classified into different categories: on-board and off-board chargers, exhibiting either unidirectional or bidirectional power flow.

This paper contains an overview of various OBCs for EVs along with a discussion of relevant standards. To meet the restrictions set on the weight, overall price, and volume, an OBC limits the power. An off-board charger’s primary advantage is that it charges batteries quickly, but requires reliable infrastructure.

In addition to simplifying connection problems and limiting hardware needs, unidirectional charging also tends to slow down battery deterioration. The bidirectional on-board chargers enable the power flow in reverse direction (from the EV to the grid). A DC-link capacitor is absent in case of a single-stage converter OBC. The overall volume of an EV charger is minimized using a single-stage charger with a higher power rating. Standards, grid codes, and EV charging strategies are thoroughly assessed to inspire the creation of innovative designs.

To conclude, this paper discusses problems that researchers are currently encountering and various trends being introduced. An OBC’s size, weight, and volume are positively affected by the introduction of wide bandgap devices. It also results in enhancement of energy levels which minimizes the charging time for an EV.