Recent Advances in DC-DC Converters for Electric Vehicles

. The emergence of electric vehicles (EVs) has become a better alternative to withstand enormous damage to the environment and human life due to the widespread use of conventional cars. As a vital element in EVs, this paper analysed and compared the key features of different DC-DC topologies from different areas, then, explored strategies to improve DC-DC converter performance. The purpose of all these studies and designs is to help researchers in the automotive industry to choose the right DC-DC converter topology for their needs to achieve the required power density. By continuously improving the design and evaluation of DC-DC converters, it can improve the performance and efficiency of electric vehicles, and further promote the use of sustainable energy. This will help reduce reliance on traditional fuels, lower carbon emissions, improve air quality and move the entire transportation industry towards a greener, more sustainable future.


Introduction
Over the past few decades, people have witnessed enormous damage to the environment and human life due to the widespread use of conventional cars.According to experts, waste products such as carbon dioxide emitted by fuel vehicles not only raise the sea level but also contribute to global warming, polluting the atmosphere and so on [1].In this context, the emergence of electric vehicles (EVs) has become a better alternative.It is an innovative solution that offers people unprecedented opportunities.Electric vehicles are mainly divided into three basic types: the electrical system of vehicles is connected to an inverter via a highvoltage direct current (HVDC) bus, powered by batteries and/or supercapacitors, driving a high-voltage threephase electric motor (EM).According to the structural diagram of EVs in Fig. 1, DC-DC converters have strong applications in the automotive industry.It not only transfers electrical energy from the battery or supercapacitor to the drive motor, but also absorbs regenerative braking energy and converts it into stored energy.Since the output voltage of different types of batteries and supercapacitors is usually unstable and low, HVDC converters are required to temporarily store the source energy and step up the low unstable voltage supply to a higher and stable level to meet the power requirements of EVs.The bidirectional DC-DC converter acts as an energy conditioner, which can switch the direction of electrical energy as needed.When a battery or supercapacitor needs to be charged, an inverter converts DC power from a generator or external power source to a voltage and current suitable for storage devices.And when it is necessary to output power from the battery or supercapacitor to the motor, the converter can convert the DC power in the storage device into high-voltage DC power suitable for motor driving.Through the precise control of bidirectional DC-DC converters, electric vehicles can achieve efficient energy transmission and conversion [2].This technology one hand improves the utilization rate of electric energy, other hand improves the energy efficiency of the overall system.At the same time, by recovering regenerative braking energy and storing it, electric vehicles can maximize driving range and reduce dependence on external power sources.
HVDC converters are highly regarded for their high voltage gain and ability to transfer power over a wide voltage range [3].Its light weight, miniaturization, low price and high efficiency and reliability make it popular in many fields.However, for the automotive industry, the requirements for DC-DC converters are more stringent, requiring converters with the characteristics of miniaturization, reliability, parameterization, light weight, controllability, scalability, and high efficiency.Especially in the electric vehicle industry, to promote the popularization of electric vehicles, researchers are also working on the study of fast charging stations (FCHAR).The goal of FCHAR is to shorten the charging time and build more charging stations to meet the demand of electric vehicles.For fast-charging compatibility, these charging stations typically employ high-power converters.In this field, HVDC converter can play an important role.Its high voltage gain can effectively increase the output power of the charging station, resulting in faster charging speed.Moreover, the miniaturization and light weight of HVDC converters make them easier to integrate into charging stations, saving space, and improving the scalability of charging stations.In addition, the high efficiency and reliability of the HVDC converter ensures the stability and safety of the charging process.Therefore, applying HVDC converters to FCHAR can meet the high requirements of the automotive industry for miniaturization, reliability, parameterization, light weight, controllability, scalability, and high efficiency.
Currently, silicon-based semiconductor technologies are mainly used in the power systems.However, this technique has some performance limitations.In contrast, wide bandgap semiconductors (WBGS) have greater potential in this regard [4].It can significantly increase power density and efficiency, and have better manufacturing performance and lower cost.Silicon carbide (SiC) semiconductors have been widely used in the design of high-power DC-DC converters.The special properties of SiC semiconductors allow it to handle higher power levels with higher efficiency and lower energy losses.In contrast, conventional silicon-based semiconductor materials tend to perform poorly in highpower applications, prone to heat and energy loss.However, the application of gallium nitride (GaN) semiconductors in the field of electric vehicles needs further research.GaN semiconductors have higher electron mobility and higher critical electric field strength, which makes them ideal for handling power conversion in high-frequency and high-temperature environments.Due to its excellent performance, GaN semiconductors are expected to play an important role in power converters for electric vehicles, improving overall system efficiency and performance.These advanced semiconductor materials will drive technological advancements in electric vehicles, enabling higher energy efficiency and more sustainable modes of transportation.Future research and innovation will further promote the development of wide bandgap semiconductor technology and accelerate the popularization and promotion of electric vehicles.
Different DC-DC converter topologies have their own unique characteristics and advantages and disadvantages.When selecting a suitable converter topology, several factors need to be considered comprehensively.The first is the requirement for power conversion efficiency.High-efficiency converters can minimize energy loss and improve the efficiency of the entire EV system.The second is the consideration of cost and device size.It is necessary to choose a converter solution with reasonable cost and appropriate size to meet the actual needs of electric vehicles.In addition, reliability is also an important consideration, especially during the operation of electric vehicles, there are higher requirements for the stability and reliability of the converter [5].
By selecting a suitable converter topology, combined with advanced control strategies and optimized design, the power density and system efficiency of electric vehicles can be improved, battery life can be extended, and users can be provided with a better driving experience.At the same time, the popularization and promotion of electric vehicles will also make positive contributions to environmental protection and the improvement of human life quality.By reducing exhaust emissions and noise pollution, electric vehicles can help improve air quality.The purpose of this article is to help automotive industry experts and ordinary people or enthusiasts of the automotive industry choose the right DC-DC converter topology to meet their needs in order to achieve the required power.To provide environmental experts with environmental solutions that can improve the performance and power of electric vehicles，extend battery life, and further promote the use of sustainable energy, which will help reduce carbon emissions and improve air quality to maintain sustainable development.This paper introduces the different DC-DC converter.They are then classified and discuss the advantage and disadvantage, and then explore the plan to improve the performance of DC-DC.

Comparison between different DC-DC Converters
As shown in Fig. 2, the converters can be divided into various types.In this section, several representative topologies are comprehensively reviewed, analysed, and compared.

Boost converter
Boost converters (BCs), as shown in Fig. 3, have the widest range of applications in electricity, suitable for applications requiring higher voltages.Boost converters typically employ Pulse Width Modulation (PWM) control techniques, featuring relatively simple control circuits that are easy to implement.In this physical element the genera function is to reduce the current, reducing the load on the input power source.By using appropriate electrical isolation components like transformers, boost converters can achieve electrical isolation between the input and output, enhancing system safety.However, boost converters introduce power losses through the inductor and switch components, leading to potentially lower system efficiency.Boost converters may exhibit some output voltage fluctuations when facing load variations, necessitating appropriate control strategies to stabilize the output voltage.Boost converters have limited adaptability to linear loads (such as resistive loads), In this physical element it has a wide range of physical variations.Boost converters find wide applications in power electronic systems, including solar battery arrays, electric vehicles, power management, and other fields.Their usually like to make high outputs makes them crucial in many applications.

Interleaved 4-phase boost converter
As shown in Fig. 4, the interleaved 4-phase boost converter (IBC) achieves these benefits by utilizing four identical levels with separate magnetic core inductors and employing a continuous phase shift based on the ratio of the switching period to the phase number [5].These features contribute to improved performance, efficiency, and stability of the converter.The IBC structure allows multiple switches to operate simultaneously, effectively distributing the input and output currents among multiple branches, reducing current and voltage stress on each branch, thus achieving higher power density.Interleaved connection of multiple phases disperses the output current among multiple branches, resulting in smaller overall output ripple current, reducing the output filter and improving the electromagnetic interference (EMI) performance.By distributing the input and output currents, power losses on each switch and inductor element are reduced, thereby improving the system's efficiency.
However, the interleaved 4-phase structure requires coordinated control of multiple phases, making the control circuit more complex compared to traditional single-phase boost converters.Due to the requirement of multiple switches, inductors, capacitors, and other components, the interleaved 4-phase structure has relatively higher hardware costs.Although the interleaved 4-phase structure can reduce output ripple current, the higher switching frequency and coupling between multiple phases may increase EMI issues in the system.

Boost Converter with Resonant Circuit
The boost converter with resonant circuit (BCRC) is the most widely used in circuit theory fundamentals [6].The Fig. 2. Classification of DC-DC converters [3].
, 02008 (2023) introduction of the resonant circuit can the switching speed of the switches, reducing EMI during switch transitions, which is beneficial for meeting electromagnetic compatibility (EMC) requirements.The BCRC converter can adjust the output voltage by tuning the parameters of the resonant circuit, providing a certain range of adjustable output voltage.Fig. 5. Boost converter with resonant circuit [3] Compared to traditional boost converters, the BCRC converter involves the design and control of the resonant circuit, requiring more complex design and analysis.The stability of the resonant circuit is an important consideration in the design of the BCRC converter, requiring the proper selection of resonant frequency, resonant capacitance, resonant inductance, and other parameters to ensure system stability and reliability.

Full Bridge Boost Converter
The full bridge boost converter (FBC) is designed with three functional stages.These stages work in conjunction to achieve efficient power conversion.The conversion of AC to DC is the main role of the rectifier.To prevent core saturation and ensure stable operation, the converter takes advantage of it.The hysteresis loop represents the relationship between the magnetic flux and the applied magnetic field.By utilizing the negative region of the loop, the converter avoids reaching the saturation point of the iron core.This allows the magnetic flux to oscillate within a desirable range.the converter can regulate the output voltage despite fluctuations in the input source voltage.
According to an expert on circuit theory, the FBC uses the most advanced technology, thereby reducing the switching losses of the switches and improving the conversion efficiency [1].Precise adjustment of the output voltage can be achieved by controlling the switching frequency.However, the design of the FBC converter is relatively complex, requiring considerations for switch selection and control circuit design.Due to high-frequency switching operations and rapidly changing currents and voltages, the FBC converter may introduce certain levels of EMI, necessitating appropriate measures to suppress EMI.

Isolated zero-voltage switching converter
As shown in Fig. 7, the isolated zero-voltage switching converter (ZVSC) employs a dual half bridge topology along with a main transformer to achieve efficient power conversion [7].To achieve soft switching, small parallel capacitors are strategically placed within the converter circuit.In addition, the converter achieves uniform zero voltage in any current direction without the need for additional components.ZVSC utilizes the zero-voltage switching (ZVS) technology, reducing switching losses of the switching devices, and improving conversion efficiency.ZVSC can provide a stable output voltage and have good suppression capability against input voltage fluctuations.By employing the ZVS technology, ZVSC reduces the EMI generated during switching transients.ZVSC converters typically use transformers for electrical isolation, providing a safe output voltage.
However, ZVSC converters have relatively complex control circuits, requiring precise control of the switching timing and frequency of the switching devices.To achieve zero voltage switching, ZVSC converters typically require high switching frequencies, which may increase switching device losses and introduce other issues such as electromagnetic interference and increased transformer size.To achieve high efficiency and stability, ZVSC converters have high requirements for component parameters and design, necessitating careful consideration of inductors, capacitors, transformers, and other components.

Sinusoidal amplitude high voltage bus converter
As shown in Fig. 8, sinusoidal amplitude high voltage bus converter (SAHVC) is a converter specifically designed for high voltage bus conversion applications [6].One of the key components in the SAHVC is the high-frequency multi-winding transformer.Through operating in SCCC mode, and incorporating common mode symmetry, the SAHVC achieves noise-free operation, high spectral purity, and ensures reliable power conversion in high voltage applications.
SAHVC converters can achieve precise control of the output voltage by adjusting the switching frequency and duty cycle of the switching devices, providing highprecision output voltage.SAHVC converters can reduce energy through appropriate control strategies and filter designs, reducing interference to other equipment.SAHVC converters employ advanced switching devices and control algorithms, achieving high conversion efficiency and reducing energy losses.SAHVCs are for high-voltage and high-power applications, incorporating suitable protection circuits and fault detection mechanisms to improve system reliability and stability.
However, the design of SAHVC converters is relatively complex, requiring considerations for switching device selection, control strategies, filter design, and other aspects, demanding high engineering skills and experience.Due to the need for precise control and low harmonic content, SAHVC converters may require expensive components and controllers, increasing system costs.SAHVC converters typically require stable power supply and low grid fluctuations, imposing certain requirements on the quality of the power grid.

Multi-port isolated DC-DC converter
The multi-port isolated DC-DC converter (MPC), as shown in Fig. 9, is a solution that caters to the requirements of systems with multiple input sources and the need for electrical isolation between these sources and the load [8].And through interleaving the switching cycles, the converter can distribute the ripple energy among multiple switching devices, thereby minimizing the overall ripple effect.In the context of Battery, this converter plays an important role in integrating and utilizing the power from multiple sources effectively, while still enjoying the benefits of a single power source.MPC converters have multiple input and output ports, allowing simultaneous handling of multiple power inputs and load outputs, enabling interconnection of multiple voltage and power levels.MPC converters efficiently convert the energy from input ports to the required electrical energy at the output ports, improving energy utilization and system efficiency.The multiple port design of MPC converters provides excellent electrical isolation, effectively isolating electrical disturbances between inputs and outputs, enhancing system stability and reliability.The multiple port structure of MPC converters allows for flexible configuration and expansion, enabling the addition or removal of input and output ports to suit different application scenarios.
The multiple port structure of MPC converters brings increased design and control requirements, necessitating careful consideration of input-output matching, power management, and signal interaction, which increases the complexity of the design.Due to the multiple port structure and complexity, MPC converters have higher costs, including component costs, control circuit costs, and system integration costs.Due to the energy conversion and matching between multiple inputs and outputs, MPC converters may experience additional power losses and efficiency reduction, requiring proper design and optimization.

Multi device integrated bidirectional DC-DC converter
The multi device integrated bidirectional DC-DC converter (MDIBC), as shown in Fig. 10, is a multi-port converter that allows for the integration of multiple power sources, including batteries and supercapacitors, into the vehicle's power system [9].By incorporating phase interleaving technology, which involves dividing the switching cycles into multiple phases, the converter can reduce the number of parallel devices required per phase, improving efficiency and power distribution.
MDIBC converters integrate multiple DC-DC converters into a single package, achieving high integration and compact design, reducing system volume and weight.MDIBC converters can perform bidirectional energy conversion, transferring energy from one input port to another load output port, while achieving energy feedback and recovery.MDIBC converters efficiently convert energy from input ports to the required electrical energy at the output ports, improving energy utilization and system efficiency.The multi-device integration structure of MDIBC converters allows for flexible configuration and expansion, enabling the addition or removal of input and output ports to suit different application scenarios.The multi-device integration structure of MDIBC converters brings , 02008 (2023) increased design and control requirements, requiring consideration of matching between devices, power management, and signal interaction, increasing the design complexity.Due to the multi-device integration structure and complexity, MDIBC converters have higher costs, including component costs, control circuit costs, and system integration costs.Due to the energy conversion and matching between multiple devices, MDIBC converters may experience additional power losses and efficiency reduction, requiring proper design and optimization [10].

Future Aspects
Table 1 summarized the advantages and disadvantages of different converters.This section provides some potential directions for improving the overall DC-DC converter.

Conversion efficiency
By selecting efficient power switching devices such as advanced MOSFETs, SiC, or GaN devices to reduce power losses, the overall efficiency can be greatly enhanced.
With their lower resistance and faster switching speeds, these devices (advanced MOSFETs, SiC, or GaN devices) outperform traditional siliconbased devices, leading to reduced power dissipation and enhanced performance.Optimizing control strategies, such as current mode control or voltage mode control, is crucial for enhancing the response speed and efficiency of the converter.By carefully selecting and implementing the appropriate control scheme, the converter can achieve faster transient response and better stability.Lowering the switching frequency and switching losses while maintaining an appropriate operating point is another key aspect of converter design.By operating the converter at a lower switching frequency, the conduction and switching losses can be minimized, leading to higher overall efficiency.Proper design techniques, such as employing soft-switching techniques or optimizing the converter's operating conditions, can help achieve the desired balance and maximize efficiency.

Output quality and stability
Adding appropriate output filtering circuits to reduce output ripple and noise is essential.These circuits, such as LC filters or low-pass filters, help smooth out the Introducing precise feedback control mechanisms, such as using high-precision sensors, is crucial for enhancing output stability and accuracy.By continuously monitoring the output voltage or current and providing feedback to the control circuit, the converter can adjust its operation to maintain the desired output and minimize deviations.Adopting suitable load adjustment strategies is important to accommodate load variations while maintaining output stability.This can include techniques such as load regulation, where the converter adjusts its output voltage or current in response to changes in the load.By dynamically adapting to different load conditions, the converter can ensure stable operation and optimal performance.

Size and cost
Implementing integrated designs that integrate key components and functions is an effective approach to reduce the size of the circuit board and the number of components.This integration not only saves space but also simplifies the manufacturing and assembly processes.Optimizing magnetic components, such as transformers and inductors, is crucial for minimizing size and power losses.By carefully designing these components with efficient core materials, appropriate winding configurations, and optimal shapes, their physical size can be reduced while maintaining high performance.This not only enhances the converter's performance but also contributes to its energy efficiency by minimizing wastage in the magnetic elements.Choosing efficient capacitors and resistors is important to reduce energy losses and volume.By selecting capacitors with low equivalent series resistance (ESR) and resistors with low power dissipation, energy losses in these components can be minimized.Additionally, using capacitors with high capacitance density and resistors with smaller physical sizes can help reduce the overall volume of the circuit, making it more compact and space-efficient.

System reliability and protection
Adding overvoltage protection circuits is crucial to prevent the output voltage from exceeding the set range.These circuits continuously monitor the output voltage and activate protective measures.By implementing overvoltage protection, the converter can safeguard connected devices and prevent potential damage or malfunctions.Implementing over-temperature protection mechanisms is essential to avoid overheating damage to circuits and devices.These mechanisms utilize temperature sensors to monitor the temperature of critical components or the overall system.Overtemperature protection ensures the longevity and reliability of the converter by preventing overheatingrelated failures.Employing short-circuit protection circuits is vital to prevent damage caused by output short circuits.These circuits monitor the output current and detect any abnormal increase or sudden surge.When a short circuit occurs, the protection circuitry responds by interrupting the power supply, limiting the current, or temporarily disabling the converter, minimizing the risk of damage or fire hazards.

Enhancing Safety
Considering isolation designs for input and output is crucial to prevent electrical isolation failures.By incorporating appropriate isolation techniques, such as using isolation transformers or optocouplers, the converter can ensure that the input and output sides are not directly electrically interconnected.This helps to protect sensitive components and systems from potential voltage spikes, noise, or ground loop issues.
Adding overcurrent protection mechanisms is important to prevent the output current from exceeding the rated value.These mechanisms monitor the output current and activate protective measures, such as reducing the current flow or interrupting the power supply, when the current exceeds a predetermined threshold.Overcurrent protection safeguards the converter and connected devices from potential damage caused by excessive currents, helping to ensure safe and reliable operation.

Conclusion
DC-DC converters are commonly used power conversion devices in EVs, converting the DC voltage from the input power source to different voltage levels of DC output.This paper introduced the principles, classifications, and key components of DC-DC converters, as well as evaluation indicators and potential improvement strategies.When selecting and designing DC-DC converters, it is crucial to understand different types of converters and their operation principles.To improve system reliability and protection, it is advisable to incorporate overvoltage, over-temperature, and shortcircuit protection circuits.In summary, by optimizing selection and design, the performance of DC-DC converters can be enhanced.High efficiency, stability, small size, low cost, reliability, and safety are crucial factors to consider when designing DC-DC converters.These improvement strategies provide guidance to engineers in designing better DC-DC converter systems that meet the requirements of various application fields.

Table 1 .
Comparison between different DC-DC converters