**1. Introduction**

Due to high fossil fuel prices, CO2 emissions and other health-threatening emissions, environmental awareness has shown a high interest in Electric Vehicles (EVs) [1]. Replacing conventional gasoline-powered vehicles with battery-powered EVs is expected to reduce fuel consumption significantly [2]. As a response to the ecological issues, a movement towards electrification is taking place in the transport sector. Many research studies have been provided to support this movement in [3–6]. To compete with conventional combustion engine vehicles and ensure the wide adoption of EVs, significant efforts are still needed. One of the main issues limiting the widespread penetration of

EVs is the long charging duration that usually exceeds 30 min [7]. In other words, one of the challenging problems limiting EVs' utilization is the lack of efficient and fast charging capabilities. Accordingly, Ultra-Fast Charging (UFC) is a necessary concept to be investigated to allow massive integration for EVs in the market, particularly when concerning long-distance travel [2]. In this regard, industrial and academic fields have been pushed to explore the Ultra-Fast EV Charging (UF-EVC) concept. This is due to the fact that fast-charging stations allow the recharging process to be done in a few minutes and hence overcoming EVs limitation problem. Studies have shown that the charging process affects EVs' adoption significantly. Therefore, future EV charging stations should have the capability of charging EVs from 10% State-of-Charge (SoC) to 90% SoC in few minutes [8–10]. Nowadays, battery technologies are evolving, enabling the EV's battery to be charged at higher power levels. Currently, offboard fast-charging stations are provided to speed up the charging process [10–14].

Conventionally, the charging methods are categorized based on the charging speed and power into three different levels, which are: level 1, level 2, and level 3 (also termed as the DC fast charging) [15]. Shortly, a new battery technology that reduces the charging time to 3 min is expected to be commercialized [15]. This introduces a new charging level termed the ultra-fast/super-fast charging level, which is different from the present DC fast charging that is commonly used in industrial and academic fields 15. Table 1 shows the electrical specifications for the EV charging stations [15].


**Table 1.** Electrical vehicle (EV) charging stations' electrical specifications.

To enable UFC for EVs, advanced power electronics converter technologies play an essential role in terms of both flexibility and efficiency. Accordingly, this paper focuses on high-power DC-DC converters that can be employed in UFC stations. As mentioned earlier, one of the main obstacles limiting the broad adoption of EVs is the long charging time. To avoid this problem, UFCs should be designed at a higher power level. Toward this direction, BMW and Porsche, as a part of the Fast Charge Consortium (a group of industrial companies engaged in the "Fast Charge" research project), have demonstrated the latest super-fast EV charger that can considerably charge EVs faster than the present charging methods [16]. Similar systems are demoed by the two companies, where the electric prototype for Porsche is capable of charging at a power level of 400 kW in less than 3 min for the first 100 km with a battery capacity of 90 kWh. While the experimental system for BMW is capable of charging at a power level of 350 kW in 15 min (10% to 80% SoC) [16]. The charging process is done using a DC-DC converter that converts a charging station input voltage of 800 V to a lower voltage of 400 V [17]. The latest prototypes by both companies can work up to 900 and 500 A, meaning that it can charge at a power level of 450 kW, which is three to nine times the capacity of the existing up-to-date DC fast-charging stations [16,17]. This capacity increase significantly reduces the duration of the charging time. Tesla has shown significant efforts to introduce fast charging for EVs. However, the current cars presented by Tesla can only charge at a maximum of 120 kW. Tesla will add enhanced superchargers in 2020. Nonetheless, the charging speed of those cars is boosted to double the charging rate of the current vehicles [16].

The Fast Charge research group is investigating the required technical specifications in terms of the fast charging infrastructure and the EV to match the high charging capacities. Siemens, as a part of the research group, has provided an energy supply system to be used in the project, allowing researchers to examine the fast-charging capacity limits demonstrated by the EV's batteries. The system can handle high voltage that reaches up to 920 V, which is the voltage level predicted for the future EVs [17]. The controller of the charger makes sure that the output is modified automatically according to the EV, allowing different EVs to be charged using a single infrastructure. To illustrate, the UFC station can be used with battery systems rated at 400 and 800 V, where the charging capacity is adapted automatically to the maximum charging capacity on the EV side. The system is characterized by its flexibility and modularity, allowing multiple EVs to be charged at the same time instant [17].

Such fast-charging stations require DC-DC converters of high-power to be designed to achieve high reliability and high efficiency for the system. One of the fast-charging stations' requirements is to develop the DC-DC converters in a modular structure since such configuration can provide redundancy, easier maintenance, as well as scalability, and ride-through capability [18].

To further illustrate, to meet the requirements of the high-voltage high-power fast charger, two approaches are established. The first approach is through semiconductor devices with high-voltage and high-current ratings to be integrated into the two-level converter topologies with series/parallel connections. However, the series connection of power switches results in the unsymmetrical sharing of voltage among the switching devices because of the switches' unequal parameters, such as the switching delays, leakage inductance, and collector-to-emitter capacitance. Accordingly, voltage balancing methods are required to avoid any failure [19].

In the second approach, power electronic converters are usually pursued to be built in a modular manner in fast-charging stations [20]. Modular converters contain several numbers of smaller modules. Building converters in a modular way is a cost-effective solution. Besides, smaller modules can be hot swapped in failure cases, which makes the maintenance of such converters easier. Moreover, the number of modules can be scaled up according to the power rating of the system.

Furthermore, by installing more modules, the concept of redundancy can be established. In other words, in modular structure-based DC-DC converters, each cell handles a small portion of the total input power. Consequently, the selected power switches are of lower voltage ratings, hence, higher switching frequency capability. Therefore, the converter efficiency is improved due to lower losses, and the transformer size is reduced due to the increase in the switching frequency [21,22]. To avoid the demerits associated with the first approach, modular converters' topologies, such as Multimodule Converters and Modular Multilevel Converters, are used to provide the modularity feature and achieve high-voltage and high-power requirements [23]. Nonetheless, the multimodule DC-DC converters are considered in this paper since the modularity concept is not limited only to the power electronics but also is extended to include the magnetics. Moreover, higher switching frequencies can be achieved in the AC link, which results in reduced weight and size of the overall power converter. Although the high-frequency transformer employment allows for a reduction in the overall converter size and weight, the overall system cost is increased, especially with the power increase. Multimodule converters are applicable for EV UFC since scalability, reliability, redundancy, and ride-through capability can be provided. Besides, such converters utilize low-power modules where each module handles only a fraction of the total required power. However, the system complexity increases with the increase in the number of modules. Moreover, the impact of EVs UFC on the grid should be considered. Therefore, coordinated charging should be devised to address challenges that might be encountered with uncoordinated EVs UFC, particularly when multiple batteries are charged through UFC mode. Furthermore, the on-going technology development in the batteries' industry supports the proliferation of EVs UFC through introducing high capacity batteries that can accept such mode of charging.

Multimodule DC-DC converters can offer a bidirectional power flow through utilizing submodules that contain Dual Active Bridge (DAB), Dual Half Bridge (DHB), or series resonant converters, where each configuration has its pros and cons [24,25]. In multimodule converters, soft switching operation, as well as higher switching frequency, can be achieved, resulting in a significant reduction in the component volume without sacrificing the efficiency [24]. The possible architectures of connecting multimodule-based DAB units are classified into four main categories, which are: Input-Series Output-Series (ISOS), Input-Parallel Output-Parallel (IPOP), Input-Series Output-Parallel (ISOP), Input-Parallel Output-Series (IPOS) [26].

A typical fast charger is usually connected to a 400 VAC grid (570 *VDC* rectified) [27]. However, this low voltage results in high current and causes high losses in the magnetic components, bus bars, and switching devices [28]. Accordingly, connecting the fast charger to a higher input voltage level (10–40 kV) will reduce the current and conduction losses in the DC-DC converter. In [29,30], a DC link with a voltage of 1000 V has been proposed to the high power, while, in [31], the fast charger is connected to a medium voltage connection of 2.4 kV. In [28], a multiport converter for EVs' fast chargers has been proposed, where the charging process is done via a 6 to 10 kV supply grid.

In [15], a power architecture for the upcoming super-fast EV charging stations has been proposed. The presented architecture in [15], is interfaced with a medium voltage grid with a voltage level of 4.8 kV. It supports the grid functionality since it can allow bidirectional power flow and can reduce during the EV charging the conduction energy loss of the grid. However, in [7], DC-DC converters for high-power EV UFC stations have been presented. The semiconductor devices voltage ratings are reduced through splitting the DC input voltage. In addition, a modulation scheme named triangular current modulation is applied to achieve Zero Voltage Switching for all the switching devices. This accordingly increases the efficiency of the system. The converter in [7] can achieve an efficiency of 99.5%. To reduce the ripple content of the output current and achieve a high power density, multiple modules are parallel interleaved. In [2], reactive power operation is investigated using an offboard charging station. In [32], two different converter architectures for UF-EVC stations are presented. The first architecture is based on low-frequency isolation (non-isolated DC-DC converter), while the second approach is based on high-frequency isolation (galvanic isolated DC-DC converter). Technical evaluation for the two architectures is carried out where the pros and cons of each topology are highlighted. Besides, simulation results elaborating on the impact of the DC fast charging station on the grid are also provided in [32]. In [1], to realize medium-voltage UF-EVC stations, a multiport power converter has been proposed. A cascaded H-bridge is utilized at the grid side. Besides, to reduce the charging station effect on the grid, the battery energy storage elements are integrated in a split manner on the level of each submodule. Talking about the DC-DC conversion stage presented in [1], multiple Dual Half Bridge units are connected in parallel, where the multiport concept at the output side is achieved by selecting different submodule configurations. This is done to charge multiple EVs at the same time instant without the need for extra chargers. Operation modes, as well as the control techniques, have been addressed in [1].

In [33], a Full-Bridge Phase-Shifted DC-DC converter that combines the features of the double inductor rectifier and the conventional hybrid switching converter has been proposed for EV DC fast chargers. The principle of operation, as well as the characteristics and design specifications, are provided. The presented DC-DC converter in [33], can achieve both: zero voltage switching and zero current switching in the leading and the lagging legs, respectively. In [31], a medium-voltage high-power isolated DC-DC converter for EVs fast chargers have been presented. The employed DC-DC converter is a modular-based structure utilizing silicon carbide switching devices to convert a single-phase rectified input voltage of 2.4 kV to a variable DC output. It is a unidirectional converter that is connected in series from the input side and connected in parallel from the output side.

In [34], the AC-DC and DC-DC stages of an EV charger have been studied. The DC-DC stage utilizes interleaved DC-DC converters to be connected to a high-voltage network (13.8 kV). The interleaved DC-DC converters are accompanied with several advantages that can be highlighted in lower input ripple and lower inductors cost and size. The charger presented in [34] allows for bidirectional power flow so that it can support renewable energy sources and smart grid applications. It is suitable for a high-power fast charger of 400 kW with an EV battery with a rated voltage level of 500 V. However, in [35], a fast charger that is based on a single stage has been proposed. In other words, the power factor correction and zero voltage switching are achieved in one single AC-DC stage. The charger utilizes DAB units that are connected in series and parallel at the input and output sides, respectively. The use of one stage resulted in the DC link capacitor elimination, allowing for higher efficiency and higher power density to be achieved. In [18], a fast-charging system for EVs is proposed. The system consists of a 15 kW multimodule converter that utilizes a three-phase rectifier and a Full-Bridge DC-DC converter. Multiple units are used in a modular manner and connected in parallel at the input side and series or parallel at the output side. To clarify, modules are connected in IPOS to provide a higher voltage that reaches up to (1000 V) or connected in IPOP to provide higher output current at a voltage level of (500 V).

In [36], a power converter that interfaces a three-phase medium-voltage grid with EV batteries has been proposed. The AC-DC stage utilizes a cascaded H-Bridge converter, in which the battery energy storage systems are integrated in a split manner to reduce the effect of the charging station on the grid. Modular DC-DC converters utilizing the Full-Bridge Phase-Shifted topology are employed to charge the EV battery. The DC-DC converters are connected in parallel to provide high currents.

According to the latest technical specifications for fast charging stations, the charging process can be done through a 400 VAC or it can be done through a medium voltage grid supply that ranges from 10 to 40 kV to avoid high current and high losses. Assuming a fast charger rated at 350 kW and EV battery rated at a high voltage level (400–920 V). Different scenarios of the mentioned specifications result in the four different architectures of the multimodule DC-DC converters, which are ISOS, IPOP, ISOP, and IPOS. In this paper, these scenarios are discussed along with the control strategy for each architecture. To support the power converter controller design, the Small-Signal Model (SSM) for the four architectures is studied in detail. Moreover, to ensure equal power sharing among the employed modules, the control scheme for the four architectures is investigated. Moreover, a generalized SSM for any multimodule DC-DC converters' connection, including Input-Series Input-Parallel Output-Series Output-Parallel (ISIP-OSOP), is provided. This is achieved by studying the SSM of Full-Bridge Phase-Shift (FB-PS) DC-DC converter, two-module IPOS, three-module ISOP, and four-module Input-Series Input-Parallel Output-Series (ISIPOS) presented in [37–42], respectively. After deriving the generalized model, the model is verified with the multimodule configurations presented in [38–42].

The main contribution of the paper can be summarized as follows:


This paper is organized such that: Section 2 presents the ISOS circuit configuration as well as the ISOS SSM. Section 3 presents the ISOP multimodule DC-DC converter referring to the work that has been previously published in [40]. Sections 4 and 5 provide the circuit configuration and SSM for the IPOP and IPOS DC-DC converter, respectively. Section 6 addresses the generalized SSM for the ISIP-OSOP multimodule DC-DC converter along with its verification. Section 7 provides the control scheme for the four architectures to achieve equal power sharing among the employed modules. Finally, Section 8 presents the conclusion.
