Comprehensive Review and Comparison of DC Fast Charging Converter Topologies: Improving Electric

Vehicle Plug-to-Wheels Efficiency Janamejaya Channegowda, Student Member, IEEE, Vamsi Krishna Pathipati, Student Member, IEEE,

and Sheldon S. Williamson, Senior Member, IEEE

General Motors-Automotive Center of Excellence (GM-ACE) Department of Electrical, Computer, and Software Engineering

Faculty of Engineering and Applied Science University of Ontario-Institute of Technology

2000 Simcoe Street North Oshawa, ON L1H 7K4, Canada

Tel: +1/(905) 721-8668, ext. 5744 Fax: +1(905) 721-3178

EML: sheldon.williamson@uoit.ca URL: http://www.engineering.uoit.ca/; http://ace.uoit.ca/

Abstract—The commercial success of electric vehicles (EVs)

relies heavily on the presence of high-efficiency charging

stations. This paper provides an overview and a comprehensive

performance comparison of the present status and future

implementation plans for DC fast charging infrastructures and

converter topologies. The paper also discusses critical

consequences of DC fast charging stations on the AC grid.

Different power converter topologies for DC fast charging are

presented, compared, and evaluated, based on the power level

requirements, efficiency, cost, and technical performance

specifications. The paper focuses specifically on Level-3 DC

fast charging converter topologies and their performance

comparison. Finally, the paper presents a detailed well-to-

wheels (WTW) analysis from an energy-efficiency standpoint.

The most important part of this analysis focuses on the effect

of usage of various charging levels and charger topologies on

the all-important plug-to-battery (P2B) energy-efficiency

within the overall context of WTW energy cycle efficiency.

Keywords—Batteries, chargers, control, dc/dc converters,

energy storage, power electronics, transportation.

I. INTRODUCTION

The increasing interest in developing vehicles running on alternate and renewable sources of energy has led to a spurt of research in the direction of improving the technologies involved in the electric and plug-in hybrid electric vehicles (EVs/PHEVs) [1]-[3]. Along with the improvement of these EV technologies, several initiatives have been undertaken by various government organizations across the globe to push for the usage of vehicles which run on alternate sources of energy. Several policies have been brought into effect to help in electrifying commercial vehicles.

The growth of the EV market has led to the obvious issue of coming up with novel and innovative ideas to charge them. Chargers are an integral part of EV plug-to-wheels (PTW) drivetrain efficiency. The intermediate stage of

charging includes the plug-to-battery (P2B) energy flow stage. Ideally, the PTW efficiency for EVs should be close to 45-50%. The PTW energy flow includes P2B and battery-to-wheels (B2W) efficiency. In order to improve the PTW energy-efficiency, a high efficiency, high reliability, high power density, and cost-effective charger design is mandatory. The terminology “plug-to-wheels (PTW)” for an EV is similar to the terminology “tank-to-wheels (TTW)” for a conventional internal combustion engine (ICE) vehicle, except that in the case of EVs, the fuel tank is replaced by the battery pack and a plug-in charging unit. Hence, for an EV charging system + drivetrain system, it is essential to analyze the PTW energy efficiency, of which P2B efficiency is most important.

Commercial EVs today have on-board chargers, which take input power from an AC wall outlet. The on-board charger system consists of an AC/DC rectifier, a DC/DC boost power factor correction stage, and a high-frequency (HF) DC/DC charger converter. It is clear that this setup leads to 3 conversion stages and facilitates a large loss of power. Thus, overall PTW energy efficiency is drastically low, and is about the same as that of conventional, gasoline vehicles. The PTW efficiency of currently available EVs ranges between 15-20%, when taking AC input from the wall. Several efforts have been made to miniaturize the onboard charger size, without compromising on charger efficiency and cost. One technique involves a 2-stage charger, which involves an AC/DC converter with an interleaved boost stage for PFC, followed by a zero-voltage switching (ZVS) DC/DC converter. Experiments were carried out on a 3.3kW battery charger and it was found that the 2-stage charger had an efficiency of 93.6%. The topology also had an added advantage of operating over a wide voltage range (200 – 450V) [4].

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Literature survey has led to believe that there has been a concerted effort in recent times to further reduce this 2-stage topology to a 1-stage resonant charger, which inherently possesses a PFC characteristic. This is utilized in the NLG5 charger, developed by Brusa Elektronik AG [5]. The presence of only one stage greatly reduces the task of controlling the charger. Additional attention has been given to ensure that all the switches operate at zero-current switching (ZCS), which in turn ensures reduction in losses. This topology not only helps in the reduction of charger size, but also eliminates the need of a large DC-link capacitor. More recently, control strategies for EV chargers have been implemented using one of the several digital controllers (DSP, microcontrollers, or FPGA) [6]-[9]. Typically, all contemporary EV chargers have a boost converter for PFC and Ground Fault Circuit Interrupter (GFCI). Following are the power levels at which charging can take place [10]:

a) Level 1 Charging: It is the most common household slow-charging method. For Level 1 charging, no extra facilities are needed. Total initial cost is about $800.

b) Level 2 Charging: This can be an on-board equipment. Level 2 charging is most preferred today, since it is quicker and has a dedicated connector. The initial cost is about $3,000.

c) Level 3 or DC fast charging: This option provides the customer the option to charge in < 1 hour to ~15 minutes of charge time. The power system for Level 3 charging is completely off-board. This is purely a commercial fast charging station and cannot be setup in residential areas. Total initial cost of such a power system would range between $40,000 to $70,000, including initial investment costs, infrastructure costs, and O&M costs.

Further information for all the levels of charging is provided in Table 1. As a test case, the Nissan Leaf® 24 kWh Li-ion battery pack is considered [11].

Table 1. Standard EV charging levels (SAE J1772).

Level Voltage Phase Power Time (h)

Level 1 120 Vac 1-phase 1.4kW 17

Level 2 240 Vac 1- and split-phase 4kW 6

Level 3 208/415 Vac 3-phase 50kW 0.5

The review of available Level 3/DC fast charging techniques are the cornerstone of this paper. Along with fast charging power converter topologies, the option of having an on-board Level 3 charger is also explored and reviewed. The advantages and limitations of the presented topologies are also highlighted for better clarity.

II. CLASSIFICATION OF DC FAST CHARGER SYSTEMS BASED

ON DIRECTION OF POWER FLOW

There are two classifications right now based on power flow direction from grid to the load and vice versa, a brief description of each charger type is given below.

A. Unidirectional Chargers

These chargers can only draw power from the grid but cannot interpose power into the grid. Usually such converters

are designed with only a single stage to reduce its size, weight and cost. Fig. 1 depicts a unidirectional full-bridge series resonant converter.

Fig. 1. Unidirectional full-bridge series resonant converter.

There has been much research being done on unidirectional chargers to obtain a reasonable control strategy which help to increase the efficiency of these chargers. These are usually preferred as they don’t affect the battery life, as the number of cycles are limited.

B. Bidirectional Chargers

These chargers typically consist of two stages namely 1) a grid-connected bidirectional AC-DC converter and 2) a bidirectional dc-dc converter. And they have the two modes, the charge and the discharge mode. Fig. 2 depicts a bidirectional charger.

Fig. 2. Typical bidirectional DC charger for EV.

The presence of two peak-current inductors tend to make the charger a little bulky and expensive. Batteries usually perish faster due to large number of cycles. Table 2 gives information in a more organized fashion regarding both the chargers.

Table 2. Comparison of chargers based on power flow direction.

Charger type Present

Status

Charger

level

Power

converter

Battery

health

Unidirectional Operational Levels 1, 2 and 3

Buck and fly back

No degradation

Bidirectional Unavailable Only for Level 2

Matrix converters

Lifespan reduces

Generally, DC fast charging stations for EVs are designed to supply about 50 kW of power [12]. The established trend is to place these chargers off-board. As these stations are bulky, keeping them off-board is convenient. The general block diagram of a DC fast charging station is as shown in the Fig. 3, the charger in the shown scenario is connected to a common AC link.

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Fig. 3. General block diagram of a DC fast charging station.

The output voltage that is fed to the load (either an ultracapacitor or a battery) may be variable or fixed. The filters placed ahead of the DC fast charging station help to maintain a healthy power factor [13]-[16].

III. DC FAST CHARGING CONVERTER TOPOLOGIES

There are a number of power converter topologies available for the purpose of rapid charging of batteries or ultracapacitors few feasible options are highlighted in this paper, they are:

A. Unidirectional Boost Converters

The unidirectional boost converter is shown in Figure 4, these are employed in situations where the output voltage has to be boosted up for loads which require higher voltage [17].

Fig. 4. Unidirectional boost converter.

The primary goal of using such a boost converter instead of a traditional diode bridge rectifier is to provide better power factor, to remove harmonics at the input end and to have an unvarying DC voltage at the output if unwanted perturbations occur at the AC end.

B. Vienna Rectifier

Another popular power converter topology is the Vienna rectifier as shown in Fig. 5. This too is a popular choice when the aim is to achieve high power factor and to attain lower harmonic distortion. As shown in Fig. 5 there is only one active switch per phase which makes the Vienna rectifier easier to control and makes it more dependable. This is essentially a pulse width modulated converter, the boost inductors present at the input play the role ascertaining power factor correction [18].

Basically, the stored energy acquired by the inductor when the switch is OFF is transmitted to the load via the diodes whenever the switch is turned ON. The advantages of employing this topology includes the absence of a neutral point connection and the lack of auxiliary commutation circuits which eliminate dead time problems.

C. AC/DC Reduced-switch Buck-Boost Converter

The main highlight of this topology is that its inexpensive, has less number of switches and most importantly since this is a buck-boost converter output voltage can be varied over a wide range. The topology is as shown in Fig. 6. There have been a few three phase front end rectifiers proposed but they are mostly boost converters which do not allow variation of voltage over wide ranges.

This converter topology can operate in buck mode when the duty ratio is below 0.5 and in boost mode when the duty ratio is above 0.5 [19].

Fig. 5. Schematic of a Vienna rectifier.

Fig. 6. 3-phase AC/DC buck-boost converter.

Level 3 DC fast charging stations are very demanding in terms of power, the available infrastructure is insufficient to meet those power demands. Usually, simultaneous operation of such charging stations tends to overburden the entire distribution system. Large scale implementation of such stations requires additional cost which has to be invested in upgrading the transmission cable and transformers. As the fast charging station penetration increases the power demand in the grid also increases proportionally. Apart from increased load demand, level 3 stations also cause drop in voltages and overload of transformers.

IV. PLUG-TO-BATTERY (P2B) EFFICIENCY

A major area of study concerning mass production and wide-spread implementation of EVs and plug-in hybrid electric vehicles (PHEVs) is the analysis of the overall well-to-wheels (WTW) efficiency. Typically, the transmission losses in transforming fossil fuel to electricity is about 7% [20]. This study helps in evaluating and comparing available EVs/PHEVs as well as in providing enhanced reasons for opting for an EV [21]. One of the reasons for reduction of plug-to-battery (P2B) efficiency in commercially available EVs such as the Nissan Leaf® and Tesla Roadster® is the presence of multiple stages in the onboard battery chargers. Moving forward, the focus of this work is to integrate the AC/DC rectifier, the DC/DC boost power-factor correction

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stage, as well as the DC/DC charger converter stage. An additional prospect is to make the charger DC (for fast chagrining) and off-board (not on-bard the vehicle). This will help increase P2B efficiency vastly, which in turn will help reduce the cost of EVs. P2B efficiency varies with varied charging levels (SAE J1772 AC Level 1, 2, and/or 3, as well as DC fast charging – Levels 2 and/or 3). The approximate P2B efficiency of a charging infrastructure can be found out by calculating the ratio of the total power supplied to the battery + supplementary loads to the total power provided by the input power source at the plug point of the charging infrastructure.

In conventional vehicles as well as plug-in hybrid electric vehicles (PHEVs), the approach to calculate WTW efficiency is to take into consideration the quantity of fuel utilized by the engine and the electrical energy drained from the grid. The energy at the wheels can be preconceived by integrating the curve under the power/time plot of the drive cycle. Fig. 7 describes the various energy conversion stages for a typical EV.

Fig. 7. Block diagram representing the various energy cycles of any typical vehicle.

The focus of this paper is to evaluate the impact of various DC fast-charging converter topologies on the P2B efficiency, which involves efficiency calculation from the DC fast charging station to the battery pack of the EV. Table 3 provides details of P2B efficiency for charging a Lithium-ion (Li-ion) battery pack, whose efficiency is considered to be around 93% [22]. The P2B efficiency can be calculated using equation 1. In practice, coupling efficiency is considered as 100% [22]. Due to poor coupling in the power circuit, higher power loss can occur with heating in couplers.

ηP2B = ηcharger x ηbattery x ηcoupling (1)

Table III. Plug-to-battery (P2B) efficiency of a Nissan Leaf® for different charger converter topologies and charging levels.

Converter

Topology

Charging

Level

Power

(kW)

Time

(h)

Charger

EFF

P2B

EFF

Buck Level 1 1.4 kW 17 88% 82%

Flyback Level 2 4.0 kW 6 92% 85.5%

Unidirectional boost

Level 3 50 kW 0.5 92% 85.5%

Vienna rectifier

Level 3 50 kW 0.5 90% 84%

3-phase AC/DC

buck-boost Level 3 50 kW 0.5 95% 88%

Efforts have been carried out to find efficiencies for on-board chargers as well as the overall P2B efficiency (including battery management system and charging efficiency). These efficiencies were found to be in the range of 80-85% [23]. The efficiency maps for various charging levels for all converter topologies are illustrated in Fig. 8.

V. PERFORMANCE COMPARISON AND EFFECTS OF DC FAST

CHARGING CONVERTERS ON THE GRID

A. Comparative Overview of Fast Charging Converter

Topologies

Few of the features of the discussed converter topologies are highlighted in Table 4. After detailed review of the three converter topologies, it can be concluded that the use of the Vienna rectifier for the implementation of the DC fast charging station is appropriate, due to the following reasons:

a) Presence of lower number of switches per phase b) Good efficiency when compared to the unidirectional

boost converter and the reduced switch Buck-Boost Converter

c) Better compensation for harmonic content d) Higher power factor, around 0.99, compared to the

unidirectional boost converter and the reduced switch Buck-Boost Converter.

Table IV. Performance comparison of DC fast charging converter topologies.

Converter Topology Mode of

Operation

Phase

Current

THD

Distinct

Feature

Unidirectional Boost Converters

Boost >5% Simple Design

Vienna Rectifier Boost <5% Highest Power Factor

AC/DC Reduced Switch Buck-boost Converter

Buck-Boost ~20% Efficiency is higher in boost mode

Due to the reasons sighted above, the Vienna rectifier is the most optimal converter topology for the DC fast charging stations from among the reviewed topologies.

B. Power Quality Effects on the AC Grid

EV charging stations have multiple impacts on the distribution system and strain it in different ways. This section enlists a few of the effects which are of prominent significance:

a) Overloading of transformers: During peak loading conditions, there exist numerous instances of transformer overloading, leading to degradation and eventually breakdown.

b) Voltage regulation: Instances of dip in the voltage level during periods of EV charging are quite common.

c) Losses: It is understandable that the overall distribution system losses will increase linearly with larger DC fast charging station penetration.

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d) Reduction in power quality: DC fast charging stations are obvious sources of harmonic distortion, which not only pollute the distribution system, but also damage fragile equipment connected to the system.

(a)

(b)

(c)

Fig. 8 Efficiency maps of the reviewed power electronic converter for all the levels of charging.

C. Vehicle-to-Grid (V2G) Power Flow

EVs can be considered as electric loads or as power sources. The charging characteristic of an EV is decided by various

factors, such as the choice of connection used (direction based on charging), the total number of EVs being charged, and the voltage/current levels of the EV energy storage system [24]. EVs equipped with adequate power electronic interface can operate as power sources, providing energy during unanticipated power outages.

Numerous factors determine successful V2G operation including the cost, effects of the grid on the EV battery pack, and presence of a smart charging controller, which ensures optimized power transfer between the EV and the grid. Efficient adoption of a smart charging controller can assist in reducing the negative repercussions of DC fast charging stations on the distribution system. Bidirectional charging can be accomplished by smart metering. Essential unidirectional and bidirectional flows between an EV and the smart grid are depicted in Fig. 9.

Fig. 9. Block diagram representing the unidirectional and

bidirectional power flow.

There are multiple benefits of V2G introduction including the possibility of employing EVs as reactive power suppliers, for balancing of loads, regulation of power, as harmonic filters, they also help in increasing the overall efficiency of the distribution system, help in reduction of the overall operating cost of the power grid distribution system and could help in possible generation of profit. As in all technologies V2G has a few drawbacks such as the slow deterioration of battery lifespan due to bidirectional charging and discharging cycles, initial infrastructure expenditure and requirement of smart communication equipment [25].

VI. CONCLUSIONS

Successful implementation of DC fast charging stations face many hurdles. Few of the issues include: high equipment cost, overloading of transformers, and lack of standard procedures and codes. Currently, Level 1 and 2 are the most popular schemes available, as they are both suitable for the available present infrastructure. Furthermore, levels 1 and 2 AC charging are more cost effective compared to level 3 DC fast charging stations. Public transportation sectors, especially mass transit systems, have a dire need for DC fast charging topologies. In addition to public transit vehicles, other commercial utility vehicles such as trams, trucks, and trains have peak power demands, such as providing starting torque for a very short duration. Peak power demands can be met if the vehicle is charged within a very short amount of time. Fast charging of electric mass transit systems is essential, since an electric city transit bus or a subway/metro train makes frequent stops (about 1 km between stops). The proposed

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DC fast charging system can be installed at major stops or train stations. DC fast charging provides tractability and a variety of charging options to the consumer. Finally, a more user-supportive charging system will allow for greater penetration of fast charging stations and greater acceptance of the technology.

The paper comprehensively discussed and reviewed an all-inclusive literature survey on the present status of DC fast charger converter topologies. The concept of DC fast charging stations and power electronic converter topologies to achieve high-power transfer from the AC grid to the EV battery were discussed in detail. The focal point of the overall discussion was off-board DC charging, as opposed to expensive and inefficient on-board AC/DC charging. It has been proven in the available literature that the Vienna rectifier is a preferred choice in high-power applications, due to superior power factor and excellent capability to cancel pout current harmonics.

In another favorable DC fast charging design scenario, the reduced-switch DC/DC buck-boost converter topology depicts excellent features, such as smooth transition between buck and boost modes of operation. However, one drawback is that the THD of phase current is very high, which makes this topology comparatively inefficient. Finally, the advantages and consequences of employing DC fast charging stations as a commercial, wide-scale infrastructure, were highlighted in the paper. Furthermore, a comprehensive overview of present day challenges in solving the undesirable effects of DC fast charging stations on the power grid distribution system were also presented.

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