Review of Non-isolated Bi-directional DC-DC Converters for Plug-in Hybrid Electric Vehicle Charge

Station Application at Municipal Parking Decks

Yu Du, Xiaohu Zhou, Sanzhong Bai, Srdjan Lukic and Alex Huang FREEDM Systems Center

Department of Electrical and Computer Engineering, North Carolina State University Raleigh, NC, 27695, U.S.A.

Email: ydu4@ncsu.edu

Abstract — There is a growing interest on plug-in hybrid electric vehicles (PHEV’s) due to energy security and green house gas emission issues, as well as the low electricity fuel cost. As battery capacity and all-electric range of PHEV’s are improved, and potentially some PHEV’s or EV’s need fast charging, there is increased demand to build high power off-board charging infrastructures. A charge station architecture for municipal parking decks has been proposed, which has a DC microgrid to interface with multiple DC-DC chargers, distributed renewable power generations and energy storage, and provides functionalities of normal and rapid charging, grid support such as reactive and real power injection (including V2G), current harmonic filtering and load balance. Several non-isolated bi-directional DC-DC converters suited for charge station applications have been reviewed and compared, as the major focus of this paper. Half bridge converter is a good candidate but it is difficult to maintain high efficiency in wide battery pack voltage range. A variable frequency pulse width modulation (VFPWM) scheme is proposed to mitigate this issue. Finally three-level bi-directional DC-DC converter is suggested to be employed in this application. A 10kW prototype verifies that 95.1-97.9% full load efficiency can be achieved in charging mode with 180-360V battery pack voltage. In addition, the inductor size is only one third of the half bridge counterpart, which is a great advantage for high power converters.

I. INTRODUCTION Nowadays about 62% of crude oil used in United States is

refined into gasoline for transportation. The associated energy security and green house gas emission problems are well known [1]. Hybrid electric vehicles (HEV’s) is one of the solutions to address these issues, because the fuel economy has been improved by optimizing internal combustion engine (ICE) efficiency, regenerating brake energy and shutting down ICE during the idle time. After more than one million HEV’s are driven on the road today, there is a growing interest on plug-in hybrid electric vehicles (PHEV’s), which is defined by IEEE-USA’s Energy Policy Committee as (1) a battery storage system of 4kWh or more, used to power the motion of

the vehicle, (2) a means of recharging that battery system from an external source of electricity, and (3) an ability to drive at least 10 miles in all-electric mode consuming no gasoline [2]. PHEV’s can be power by electricity from various sources, including emerging renewable power generations, and benefit from lower fuel (electricity) cost. Green house gases such as CO2 emission is expected to be greatly reduced due to much less petroleum consumption for daily commuters who drive PHEV’s mainly in all-electric mode.

Major automakers are preparing to launch the first models in a new generation of PHEV’s in 2010. Several global and U.S. market research reports [3-4] indicate there will be a rapid growth on PHEV’s sales. Reference [4] forecasts that PHEV’s will follow a similar adoption pattern as that of HEV’s over the past few years, and by 2015, a total of 1.7 million PHEV’s will be on the road worldwide.

Currently major PHEV’s are designed for an all-electric range of several tens miles to meet daily commute requirement, due to the high cost of on-board battery energy storage system. An on-board charger is usually employed for slow overnight charging in home garage. However, as battery capacity and all-electric range of PHEV’s are improved, and potentially some PHEV’s or EV’s in the future need fast charging to extend all-electric drive range, there is increased requirement to build off-board charge station infrastructures. In Part II the motivations to build municipal parking deck charging stations are discussed and a charge station architecture has been proposed with the functionalities of normal and rapid charging, grid support such as reactive and real power (V2G mode) injection, current harmonic filtering and load balance. The proposed charging station has a DC power distribution bus, which can be considered as a microgrid, to interface with DC-DC chargers, distributed renewable power generations and energy storage system. In Part III, several non-isolated bi-directional DC-DC converters suited for charge station application have been reviewed and compared. Half bridge converter is considered to be a good

This work made use of ERC shared facilities supported by the National Science Foundation under Award Number EEC-08212121.

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candidate but it is difficult to maintain high efficiency in wide battery pack voltage range. In Part IV, a variable frequency pulse width modulation (VFPWM) scheme is proposed to improve the power efficiency of half bridge converter at lower battery pack voltage, because major portion of energy is delivered to battery in low pack voltage range. The efficiency improvement is verified by experimental test. In Part V, a three-level bi-directional DC-DC converter is proposed to be employed in charging station applications. A 10kW prototype verifies that 95.1-97.9% full load efficiency can be achieved in charging mode with 180-360V battery voltage. In addition, the inductor size is only one third of the half bridge counterpart.

II. ACHITECTURE OF MUNICIPAL PARKING DECK CHARGING STATION

A. Motivations to Build Charge Station Infrastructure Currently most PHEV’s use single-phase on-board charger

to refuel their batteries, which is the common practice for both converted PHEV’s and several ones that will be commercialized soon. On-board charger can either use independent power converter, or leverage the power stage of drive train and motor inductance [5]. The power rating of on-board charger is low, limited to the current rating of wall plug. For example, 120V/12A (Level I) or 240V/32A (Level II) single-phase AC input according to SAE-J1772, and so it is suited for slow overnight recharging. However, with advancement of battery technology, the energy density and power density of battery packs are improved and battery cost decreases, the desire of more on-board battery capacity for more all-electric range and less gasoline consumption is possible for future PHEV’s or EV’s. Therefore there will be a trend for battery chargers to shift from compact low power on-board installation to shared large high-power off-board charging station in the future.

The second motivation to build this charge station infrastructure is to use a DC link to interface with distributed renewable power generations, which can be considered as a microgrid. With high penetration of PHEV’s, it is necessary to install new power generation capacity. For municipal parking deck applications, it is quite possible that PHEV’s will be charged during daytime and refueled with peak power electricity. The distributed renewable power generations can potentially provide a solution for this issue because they will not only reduce the power demand from the grid during peak time, or even inject real power back to support the grid, but also recharge PHEV battery packs with green energy source and make transportation industry cleaner. The charge station can install its own solar power generation, fuel cell power generation and wind turbines [6-7]. For example, PV panels generate power during daytime and can reduce the power demand from grid during peak load time. In addition, there is less safety or noise concern to install PV in municipal parking decks. On the other hand, vehicle to grid (V2G) operation provides a potential solution of using PHEV batteries as energy storage devices for high penetration of distributed wind and solar power generations and smoothes their intermittent power [8].

The third motivation is to fast charge a PHEV/EV battery pack. In order to improve the all-electric range of PHEV’s and EV’s, one will either increase the on-board battery capacity or recharge the battery pack in very short time at a fast charging station. However, all-electric range will be eventually limited by battery capacity due to the size and weight as well as the cost of batteries. Therefore, high power fast charging will potentially solve EV range problem by recharging its battery in 10 to 30 minutes, like what people do in today’s gasoline station, as long as the battery packs are capable of accepting high rate charging current.

B. Charging Station with DC Power Distribution The proposed architecture of municipal charge station at

parking decks is shown in Fig.1. Compared to discrete AC-DC and DC-DC chargers, the proposed charge station uses 750V or higher DC distribution bus with one high power three-phase AC-DC converter as grid interface. This architecture has several advantages. The specific cost of high power AC-DC stage is lower than that of discrete low power AC-DC converters if AC power distribution bus is used. The three-phase rectifier is rated for average power rather than peak power if ultra capacitor energy storage is installed to filter the ripple power. DC distribution bus is easy to integrate distributed renewable power generations such as wind, PV, fuel cells and other energy storage devices. The power of each DC-DC channel is rated for normal slow charge rating to minimize cost. On the other hand, the parallel of several DC-DC stages provides a high power fast charging channel, assuming only small portion of PHEV’s will require this service. With bi-directional DC-DC converters, energy stored in PHEV batteries can be fed back to grid, which is called V2G operation. An intelligent energy management system (iEMS) with wireless Zigbee communication platform can coordinate system operation [9]. The charge station can provide several grid support functions such as reactive power injection, peak power generation, harmonic current filter, and load balance [10].

Figure 1: Proposed Charging Station Architecture with DC Power Distribution and Renewable Generations

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The bi-directional DC-DC converters are basic building blocks for municipal charging stations and they are the interface between PHEV battery system and the DC distribution grid. Bi-directional DC-DC converters with low cost, high efficiency and high reliability are important for the charging stations. The non-isolated bi-directional DC-DC converters can be considered for this application and their performance is compared in the following sections.

III. NON-ISOLATED BI-DIRECTIONAL DC-DC CONVERTERS

Non-isolated bi-directional DC-DC converters generally have advantages of simple structure, high efficiency, low cost, high reliability, etc. Several non-isolated bi-directional DC-DC converters that are reported in literature [11-14] are shown in Fig.2. They can be categorized into basic topologies such as Half-bridge converter, Cuk converter, SEPIC/Luo converter and derived topologies such as cascaded half-bridge converter and interleaved half-bridge converter. One widely used topology is Half-bridge converter which operates either in Buck or in Boost mode. Cuk and SEPIC/Luo can convert power bi-directionally by using two active switches. The cascaded half bridge and interleaved half bridge can be considered as derived topologies from the basic half bridge, and their performance can be evaluated based on the performance of half bridge converter. Therefore, Half Bridge, Cuk and SEPIC/Luo converters are compared based on the

example specification of bi-directional DC-DC converters in charging station.

The example specification and converter inductor values are listed in Table I. The power rating is 10kW. The high voltage DC link is 750V and battery side voltage ranges from 180-360V. The inductor size is calculated based on 20kHz switching frequency and 30% maximal inductor current ripple referred to the inductor DC current. It can be found that Cuk and SEPIC/Luo converter use two larger inductors and one more capacitor compared with Half Bridge converter.

For all three basic topologies shown in Fig.2, the battery is connected to C1 through a common mode choke and ground fault interrupter (GFI) to limit the leakage current. C2 is connected to DC link side. When bi-directional DC-DC converters work in charging mode, the ratio of output voltage Vo to input voltage Vin is lower than 1; when they work in discharging or V2G mode, the ratio of Vo to Vin is larger than1. The inductor RMS current and switch current stress are shown

Figure 2: Several Non-isolated Bi-directional DC-DC Converters Reported in Literatures

Figure 4: Comparison of Switch Current Stress in Half Bridge, Cuk and SEPIC/Luo Converters

Figure 3: Comparison of Inductor RMS Current in Half Bridge, Cuk and SEPIC/Luo Converters

TABLE I. SPECIFICATIONS FOR BI-DIRECTIONAL DC-DC CONVERTERS IN MUNICIPAL PARKING DECK CHARGE STATION

Rated Power 10kW DC-link Voltage 750V

Battery Pack Voltage 180V-360V

Maximal Inductor Current Ripple 30% (Peak to Peak) Switching Frequency 20kHz

Half Bridge CUK SEPIC

/LUO L1 (mH) 1.12 1.46 1.46 L2 (mH) - 3.04 3.04

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in Fig. 3 and 4 respectively. The RMS current in inductor L1 is similar for three topologies, but inductor L2 in Cuk and SEPIC/Luo converters consume additional power, although the current stress is much lower than that in L1. From Fig.4, the current stress for active switches and diodes in Cuk and SEPIC/Luo converter are larger than that in Half Bridge converter under same input/output voltage and power conditions. Therefore, Half Bridge is expected to be more efficient and it also has less number of inductor and capacitors. Half Bridge converter is a better candidate in this scenario.

IV. VFPWM SCHEME FOR HALF BRIDGE CONVERTER EFFICIENCY IMPROVEMENT

The battery side voltage can change in wide range from 180V to 360V (2:1). When battery pack voltage is high, the efficiency of Half Bridge is better because the current stress for inductor L1 and switches are lower. However, if battery side voltage is low and constant power (10kW) charging is assumed to be the worst scenario, current stress in converter increases and the efficiency drops quickly. It is even worse for low power conversion efficiency in lower battery voltage range because during charging process the major portion of charge is injected to battery in lower voltage range. This means that a big portion of energy is delivered to battery pack with low efficiency.

This problem can be mitigated by the proposed variable frequency pulse width modulation (VFPWM) scheme. Fig.5 shows the inductor current ripple ratio, which is referred to the DC current component of inductor L1, in the wide battery pack voltage range at constant full power (10kW). If the switching frequency is fixed at 20kHz, in low battery voltage range the current ripple ratio is reduced by roughly half and much lower than the specified maximal 30% current ripple ratio. This is not necessary because there is little benefit, and current stress of output capacitor is already very low because only the inductor ripple current goes through capacitor C1. Instead, maintaining a constant inductor current ripple ratio of 30% in full voltage range at 10kW allows lower switching

frequency in low battery voltage range, which can help reduce switching loss and improve efficiency of Half Bridge converter in wide voltage range. The switching frequency is dependent on battery pack voltage in VFPWM scheme and is shown in Fig.6. The inductor ripple current will increase due to the reduced switching frequency in lower voltage range. Since the inductor ripple current is filtered by battery side capacitor C1, the increased ripple current will have no adverse impact on PHEV battery pack.

Experiment setup is built to test the efficiency of conventional fixed frequency (20kHz) PWM and proposed VFPWM Half Bridge converter. Two 1200V/300A IGBT’s APTGF300A120G are used in the half bridge converter prototype and inductor value is L1=1.12mH. The DC choke is built with KoolMu magnetic core material and E-E shape with distributed air gap. Two pairs of 00K145LE E-E cores are used and the relative core permeability is 26. DC-link voltage is 750V. The experimentally tested efficiency in wide battery side voltage range from 180V to 360V at full load 10kW is

Figure 5: Comparison of Inductor Current Ripple Ratio in Conventional PWM and VFPWM

Figure 7: Experimentally Tested Converter Efficiency of PWM and VFPWM Half Bridge Converter

Figure 6: Switching Frequency Dependency on Battery Pack Voltage in VFPWM Half Bridge

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shown in Fig.7. The efficiency curves for bi-directional operation, charging or Buck mode, and discharging or Boost mode, are provided. Full load efficiency is more important because most of the energy injected into battery pack is during large current and high power charging stage. Experiment result shows that the efficiency of Half Bridge converter in both Buck/Charge mode and Boost/Discharge mode is improved by 1-2.5% by proposed VFPWM scheme in lower battery voltage range, compared to conventional PWM scheme with fixed switching frequency.

V. BI-DIRECTIONAL THREE-LEVEL CONVERTER

Three-level converters have been proposed for high DC-link voltage applications [15-18]. Three-level converters are characterized of low switch voltage stress and smaller energy storage devices such as inductor and capacitor. Topologies of three-level converter can be categorized into Neutral Point Clamped (NPC), Flying Capacitor and Diode Clamped converter, etc.

The topology of a neutral point clamped three-level bi-directional converter is shown in Fig.8. The operation waveforms of this three-level (TL) bi-directional DC-DC converter for Buck or charging mode, and Boost or

discharging mode are shown in Fig.9 and Fig.10, respectively.

The Buck mode equations are listed from (1) to (6):

2:ratioduty switch The LD

DVBV

Ds == (1)

DVBV

LD⋅

=2

:ratioduty effective The (2)

)2(

)21( :current rippleinductor The

sfLsDsDdV

⋅⋅

⋅−⋅⋅ (3)

sDdI

LDdI

=⋅2

:current DCinductor The (4)

12

21 :current RMSswitch The Lr

sDsD

dI+⋅⋅ (5)

12

211 :current RMS diode The Lr

sDsD

dI+⋅−⋅ (6)

Boost mode equations are listed from (7) to (12):

2

1

/

11 :ratioduty switch The LD

BVDVsD +=−= (7)

BVDVLD

/

21:ratioduty effective The −= (8)

)2(

)12()1(:current rippleinductor The

sfLsDsDdV

⋅⋅

−⋅⋅−⋅ (9)

sDdI

LDdI

−=

−

⋅

11

2:current DCinductor The (10)

12

21

1 :current RMSswitch The Lr

sDsD

dI+⋅⋅

− (11)

Figure 8: A Neutral Point Clamped Three Level Bi-directional DC-DC Converter

Figure 10: Boost Mode Waveforms for TL Converter

Figure 9: Buck Mode Waveforms for TL Converter

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12

211

1:current RMS diode The Lr

sDsD

dI+⋅−⋅

− (12)

To meet the specifications which are listed in Table I, parameters of three-level (TL) converter are listed in Table II and compared to conventional half-bridge (HB) converter as well.

A 10kW experiment prototype, as showed in Fig.11, was built to evaluate the performance of three-level bi-directional DC-DC converter in municipal parking deck charge station application. Due to reduced switch voltage stress, 600V IGBT can be employed instead of 1200V IGBT in half bridge converter. Generally 600V IGBT has much lower on-state voltage and switching loss than 1200V IGBT. Two CM150DY-12NF IGBT half-bridge modules are used in the prototype. Also the total IGBT’s cost is lower compared to 1200V IGBT’s. The inductor size is only one third of that in half bridge converter. This is a great benefit for high power converters because inductors with higher inductance and high current are very bulky, expensive and inefficient.

The experiment waveforms for 180V battery side voltage and 10kW power are shown in Fig.12 and 13, which are corresponding to Buck and Boost mode respectively.

The efficiency of three-level bi-directional DC-DC converter prototype is measured with full power rating 10kW

and wide battery side voltage range from 180V to 360V, as shown in Fig.14. The efficiency of three-level converter in charge mode varies from 98% to 95%. The discharge efficiency is about 1% lower than charging mode because IGBT’s conduct for a longer time than diodes in each

Figure 11: 10kW Prototype of Bi-directional Three-level Converter

Figure 13: Test Waveforms of TL converter in Boost Mode

TABLE II. COMPARISON OF SPECIFICATION OF HB AND TL

Rated Power 10kW

DC-link Voltage 750V Battery Pack Voltage 180V-360V

Maximal Inductor Current Ripple 30% (peak to peak)

HB TL

Switching Frequency 20kHz 10kHz

Inductor Size 1120uH 348uH

Figure 12: Test Waveforms of TL converter in Buck Mode

Figure 14: Efficiency Test Results for TL and HB

Vce1 (200V/div) Vce4 (200V/div)

VL (200V/div) IL (20A/div)

Time (40uS/div)

Vce2 (200V/div) Vce3 (200V/div)

VL (200V/div) IL (20A/div)

Time (40uS/div)

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switching cycle. The tested efficiency of half bridge converter in the same condition is also shown in Fig.14. Test results indicate that about 2-3% efficiency improvement can be achieved by three-level DC-DC converter for Buck mode and Boost mode, respectively.

VI. CONCLUSION Several low cost non-isolated bi-directional DC-DC

converters suited for municipal packing deck charge station applications have been reviewed and compared. Half bridge converter is better than Cuk and SEPIC/Luo converter in this scenario due to smaller number of passive components, lower switch current stress and higher efficiency. However, in wide battery side voltage range, efficiency of half bridge converter drops quickly with lower battery pack voltage. This problem can be mitigated by proposed VFPWM scheme which obtains 1-2.5% improvement. Three-level bi-directional DC-DC converters have been investigated for charge station application. Experiment results show 2-3% higher efficiency than that of half bridge. In addition, much smaller inductor is required and there is no audible noise in three-level converter compared with VFPWM half bridge converter, either. Therefore, the bi-directional three-level DC-DC converter is recommended for municipal parking deck charge station application.

ACKNOWLEDGMENT This work is also supported by Advanced Transportation

Energy Center (ATEC) at North Carolina State University and we wish to acknowledge both FREEDM Systems Center and ATEC for the financial support.

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