A Bridgeless PFC Converter for On-Board Battery Charger

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Abstract— This paper presents a bridgeless SEPIC topology with Power Factor Correction (PFC) to charge electric vehicle battery. The converter offers a few ...
A Bridgeless PFC Converter for On-Board Battery Charger Pui Yee Kong, Aziz, J. A, M. R Sahid, Low Wen Yao Power Electronics and Drives Research Group, Department of Electrical Power Engineering, Faculty of Electrical Engineering, Universiti Teknologi Malaysia 81300 Skudai, Johor, Malaysia. [email protected], [email protected], [email protected],[email protected] Therefore, battery charger has great influential in battery charging time, lifetime and performance.

Abstract— This paper presents a bridgeless SEPIC topology with Power Factor Correction (PFC) to charge electric vehicle battery. The converter offers a few advantages such as reduce total component count and reduction of conduction loss during the conversion AC to DC, hence higher overall system efficiency. The PFC converter operated under continuous conduction mode (CCM) with average current mode control strategy. The model is built and simulated in MATLAB. From the simulation study, it is found that the proposed topology and the proposed control strategy provide a promising result.

A typical lithium-ion battery charging profile is illustrated in Fig. 1. Firstly, the battery is charged under constant current (CC) phase and then followed by constant voltage (CV) phase. During CC stage, battery is charged by constant current while the battery voltage increases progressively. When the cell voltage reaches its upper voltage limit, CV phase will take place. In CV phase, the charging voltage is constant while the charging current is gradually decreased and it is commenced at 85% of its full capacity [7]. The charging process terminate when current reaches predetermined minimum current point which is around 0.1C [8]. It is found that the CC-CV charging strategy is able to avoid overcharging and achieve full stage of charge [9]. The maximum charging rate is stated based on manufacturer specifications in order to avoid excessive current flow which may lead to permanent battery damage.

Keywords—Electric Vehicle; Power Factor Correction; Battery Charger; Bridgeless SEPIC.

INTRODUCTION

Electric vehicle (EV) are getting tremendous interest over decades due to world energy crises, greenhouse gas emissions, high fuel price [1] [2]. Zero emissions of EVs and high efficiency of electric motor relatively to internal combustion engine have resulted in great effort to develop EVs in automobile industries. Moreover, zero or low emission vehicle regulations has been set and started in some countries to encourage the promotion of EVs as well as to reduce air pollution. Electricity used to recharge batteries could be generated from clean and renewable energy such as solar, hydroelectric and wind energy. Development of EVs has been started since 19th century before the invention of internal combustion engine (ICE). Nevertheless, it experienced dramatically decline when the ICE technology is matured and the fuel cost is cheaper [3]. The advancement of power electronics and battery technology has stimulated again the development of EV.

Voltage, ( V)

Voltage Current

Travel distance of EVs greatly relies on battery performance. Battery performance is affected by some factors, such as temperature, current rate and life cycle. Extensive research on battery technology and battery management system has been carried out to increase the performance of battery. However, battery performance not only relies on battery design and battery management system, but also the charging method. Different type of battery required different charging method. Proper charging method will prolong the battery life and optimize its performance [4] [5]. On the contrary, improper charge the battery will degrade the performance and time span of battery or even causes the permanent damage on battery [6].

Constant Voltage Mode

Time, t (hours)

Fig. 1. Typical Lithium-ion battery charging profile

Battery charger can be divided into on-board charger (i.e. battery charged at lower power or slow charger) or off-board charger (i.e. battery charged at higher power or fast charger). The on-board charger consume longer charging time make it suitable for charging the vehicle at home/workplace or shopping mall through utility supply [2]. On-board charger offer convenient and easy access of charging port to user.

This Project was supported by Exploratory Research Grant Scheme (ERGS) of Malaysia and Universiti Teknologi Malaysia under project “Smart Battery Management System” (Vot No: 00G46).

978-1-4799-4848-2/14/$31.00 ©2014 IEEE

Constant Current Mode

Current, (A)

I.

383

L1

Besides, off-board charger proposes for rapid charging process, installed in public or commercial areas to serve as the role of gas station for ICE vehicle today. Charger also classified in term of power levels and time of charging. Society of Automotive Engineering (SAE) of United State has been classified charging stations into three categories depend on available power levels [10].

D2 Vg S2

(c) Fig. 2. (a) Shematic of Bridgeless SEPIC circuit, at (b) positive half-line cycle, at (c) negative half-line cycle

Bridgeless Single-Ended Primary Inductance Converter (SEPIC) is proposed to be used as power converter in this project [15]. Advantages of bridgeless topology are less component count and less conducted component during each mode of operation within one switching period. Fig. 2(a) shows the bridgeless SEPIC circuit. The equivalent circuit during positive and negative half-line circuit is shown in Fig. 2(b) and Fig. 2(c) respectively. During positive half-line cycle, L1, Cb1, L2, D1, Co, S1 on while the body diode of MOSFET S2 will allow the flow of current, while at negative half-line cycle, L1, Cb2, L3, D2, Co, S2, body diode of MOSFET S1 will conduct. Opration mode during negative half-line cycle same as the positive half-line cycle. Detail description operation mode and analysis of bridgeless SEPIC topology of the project has been discussed in [15][16]. SEPIC is operating in continuous conduction mode (CCM) due to high power application. There are two stage of operation in CCM. First stage, two MOSFET S1 and S2 are on to allow current passed through switch S1 and D2, body diode of S2 meanwhile inductor voltage L1 and L2 are increases linearly. As for second stage, two MOSFET S1 and S2 are off and diode D1 is then on to conduct current. Both inductors L1 and L2 are discharge to transfer energy to intermediate capacitor Cb1, output capacitor Co and load. Fig. 3 shows the converter operating in CCM for both switches in on and off stage. Extensive of analysis of SEPIC operated in CCM presented in [17]-[19]. [18]

PROPOSED CHARGER OVERVIEW

A. Converter Topology L1

L1 C b1

L2

D1

D2

Co

IL1

Vg

C b1 S1

Icb1

L2

Co

Vg

IDs2

L3

C b2

L1

L1

L2

D1

Co

IL1

Li-ion

Cb1

D1

Icb1

L2 I D1

Ico

Cb1

Li-ion

(a)

(a)

S1

+ Vo -

D s2

IL2

S2

 

Ico

S1

Li-ion

IL2

The rest of this paper is organized as follow. Section II presents the proposed charger overview. Section III demonstrates design of power stage parameters. Simulation results of the proposed method are present in section IV and section V concludes the paper.

 

L3

Cb2

Battery charger draw power from utility and delivered to battery pack of EVs. Rectification of AC power to DC power is processed within the battery charger. Power factor correction (PFC) has to be considered in charging system, not only to comply the required international standards but also to reduce the impact to power grid that connected. PFC can be achieved by using passive filter or active control. According to industrial standard, the charger must comply with European standard, IEC 1000-3-2 or IEC 1000-3-12 for the harmonic harmonics content depending on its power rating [11]. In this sense, it is mandatory for the charger to regulate output voltage and current together with high power factor. PFC provides an attractive solution in reducing or eliminating low order harmonics thus improving power factor. Active PFC control the flow of power from input to output by making current waveform close to the shape and phase of applied voltage waveform. The purpose is to make the load to be appeared as purely resistive which satisfied high power factor. Active PFC can be operated in single-stage or multi-stage, depending on the requirement of the application. Most of PFC topology consists of bridge rectifier and a DC-DC converter. However, high conduction loss of input bridge converter triggers the need of an alternative way to eliminate the diode bridge, namely bridgeless topology [12]-[14]. [13]

II.

Li-ion

Co

Co

+ Vo -

Li-ion

Vg

IDs2

Vg

(b) (b)

Fig. 3. Bridgeless SEPIC circuit operating in CCM, when (a) S1 and S2 is on and (b) S1 and S2 is off

384

Iref Vref

Outer Loop Current Controller Outer Loop Voltage Controller

Iˆ Lref

|Sin ωt|

ib

iL(t)* IˆLref

Inner Loop Current Controller

Vc(t)

q(t)

Switch Switch

Vr

Power Stage

iL vb

Fig. 4. Block diagram of average current mode control strategy

Electrochemical models has higher accuracy in term of predict battery behaviors, it focus in batteries physical design and parameters but they are complex where a lot of physical and chemical parameters are required, make it time consuming for battery modeling. Mathematical model is useful for system designers to adopt mathematical equations to predict battery characteristics such as runtime, efficiency and capacity. However, it only work for certain specific applications and gave least accurate. Electrical model are equivalent circuit based on combination voltage sources, resistors, and capacitors to predict the electrochemical process and battery dynamics. Accuracy of electrical model lies between electrochemical model and mathematical model [22] [23].

B. Converter Controller CC-CV charging strategy is used in this paper as shown in Fig. 1. In the beginning of charging process, battery pack is charge under constant current until the battery voltage reaches predefine voltage, follow charged by constant voltage until current reaches predetermined minimum current which is around 0.1C. The charging current is gradually decreased during CV stage to prevent battery overcharge. Average current mode control is employed in this research project to achieved both high power factor as well as regulate required output current and output voltage of battery during CC and CV stage. Average current mode control able to perform good tracking over an average current with high degree of accuracy, no slope compensation, and less sensitive to commutation noises.

In this paper, 31 cells are connected in series with capacity of 18AH to use as battery pack. In order to model the battery, charging profile is taken from experiment data of Lithium Ferro Phosphate Prismatic (LiFePO4) battery with charging method of constant current at 9A (0.5C). Battery charging profile is modelled based on mathematical equations and least squares algorithm. This mathematical model can be easily realized by simple transfer function blocks. Mathematical equation is obtained by utilized MATLAB curve fitting toolbox.

The operation principle of average current mode control strategy is illustrated as Fig. 4. There are two feedback loops in this control method at each phase, outer loop and inner loop. Selection of switch enables CC-CV charging control strategy. Constant current is supply to charge the battery during CC phase. Feedback current, ib is regulated by slower response (outer loop) to follow reference current, Iref to provide constant current. IˆLref signal is generated from outer loop controller and

Applying least squares method to 0.5C charging profile, the fitted equation is:

multiply with sinusoidal reference waveform, sinωt thus provide reference iL(t) to shape the input current through inner loop. Control signal Vc(t) is generated to control the input current in PWM stage to achieve high PFC. When the battery voltage reach a predetermine value which is equal to stage of charge (SOC) 85% of battery pack, it will enter CV mode. Feedback output voltage, vb is regulated to follow reference voltage, Vref until the output current reaches 0.1C. The control of PFC is same as CC phase.

V(T) = a o + a 1T + a 2 e bT

(1)

The corresponding parameters can are a0=3.285, a1=0.1474, a2=0.2681, b=24.99, where T is charging time in hours and V(T) is voltage across the charging battery. Fig. 5 shows the mathematical equation, V(T) and lithium-ion battery charging profile at 0.5C. The fitted equation able to captures the behavior of battery. Equivalent circuit model of equation (1) demonstrates in Fig. 6. The battery model consists of one capacitor Cs connected parallel with resistor Rs which represent transient element of battery and it series with one resistor Rp (DC resistance of battery) and a storage element, capacitor Cp [21].

C. Battery Model Low lithium-ion battery cell voltage required several battery cells connected in series or in parallel to serve as energy storage device in EV application. Battery model is necessary in describing the battery behavior and characteristics. A variety of models have been developed with varying degrees of accuracy. Generally, battery model can be classified into three types: electrochemical, mathematic and electric circuit-based [20] [21].

385

L1L 2 L1 + L 2

L eq =

(5)

Rl is minimum load resistance, fs is switching frequency.

L1 =

d=

dri

M M +1

(6)

(7)

where d is duty ratio, ri is the percentage of input current ripple, value of L1 and L2 can be compute from (5) – (7). When the converter operating as PFC, the voltage intermediate capacitor, Cb has to follow the input voltage and has to obtain almost constant value in switching period.

Fig. 5. Charging profile of Lithium Ferro Phosphate Prismatic (LiFePO4) battery at 0.5C

I

Rs

2L eq

Cb =

Cs V(T)

1 w (L1 + L 2 )

(8)

2 r

resonant frequency wr must be greater than line frequency wl but less than switching frequency ws. Initial value of C1 is obtained from (7) and adjusted for a better response through simulation.

Rp Cp

Co =

Fig. 6. Equivalent circuit model of lithium-ion battery

III.

DESIGN OF POWER STAGE PARAMETERS

K > K crit

Vo Vg

1 = 2 2(M + 1)

IV.

KR l 2f s

RESULTS AND DISCUSSION

The proposed topology and control strategy has been verified by MATLAB® simulation. The bridgeless SEPIC is design based on parameters shown in TABLE I and the lithium-ion battery specification is summarized in TABLE II.

(2)

Fig. 7 shows the input voltage and input current at two different supply input voltage, 110Vrms and 230Vrms respectively under CC phase. It can be seen that the input current and input voltage are in phase. Output voltage and output current during CC phase presented in Fig. 8 which meets the desired parameters. The ripple voltage and current is about 3% respectively. During CV phase, Fig. 9 shows the input voltage and input current versus the supply voltage of 110Vrms and 230Vrms. The current decrease gradually during CV stage as mentioned in previous section is shown in Fig. 10.

(3)

CCM operation can be guaranteed for value of K > Kcrit, otherwise, the converter will operate in DCM where M is voltage conversion ratio, Vo is output voltage, Vg is the peak value of input voltage, Kcrit is critical conduction parameter and K is conduction parameter.

L eq =

(8)

rv is the output voltage ripple in percentage.

Converter parameters are designed to meet the desired performance. Following condition should be satisfied to ensure the converter operate in CCM:

M=

1 w l rv R

(4)

386

DESIGN PARAMETERS OF CIRCUIT Values

Input Voltage, Vg

110-240Vrms

Line Fequency, fL

50Hz

Switching Fequency, fs

80kHz

Input Inductor,L1

2.65mH

Intermediate Inductors, L2&L3

116uH

Intermediate Capacitors, Cb1&Cb2

1.5uF

Output Capacitor, Co

50mF

Output Voltage, Vo

113.15V

TABLE II.

Output Voltage Output Current 100

10 8 6 40

40.05

40.1

40.15

40.2

Time (sec)

40.25

40.3

40.35

GP18EVLF (LiFePO4) 18Ah

Norminal Voltage

3.2V

Charge Chracteristic

Charging Termination

9A (0.5C) to 3.1V Taper Current 0.9A (0.05C) at 3.1V

200

Input Current Input Voltage

100 200

20 10

0

0

Ampere (A)

0

0

-2 -4

-100

-10 -100

-200 3.6

-20

3.62

3.64 3.66 Time (sec)

3.68

-200 4.2

4.22

4.24 4.26 Time (sec)

400

Input Voltage Input Current

300

Voltage (V)

8 6 4 2 0 -2 -4 -6 -8

Ampere (A)

0 -100

Input Voltage Input Current

200

200 100

4.3

(a)

3.7

(a) 400

4.28

2 1 0 -1 -2

Ampere

Voltage (V)

100

4 2

Voltage (V)

Input Current Input Voltage

Ampere (A)

Cell Capacity

Charging Method

Voltage (V)

40.4

Fig. 8. Output voltage versus output current at supply input voltage 110V under CC phase

SPECIFICATIONS OF CELL

Cell Model

Ampere (A)

Parameters

Voltage (V)

TABLE I.

0

-200

-200 -300 -400 3.6

-400 4.2 3.62

3.64

3.66

Time (sec)

3.68

3.7

4.22

4.24 4.26 Time (sec)

4.28

4.3

(b)

(b)

Fig. 9. Input voltage versus input current at (a) input voltage 110V (b) input voltage 230v under CV mode

Fig. 7. Input voltage versus input current at (a) input voltage 110V (b) input voltage 230V during CC phase

387

Output Voltage Output Current

[3]

Voltage (V)

Ampere (A)

113

[4]

[5]

5

48

48.05

48.1

48.15 48.2 48.25 Time (sec)

48.3

48.35

[6]

48.4

Fig. 10. Output voltage versus output current at supply input voltage 110V under CV phase 120

[7]

Vbat

[8]

110

Voltage (V)

100

[9]

90 80

[10]

70 60 0

[11] 2000

4000

[12]

6000 8000 10000 12000 14000 16000 Time (sec)

[13]

Fig. 11. Battery voltage undergo charging process

Fig. 11 shows the simulation of battery voltage under charging. During CC phase, the battery voltage increase progressively and it regulated at constant voltage during CV phase before charging process was terminated. V.

[14] [15]

CONCLUSION [16]

A design of bridgeless PFC battery charger is presented in this paper. The AC/DC converter is a single stage bridgeless SEPIC operating in CCM with average current control strategy to transfer the require power from utility to the battery pack. The converter reduces the total components conducted at each half-line cycle compared to conventional SEPIC converter. The theoretical calculation for power stage is elaborated. It is proven that the closed loop PI controller successfully regulates the require parameters to charge the battery pack.

[17]

[18]

[19]

ACKNOWLEDGMENT The authors gratefully thank to Ministry of Education Malaysia for Exploratory Research Grant Scheme (Vot No: 4L126) and Universiti Teknologi Malaysia under project “Smart Battery Management System” (Vot No: 00G46) for financial support.

[20] [21]

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