A Modified Battery Charger with Power Factor Correction for Plug-In Electrical Vehicles ^†

Abstract

Electric vehicles are becoming famous worldwide for having the striking benefits of improved energy efficiency and reduced carbon footprints, etc. Plug-in electrical vehicles (PEVs) are powered by batteries. These batteries need to be recharged after every usage cycle. A lithium-ion battery consisting of 100 cells has a terminal voltage of 420 V when fully charged and 300 V in a discharged state. Therefore, such batteries require a battery charger with an output voltage range from 300 V–420 V to fully charge the batteries. There are two types of on-board battery chargers: two-stage and single-stage battery chargers. The former has the drawbacks of larger component count, larger size, and lower efficiency. However, there are few works on the latter, which has not been well-explored for EV applications. Hence, in this research project, a single-stage battery charger is proposed. The proposed charger consists of two integrated stages. The 1st stage is the power factor correction (PFC) converter and the later one is the LLC resonant DC–DC converter. Both stages are driven by a common half-bridge network. The proposed charger is designed for charging a 100 cell 3.2 kW Lithium-ion battery. The operation and performance of the proposed charger is evaluated by a Pspice-based computer simulation. The simulation results showed that the proposed charger can efficiently charge the battery from a depleted state to a fully charged state.

1. Introduction

A worldwide reality that must be tackled with is matters concerning the environment, which includes polluted air that is caused by the gasses of fossil fuel utilized in transportation, exhaust gasses of industrial waste and power stations. The largest percentage of fossil fuel utilization is gasoline- and diesel-based transportation, which has increased the level of environmental pollution by burning of these harmful materials. Therefore, the solution to these problems is the utilization of fossil-safe energy means, for which electric vehicles are the best solution to reduce fossil emissions and environmental pollution [1]. Due to the benefits possessed by electric vehicles in comparison to the fuel-engine based vehicle, the researchers are focusing on the development of electric vehicle battery chargers, which have improved efficiency and the best performance. A single-stage battery charger with integrated two-stages is introduced in [2]. The topology shares the same switching devices for PFC and resonant dc–dc circuits and works for fixed input and fixed output voltage. The topology has limited utilization, as it cannot be used for variable input/output applications. Another type of single-stage battery charger with a lower component count and without the need of an electrolytic capacitor is proposed in [3]. The circuit performs the charging operation with ZVS and ZCS of switches and diodes and gives better PFC; however, it cannot operate for the entire load range and gives the fixed output power. A single-stage LED driver circuit for the application of street lighting is presented in [4]. The topology consists of the integrated two stages, in which the half-bridge inverter combines the two boost circuits and the LLC resonant converter. The switches operate with ZVS and the circuit is implemented for a 100 W prototype, which gives the maximum efficiency of 91%. In this paper, an integrated single-stage EV battery charger topology with variable input and variable output power range is proposed. The topology of the LED driver (given above) has been modified and utilized for the single-stage EV battery charger application.

2. Proposed Single-Stage Battery Charger

Figure 1 presents the circuit topology of the proposed single-stage electrical vehicle battery charger. It comprises two power factor corrected isolated ac–dc converters, which have parallel connection at the input side with an ac source and series connection at the output side with the load. Each of the converters performs the battery charging operation in two stages: The first stage is the ac–dc with PFC converter, along with the dc–dc resonant converter stage. For example, the ac–dc stage of converter-1 consists of a bridge rectifier (D₁–D₄), input inductor L₁, capacitors C₁ and C₂ working as a voltage divider, boost operation diodes D_b1 and D_b2, inductor L_b1 for boost operation, power processing switches S₁ and S₂, and capacitor C_bus1 for bus voltage, while the dc–dc stage comprises of C_r1 as a resonant capacitor, L_r1 as resonant inductor and L_m1 as magnetizing inductor, T₁ as power transformer, output bridge rectifier (D_o1–D_o4) and output capacitor C_o1. The power mosfets S₁ and S₂ have the parasitic capacitors and diodes C_S1 and C_S2 and D_S1 and D_S2, respectively. The power switches S₁ and S₂ are common for both stages; therefore, these switches basically combine two stages, which is why the proposed converter is called a single stage converter. Similarly, the converter-2 is a replica of converter-1; both are in parallel connection with a common input supply V_S at the source side and are connected in series with the load resistor R_L at the output, as given in Figure 1. The inductor L₁ serves as a filter inductor and the voltage divider capacitors have a dual role in the circuitry, i.e., they serve the purpose of input capacitor in addition to dividing the input voltage in half before supplying the boost circuits. Solving the circuit for gain analysis, the dc gain of the llc resonant converter is given in Equation (1).

$$G_{dc}=\frac{V_o}{V_{bus1}}=\frac{1}{n}\frac{K_1}{\sqrt{{\left(1+K_1-\frac{1}{f_n^2}\right)}^2+Q_1^2{K}_1^2{\left(f_n-\frac{1}{f_n}\right)}^2}}$$

(1)

3. Design and Implementation

The proposed battery charger was designed for charging a 100 cell, 3.2 kW lithium ion battery. The input supply voltage to the charger was 220 Vrms at 50 Hz. The output voltage of the charger ranges from 300 V to 420 V to charge the lithium-ion battery. The charging properties are given in the

Table 1. Battery charging current, voltage, power and C-rate in CC and CV modes [5].

Operating Mode	Key Point	Vbat	Ibat	Po	Equivalent Resistance	C Rate
CC mode	A	300 V	7.61 A	2.28 KW	39. 42 Ω	1 C
	B	360 V	7.61 A	2.74 KW	47.30 Ω	1 C
	C	420 V	7.61 A	3.2 KW	55.19 Ω	1 C
CV mode	C	420 V	7.61_15 A	3.2 KW	55.19 Ω	1 C_0.02 C
	D	420 V		630 W	2763 Ω

.

4. Simulation Results

The charging capability can be estimated by operating the converter at five main operating points (A, B, C, D, and E). To charge the intensely depleted battery, constant current is required at the charge rate of 0.07 C with the current of 0.357 A, with the voltage range of 100 V–250 V. The charging range of the moderately depleted battery can be defined with a charge rate ranging from 0.7 C_.05 C and a current range of 3.57 A_0.255 A. The proposed EV battery charger was simulated in PSpice software to evaluate its performance at three key points, namely point A, point B and point C. The results obtained for three key points are presented in the Figure 2, Figure 3 and Figure 4 respectively, which represents the output power, battery voltage, output voltage of bridge rectifier and current through the input inductor.

5. Conclusions

The preferred use of Electric vehicles for transportation is going to increase the use and development of battery chargers. In this research, the single-stage battery charger for electric vehicles was proposed. The results showed the wide output range of 0.63 kW at $V_{bat}=420 \mathrm{V}$, 2.2 kW at $V_{bat}=300 \mathrm{V}$ and 3.4 kW at $V_{bat}=420 \mathrm{V}$. The charger operates for greater than 80% efficiency, with a 0.99 power factor. The charger also achieves the ZVS and ZCS operation, so the switching losses are negligible, thus improving the efficiency of the charger.