A 3 kW Bidirectional DC-DC Converter for Electric Vehicles

Abstract

A bidirectional DC-DC converter (BDC) is an indispensable electrical unit for the electric vehicles (EVs). High efficiency, high power density, isolation, light weight and reliability are all essential requirements for BDC. In this paper, a 3 kW BDC for the battery charger of EVs is proposed. The proposed converter consists of a half-bridge structure on the primary side and an isolation transformer and a synchronous rectifier structure on the secondary side. With this topology, minimum number of switching devices are required for bidirectional power flow between the two dc buses of EVs. The easy implementation of the synchronous rectification gives advantages in terms of efficiency, cost and flexibility. The proposed BDC achieves high efficiency when operating in both modes (step-up and step-down). A 3 kW prototype is implemented to verify theoretical analysis and the performance of the proposed converter.

1. Introduction

With the global energy crisis the conventional vehicles (internal combustion engines) face the increasingly serious problems of energy. In contrast, the EVs especially battery electric vehicles (BEVs) depend on variety of options for its driving power. BEVs offer the advantages of safety, silent operation and no emissions when powered by renewable energy sources such as wind or solar which are virtually emission free [1]. These vehicles can also make efficient use of energy by storing energy recovered during braking or deceleration cycle in the batteries. The storage or charging process of the battery is achieved by a BDC, which is the key block in EV energy system to link high voltage (HV) dc bus and low voltage (LV) dc bus as shown in Fig. 1. This BDC should have high power density and high efficiency to meet the desired goals for EV’s battery charger. When the EV is parked, the battery can be charged by the household utility outlet from the grid through the BDC. For the other case when the EV is in the driving state, the BDC provides the electrical power from LV battery to the motor through DC-AC inverter and also DC loads in the EV.

Fig. 1.The energy system of EV

BDCs are broadly classified into isolated and non-isolated types. The conventional non-isolated buck/boost BDC cannot operate in the wide voltage conversion range [2]. The isolated BDC are preferred for EVs due to the advantages of high voltage conversion ratio and safety. Many different types of isolated BDCs [3-6] have been proposed due to these advantages, some full-bridge BDCs [7-10] have also been proposed in recent years. However, full-bridge converters have the disadvantage of high voltage ripples if not employing an extra voltage clamping circuit [11]. By contrast, half-bridge converters [12-14] have a simple structure and a better anti-imbalance ability in the transformer. In some topologies of halfbridge converters, MOSFET body diodes are applied for synchronous rectification in both buck/boost modes [15], but high conduction losses result in low efficiency, thus limiting the use of these converters to only low power applications.

This paper describes the development of a 3 kW BDC for EVs. The converter consists of a half-bridge topology, an isolation transformer and a synchronous rectifier. The isolation transformer provides the advantages of wide conversion range and safety, and the easy implementation of synchronous rectification offers the benefits in terms of efficiency, cost and flexibility. However, this structure has been mostly proposed for less than 1.5 kW application [3, 11-13, 16-17], so the operation of more than 3 kW in both step-up and step-down modes has the practical significance for the EV battery charger products.

 

2. Topology Configuration and Operational Principles

2.1 Topology configuration

Fig. 2 shows the circuit diagram of the proposed halfbridge BDC. The design uses a half-bridge connected with the DC power supply on the primary side and a center-tapped transformer and a synchronous rectification on the secondary side. The converter can operate in two modes, namely, step-down mode and step-up mode. All of the four switches Q1–Q4 are gated in both modes. Switch Q1 is complementary with switch Q4, and switch Q2 is complementary with switch Q3.

Fig. 2.Circuit diagram of the proposed converter

In the step-down mode, the primary side DC power supply VH (210–380 V) charges the secondary side battery VL (21–29 V), and Q3 and Q4 provide rectification. In contrast, Q1 and Q2 operate as rectifiers in the step-up mode when VL supplies the high side battery VH. For mathematical insight into the proposed converter, some assumptions are made as: (i) the ON-state resistance RDS(ON) of all switches is ignored; (ii) the capacitors C1, C2 and Co are large enough, and the voltage across the capacitors can be taken as constant; and (iii) the capacitance of the capacitors C1 and C2 is equal i.e. C1=C2 =C. Thus, VC1 =VC2 =VH /2; and (iv) D1–D4 are the body diodes of Q1–Q4, and the diode forward resistance is zero.

2.2 Operational principles of the proposed BDC

2.2.1 Step-down mode

This is similar to a buck converter operational mode in which VH supplies VL with the charging current, iL. The equivalent circuits are shown in Fig. 3. The pulse-width modulation (PWM) technique is used to control the switches Q1–Q4. In both modes Q1 and Q2 are gated, with the duty cycle less than 0.5, while the duty cycle of Q3 and Q4 is more than 0.5. Fig. 4 shows some typical waveforms for the step-down mode. The operating principles during one switching period are described as follows:

Stage 1 [t0–t1]: Switch Q1 turns on and switch Q4 turns off at t0, while switch Q3 remains on. The current flow path for this stage is shown in Fig. 3(a). In this stage vAB=VH /2, the current i1 flows through Q1 as ip, which is reflected from inductor current iL. The current iL increases linearly and flows totally through switch Q3 to charge the battery VL

Stage 2 [t1–t2]: Switch Q1 turns off and switch Q4 turns on at t1, while switch Q3 still remains on. Because of the transformer leakage inductance lk1, there is freewheeling current through D2 [Fig. 3(b)], vCA=VH /2 and ip decreases linearly to zero. In this stage vA is clamped to ground so vQ2=0 and vQ1=VH . Meanwhile i4 increases and i3 decreases linearly and at t2, i3=i4= iL/2.

Stage 3 [t2–t3]: Switch Q1 and Q2 are in off state and vQ1= vQ2= VH /2. In this stage, vAB=0, no power is transferred to secondary side and the energy stored in the inductor L charges the low side battery VL [Fig. 3(c)]. The current iL is shared equally by switches Q3 and Q4.

Stage 4 [t3–t4]: Switch Q2 turns on and switch Q3 turns off at t3, while switch Q4 remains on. The current flow path is shown in Fig. 3(d). This is a similar operation to stage 1[t0-t1], but the voltage vAB=−VH /2. The current i2 is built as −ip. In this stage, Q4 is conducting and i4 increases linearly as iL.

Stage 5 [t4–t5]: Switch Q2 turns off and switch Q3 turns on at t4. Switch Q4 still remains on. Because of lk1, D1 conducts and vA is clamped as VH, therefore vQ2= VH and vQ1=0 [Fig. 3(e)]. Meanwhile i3 increases and i4 decreases linearly and at t3, i3=i4= iL/2.

Stage 6 [t5 –t6]: The operation of this stage is the same as stage 3. The current path for this stage is shown in Fig. 3(f).

Fig. 3.Equivalent circuits for the step-down mode

Fig. 4.Theoretical waveforms for the step-down mode

2.2.2 Step-up mode

For the step-up mode, the equivalent circuits considering the leakage inductance of the proposed converter are shown in Fig. 5. In this operational mode, VL discharges to supply the primary side output voltage of VH with current i1 or i2. Because of the existence of the transformer secondary side leakage inductance lk2 and lk3, there will be current stress on Q3 and Q4. For protection of Q3 and Q4, the RC snubber circuit is necessary in parallel connection with Q3 and Q4. The theoretical waveforms are shown in Fig. 6, and modes of operation in one period (t0–t6) are described as follows:

Stage 1 [t0–t1]: Switch Q3 is turned on at [t0, with switch Q4 remaining on while Q1 and Q2 are in the off state. The current flow path for this stage is shown in Fig. 5(a). The secondary side of the transformer is effectively shorted, and vAB=0. Meanwhile, the energy is stored in the inductor L, while no energy is transferred to the primary side. In this stage, iL increases linearly and is divided equally between Q3 and Q4. The primary side battery VH is charged by the capacitors C1 and C2.

Stage 2 [t1–t2]: Switch Q4 is turned off while switch Q1 is turned on at t1, with switch Q3 remaining on. Because of the leakage inductance lk2, there is stress on Q4 and the RC snubber is charged by i4 [Fig. 5(b)]. The current i4 decreases linearly to zero and i3 increases linearly to iL, building i1 as ip which increases linearly.

Stage 3 [t2–t3]: In this stage vAB=0 [Fig. 5(c)], the energy stored in L is transferred to the primary side, iL and decrease in linearity. The capacitor C2 is discharged and capacitor C1 is charged.

Stage 4 [t3–t4]: Switch Q4 is turned on, with switch Q3 remaining on while switch Q1 is turned off at t3. This stage is similar to stage 1 in which the inductor L stores energy again, and the inductor current iL is equally shared by switches Q3 and Q4 [Fig. 5(d)]. Capacitor C1 and C2 discharge to supply the primary side DC source VH.

Stage 5 [t4–t5]: At t4 Switch Q3 is turned off, with switch Q2 turned on and switch Q4 remaining on. Because of the leakage inductance lk3 [Fig. 5(e)], there is stress on Q3 and the RC snubber is charged by i3. The current i3 decreases linearly to zero with i4 increasing linearly to iL, building i1 as −ip.

Stage 6 [t5–t6]: In this stage [Fig. 5(f)], i4 is built as iL which decreases in linearity and energy is transferred to primary side. The capacitor C2 is charged by i2 conducted by Q2, which decreases linearly.

Fig. 5.Equivalent circuits for the step-up mode

Fig. 6.Theoretical waveforms of the step-up mode

 

3. Circuit Design Analysis

The BDC operates in step-down and step-up modes. Design parameters for the step-down mode will be discussed in detail. The parameters obtained can be used for step-up mode as well. The theoretical analysis and design guidelines will be discussed in this section.

3.1 Step-down mode

When the number of turns on the secondary windings are equal, that is, N2=N3, then N can be defined as the transformer turns ratio, as N1/N2 or N1/N3. The relationship between VH and VL is expressed as:

where D is the duty ratio of Q1 and Q2.

To design the inductor L, the inductor current ripple ΔiL and the minimum duty ratio Dmin should be considered. The inductance L can be calculated as:

When designing output capacitor Co, the transient overshoot should be taken into account. Because of the inductor L, the energy stored in L will be transferred to Co if there is a sudden change of load, causing a sudden change of VCo. According to the design specification, the overshoot voltage should be less than 3% of VL, so that for a 50% to full load situation, Co can be calculated as:

and for the equivalent series resistance (ESR) of Co, the value of ESR should be limited by the following equation:

So several capacitors may be connected in parallel, if necessary, to meet the requirements of ESR.

The two capacitors C1 and C2 should be large enough to constrain the input current ripple and equally share VH for Q1 and Q2.

In practical applications, there are many reasons causing the voltage imbalance. A few of those are: 1) the conduction periods of the two high side switches Q1 and Q2 are not strictly equal, 2) C1 and C2 are charged and discharged in turns, and if the capacitance value is not big enough, there will be big voltage ripple which may cause voltage imbalance.

To avoid the voltage imbalance, it is necessary to keep the equal conduction periods for Q1 and Q2 to the maximum possibility, and it should be noted that the switch Q4 is complementary with Q1 and it is the same situation for Q2 and Q3. For C1 and C2, there is

where ip is the transformer primary side current.

It can be seen that if C is big enough, the ripple will be small and it will not affect the circuit operations.

The maximum voltage stress and RMS/max current ratings should be considered when selecting the switches of both sides. Q1 and Q2 have the ratings of:

Q3 and Q4 have the ratings of:

3.2 Step-up mode

For this mode, VL is the input voltage, and VH is output voltage. Therefore:

where D’ is the duty ratio of Q3 and Q4.

Because the BDC operates just as the current flows inversely, but the voltage polarity remains unchanged, so the design parameters of all components of step-down mode can be employed in the step-up mode.

 

4. Experimental Results

A 3 kW prototype, as shown in Fig. 7, was built and tested to evaluate the performance of the proposed BDC. The experimental parameters and circuit components are given in Table 1. The experimental waveforms of stepdown and step-up modes are shown in Fig. 8 to Fig. 13.

Fig. 7.Detailed photograph of the 3 kW proposed BDC

Table. 1.Experimental parameters

Fig. 8.Step-down mode experimental waveforms at light load

Fig. 9.Step-down mode experimental waveforms at full load

Fig. 10.Step-down mode experimental waveforms at 1 kW

Fig. 11.Step-up mode experimental waveforms at light load

Fig. 12.Step-up mode experimental waveforms at full load

Fig. 13.Step-up mode experimental waveforms at 2 kW

Fig. 8 shows the voltage across two switches Q1 and Q4, which is VQ1 and VQ4 , respectively, and the input and the output voltage VH of 380 V and VL of 29.6 V at light load. One can see that the voltage stress on switch Q1 is equal to VH, and that on switch Q4 is equal to VH /N. The output voltage VL is well regulated at 29 V. In this situation, the duty ratio of switches Q1 and Q2 is 23.4%.

Fig. 9 shows the inductor current iL and the switch voltages VQ1 and VQ4 at full load. It can be seen that the average output current iL,Avg is 107.75 A, achieving 3.1 kW output power.

Fig. 10 shows the step-down 1 kW experimental waveforms. Channel 1 and channel 3 show the drain-to-source voltage VQ3 and VQ2 . The input voltage can be seen from the maximum value of VQ2(max) as 377 V and the output voltage is 29 V. In this situation the duty ratio of Q1 and Q2 is about 23.1%. Channel 2 shows transformer primary side current which changes the direction accordingly when Q1 or Q2 are turned on. The average value of every conduction period is about 13 A. Channel 4 shows the voltage across L.

Fig. 11 shows VQ1 , VQ4 , VL and VH for the step-up mode at light loads. It should be noted that the output voltage is well regulated at 380 V, and the maximum voltage on switch Q1 is equal to VH and that on switch Q4 is VH/N. In this situation, the duty ratio of Q3 and Q4 is 77.2%.

Fig. 12 shows the average input current of −108.08 A, output current of 7.26 A, and the voltage across switches Q1 and Q4 at full load conditions. The input and output power obtained are 3.13 kW and 2.76 kW, respectively.

Fig. 13 shows the step up mode 2 kW experimental waveforms. The input voltage is 29 V and the output voltage can be seen from VQ2(max) as 351 V. In this situation the duty ratio of Q3 and Q4 is about 75.2%. Channel 2 shows transformer primary side current iP with the average value of every conduction period about 23 A. Channel 4 shows the voltage across the inductor L.

Fig. 14 shows the measured efficiencies of the proposed BDC when it operates in step-down mode. The converter achieves efficiency of higher than 90% from 30% to full load.

Fig. 14.System efficiency in step-down mode

Fig. 15 shows the measured efficiencies of the proposed converter when it operates in step-up mode. It can be seen that more than 88% of the efficiency is achieved from 30% to full load.

Fig. 15.System efficiency in step-up mode

 

5. Conclusion

A 3 kW BDC for EVs is proposed in this paper. With a simple topology structure, the converter consists of a half-bridge on the primary side and a synchronous rectifier on the secondary side. Four switches are employed in both the step-down mode and step-up mode. The theoretical analyses have been proved by the experimental results of the 3 kW prototype circuit. When operating in step-down mode, an efficiency was achieved of more than 90% from 30% to the full load, while an efficiency of 88.2% is achieved at full load for the step-up mode.