Abstract In this paper, a mode change method is proposed to charge and discharge a battery safely which is the main energy source of electric vehicle. To verify the proposed mode change method, a simulation is performed with a bi-directional converter of a multi-phase which is using vehicle system. The three phase interleaved bi-directional DC/DC converter performs average current control for each phase inductor current. And the proposed mode change method is applied for the operation from step-up(boost) to step-down(buck). The mode change method is proposed that can be limited overshoot current for transition and improving a dynamic current characteristic. To confirm the validity of the proposed method, its operation and performance are verified through a simulation and experiment with inverter for IPMSM.

Index Terms—Bi-directional converter, mode change,

look-up table, EV

I. INTRODUCTION Recently, it has been researching a countermeasure for

environmental pollution and fossil fuels exhaustion. Especially, in the automotive industry, the research of electric vehicle using battery for main energy source has been concentrated. In general, EV systems need a bi-directional power converting system which is using to step-up and step-down for the battery and inverter DC_Link voltage.

A bi-directional DC/DC converter is located between a battery and an inverter. It is separated two modes. The

step-up mode is that the power converter (bi-directional converter and inverter) transfers a charged energy of battery to motor in driving a vehicle. The step-down mode is that the power converter transfers a regenerated energy of motor to battery in reducing vehicle speed. However, when converting the mode of the bi-directional DC/DC converter, the control isn’t performed well according to the controller gain values. Because it can adversely affect the transient current according to a rapidly changing load should be limited.[1-2]

The proposed mode change method was applied to the three phase interleaved bi-directional DC/DC converter as shown in Fig. 1. To limit a transient current and improve a dynamic current characteristic during mode changing from step-up mode to step down mode, initial value of the PI controller is set to that of look-up table obtained by experiment.

II. BI-DIRECTIONAL DC/DC CONVERTER A bi-directional power converter for EV should be

considered lifetime, stability and volume. A battery and capacitor of DC_Link current relate to the lifetime of the EV used to power the electric energy.

Unlike the parallel type, the interleaved type distributes the current to multiphase. Therefore, each of the current stress on the power device is reduced. As a result power

Mode Change Method of Bi-directional DC/DC Converter for Electric Vehicle

Ki-Man Kim, Sang-Hoon Park, Jung-Hyo Lee, Chul-Ho Jung, Chung-Yuen Won School of Information and Communication Engineering,

Sunkyunkwan University, Suwon, South Korea. kamddy@skku.edu

Fig. 1. Scheme of the converter using a proposed mode change method

8th International Conference on Power Electronics - ECCE Asia

May 30-June 3, 2011, The Shilla Jeju, Korea

978-1-61284-957-7/11/$26.00 ©2011 IEEE

[WeP1-046]

Fig. 2. Operating mode of the converter according

to DC_Link voltage variation

device size and rated current is reduced. An input current and output voltage ripple has the advantage of being reduced.[3-5]

The disadvantage is that switch and inductor are proportionally increased as an increased number of phase. However each device would have to handle the rated current is decreased. Also the DC_Link capacitor design is easier because of the current ripple frequency of three times.[6] A bi-directional DC/DC converter used in this paper is

operating step-up mode or step-down mode depending on the driving situation of vehicle. It is separated total of 5 modes that can be divide a 2 modes to the motoring and regenerating modes. At that time, the converting reference of operation mode is decided by the DC_Link voltage variation inside of hysteresis band width. In other words, according to the DC_Link rated voltage of the inverter, if the DC_Link voltage is higher than upper level threshold by the regenerated energy in motor for reducing vehicle speed, the converter operates buck mode. If the DC_Link voltage is lower than lower level threshold by heavy load condition or vehicle driving mode, the converter operates boost mode. Fig. 2 shows the operating mode of the converter according to DC_Link voltage variation inside of hysteresis band width.

The mode 1 is a section of establishing the DC_Link voltage while driving a vehicle system in initial state, the mode 2 is a section of maintaining the DC_Link rating voltage (200V) while driving a constant speed. The mode 3 is a section of raising the DC_Link voltage while regenerating from a motor.

The mode 4 and 5, when the DC_Link voltage is higher than the upper level by the regenerated energy, are a section that the converter is operating buck mode to charge a battery. During this section, the DC_Link voltage is starting to decrease, when energy regeneration is completed or voltage level is below than the lower level threshold, the boost mode operates to hold the constant range of DC_Link voltage.

III. THE PROPOSED MODE CHANGE METHOD

A. Problems of conventional PI controller As shown in Fig. 1, the topology of this proposed

algorithm has two modes for operation. The one is buck- mode for charging the battery and the other is boost mode for suitable voltage of DC_Link. Although insufficient voltage of DC_Link makes the limit of motor drive operation, it does not make the system in danger.

ip

KK

s1

Ls R

*I oI

PI Controller ( )cG s

1

carrierVcV

Fig. 3. Constant current mode control block diagram

Of course it is also important to fast response in boost mode operation, PI controller generates enough speed to compensate the DC_Link voltage error. However, in buck mode operation, stable voltage control is more important than boost mode operation because the control response guarantees the system stability. If high regenerating current comes from motor makes the radical DC_Link voltage variation, it causes the system to out of control. Therefore, in buck mode operation, control speed to make the stable DC_Link voltage is very important. Fig. 3 shows the block diagram of control system according to the buck mode current control. This transfer function could be described by G(s) in equation (1).

2

2 2( )2

n

n n

G ss s

(1)

Damping ratio and natural frequency n which

are directly affected by PI gains are important factors for response analysis. Addition of the PI controller makes the system has the point of zero and pole. These points change the characteristics of transient state according to the damping ratio and natural frequency. Among them, maximum current at transient state is affected by

maximum overshoot pM , and the end time of transient

state is defined as settling time st .

First, maximum overshoot pM is calculated from

transfer function G(s) as below.

2/ 1pM e (2)

0.2

0.4

0.6

0.8

1

Fig. 4. Unit step response with G(s)

On the other hands, settling time st is varied by the

allowance which is the error from the reference. If the allowance makes 1%, settling time st can be obtained

by this equation (3).

0.01n ste (3)

As shown in these equations, the relation between

maximum overshoot pM and settling time st is proportion. Therefore, ideal control; small maximum overshoot and short settling time is difficult to realize by PI controller. Fig. 4 shows the step response according to the variable damping ratio . As shown in the Fig. 4, as

the settling time st is shorter, the maximum overshoot

pM is larger. If the current controller has the slow transient time by

its small gains, it means that the dynamic of current is slow, this case can occur the over voltage of DC-Link by regenerating current from motor. Also, despite large gains for increasing the dynamic of current can control the DC-Link voltage inside the band of hysteresis, it causes the large overshoot current at battery. Because the gains of PI controller determine the damping ratio and natural frequency. The dynamics of the system and transient characteristics are sensitive these gains.

Fig. 5. Hysteresis control block diagram

B. Proposed method for transition

Fig. 6. Look-up table for the proposed mode change method

Fig. 5 shows the proposed block diagram, which shows operating mode change method for three phase interleaved bi-directional DC/DC converter.

As already mentioned, operation mode of converter is determined by inverter DC-Link voltage. According to the chosen operation mode, DC-Link voltage and converter current are controlled at boost mode, and only converter current is controlled at buck mode.

In current control of bi-directional DC-DC converter, each phase current is independently controlled by each current controller for balancing the phase currents.

Applying average current mode control method, it was controlled to minimize the error between the current reference value of converter and real current value (according to ADC, each current sampling time is the same as each average current value).

In EV system, the fast dynamic is needed for frequently varied load according to the driving circumstance. To obtain the fast transient response without overshoot, initial PI controller is set by look-up table value except reset the PI controller. Fig. 6 shows the look-up table data is obtained by battery voltage and reference current.

From this initialization of the PI controller that the duty ratio is obtained at steady state, DC-Link voltage can be controlled in the band of hysteresis and reducing the peak current. This initial value of the PI controller is estimated by the battery impedance and voltage.

IV. SIMULATION RESULTS

Table 1 shows the simulation parameters used in this paper. The simulation has been implemented under the vector control of inverter using a 10kW IPMSM.

Fig. 7 shows the simulation results which do not consider the initial condition of controller. First, DC_Link voltage of the inverter is charged by boost

TABLE

SPECIFICATION OF THE SIMULATION PARAMETERS

Converter specification

Rated Power: 15kWBattery voltage: 96V

DC_Link voltage: 200VEach phase inductance: 500μH

Motor specification

Rated power: 10kW

Rated torque: 40NmPhase voltage: 100VPhase current: 100A

d-axis inductance: 0.848mHq-axis inductance: 1.484mHPhase resistance: 30m

Pole: 16pole

control of DC/DC converter. The DC_Link voltage is kept up 200V by converter and the motor is operated by inverter at rated speed. And to implement the motor to be regenerate, speed reference higher than rated speed is inserted on the controller. At 0.2[msec] of simulation time, constant speed load is applied. Then the speed controller operates for reducing the speed but it only generates maximum current reference of speed controller because the motor speed does not follow the reference. This current reference makes the power regeneration, and the DC_Link voltage is increasing. And then, if the voltage achieves the target value of hysteresis controller in Fig.3, the DC/DC converter operates as the buck mode.

In Fig. 7(a), if the gains of buck mode current controller are limited for reducing the current noise,voltage is increased over the limitation because of the slow dynamics of current controller. Meanwhile, if the gain is increased for limitation of DC_Link voltage, the noise is also increased as shown in the Fig. 7(b). Fig. 8 shows the simulation results with proposed mode change method though the dynamics of the controller is slow, it can make the voltage in the region of hysteresis controller by determination of initial value of

(a) The gain value of PI current controller

with pK = 0.1 and iK = 10

(b) The gain value of PI current controller

with pK = 0.5 and iK = 10

Fig. 7. Simulation results without a mode change method

Fig. 8. Simulation results with the proposed mode change method

Fig. 9. Block diagram of experimental setup

the PI controller. This initial value is determined by current reference and battery impedance. The current reference is obtained by BMS (Battery Management System) and battery impedances are detected by the observer at boost mode operation.

V. EXPERIMENTAL RESULTS

In Fig. 9, the experimental setup is presented, which has inverter of Yaskawa corporation, AC/DC converter of Fuji, induction motor of Siemens to implement the regenerating mode. 11kW Yaskawa inverter will be controlled IM by three phase grid connected Fuji AC/DC converter. AC/DC converter supplies power source from grid to inverter. The inverter 2 has to operate the IM with vector control for speed.

The hysteresis voltage control of DC_Link has to be performed because mode changing is determined by DC_Link voltage. It is possible to operate for inverter 1 the torque control for IPMSM.

The bi-directional DC/DC converter consists of shown in Fig. 10. To control the converter, the control parts

Fig.10. Prototype of the bi-directional DC/DC converter.

Fig. 11. Prototype of the three phase voltage source inverter.

(control board based on TMS320F28335, sensor board, gate driver) are operated for sensing the voltage and current that will be used as a PI controller. Three inductors distribute the battery and regenerated DC_Link current as a result the volume can be reduced.

Fig. 11 shows the configuration of prototype inverter. The output voltage of bi-directional converter is used as an input voltage of the inverter at step-up mode. To implement bi-directional power transfer, a torque control has been used with 10kW IPMSM. And then coupling the IPMSM, IM makes to be operated at higher than the rated speed for changing a mode from step-up to step-down.

Fig. 12 shows the experimental results in boost mode that are showing the DC_Link voltage, A-phase inductor current and PWM gate signal as shown in Fig. 12(a). Three phase current balance control can be confirmed as shown in Fig. 12(b). The each phase current of inductor is controlled independently to operate with 120 phase difference.

In Fig. 13, the steady state waveforms are presented, when a bi-directional converter is operating as buck converter. Experimental results are as shown in Fig. 14 for the three phase inductor current, A pahse inductor current and PWM waveforms. The battery current ripple is reduced which is the sum of three phase inductor current.. In addition, the parallel capacitor reduces the ripple even more.

(a) DC_Link voltage, A-phase inductor current and PWM gate signal

(b) Three phase inductor current.

Fig. 12. Experimental waveforms in boost mode.

(Y-axis = DC_Link voltage : [60V/div], Inductor current : [10A/div], PWM signal : [10V/div], X-axis = 50[ s/div])

(a) Three phase inductor current, A-phase inductor current,

PWM gate signal.

(b) Three phase inductor current.

Fig. 13. Experimental waveforms in buck mode.

(Y-axis = Inductor current : [10A/div], PWM signal : [10V/div], X-axis = 50[ s/div])

(a) Conventional PI controller with low gain value.

(b) Conventional PI controller with high gain value.

(c) PI controller with proposed method.

Fig. 14. Experimental waveforms at mode change.

(Y-axis = DC_Link voltage : [50V/div], Battery current : [1A/div], X-axis = 50[ s/div])

Fig. 14 shows the experimental results that the

DC_Link voltage and battery current during a mode change. When using a low gain value, the DC_Link voltage can be overcharged because of slow dynamic current as shown in Fig. 14(a). To improve the dynamic current characteristic using high gain value, the battery current can cause overshoot current as shown in Fig. 14(b). In order to solve these problems by using the proposed mode change method, and results were as shown in Fig. 14(c). When a mode changes from step-up to step-down using a look-up table, as a result DC_Link voltage is controlled between upper level and lower level and the overshoot battery current can be limited.

VI. CONCLUSION

In this paper, when a mode is changed from step-up to step-down, the method for limiting the transient current by using a proposed mode change method is proposed. The proposed mode change method is proved through a simulation and experiment which uses a three phase interleaved bi-directional DC/DC converter with the inverter for IPMSM.

ACKNOWLEDGMENT

This work is the outcome of a Manpower Development Program for Energy & Resources supported by the Ministry of Knowledge and Economy (MKE).

REFERENCES [1] H. S. Bae, J. H. Yang, J. H. Lee and Bo H. Cho, "Digital

State Feedback Current Control using Pole Placement Technique for the 42V/14V Bi-Directional DC-DC Converter Application”, Applied Power Electronics Conference, pp. 3 7, Feb. 2007.

[2] Chang-Gyu Yoo, Woo-Cheol Lee, Kyu-Chan Lee, Cho, B. H., "Transient current suppression scheme for bi-directional DC-DC converters in 42 V automotive power systems", IEEE APEC, pp. 1600 1604, Mar. 2005.

[3] D a v i d J . P e r r e a u l t a n d J o h n G . K a s s a k i a n , “Distributed Interleaving of Paralleled Power Converters”, IEEE Trans. On Circuits and Systems-I: Fundamental Theory and Applications, Vol. 44, NO. 8, pp. 728 734, Aug. 1997.

[4] Arash A Boora, Firuz Zare, Gerad Ledwich, “Bidirectional Positive Buck-Boost Converter”, Power Electronics and Motion Control Conference, pp. 723~727, Sept. 2008.

[5] Junhong Zhang, Jih-Sheng Lai, Rae-Young Kim, Wensong Yu, “High-Power Density Design of a Soft-Switching High-Power Bidirectional dc-dc Converter”, IEEE Trans. Power Electron., Vol. 22, NO. 4, pp. 1145~1153, July, 2007.

[6] Xin Guo, Xuhui Wen, Ermin Qiao, "A DSP Based Controller for high Power Dual-Phase DC-DC Converters”, Power Electronics and Motion Control Conference, pp. 1 5, Aug. 2006.