Performance Analysis of Bi-directional DC-DC Converters for Electric Vehicles and Charging

Infrastructure

Adeeb Ahmed(1) Mehnaz Akhter Khan(2) Mohamed Badawy(1) Yilmaz Sozer(1) Iqbal Husain(2)

Department of ECE Department of ECE

The University of Akron (1)

Akron, Ohio, USA North Carolina State University (2)

Raleigh, North Carolina, USA

Abstract—This paper presents the performance analysis and comparison of two different types of bidirectional DC-DC converters - Cascaded Buck-Boost (CBB) and Combined Half- Bridge (CHB) for use in Plug-in EV and HEV. The comparison of the two converters is based on device requirements, rating of switches and elements, control strategy and performance. Each of the converter topologies has some advantages over the other in certain aspects. Feasibility studies are carried for practical applications in specific scenarios. The simulation and experimental results are provided for both converter types to support the theoretical findings.

I. INTRODUCTION The DC-DC converter between the energy storage device

and the inverter in an electric powertrain of an EV/HEV is used to condition the voltage levels and provide stable DC bus voltage [1]. Furthermore, the DC-DC converter need to have bi-directional power flow capability so that regenerative energy can be captured processed and then stored in the energy storage. In addition, some applications may require overlapping input-output voltage ranges.

The two DC-DC converters analyzed and compared in this research can also be used in an EV charging station infrastructure. Since the on-board charger do not provide fast charging capability, DC charging is required to ensure fast charging for extending the all-electric drive range. A municipal parking deck charging station with DC power distribution bus can employ bi-directional DC-DC charger to allow Vehicle to Grid (V2G) operation [3]. V2G operation can be useful to inject real or reactive power to the grid to ensure current harmonic filtering or load balancing. A bi-directional converter with overlapping input output voltage range would enhance the operational flexibility for G2V or V2G applications.

Several different types of bi-directional DC-DC converters along with their comparison appear in the literature [2-4]. Most of them require fewer components and simple control techniques but cannot provide bi-directional buck-boost power flow capability. In this research, a comparison between two bi-directional buck-boost converters is presented to analyze the benefits and the drawbacks of the topologies for EV applications. The comparison is based on system stability, component sizing and ratings. The first converter is the Cascaded Buck Boost (CBB) converter proposed in [6] and shown in Fig.1. This converter has been adopted for different applications and research is available on comparison, study and other modifications of this converter [2-5]. The other converter shown in Fig.2 is the one proposed in [7] and is called the Combined Half Bridge (CHB) converter.

II. CONVERTER TOPOLOGIES OF INTEREST Fig.1 and Fig.2 show the two different converters of

interest. Fig.1 presents the conventional Cascaded Buck Boost topology (CBB) having an interfacing inductor between the input and output sides [6]. Fig.2 on the other hand presents the unconventional Combined Half Bridge (CHB) topology where the two half bridge converters are cascaded together with a common dc bus capacitor [7]. Both converters provide an input and output voltage overlapping capability.

Fig.1: Cascaded Buck Boost Converter

1401978-1-4799-0336-8/13/$31.00 ©2013 IEEE

Fig.2: Combined Half Bridge Converter

III. ANALYSIS OF THE CBB AND CHB TOPOLOGIES DC-DC converter with multi-input and multi-output

capability is useful for electric and hybrid vehicles that use multiple sources or require multiple auxiliary outputs. For example, multiple output options are needed to connect two sources such as an ultracapacitor-battery storage combination. In heavy hybrid vehicles such as fleet trucks, multiple auxiliary outputs would require different output voltage levels. The following provides the analysis of the CBB and CHB topologies in different operating modes.

A. CHB Topology The CHB topology in Fig.3(a) can be used for a

combination of battery and ultracapacitor storage which would be connected to and of the figure respectively. The traction inverter would be connected across the DC-link provided by the capacitor . is the charging port and connected to rectified DC sources during charging of battery or ultra-capacitor at the and ports.

(a)

(b)

Fig.3: (a) Multi output case in CHB (b) < (Boost mode) and > and (Buck mode)

The duty cycles , and for the gate switching

signals of , and , respectively can be represented based port voltages such as : 1

The CHB converter can also be used in the V2G mode with the battery pack and ultracapacitor bank serving as multiple input sources, and a DC external load connected across C4 at Vout in the circuit shown in Fig.4(a). The duty cycles , and for the gate switching signals of S , S and S , respectively are as follows: 1 1

where is the intermediate stage voltage.

(a)

(b) Fig.4: (a) Multi input case in CHB (b) and < (Boost mode)

and > (Buck mode)

B. CBB Topology The CBB converter can be used with one input and

multiple outputs similar to the CHB topology. The duty cycles and for the gate switching signals of and

, respectively are 1 1

The gate signal control of the CBB topology is complex for some operating modes. In case of multiple outputs buck operation an extra freewheeling mode is required for the lower output branch as given in Fig.5(c).

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IV. COMPARISON OF THE CBB AND CHB TOPOLOGIES Comparisons of the two converter topologies are done

for the following aspects: i) Switching mechanism ii) Stresses on switches and diodes, iii) Ratings of the passive components, iv) Size of the passive components, v) Interleaving capability, and vi) Multi input output capability.

(a)

(b) (c)

Fig.5: (a) Multi output case in CBB (b) V <V and V (Boost mode) (c) V >V and V (Buck mode)

A. Switching Mechanism Both the converters basically require only one switch to

be switched at a particular frequency to operate either as buck or boost converter. Another switch is required to be in the ON mode for the full switching period for current conduction [6]. An alternative strategy for switching both the switches with different duty ratios and maintaining a particular intermediate voltage for CHB appears in [1]. This strategy results in a higher intermediate voltage across the central capacitor which can be used as for a multi-output converter.

B. Switch Rating and Size Stress on the switches is one of the prime concerns when

going for final implementation of the converters. Equipment size, weight and cost are largely dependent on the ratings of the switches. Comparison tables including the component ratings are given in this section. All the tables presented in this section are based on single input single output circuits shown in Fig.1 and Fig.2.

Both the basic converters with single input single output configuration comprise of four switches with freewheeling diodes. Although all four of them are never used together, all of them must be there to ensure complete flexibility. For the CBB, peak voltage across any switch depends on the operation mode as provided in Table I. For CHB, all the switches and diode experience the same voltage stress which is equal to the voltage across central capacitor. Therefore, voltage across the central capacitor must be limited up to the maximum voltage that the switches are designed to withstand. Average currents through all switches and diodes are provided in Table II; these currents play a significant role

in the decision making for choosing the converter topology. Significant losses occur during switching in the circuit and switching losses increase if amount of current in the element is high while switching takes place. Thus, lower current and voltage across switches are desirable. Table II shows that both of the converters have essentially the same type of average current through the switches if same sort of control mechanism is applied (maintaining duty cycle at 100% for one of the switches in all operating modes).

TABLE I. PEAK VOLTAGE FOR CBB to

Buck to

Boost to

Buck to

Boost

D1/ S1 V1 0 0 V1

D2/ S2 V1 V1 V1 V1

D3/ S3 0 V2 V2 0

D4/ S4 V2 V2 V2 V2

TABLE II. AVERAGE CURRENT THROUGH SWITCHES AND DIODES(CBB AND CHB)

V1 to V2

Buck V1 to V2 Boost

V2 to V1 Buck

V2to V1 Boost

CBB CHB CBB CHB CBB CHB CBB CHBD1 0 0 0 P/V1 P/V1 0 P/V2 0

D2 P/V2 0 0 0 0 P/V1 0 0

D3 P/V2 0 P/V1 0 0 0 0 P/V2

D4 0 P/V2 0 0 P/V1 0 0 0

S1 P/V2 P/V1 P/V1 0 0 P/V1 0 P/V1

S2 0 0 0 P/V1 0 P/V2 P/V2 0

S3 0 P/V2 0 P/V2 P/V1 0 P/V2 0

S4 0 0 P/V1 0 0 0 0 P/V2

C. Inductor and Capacitor Ratings and Sizes Inductors are the heaviest and most expensive passive

building element in any high power converter. The inductor size can strongly influence the selection of one topology over the other. Cascaded buck-boost requires only one inductor whereas combined half-bridge requires two. For the final selection decision, inductor ratings and sizes must be calculated. In both circuits, required inductor rating depends on the operating condition. For successful operation, inductor must be sized and designed considering the worst case scenario. Table III provides the inductor ratings of the two converters for different modes of operation.

The required inductance values for the high power converters are plotted in Fig. 6 to give a better idea about the relative sizes. Though the expressions appear to be different, the required duty ratio for the converters were such that it resulted in same inductance level for both the converters. The required inductance increases with the conversion ratio; for unit conversion ratio theoretically no inductance is required (in this case, Switch 1 and Switch 3 can be closed with unity duty ratio making a direct path for

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the current). Table III also provides the values for average inductor currents. For CBB, average current through inductor largely depends on mode of operation and operating voltages at any side. Considering the worst case design scenario, the inductor must be capable of carrying the maximum of four possible currents provided in the designated column for CBB in Table III. On contrary, the inductor currents in CHB are fixed for each inductor depending on the operating voltage at the side it is connected to.

Table IV provides the required capacitance values for both the converters. Both the converters have the same expression for minimum required capacitance. The required values are calculated considering certain allowed voltage ripple across the capacitor. Fig.7 shows the capacitance variation with conversion ratio. A comparison of Fig.7(a) with Fig.7(b) shows that boost operation with smaller conversion ratio requires higher capacitance for maintaining a constant voltage ripple.

TABLE III. INDUCTOR RATINGS AND AVERAGE CURRENTS

Inductance Average Inductor

Current CBB CHB CBB CHB

L L1 L2 L L1 L2

V1 to V2 Buck

1∆

V 1 Df ∆ LIL P

P/V2 P/V1 P/V2

V1 to V2 Boost

∆

P/V1 P/V1 P/V2

V2 to V1 Buck

1∆

V 1 Df ∆ LIL P

P/V1 P/V1 P/V2

V2 to V1 Boost

1 ∆

P/V2 P/V1 P/V2

TABLE IV. Capacitor Ratings V1 to V2

Buck V1 to V2

Boost V2 to V1

Buck V2 to V1

Boost

C1

18 ∆ 1VG ∆C1

18 ∆ 1 ∆

D. Interleaving Capability Interleaving technique can be applied to both converter

topologies to reduce the switching stresses and the voltage and current ratings of the switches. Effective switching frequency also increases with the introduction of interleaving which in turn helps in reducing ripples in output voltage and inductor currents [8-10]. In both converters, there is the flexibility of applying input side interleaving or output side interleaving or both. The CHB can be used as DC to AC

converter by adding one extra half-bridge leg, but for CBB an extra H- bridge converter is needed.

(a)

(b)

Fig.6: Inductance variation with the change in conversion ratio (For CHB and CBB). (a) Buck operation, (b) Boost operation

(a)

(b) Fig.7: Capacitance variation with the change in conversion ratio (For

CHB and CBB). (a) Buck operation, (b) Boost operation

E. Multiple Inputs and Outputs Both the converters were found to be suitable for

multiple output capabilities as seen in Section III. But CBB topology is not suitable for multi-input operation since there will be circulating current in the input loop due to the common connection point of the two inductors in the output leg of the converter.

0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Vout/Vin

Req

uire

d In

duct

ance

(m

H)

Inductance variation for 75KW Converter with Vout

=300V, 10% Current Ripple (Buck)

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 30

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Vout

/Vin

Req

uire

d In

duct

ance

(m

H)

Inductance variation for 75KW Converter with Vout

=300V, 10% Current Ripple (Boost)

0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 10

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Vout/Vin

Req

uire

d C

apac

itanc

e ( μ

F)

Capacitance variation for 75KW Converter Vout

=300V, 10% Voltage Ripple (Buck)

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3200

300

400

500

600

700

800

900

Vout/Vin

Req

uire

d C

apac

itanc

e ( μ

F)

Capacitance variation for 75KW Converter Vout

=300V, 10% Voltage Ripple (Boost)

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V. STABLITY ANALYSIS System stability is a major concern in case of high

power converters and should be considered in the design procedure. The inductor and capacitor sizing listed in Section IV are derived considering ideal circuit elements neglecting the effective series resistance (ESR) of the capacitors and resistance of the inductor coils. In this section, stability analysis of the open loop system will be provided in terms of state space model [11-13]. The basic CHB and CBB topologies with single-input single-output mode have the following transfer function containing the unavoidable non-idealities.

For CHB, the following state space matrix were computed considering the switching of and

0

0 0 00

B L000 C 0 RLRL 0 RLRL , D 0

, 1

where , , , , are the resistances of the passive elements.

The CBB on the other hand has a second order transfer function since at most two energy storing elements experience the switching at a time. The following state space model is applicable for CBB considering switching of

and .

B DL000 , C RLRL RLRL , D 0

, 1

For analyzing the stability of the open loop circuit, three different situations are considered. The first test is performed to observe the effect of input voltage change

while maintaining a constant output level at constant load current. For CHB, the input voltage was considered to be varying from 100V to 1800V while maintaining output voltage at 300V and desired intermediate capacitor voltage at 500V when possible. This will cover a wide operating range containing both buck and boost operation. Since stage 1 of CHB is only capable of boost operation, the intermediate voltage across was kept equal to the input voltage when input voltage level exceeded the desired intermediate voltage of 500 V. For CBB, similar input voltage range was considered with desired intermediate voltage of 500 V. Since the first stage of CBB is only capable of performing buck operation, the intermediate voltage was kept equal to the input voltage when input voltage was smaller than 500 V.

State matrix of CHB provided two complex conjugate poles with real part trajectories shown in Fig. 8(a) and Fig. 8(b) which are always in the negative axis. Although passive elements were designed considering the rated conditions using Table III and Table IV, the system seems to be stable at operating points well beyond the nominal region. Real part of the pole trajectory for CBB is presented in Fig. 8(c) and is found to be in the stable region as well.

(a)

(b)

(c) Fig. 8: Effect of input voltage change in system poles (a) pole 1 of CHB

(b) Pole 2 of CHB (c) CBB

200 400 600 800 1000 1200 1400 1600 1800-11

-10

-9

-8

-7

-6

-5

-4

Input Voltage (V)

Rea

l Par

t of S

ystem P

ole

Effect of the input battery voltage changing on the real part of the pole 1

200 400 600 800 1000 1200 1400 1600 1800-19

-18

-17

-16

-15

-14

-13

-12

Input Voltage (V)

Rea

l Par

t of S

ystem P

ole

Effect of the input battery voltage changing on the real part of the pole 2

200 400 600 800 1000 1200 1400 1600 1800-46

-44

-42

-40

-38

-36

-34

-32

-30

-28

Input Voltage (V)

Rea

l Par

t of

Sys

tem

Pole

Effect of the input battery voltage changing on the real part of the pole 1

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To assess the system performance in case of any discrepancy in the chosen inductance value, the system pole trajectory was observed for varying the inductance at input side for CHB and the central inductance for CBB. Real part of the system pole trajectories for CHB are presented in Fig. 9(a) and Fig. 9(b). Real part trajectories of the pole for CBB is shown in Fig. 9(c). Both the systems were found robust against any variation in the inductance value over a wide range.

The third test was done to observe the effect of changes in output power of the system. Since the converter output needs to be varied to meet the required power level, the system must be robust over a wide range of power level. At 300 V output voltage, the load resistance was varied from 1Ω to 100 Ω which correspond to an output power level of 90 kW to 0.9 kW. Real part pole trajectories of CHB are shown in Fig. 10(a) and Fig. 10(b). Real part pole trajectory for CBB is shown in Fig. 10(c). Both the converters are found to have poles in the negative axis at all power levels.

(a)

(b)

(c) Fig. 9: Effect of change in inductance in system poles (a) pole 1 of

CHB (b) Pole 2 of CHB (c) CBB

VI. SIMULATION RESULTS A simulation exercise was carried out for the CHB with

converter specifications chosen for a typical passenger sedan type hybrid electric vehicle. For the charging mode, multi-output case, the input chosen for charging is rectified DC 110√2 V. The multiple storage devices are

ultracapacitor bank at voltage, V 400 V and battery pack at voltage, V 200 V. The charging loads considered are 4kW and 10 kW for ultracapacitor and battery, respectively. The results in Fig.11 show satisfactory operation of the CHB topology with multiple outputs.

(a)

(b)

(c) Fig. 10: Effect of output load power level in system poles (a) pole 1 of

CHB (b) Pole 2 of CHB (c) CBB

Fig.11: Simulation results for CHB with multiple outputs

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

x 10-4

-12

-10

-8

-6

-4

-2

0

V1 Side Inductance (mH)

Rea

l Par

t of S

ystem P

ole

Effect of the V1 side inductance changing on the real part of the pole 1

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

x 10-4

-24

-22

-20

-18

-16

-14

-12

-10

V1 Side Inductance (mH)

Rea

l Par

t of S

ystem P

ole

Effect of the V1 side inductance changing on the real part of the pole 2

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

x 10-4

-1000

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

V1 Side Inductance (mH)

Rea

l Par

t of S

ystem P

ole

Effect of the V1 side inductance changing on the real part of the pole 1

10 20 30 40 50 60 70 80 90 100-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Load Resistance (Ω)

Rea

l Par

t of S

ystem P

ole

Effect of output power change on the real part of the pole 1

10 20 30 40 50 60 70 80 90 100-140

-120

-100

-80

-60

-40

-20

0

Load Resistance (Ω)

Rea

l Par

t of S

ystem P

ole

Effect of output power change on the real part of the pole 2

10 20 30 40 50 60 70 80 90 100-180

-160

-140

-120

-100

-80

-60

-40

-20

Load Resistance (Ω)

Rea

l Par

t of S

ystem P

ole

Effect of output power change on the real part of the pole 1

5.8 5.8 5.8001 5.8001 5.8002 5.8003 5.8003 5.8003 5.80040

0.51

1.5

Gat

e2

5.8 5.8 5.8001 5.8001 5.8002 5.8003 5.8003 5.8003 5.80048090

100

IL1

5.8 5.8 5.8001 5.8001 5.8002 5.8003 5.8003 5.8003 5.80040

0.51

1.5

Gat

e3

5.8 5.8 5.8001 5.8001 5.8002 5.8003 5.8003 5.8003 5.80045

1015

IL2

5.8 5.8 5.8001 5.8001 5.8002 5.8003 5.8003 5.8003 5.80040

0.51

1.5

Gat

e5

5.8 5.8 5.8001 5.8001 5.8002 5.8003 5.8003 5.8003 5.8004405060

IL3

Time(sec)

1406

For performing the simulation for multi-input case, the voltages considered are: ultra-capacitor voltage, V400 V, battery voltage, V 200 V, and intermediate stage voltage, V 600 V. The load is assumed to be 20 kW. The results in Fig.12 show satisfactory operation of the CHB topology with multiple inputs.

Fig.12: Simulation results for CHB with multiple inputs.

The charging operating condition for the CBB is considered the same as that of the CHB for the simulation exercise. The input is chosen as rectified DC 110√2 V, ultracapacitor voltage V 400 V and battery voltage V 200 V. The charging loads are 4 kW and 10 kW, respectively. The results in Fig.13 show satisfactory operation of the CBB topology with multiple outputs.

Fig.13: Simulation results for CBB with multiple outputs.

VII. EXPERIMENTAL RESULTS Fig. 14 shows the setup experimental setup developed

for evaluating the CHB and CBB converter topologies. Converters with interleaving capability was developed for both topologies.

For CHB, 450µH inductance was used both at input and output sides. For C , C and CM, 3300µF capacitors where used for each. Microchip dSPIC33 was used for the controller implementation. For the CBB, a larger system was developed with 4950 µF capacitors at both input and output terminals. 800 µH inductance was used as the central inductor while TI2812 processor was chosen for controller implementation.

Experimental results for CHB and CBB are provided in Fig. 15 and Fig. 16, respectively. Steady state and transient responses are provided for both topologies. Fig. 15(a) shows steady state output voltage (Ch3) for CHB at 183.1 V while supplying a 3.8 kW load with 20.75 A current (Ch4). The converter was operated in buck mode with 197 V input voltage (Ch1) with 292 V intermediate voltage (Ch2) across the central capacitor. Another test run performed to observe the transient response is presented in Fig. 15(b). In Fig. 15(b), output voltage (Ch3) changes from 0 to 120 V while intermediate voltage (Ch2) changes from 0 to 170 V.

Fig. 16(a) shows steady state response of CBB with 630 V input (Ch2) and 410V output (Ch1) voltage. Total output current of 174 A (ChM) was maintained which was measured using two separate current probes (Ch3, Ch4) as seen in the figure. Two separate signals Iout1 and Iout2 were added and shown in Fig. 16(a) indicated as ‘M’ and labeled Icharging. Fig. 16(b) shows the transient response of the system while charging current (Ch3) changed from 0 to 90 A and then from 90 A to 50 A while maintaining average output voltage (Ch1) close to 360 V. The battery voltage labeled as ‘Vout’ in Fig. 16(b) is found to increase when 90 A current was flowing.

Fig. 14: Experimental Set up

1.2 1.2001 1.2001 1.2001 1.2002 1.2003 1.2003 1.2003 1.20040

0.51

1.5

Gat

e2

1.2 1.2001 1.2001 1.2001 1.2002 1.2003 1.2003 1.2003 1.20040

10

20

IL1

1.2 1.2001 1.2001 1.2001 1.2002 1.2003 1.2003 1.2003 1.20040

0.51

1.5

Gat

e4

1.2 1.2001 1.2001 1.2001 1.2002 1.2003 1.2003 1.2003 1.200470

80

90

IL2

Time(sec)

6.5 6.5001 6.5002 6.5003 6.50040

0.5

1

1.5

Gat

e4

6.5 6.5001 6.5002 6.5003 6.500420

30

40

IL1

6.5 6.5001 6.5002 6.5003 6.50040

0.5

1

1.5

Gat

e6

6.5 6.5001 6.5002 6.5003 6.500460

65

70

IL2

Time(sec)

Control Circuit for CHB

Whole set up for CHB

Inductors

Control Circuit

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(a)

(b) Fig. 15: Experimental results for CHB. (a) Results for 3.8 kW (Ch1- input voltage, Ch2-intermediate stage voltage, Ch3- output voltage, Ch4- output current). (b) Experimental result shows the initial transient response of voltages and current (Ch1- input voltage, Ch2- intermediate stage voltage, Ch3- output voltage, Ch4- output current).

(a)

(b) Fig. 16: Experimental results for CBB at (a) Results for 71.3 kW (Ch1- output voltage, Ch2- input voltage, (ChM=Ch3+Ch4)- charging current at steady state) (b) Response while battery charging current changes from 0A to 90 A and 90 A to 50 A(Ch1-output voltage, Ch2-input voltage, Ch3-charging current).

VIII. CONCLUSION The performance analysis and comparison for two bi-

directional DC-DC converters for EV applications are presented. Both the converters have their own advantages and disadvantages. The appropriate converter can be chosen based on the specific application. For EV charging station, multi-input and multi-output case, CHB can have better performance since input side and output side controls are independent. System control flexibility and reliability is better with CHB. CBB on the other hand requires fewer components and would be a good choice for EV powertrain.

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