hybrid electric vehicle

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 12, DECEMBER 2013 5489

A Fully Directional Universal Power ElectronicInterface for EV, HEV, and PHEV Applications

Omar C. Onar, Member, IEEE, Jonathan Kobayashi, Student Member, IEEE,and Alireza Khaligh, Senior Member, IEEE

Abstract—This study focuses on a universal power electronic in-terface that can be utilized in any type of the electric vehicles, hy-brid electric vehicles, and plug-in hybrid electric vehicles (PHEVs).Basically, the proposed converter interfaces the energy storage de-vice of the vehicle with the motor drive and the external charger,in case of PHEVs. The proposed converter is capable of operatingin all directions in buck or boost modes with a noninverted out-put voltage (positive output voltage with respect to the input) andbidirectional power flow.

Index Terms—Bidirectional dc/dc converters, electric vehicles(EVs), energy storage system, hybrid electric vehicles (HEVs),plug-in hybrid electric vehicles (PHEVs), universal dc/dc converter.

I. INTRODUCTION

E LECTRIFICATION of the transportation industry is es-sential due to the improvements in higher fuel economy,

better performance, and lower emissions [1]–[6].In vehicular applications, power electronic dc/dc converters

require high power bidirectional flow capability with wide inputrange since the terminal voltage of energy storage devices varieswith the state of charge (SoC) and load variations [7]. In the caseof a hybrid electric vehicle (HEV), a bidirectional dc/dc con-verter interfaces the energy storage device with the motor driveinverter of the traction machine; i.e., the converter is placed be-tween the battery and the high-voltage dc bus. In accelerationor cruising mode, it should deliver power from the battery to thedc link, whereas in regenerative mode, it should deliver powerfrom the dc link to the battery. In the case of an EV or plug-inhybrid electric vehicle (PHEV), while accomplishing the afore-mentioned task, the bidirectional dc/dc converter also interfacesthe battery with the ac/dc converter during charging/dischargingfrom/to grid [8]. Therefore, the bidirectional dc/dc converter

Manuscript received June 29, 2012; revised September 10, 2012 and Novem-ber 28, 2012; accepted December 17, 2012. Date of current version June 6, 2013.This work was supported by the U.S. National Science Foundation under Grants0801860, 1157633, and 0852013. Recommended for publication by AssociateEditor A. Sathyan.

O. C. Onar is with the National Transportation Research Center, Oak RidgeNational Laboratory, Oak Ridge, TN 37831 USA (e-mail: [email protected]).

J. Kobayashi is with the Department of Mechanical Engineering, Universityof California, Berkeley, CA 94720 USA (e-mail: [email protected]).

A. Khaligh is with the Power Electronics, Energy Harvesting and Renew-able Energies Laboratory, Department of Electrical and Computer Engineering,University of Maryland, College Park, MD 20742 USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2012.2236106

Fig. 1. Power electronic interfaces in an electric vehicle.

should interface the battery with the charging converter, as well.Fig. 1 illustrates the role of the bidirectional dc/dc converter inthe electrical power system of a plug-in electric vehicle [9].

In grid-connected mode, the bidirectional dc/dc convertermust have the capability to convert the output voltage of theac/dc converter into a suitable voltage to recharge the batteriesand vice versa when injecting power to the grid. In driving mode,dc/dc converter should be able to regulate the dc link voltage forwide range of input voltages. In driving mode, usually the bat-tery voltage is stepped-up during acceleration. DC link voltageis stepped-down during braking, where Vdc > Vbatt . However, ifmotor drive’s nominal voltage is less than battery’s nominal volt-age, Vdc < Vbatt , the battery voltage should be stepped-downduring acceleration and the dc link voltage should be stepped-up during regenerative braking. In addition to these cases, in anHEV to PHEV conversion, the grid interface converter’s out-put voltage might be less or more than the battery’s nominalvoltage [10], depending on the grid’s Vac voltage and the gridinterface converter’s topology. The rectified grid voltage shouldbe stepped-up if Vrec < Vbatt in V2G charging mode or the bat-tery voltage should be stepped-up for V2G discharging mode.If the rectified grid voltage is more than the battery’s nomi-nal voltage, i.e., Vrec > Vbatt , the rectified voltage should bestepped-down in V2G charging mode and the battery voltageshould be stepped-up in V2G discharging mode.

When all these possibilities are considered, the need for auniversal bidirectional dc/dc converter is obvious which shouldbe capable of operating in all-directions with stepping-up andstepping-down functionalities. Such a universal converter wouldmeet all the needs of the auto industry. The proposed converterin this manuscript not only fulfills these conditions, but alsocan be utilized for retrofit conversion of conventional cars toHEVs as well as the HEV to PHEV conversions. It can beplaced between the energy storage device and the high-voltagebus of the vehicle regardless of the nominal voltage ratings of

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5490 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 12, DECEMBER 2013

Fig. 2. Proposed fully directional universal dc/dc converter.

the battery, motor drive, and the grid interface converter inputsand outputs. Therefore, the proposed converter is called a fullydirectional converter.

This paper is organized as follows. In Section II, the topo-logical overview and the operation modes are presented. Theanalytical model of the converter and the control system de-velopment is given in Section III. Section IV focuses on thesimulation and experimental results to evaluate and validate thecapabilities of the proposed converter. Finally, the conclusionremarks and future work are provided in Section V.

II. SYSTEM DESCRIPTION AND OPERATING MODES

The circuit schematic of the proposed converter is depictedin Fig. 2. The converter has five power switches (T1-5) withinternal diodes and five power diodes (D1–D5), which are goingto be properly combined to select buck and boost modes ofoperation. Here, Vdc represents the motor drive nominal inputvoltage during driving mode or the rectified ac voltage at theoutput of the grid interface converter during plug-in mode (alsothe input voltage of the grid interface converter to be inverted toac). The nominal voltage of the vehicle’s ESS is represented byVbatt .

The proposed converter is capable of operating from Vdc toVbatt boosting, Vdc to Vbatt bucking, Vbatt to Vdc boosting, orVbatt to Vdc bucking, all with positive output voltage. In anyof the four modes, only one of the power switches is operatedin pulse width modulation (PWM) mode, while all the otherswitches are completely ON or OFF. Therefore, the switchinglosses are not more than that of any conventional buck or boostconverter. In addition, the proposed converter requires only onehigh-current inductor unlike some of the existing buck and boostconverter combinations or the cascaded configurations.

Conventional buck–boost converters can step-up or step-down the input voltage. However, they are not capable of pro-viding bidirectional power flow. Moreover, their output voltageis negative with respect to the input voltage, which needs aninverting transformer to make the output voltage positive [11].The noninverted operation capability of the proposed convertertotally eliminates the need for an inverting transformer, whichreduces the overall size and cost. Although there are some non-inverted topologies [12]–[22], some of them require two or moreswitches being operated in PWM mode that causes higher total

TABLE IOPERATION MODES OF THE PROPOSED CONVERTER

switching losses [12]–[14], [16]–[23]. Among these topologies,although they provide buck or boost operations, bidirectionalpower flow cannot be achieved in the topologies of [12], [16],[19], and [24]–[26]. The conventional two-quadrant bidirec-tional converters would operate buck mode in one direction andboost mode in the other direction; however, they cannot operatevice versa. They would not step-up the voltage in the directionthat they can step-down [15], [18], [27], [28]. Two cascadedtwo-quadrant bidirectional converters may achieve bidirectionalpower flow with bucking or boosting capabilities; however, theyrequire more than one high-current inductor [13], [17]. In [18],although two switches and two inductors are used, only uni-directional bucking or boosting can be achieved. In the caseof a dual-active bridge dc/dc converter, all switches are oper-ated in PWM mode; therefore, switching losses are four timeshigher in the half-bridge case or eight times higher in full-bridge case than that of the proposed converter. Dual-activebridge dc/dc converters [29]–[41] also require a transformer atthe middle stage which would increase the overall losses, size,and cost [20]–[23]. In [20], two inductors are required in ad-dition to the transformer, and in [21] the number of inductorsis three. In [22], bidirectional power flow is possible with tenswitches and two inductors. Although soft switching strategiescan be considered for dual-active bridge dc/dc converters in or-der to reduce the switching losses such as in [23], there shouldbe eight power switches and eight power diodes with three in-ductors; therefore, a high number of components would not beeconomical. Moreover, having more than one switch operatingin PWM mode would make the control system more compli-cated. However, in the proposed converter, the controls are assimple as the conventional buck or boost dc/dc converters inspite of all the competences. Finally, in [24], the proposed dc/dcconverter requires two transformers with one being multiwindedwhich complicates the structure, adds up to cost, and it does nothave the bidirectional operating capability.

The operation capabilities of the proposed converter signifi-cantly increases the flexibility of the converter while offering abroad range of application areas in all HEV and PHEV appli-cations as well as their conventional to HEV or HEV to PHEVconversions with add-on batteries regardless of the voltage rat-ings of the motor drive, battery, and the grid interface converter.

The different operation modes of the converter, includingthe status of the corresponding switches in each mode and thedirection of power flow, are mapped in Table I.

T2 and T4 serve as simple ON/OFF switches to connect or dis-connect the corresponding current flow paths, whereas T1 , T3 ,and T5 are either ON/OFF or PWM switches with respect to the

ONAR et al.: FULLY DIRECTIONAL UNIVERSAL POWER ELECTRONIC INTERFACE FOR EV, HEV, AND PHEV APPLICATIONS 5491

Fig. 3. Vdc -to-Vbatt boost mode of operation.

Fig. 4. Vbatt -to-Vdc buck mode of operation.

corresponding operating mode. Different cases and operatingmodes of the converter are detailed in following sections.

A. Case 1: Vdc < Vbatt

If the rated dc link voltage is less than battery’s rated voltage,the dc link voltage should be stepped-up during charging in grid-connected mode and in regenerative braking during driving.Under the same voltage condition, the battery voltage shouldbe stepped-down during plug-in discharging in grid-connectedmode, and in acceleration or cruising during driving.

Mode 1) Vdc → Vbatt Boost Mode for Plug-in Chargingand Regenerative Braking: In this mode, T1 and T4 are keptON, while T2 and T3 remain in the OFF state, as shown inFig. 3. The PWM switching signals are applied to switch T5 .Therefore, from Vdc to Vbatt , a boost converter is formed byD1 , T1 , L, T5 ,D4 , and T4 . Since D1 and D4 are forward-biased,they conduct; whereas D3 and D2 do not conduct. Since T5 is inPWM switching mode, when it is turned ON, the current fromVdc flows through D1 , T1 , L, and T5 while energizing the induc-tor. When T5 is OFF, both the source and the inductor currentsflow to the battery side through D4 and T4 .

During this mode, Vdc and Vbatt sequentially become the in-put and output voltages. Since the inductor current is a state vari-able of this converter, it is controllable. Therefore, the chargingpower delivered to the battery in plug-in mode or high-voltagebus current in regenerative braking can be controlled.

Mode 2) Vbatt → Vdc Buck Mode for Plug-in Dischargingand Acceleration: The circuit schematic of this operation modeis provided in Fig. 4. In this mode, T1 , T4 , and T5 remain OFF,while T2 is kept in ON state all the time. The PWM switchingsignals are applied to switch T3 . Therefore, from Vbatt to Vdc , abuck converter is formed by T3 ,D3 ,D5 , L, T2 , and D2 . WhenT3 is turned ON, the current from the battery passes throughT3 ,D3 , L, T2 , and D2 , while energizing the inductor. When T3is OFF, the output current is freewheeled through the D5 , T2 ,and D2 , decreasing the average current transferred to the load

Fig. 5. Vdc -to-Vbatt buck mode of operation.

side. D3 and D2 are forward-biased, whereas D1 and D4 do notconduct. D5 only conducts when T3 is OFF.

In this mode, Vbatt and Vdc are the input and output voltages,respectively. During stepping-down the battery voltage whiledelivering power from battery to the dc link, the inductor isat the output and its current is a state variable. Therefore, thedc link voltage and the current delivered to the dc link can becontrolled in driving mode.

B. Case 2: Vdc > Vbatt

If the rated dc link voltage is more than the battery’s ratedvoltage, dc link voltage should be stepped-down during chargingin grid-connected mode and in regenerative braking while thevehicle is being driven. Under the same voltage condition, thebattery voltage should be stepped-up during plug-in dischargingin grid-connected mode and in acceleration or cruising whiledriving.

Mode 3) Vdc → Vbatt Buck Mode for Plug-in Charging andRegenerative Braking: In this mode, T1 is in the PWM switch-ing mode. Switches T2 , T3 , and T5 remain in OFF state whileT4 is kept ON all the time. Therefore, from Vdc to Vbatt , a buckconverter is made up by D1 , T1 ,D5 , L,D4 , and T4 as shownin Fig. 5. When T1 is turned ON, the current from Vdc passesthrough D1 , T1 , L,D4 , and T4 while energizing the inductor.When T1 is OFF, the output current is recovered by freewheel-ing diode D5 decreasing the average current transferred fromdc link to the battery. Since diodes D1 and D4 are forward-biased, they conduct whereas D2 and D3 do not conduct. D5only conducts when T1 is OFF.

In this mode, Vdc and Vbatt are the input and output voltages,respectively. The dc link voltage can be regulated in drivingmode (regenerative braking) by controlling the current trans-ferred to the battery. In plug-in charging mode, the current orpower delivered to the battery is also controllable.

Mode 4) Vbatt → Vdc Boost Mode for Plug-in Dischargingand Acceleration: During this mode, T1 and T4 remain OFF,whereas T2 and T3 remain ON all the time. Switch T5 is operatedin PWM switching mode. Therefore, from Vbatt to Vdc , a boostconverter is formed by T3 ,D3 , L, T5 , T2 , and D2 , as illustratedin Fig. 6.

When T5 is turned ON, the current from Vbatt passes throughT3 ,D3 , L, and T5 while energizing the inductor. When T5 isOFF, both inductor and the source currents pass through T2 andD2 to the dc link. In this mode, D3 and D2 are forward-biased


Fig. 6. Vbatt -to-Vdc boost mode of operation.

Fig. 7. State-space model of the simplified converter in boost mode.

Fig. 8. State-space model of the simplified converter in buck mode.

and they conduct, whereas D1 ,D4 , and D5 are reverse-basedand do not conduct.

In this mode, Vbatt and Vdc are sequentially the input andoutput voltages. The dc link voltage can be regulated in drivingmode (regenerative braking) by controlling the current drawnfrom the battery. In plug-in charging mode, the current or powerdrawn from battery is also controllable.

III. CONTROL SYSTEMS

For the control system of the proposed topology, an all-electric range focused operating strategy has been considered[25]. As described in Section II and shown in Figs. 3–6, alloperation modes of the proposed converter are combinationsof buck and boost operations with different configurations andinput/output voltages, as expressed in Table I. Therefore, simpli-fied state-space averaged large-signal transfer functions of thebuck or boost modes of operations can be derived. The state-space block diagrams for the boost and buck modes of operationsof the proposed converter are shown in Figs. 7 and 8.

Two different controllers are incorporated for the proposedsystem: one employed in plug-in charging/discharging and theother is for acceleration/deceleration during driving. In plug-in

Fig. 9. DC/DC converter charge/discharge power controller.

Fig. 10. DC/DC converter’s cascaded controller for driving mode.

TABLE IIEXPERIMENTAL CONDITIONS AND CIRCUIT PARAMETERS

mode, generally, it is desired to control the charging or dis-charging power of the battery, whereas in driving mode it isimportant to provide a regulated dc link voltage to the motordrive. Therefore, a power controller is used for plug-in modesand a double-loop voltage and current controller is employed foracceleration/braking modes of the driving. The battery powercontroller, shown in Fig. 9, allows the reference charge or dis-charge power to/from the battery to be tracked This referencepower can be determined based on the SoC of the battery, userrequirements, and the state of the grid. The cascaded voltage andcurrent controller, shown in Fig. 10, allows the high-voltage busto be kept at the proper voltage while also accommodating thepower demanded or supplied by the dc link. This enables regen-erative recharging of the battery from the dc link and dischargingof the battery to the dc link, while maintaining the proper dclink voltage level for the hybrid vehicle.

IV. EXPERIMENTAL SETUP, RESULTS, AND DISCUSSIONS

The details of the experimental setup of the proposed con-verter are provided in Table II. Since the proposed topology isnew and has not been built or tested before, it is more appropri-ate to build the small-scale prototypes rather than the full-scalehigh power converters. Moreover, due to the safety purposes andto protect the students and the laboratory equipment, a smaller


Fig. 11. Experimental setup of the proposed converter.

scale prototype with lower voltage rating is preferred to serveas a proof of principle.

As given in Section II, there can be two cases, namely, Case 1:Vdc < Vbatt and Case 2: Vdc > Vbatt . Therefore, in order to rep-resent these cases for two different application scenarios, the Vdcand Vbatt voltages selected to be 24 and 42 V, respectively, inCase 1. In Case 2, since rated dc link voltage is higher thanthe rated voltage of the battery, Vdc and Vbatt are selected tobe 42 and 24 V, respectively. The picture of the experimen-tal setup of the proposed converter is shown in Fig. 11. Asa feedback and control system interface, TMS320F2812 DSPmodule from Texas Instruments has been employed. For pro-gramming the DSP, processing the feedback signals, and controlrealizations, Target Support Package 4.1 for TMS320C2000 andEmbedded IDE Link for Code Composer Studio from Math-Works Inc have been used that allows deploying generated codeonto the real-time embedded processors, microcontrollers, andDSPs.

For the experimental tests, in each mode, voltage is applied toone terminal, representing the charging voltage or regenerativebraking while output is a load, representing the battery chargingload or regenerative braking power of the motor drive. In theother mode, battery is the source while the dc link is the load,representing the plug-in discharging or acceleration mode.

A. Case 1. Mode I: Vdc → Vbatt Boost

The experimental results for this mode of operation are pre-sented in Fig. 12 where channel 1 is Vdc , channel 2 is Vbatt ,channel 3 is input current before the capacitor, and channel 4 isthe switching signal of switch T5 . As shown in Fig. 12, 24-VVdc voltage is boosted to slightly more than the 42 V, batteryrated voltage Vbatt .

Fig. 12. Experimental results for Vdc -to-Vbatt boost mode.

Fig. 13. Experimental results for Vbatt -to-Vdc buck mode.

B. Case 1. Mode 2: Vbatt → Vdc Buck

The experimental results for this mode of operation are pre-sented in Fig. 13, where channel 1 is Vdc , channel 2 is Vbatt ,channel 3 is output current after the capacitor, and channel 4is the switching signal of switch T3 . The input voltage Vbattis stepped-down to about 24 V (Vdc terminals), as shown inFig. 13.

C. Case 2, Mode 3: Vdc → Vbatt Buck

The experimental results for this mode of operation are pre-sented in Fig. 14 where channel 1 is Vdc , channel 2 is Vbatt ,channel 3 is output current after the capacitor, and channel 4 isthe switching signals of the switch T1 . From Fig. 14, it is seenthat the input voltage of Vdc is stepped down to 24 V of thebattery terminal voltage.

D. Case 2, Mode 4: Vbatt → Vdc Boost

The experimental results for this mode of operation are pre-sented in Fig. 15, where channel 1 is Vdc , channel 2 is Vbatt ,


Fig. 14. Experimental results for Vdc -to-Vbatt buck mode.

Fig. 15. Experimental results for Vbatt -to-Vdc boost mode.

channel 3 is input current before the capacitor, and channel 4 isthe switching signal of switch T5 . It can be seen from Fig. 15that the modified converter is capable of boosting the 24 V ofVbatt voltage to about 42 V of Vdc output voltage.

The capacitors at the input and output of the converter, Cdc =Cbatt = 2200 μF, possess a portion of energy stored at theinput and output of the converter; therefore, the input or outputcurrents of the proposed topology are not necessarily equal tothe inductor current. For boost modes of operation, the inputcurrents and for the buck modes of operations the output currentsare presented in the results.

In order to present the long-term performance of the proposedconverter over a drive cycle and to show the capability of switch-ing between the modes for each of the cases, simulations wereperformed in addition to the experiments. For the simulations,a portion of Urban Dynamometer Driving Schedule drive cy-cle has been implemented that includes driving conditions suchas acceleration, braking, and idling. In order to make an accu-rate analysis, the simulations were also down scaled assuminga rated dc link voltage of 24 V and rated battery voltage of

Fig. 16. Simulation results for Case 1, Modes 1 and 2. (a) Load current andswitching between respective modes. (b) Switches in PWM mode with respectto the operating mode. (c) Regulated load bus voltage.

42 V for Case 1, and assuming a rated load bus voltage of 42 Vand rated battery voltage of 24 V for Case 2, respectively. Theload demand of the drive cycle is also scaled-down, keepingthe same load demand profile. Simulation results for Case 1 arepresented in Fig. 16. In Fig. 16(a), the load current that cor-responds to the load demand of the drive cycle is presented.When the load demand is positive (Mode 2), the vehicle is ac-celerating and battery power should be delivered to the dc link.Since Vdc < Vbatt , battery voltage should be stepped-down inthis situation. When the load demand is negative (Mode 1), mo-tor drive delivers power from the traction machine to the dclink. Therefore, power should be recovered from the dc link tothe battery by stepping-up the dc link voltage and supplyingthe braking energy to the battery. The switches receiving PWMsignals are mapped in Fig. 16(b) with respect to the operationmode. Finally, regulated dc link voltage at 42 V is presented inFig. 16(c).

Simulation results for Case 2 are presented in Fig. 17 underthe same load conditions. In Fig. 17(a), the load current thatcorresponds to the load demand of the drive cycle is presented.When the load demand is positive (Mode 4), the vehicle isaccelerating and battery power should be delivered to the dc link.Since Vdc <Vbatt , the battery voltage is boosted in this situation.When the load demand is negative (Mode 3), the vehicle isbeing braked and the motor drive delivers power from tractionmachine to the dc link. Therefore, power should be recoveredfrom the dc link to the battery by stepping-down the dc link.


Fig. 17. Simulation results for Case 2, Modes 2 and 3. (a) Load current andswitching between respective modes. (b) Switches in PWM mode with respectto the operating mode. (c) Regulated load bus voltage.

The switches receiving PWM signals are mapped in Fig. 17(b)with respect to the mode of operation. Finally, regulated dc linkvoltage at 24 V is presented in Fig. 17(c). In both Figs. 16and 17, converter switches from buck to boost or boost to buckmodes of operation as the load current changes its direction. Inall cases, converter performs well and the dc link voltage is notaffected by these changes as it is regulated continuously. Thetransition from acceleration to/from regenerative braking doesnot affect the converter’s stability as well.

V. EFFICIENCY-LOSS ANALYSIS AND COMPARISONS WITH

EXISTING APPROACHES

The switching losses of the proposed fully directional nonin-verted buck–boost converter are very similar to that of a regularnoninverting buck–boost converter. To make the criteria of com-parison clear, the compared converter should have noninvertedoutput and have a relatively wider voltage for both the batteryand the dc link. Even if the proposed converter is comparedwith the simplest fundamental buck or boost dc/dc convertersfor each of its modes, the switching losses are identical sincethe proposed converter has only one switch in PWM mode inall of the modes. Since the number of switches operating inPWM is the same for the proposed and the conventional con-verters, one can obtain that there is no increase in switchinglosses. However, it can be stated that the proposed converter hasrelatively slightly more conduction loss in all operating modes.The additional conduction loss is mainly due to the additionalswitches or diodes in the current flow paths of the proposedconverter. Since the proposed dc/dc converter has four different

operating modes, losses should be examined separately as allmodes introduce different loss components.

When the converter is operated in boost mode from dc linkto the battery (for example, in plug-in charging mode), it canbe found that the difference in loss is the conduction losses of apair of an insulated-gate bipolar transistor (IGBT) (T1 and T4)and a diode D1 , whereas the switching losses are the same ifthe bottom line is a regular boost dc/dc converter. In dc analysis,diode conduction loss would be PD = vF .IF , while the IGBTconduction loss would be PT = vCE(SAT) .ICE . Therefore, inthis mode, change in losses can be expressed as

ΔPloss,1 = PD1 + PT 1 + PT 4 . (1)

When the dc/dc converter is operated in buck mode frombattery to the dc link (for example, in plug-in discharge mode),it is seen that the additional conduction losses in compare toa conventional buck converter are due to a pair of additionaldiode (D2 and D3) and the switch T2 . Therefore, in this mode,additional conduction losses can be calculated by

ΔPloss,2 = PD2 + PD3 + PT 2 . (2)

Similarly, when dc/dc converter is operated in buck modefrom dc link to the battery (for example, the regenerative brakingin driving mode), additional conduction losses occur due to theforward biased diodes D1 and D4 and the conducting switch T4as given by

ΔPloss,4 = PT 4 + PD1 + PD4 . (3)

When the dc/dc converter is operated in boost mode frombattery to the dc link (for example, the acceleration in driv-ing mode), the additional conduction losses as compared to aconventional boost converter are due to a pair of additional con-ducting switches (T2 and T3) and the diode D3 . This results inadditional losses to be

ΔPloss,3 = PT 2 + PT 3 + PD3 . (4)

To estimate comparative change in efficiency, η is identifiedas efficiency of the conventional buck or boost mode and η′

is defined as the efficiency of the proposed converter with ad-ditional losses. If Po is the output power and Pin is the inputpower, the change in the efficiency can be obtained as

Δη = η − η′ =Po

Pin− Po

Pin + ΔPloss(5)

The comparative change in efficiency for all of the four modesis formulated as a function of the Pin , Pout , Ploss , and ΔPloss ;as ΔPloss for different modes are given by (10)–(13). For theseanalyses, as used in the experiments, HGTG30N60A4D IGBTmodules and FFPF30U60STTU power diodes from FairchildSemiconductor are used where IGBT’s VCE(SAT) is 1.6 V,whereas diode’s VF is 2.1 V as given in the manufacturers’datasheets. Change in efficiency values is summarized in Ta-ble III with respect to different operating modes.

For a typical plug-in vehicle (Chevrolet Volt) in Vdc → Vbattboost mode, where the input voltage is 120

√2 = 169.7 V, out-

put voltage is 300 V, and the average input and output currentsare 15.48 and 8.5 A, respectively (scaled-up proportionally with


TABLE IIICOMPARATIVE CHANGE IN EFFICIENCY OF DC/DC CONVERTER FOR

DIFFERENT MODES

respect to the experimental average currents). Therefore, thechange in efficiency for this mode can be expressed as

Δη1 = η1 − η′1

=300 × 8.5

169.7 × 15.48

− 300 × 8.5169.7×15.48+[(1.6×15.48)+(1.6×8.5)+(2.1×15.48)]

= 0.9707 − 0.9452 = 2.55%. (6)

When stepping-down the battery voltage, in Vbatt →Vdc buckmode, input voltage is 300 V, output voltage is 169.7 V, and theinput and output currents can be scaled-up to 8.5 and 14.62 A,respectively. Under these conditions, the change in efficiencycan be calculated by

Δη2 = η2 − η′2

=169.7 × 14.62

300 × 8.5

− 169.7 × 14.62300×8.5+[(2.1×8.5)+(2.1×14.62)+(1.6×14.62)]

= 0.9729 − 0.9462 = 2.67%. (7)

For the rest of the operation modes, the analyses are the same,as detailed in (3) and (4). Therefore, change in efficiencies inmodes 1 and 2 are identical to that of modes 3 and 4.

The proposed fully directional dc/dc converter is also capa-ble of providing potential cost reductions as compared to theconventional or commercially available approaches. For exam-ple, if the ac grid voltage is 120 V, the rectified dc link voltagecould be 120

√2 = 169.7 V. Since the battery voltage in com-

mercially available EV or PHEV (346 V for Prius Plug-in and300 V for Chevy Volt) is higher than this voltage, dc link voltageshould be boosted to recharge the battery. In order to dischargethe battery to the grid, a buck converter operation is necessaryfrom battery to the dc link (buck from 346 or 300 V to 169.7 V).Therefore, in plug-in mode, one boost and one buck converteror a two-quadrant dc/dc converter would be required. In driv-ing mode, again the battery voltage is around 346 or 300 V;however, the input voltage for the motor drive inverter is 650 Vfor Prius Plug-in and 500 V for Chevy Volt. Therefore, batteryvoltage should be boosted in acceleration or cruising modes. Onthe other hand, buck converter operation is necessary in orderto recharge the battery during regenerative braking conditions(buck from 650 or 500 V to 346 or 300 V battery terminals).Therefore, in driving mode, one boost and one buck converter ora two-quadrant dc/dc converter would be required. Considering

TABLE IVCOMPARATIVE ANALYSES OF THE PROPOSED DC/DC CONVERTERS WITH

CONVENTIONAL APPROACHES

these buck and boost functionalities, the proposed converter re-duces the number of dc/dc converters in a typical plug-in hybridor all-electric vehicle.

In order to provide the same functionality, four dc/dc con-verters would be needed with conventional converters: two ofthem would be boost dc/dc converters (one for plug-in and onefor driving modes) and other two of them would be buck dc/dcconverters (one for plug-in and one for driving modes). In thiscase, instead of one inductor, four inductors would be neededfor each of the converters. In commercially available EVs andPHEVs, currently the capability of injecting power back to thegrid does not exist. In plug-in charging, there is a boost con-verter employed after the rectifier and for the driving mode, theyutilize a two quadrant converter to provide both the boost andbuck functions either for acceleration or regenerative brakingmodes. The boost converter after the rectifier can be replaced bya two-quadrant converter in order to have both the grid charg-ing and discharging functionalities. In Table IV, the proposedconverter is compared with the conventional approach with fourconventional dc/dc converters and the commercially availablevehicle power electronics with and without the grid dischargingcapability.

As seen from Table IV, the proposed converter adds only twomore semiconductor devices; however, it reduces the numberof inductors from four to one as it is compared to the two-buck–two-boost converter’s approach. Since the inductor coreand winding materials are extremely more expensive than thesemiconductor devices, it is always desirable to add two moresemiconductor devices for reducing the number of inductors bythree. Moreover, inductors would require much more space asit is compared to the space requirement of two switches. There-fore, one can state that the proposed dc/dc converter wouldreduce both the cost and the size of the conventional approachfor the same functionality basis. As compared to the commercial


approach without the grid discharge mode, the proposed con-verter has six more semiconductor switches. However, it reducesthe size, cost, and space requirements of the inductors by 50%while adding the grid discharge functionality.

VI. CONCLUSION

This study presents a novel dc/dc converter structure that issuitable for both industrial needs and the retrofit electric vehicleconversion approaches for all EV, HEV, and PHEVs regardlessof their rated dc link voltage and motor drive inverter voltage aswell as the battery nominal voltage. The functionalities of theproposed converter provide a broad range of application areas.Due to the operational capabilities, the proposed converter is oneof a kind plug-and-play universal dc/dc converter that is suitablefor all electric vehicle applications. The proposed topology issuitable not only for conversion approaches but also is a goodcandidate to reduce the number of dc/dc converters from twoto one in commercially available vehicles such as Toyota Prius.Through the simulation results and experimental prototype, thefunctionalities for two different cases with four different modeshave been verified. In each case, bidirectional power flow is pro-vided with fully directional bucking and boosting capabilities. Inthe future, a full-scale dc/dc converter will be built for a typicalmid-size sedan vehicle and the converter will be implementedfor a real-world application.

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Omer C. Onar (S’05–M’10) received the Ph.D. de-gree in electrical engineering from Illinois Instituteof Technology (IIT), Chicago, in July 2010.

At IIT, he was a Doctoral Research Assistant at theEnergy Harvesting and Renewable Energies Labora-tory (EHREL), Electric Power and Power ElectronicsCenter (EPPEC). His research interests include powerelectronics, energy storage systems, electric, hybridelectric, and plug-in hybrid electric vehicles. He isthe principle author/coauthor of over 45 journal andconference papers as well as three books including

Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems (CRCPress, 2009).

Dr. Onar received the IEEE-Vehicular Technology Society’s 2008-2009“Transportation Electronics Fellowship” (world-wide single award) and theIEEE-Power Electronics Societys 2009 “Joseph J. Suozzi INTELEC Fellow-ship in Power Electronics” (world-wide single award). In July 2010, he wasawarded the distinguished Alvin M. Weinberg Fellowship at the U.S. Depart-ment of Energy’s Oak Ridge National Laboratory, where he joined the Energyand Transportation Science Division. He is a Member of the IEEE VehicularTechnology Society, IEEE Power Electronics Society, IEEE Industrial Electron-ics Society, IEEE Industry Applications Society, and IEEE Power and EnergySystems Society. He has been the “Top Reviewer” of the IEEE TRANSACTIONS

ON VEHICULAR TECHNOLOGY, elected by the Editor-in-Chief and EditorialBoard of the Vehicular Technology Society. He is also a Guest Associate Editorfor the IEEE TRANSACTIONS ON POWER ELECTRONICS Special Issue on Trans-portation Electrification and Vehicle Systems.

Jonathan Kobayashi (S’08) received the B.Sc. de-gree in electrical engineering from the Illinois Insti-tute of Technology (IIT), Chicago, in 2011. He iscurrently working toward the M.S. degree in controlsand dynamics at the Mechanical Engineering Depart-ment, University of California Berkeley, Berkeley.

He worked as a power electronics researcher at theEnergy Harvesting and Renewable Energies Labora-tory (EHREL), Electric Power and Power ElectronicsCenter (EPPEC), IIT, where he has worked on an ex-perimental bi-directional AC/DC and DC/DC power

electronic converters for plug-in hybrid electric vehicles. He was with the IITFormula Hybrid Team from August 2008 to May 2009, where he was work-ing on control systems and data acquisition. Earlier, he was with the HawaiianElectric Company in Honolulu, Hawaii, working on various projects related totransmission and distribution substations, system protection, and communica-tions. He is an expert in computer, web, and electrical power technologies.

Mr. Kobayashi received the First Hawaiian Bank Scholarship in 2007-2008;received the Heald Scholarship, and the FIRST Robotics Scholarship in 2007.He was included in the Illinois Institute of Technology’s Dean’s List in Fall2007 and Fall 2009.

Alireza Khaligh (S’04–M’06–SM’09) is an Assis-tant Professor and the Director of Power Electronics,Energy Harvesting and Renewable Energies Labora-tory (PEHREL), Electrical and Computer Engineer-ing Department (ECE), and the Institute for Sys-tems Research (ISR), University of Maryland at Col-lege Park (UMCP). Prior to UMCP, he was a Post-Doctoral Research Associate in the Grainger Centerfor Electric Machinery and Electromechanics in theUniversity of Illinois at Urbana-Champaign (UIUC),Urbana, IL, and also an Assistant Professor at Illinois

Institute of Technology (IIT), Chicago, IL. He is the author/coauthor of over 100journal and conference papers. His major research interests include modeling,analysis, design, and control of power electronic converters, electric and plug-inhybrid electric vehicles, biomechanical energy harvesting, and miniature powerelectronic interfaces for microrobotics.

Dr. Khaligh is the recipient of the Best Vehicular Electronics Paper Awardfrom the IEEE Vehicular Technology Society in 2012, Ralph R. Teetor Ed-ucational Award from Society of Automotive Engineers in 2010, the ArmourCollege of Engineering Excellence in Teaching Award from IIT in 2009, and theDistinguished Undergraduate Student Award from Sharif University of Tech-nology in 1999. He is the General Chair of the 2013 IEEE TransportationElectrification Conference and Expo., and the Grants and Awards Chair for the2013 IEEE Applied Power Electronics Conference and Expo. He was the Pro-gram Chair of the 2011 IEEE Vehicle Power and Propulsion Conference, and theProgram Cochair of the 2012 IEEE Transportation Electrification Conferenceand Exposition.

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