Buck Boost Converter

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 5, MAY 2009 1267

Digital Combination of Buck and Boost Convertersto Control a Positive Buck–Boost Converter and

Improve the Output TransientsYoung-Joo Lee, Student Member, IEEE, Alireza Khaligh, Member, IEEE, Arindam Chakraborty,

and Ali Emadi, Senior Member, IEEE

Abstract—A highly efficient and novel control strategy for im-proving the transients in the output voltage of a dc–dc positivebuck–boost converter, required for low-power portable electronicapplications, is presented in this paper. The proposed control tech-nique can regulate the output voltage for variable input voltage,which is higher, lower, or equal to the output voltage. There are sev-eral existing solutions to these problems, and selecting the best ap-proach involves a tradeoff among cost, efficiency, and output noiseor ripple. In the proposed method, instead of instantaneous tran-sition from buck to boost mode, intermediate combination modesconsisting of several buck modes followed by several boost modesare utilized to distribute the voltage transients. This is unique of itskind from the point of view of improving the efficiency and ripplecontent in the output voltage. Theoretical considerations are pre-sented. Simulation and experimental results are shown to provethe proposed theory.

Index Terms—Control, efficiency, Li-ion battery, positive buck–boost converter, transients.

I. INTRODUCTION

AVERY common power-handling problem, especially forportable applications, powered by batteries such as cel-

lular phones, personal digital assistants (PDAs), wireless anddigital subscriber line (DSL) modems, and digital cameras, isthe need to provide a regulated noninverting output voltagefrom a variable input battery voltage. The battery voltage, whencharged or discharged, can be greater than, equal to, or lessthan the output voltage. But for such small-scale applications,it is very important to regulate the output voltage of the con-verter with high precision and performance. Thus, a tradeoffamong cost, efficiency, and output transients should be consid-ered [1]–[5].

A common power-handling issue for space-restrained appli-cations powered by batteries is the regulation of the output volt-age in the midrange of a variable input battery voltage. Some ofthe common examples are 3.3 V output with a 3–4.2 V Li cell

Manuscript received August 2, 2007; revised May 29, 2008, and January 12,2009. Current version published May 1, 2009. This paper was presented in partat the 2006 IEEE Power Electronics Specialists Conference. Recommended forpublication by Associate Editor J. A. Cobos.

The authors are with the Electric Power and Power Electronics Center,Electrical and Computer Engineering Department, Illinois Institute of Tech-nology, Chicago, IL 60616 USA (e-mail: [email protected]; [email protected];[email protected]; [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.2009.2014066

input, 5 V output with a 3.6–6 V four-cell alkaline input, or a12 V output with an 8–15 V lead–acid battery input [6]–[9].

With an input voltage range that is above and below the out-put voltage, the use of a buck or a boost converter can be ruledout unless cascaded. Cascaded combination of converters re-sults in cascaded losses and costs; therefore, this approach isseldom used. In such a range of power demand, the transitionof dc voltage from one level to another is generally accom-plished by means of dc/dc power converter circuits, such asstep-down (buck) or step-up (boost) converter circuits. Thereare various topologies such as inverting buck–boost convert-ers, single-ended primary inductance converters (SEPICs), Cukconverters, isolated buck–boost converters, and cascaded buckand boost converters, which can be implemented to maintain aconstant output voltage from a variable input voltage [10]–[13].

The important points of concern for such low-voltage-rangepower supplies are output ripple, efficiency, space, and the cost.The aforementioned topologies are generally not implementedfor such power supplies due to their lower efficiency, highersize, and cost factors. The most difficult problem is the spikes inthe output voltage, which causes the converter to lose efficiencyduring the transition from buck mode to the boost mode. Cost,size, switching speed, efficiency, and flexibility all need to beconsidered in designing such power supplies. The advantage ofhaving higher efficiency is longer runtime at a given brightnesslevel from the same set of batteries [14]–[20].

In this paper, a novel theory is presented in order to fulfillthe requirements of energy-efficient power supplies for battery-powered portable applications. This manuscript extends [1]and [2] by adding the experimental results, to justify the theoret-ical results. The two main important factors, the efficiency andthe voltage regulation, which are derived numerically from theexperimental results, are shown, and their comparison with theconventional methods is tabulated. The proposed method im-proves the transition problem and tries to reduce the transientshappening during the transition from the buck mode to the boostmode. The paper is organized as follows. Section II discussesthe operation of the converter in different modes and issuesassociated with transition from buck to boost. Existing topolo-gies and solutions to deal with this problem are addressed inSection III. The proposed new method of transient improvementis presented in Section IV. Section V addresses the simulationresults applying the conventional and proposed methods of con-trol. Analytical studies for each control technique are shown,and the method of control is elaborated. Experimental results

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Fig. 1. Input–output curve for the power supply.

Fig. 2. Duty ratio variation for the buck and boost modes.

are presented in Section VI to verify the simulations that arecarried out for different control techniques. Finally, conclusionsare drawn based on each method in Section VII.

II. TRANSITION PROBLEM

An example for a battery-powered application is shown inFig. 1. The input voltage of the battery when fully charged is6 V and when discharged is 3.6 V [21]. This supply needs tocontinuously provide a steady output of 5 V. Thus, the converterneeds to operate in the buck mode for the period “TA ,” followedby the buck–boost mode for “TB ,” and finally in the boost modefor “TC .”

The change in the duty ratio for different modes of operationbased on different ranges of the input voltage is shown in Fig. 2.Referring to Fig. 1, for the period “TA ,” the converter worksin the buck mode, where the duty changes from the minimumvalue when the input is 6 V to the maximum when the input isapproximately between 5.1 and 4.9 V, as shown in Fig. 2; for thetime “TB ,” the converter is in buck–boost mode; and, finally,for the period “TC ,” it is working in the boost mode. The points“A” and “B” in Fig. 2 are in reference to those in Fig. 1.

A. Analytical Studies

In buck topology, when Vin is equal to Vout , the duty cyclewill approach to 1, Dbuck = Vout/Vin . In the boost operatingtopology, when Vin approaches Vout , the duty cycle moves to-

Fig. 3. Variations of duty cycle of buck, buck–boost, and boost convertersversus Vin − Vout .

Fig. 4. (a) General block diagram of error amplifier and drivers. (b) Operationmodes.

ward zero, Dboost = 1 − (Vin/Vout). In the buck–boost operat-ing condition, since Dbuck−boost = Vout/Vin + Vout , therefore,for Vin equal to Vout , duty cycle becomes 0.5. In other words,when Vin decreases toward Vout , the duty cycle should followthe pattern of 1 for buck to 0.5 for boost and then zero in theboost operating topology. This is demonstrated in Fig. 3. Instan-taneous transition from buck to buck–boost or boost incorpo-rates a sudden change in the duty cycle from Dbuck,max = 1 toDbuck-boost = 0.5 or Dboost,min = 0. Output voltage variationsare associated with the sudden changes in the duty cycle change,due to the fact that output voltage is a nonlinear function of theduty cycle. This is shown in Section III.

Transition between different modes of operation is a resultof comparing the output of the error amplifier (EAout) andits level shifted value (VLS) with the sawtooth waveform, asshown in Fig. 4. Since during a transition from buck to buck–boost the duty cycle changes from 1 to 0.5, the EA outputsignal should be level shifted by 0.5 Vpp . However, to pre-vent toggling between buck mode and buck–boost mode, theVLS should be less than 0.5 Vpp . In buck mode, Dbuck =EAout . Vin approaches Vout , and Dbuck approaches 1. In buck–boost mode, Dbuck-boost = EAout − 1 + 0.5. When Vin is equalto Vout , Dbuck-boost becomes 0.5. In boost mode, Dboost =EAout − 1 + ∆, where ∆ = (Vout,max − Vin)/Vout,max from


LEE et al.: DIGITAL COMBINATION OF BUCK AND BOOST CONVERTERS TO CONTROL A POSITIVE BUCK–BOOST CONVERTER 1269

buck–boost mode. Therefore, in each mode, the sensi-tivity of the control parameter EAout with respect totime is (∂Dbuck/∂t) = (∂EAout/∂t), (∂Dbuck−boost/∂t) =(∂EAout/∂t), and (∂Dboost/∂t) = (∂EAout/∂t).

Based on the relationship of the input–output voltageof the buck (Dbuck = Vout/Vin ), buck–boost ([Vout/Vin ] =[Dbuck−boost/1 − Dbuck−boost ]), and boost ([Vout/Vin ] =[1/1 − Dboost ]) operation topologies, we have

∂ (Vout/Vin)∂Dbuck

∂Dbuck

∂t= 1 (1)

∂ (Vout/Vin)∂Dbuck-boost

∂Dbuck-boost

∂t=

1(1 − Dbuck-boost)

2 (2)

∂ (Vout/Vin)∂Dboost

∂Dboost

∂t=

1(1 − Dboost)

2 . (3)

Based on (1), output voltage of the converter in buck modeis a linear function of the duty cycle. In other words, for thecase of switching frequencies higher than natural frequency ofthe system, the controller is able to regulate the transient in theoutput voltage. However, the output voltage of the converterin boost and buck–boost modes is a nonlinear function of theduty cycle, based on (2) and (3). This means that the directtransition from buck to buck–boost or boost will incorporateoutput voltage variations, which cannot be well regulated byconventional control techniques.

III. EXISTING SOLUTIONS

A. Demerits of Using an Ordinary Buck–Boost Converter

There are various issues about an ordinary buck–boost con-verter, which prevents its use for such specific applications. Thebiggest problem associated with such a converter is that the out-put of such converter is inverted. Of course, it can be inverted,but it requires a transformer, which adds to the cost and spaceand sacrifices the efficiency of the converter.

B. Demerits of Using SEPIC Converter

A very popular buck–boost topology that requires more com-ponents but produces a noninverting output is the single-endedprimary inductance converter (SEPIC). SEPIC, a popular buck–boost circuit has limited efficiency (usually maximum about85%) and requires either a transformer or two inductors. Thus,including a transformer or two inductors would occupy morespace and, thereby, increase the size and the cost. The use ofthese components would add to the losses, and thereby degradethe efficiency of the converter [10], [11].

C. Disadvantages of a Cascaded Buck and Boost Converter

Such a topology applies two dc–dc converters cascaded to-gether (see Fig. 5); hence, the loss of the whole single converteris actually doubled in this case, which results in poor efficiency.The number of external components such as inductors, decou-pling capacitors, and the compensation networks needed forboth controllers in this case is more. Due to more components,more space is occupied, which results in higher cost [12]. In

Fig. 5. Cascaded boost and buck converter.

Fig. 6. Cascaded boost converter and LDO regulator.

Fig. 7. Combined-method-based control logic for deciding modes ofoperation.

addition, subharmonic problem [22] is another issue, whichprevents utilizing cascaded converters.

D. Boost Converter Cascaded to a Low-Dropout (LDO)Voltage Regulator

This topology is another cascaded topology composed of twodifferent converter circuits. The first one is a boost converter,followed by an LDO voltage regulator (see Fig. 6). Here, thevarying voltage of the battery is stepped up using the boostconverter, and then the output of the boost converter is regulatedusing the LDO to obtain a voltage in the middle range of thevarying battery voltage. The biggest disadvantage of an LDOcircuit is lower efficiency, due to the fact that it is a cascadednetwork of two converters, which, in turn, double the number ofloss components used in the proposed converter stage. Anotherconcern here is the cost; thus, this topological control is notmuch desired [12].

IV. PROPOSED NEW METHOD

The proposed method is to add interface modes, which area combination of buck and boost operating topologies (Figs. 7and 8). As shown in Fig. 8, when the input voltage is consider-ably higher than V1 , the converter operates in purely buck mode.However, during the time period, where the input voltage is be-tween V1 and V2 , threshold voltage, the combination mode Acomes into operation, followed by the buck–boost mode for thevoltage range V2 and V3 . In the voltage range V3 and V4 , the con-verter operates in the combination mode B. Finally, for the inputvoltages below V4 , the converter operates purely in the boost op-erating mode. By adding the combination modes A and B duringthe time periods “T1” and “T3” just before and after the stage,where vin ≈ vout , the transient at the output of the converter can



Fig. 8. Voltage curves for combined-method-based control.

Fig. 9. Block diagram of the theory of digital combination of power converters.

be improved significantly. Operation of the converter in buck–boost mode decreases the efficiency of the converter. In order toimprove its efficiency, buck–boost mode should be eliminated.This is another major contribution of this paper. In that case,time periods T2 will be eliminated and the operation mode willchange from buck to the boost through intermediate combinationmodes.

Each combination mode is a combination of several buck andboost operating topologies. In the proposed method, instead ofa sudden transition from buck operating mode to a buck–boostoperating mode, a combination of buck and boost operatingtopologies is applied to distribute the voltage transient and,therefore, obtain smoother output waveform. This is the conceptof digital combination of power converters (DCPCs), which isapplied to a noninverting buck–boost converter in this paper. Incombination mode A, the number of buck operating topologies isusually higher than the number of boost operations. Similarly, inthe combination mode B, the number of boost operating modeswill be higher than buck operations. The concept of DCPChas been introduced in Section IV-A, and then Sections IV-Band IV-C present the analyses and operation of the positivebuck–boost converter.

A. Digital Combination of Power Converters

Control approach based on DCPC improves the dynamic re-sponse of the converter during transients by switching betweendifferent converter topologies to spread out the voltage spikesthat are an inevitable part of the transients. Fig. 9 demonstratesthe block diagram of the proposed approach. In this figure, con-verter topology control (CTC) controls the operating topologies

Fig. 10. Circuit topology of a positive buck–boost converter.

of the converter. In addition, transition control (TC) unitscontrol the transitions between various topologies. For instance,TCi controls the transition behavior of the converter from the ithconverting topology to the (i + 1)th topology. In the transitionfrom ith topology to the (i + 1)th topology, instead of instanta-neous transition from ith topology to (i + 1)th topology, for αi

switching cycle, converter operates in ith topology and for βi

switching cycle, it operates in the (i + 1)th topology. Operationwith αi and βi switching cycles in transition mode will be re-peated for γi times. For a specific case, where αi = mi, βi = 1and γi = mi − 1, in the transition from the ith topology to the(i + 1)th topology, for the mi switching cycle, the converteroperates in ith topology and for one switching cycle, it switchesto the (i + 1)th topology. Then, in the next cycle, TCi triesto increase the number of cycles of operation in the (i + 1)thtopology and decrease the number of cycles of operation inthe ith topology. Then, in the next cycle, TCi tries to increasethe number of cycles of operation in the (i + 1)th topology anddecrease the number of cycles of operation in the ith topology.Finally, before the complete transition to the (i + 1)th topology,for one switching period, it operates in the ith topology and, formi switching cycle, it switches to the (i + 1)th topology. DCPCimposes the fact that the proposed converter topology shouldhave the capability of operating in various converter topologieswith only controlling the switching components of the circuit.

B. Operation of Positive Buck–Boost Converter

The circuit topology of a positive buck–boost converter isshown in Fig. 10. In buck–boost operating mode, always, twoswitches, Q1 and Q2 , and two diodes, D1 and D2 , are switchingin the circuit. A positive buck–boost converter can operate as abuck converter by controlling switch Q1 and diode D1 , whenQ2 is OFF and D2 is conducting. It can also work as a boostconverter by controlling switch Q2 and diode D2 , while Q1 isON and D1 is not conducting. When the voltage of the batteryis more than the output reference voltage, converter operates asa buck converter. As soon as the voltage of the battery dropsto a value less than the output reference voltage, the convertershould switch to boost mode. The added advantage of the con-verter is that the output of such a converter is always positive[16]–[20].

Table I presents the values of α1 , β1 , and γ1 for the conven-tional method of transition from buck to buck–boost and thevalues of α2 , β2 , and γ2 from buck–boost to the boost operatingconditions.



TABLE ITRANSITION MODE CONTROL PARAMETERS (α, β , AND γ) FOR THE

CONVENTIONAL METHOD OF TRANSITION FROM A BUCK TO BOOST

OPERATING TOPOLOGY

Fig. 11. Closed loop control strategy for the proposed method.

The overall system level closed loop control strategy of theproposed method is shown in Fig. 11. The control logic fordeciding the modes of operation is based on the idea of Fig. 8.Here, both the input and the output voltages are sensed and theproper duty ratios are applied to the switches Q1 and Q2 basedon proper duty setting and the desired mode of operation.

C. Analysis of the Combined Method

In period combination mode A, combining α1 buck samplesand β1 boost samples, the output voltage variation can beexpressed as

∂

∂tvout

∣∣∣∣∣CombA

=1

C(α1 + β1)

×(α1

(

i L,buck − 1R

vout

)

+β1

((1 − dboost)

i L,boost −1R

vout

))

(4)

where

i L,buck = (vin − vout/2L) dbuckT + I0,buck ,

i L,boost= (vin/2L) dboostT + I0,boost , and I0,buck and I0,boost repre-sent the initial currents of the inductor in the buck and boostmodes, respectively

∂

∂tvout

∣∣∣∣CombA

=1

C(α1 + β1)

×

α1

(vin−vout

2LdbuckT + I0,buck − 1

Rvout

)

+β1

((1−dboost)

vin

2LdboostT + I0,boost −

1R

vout

)

(5)

TABLE IIPARAMETERS OF POSITIVE BUCK–BOOST CONVERTER

Now, for the output voltage variation to be minimum or zero,s (∂/∂t) vout |CombA = 0 ; thus

α1

β1=− (1−dboost)

2 (vout/2L) dboostT−I0,boost+ (1/R) vout

(1−dbuck) (vout/2L) T+I0,buck− (1/R) vout.

(6)Similarly, for the combination mode B, the ratio of boost and

buck pulses can be defined in terms of circuit parameters. Byimplementing the proposed combination modes, since transientsget almost evenly distributed due to the application of thesecombined modes, the output voltage transients will significantlyimprove. Fig. 12 presents the control signals for the combinationmodes A and B.

V. SIMULATION RESULTS

The simulation results have been obtained for the converterbased on the parameters shown in Table II.

A. Conventional Method of Solving the Transition Problem

Simulations are carried out on the positive buck–boost con-verter using the conventional methods. Fig. 13 presents theoutput voltage waveform, buck and boost pulses for a directtransition from buck to boost mode. There is about 12%–14%ripple in the output voltage during direct transition from buckto boost. Fig. 14 shows the output voltage, input voltage, andbuck and boost pulses for a transition from buck to boost with anintermediate buck–boost mode. In this method, the converter ini-tially works in the buck mode, when the input voltage is greaterthan the output voltage, followed by the buck–boost mode whenthe voltages are almost equal. Finally, the converter works in theboost mode when the input voltage is lower than the output volt-age. The simulation results indicate that the presence of spikes inthe output voltages during transitions through different modes isabout 6%.

B. Simulation Results of the Proposed Method

Applying the parameter values from Table II for the calcu-lation of buck and boost samples and using (6), just beforevin ≈ vout , the rounded ratio of α1 and β1 is 3:1. Similarly, justafter vin ≈ vout , the ratio of α2 and β2 is found to be 1:2. Thus,we choose α1 = 3 or three buck cycles and β1 = 1 or one boostcycle for the time period in combination mode A and α2 = 1



Fig. 12. Buck, boost, and buck–boost pulses in (a) transition from buck mode to combination mode A, (b) combination mode A, (c) transition from combinationmode A to buck–boost mode, (d) transition from buck–boost mode to combination mode B, (e) combination mode B, and (f) transition from combination mode Bto boost mode.



Fig. 13. Input and output voltages and buck and boost pulses for direct tran-sition from buck to boost mode.

Fig. 14. Input and output voltages and buck and boost pulses for transitionfrom buck to buck–boost and then to boost mode.

Fig. 15. Proposed number of cycles for the addressed application.

Fig. 16. Input and output voltages for the transition from buck to boost throughcombination mode A, buck–boost, and combination mode A.

or one buck cycle and β2 = 2 or two boost cycles for the timeperiod in combination mode B. This ratio is presented in termsof block diagram in Fig. 15 for solving the addressed problem.

1) Proposed Combination Method With Buck–Boost Mode inthe Middle: The simulations were carried out on the converterusing the exact combination method along with the buck–boostin the middle. This method improves the ripple content in theoutput voltage of the converter when the input voltage becomesalmost equal to the output voltage and during other transitionmodes. The waveforms are shown in Fig. 16. It is seen that thepeak transient happening during the transition is about 4%.

2) Without Buck–Boost Mode in the Middle: The buck–boost mode, in the middle, was neglected to save the efficiencyof the converter, since during this mode of operation, both theswitches are operated simultaneously.

By applying this combination method of control andsimulating the converter, the results shown in Fig. 17 areobtained. The simulation results show that output voltage tran-sients during transition from combination mode A to com-bination mode B are somehow similar to transients avail-able in transition from combination mode A to buck–boostmode. This voltage variation in this method is about 4%; how-ever, canceling the buck–boost operating mode in between



Fig. 17. Input and output voltages and buck and boost pulses during transitionfrom buck to combined mode A and then to combined mode B and boost withoutbuck–boost mode.

Fig. 18. Output voltages for all four different transition methods.

significantly improves the efficiency of the converter. This isalso proved through the measurements in Section VI. Fig. 18presents the output voltages for all different transition methodsdiscussed.

Fig. 19. Experimental circuit configuration.

VI. EXPERIMENTAL RESULTS

The hardware of a positive buck–boost converter is designedbased on the parameters listed in Table II. Then the converteroperates at 100 kHz switching frequency. Two n-type MOSFETswitches and two Schottky barrier diodes are used for positivebuck–boost converter configuration. The MOSFET switches anddiodes are IRF540 and 1N5817, respectively. Controller hasbeen implemented using a Texas Instrument digital signal pro-cessor (DSP) (320F2812). The output voltage reference is set to5.0 V, and input voltage varies from 6.0 to 3.6 V.

Two high-side gate drivers are designed for IRF540. Thededicated gating logic is applied to the converter for properoperation as buck, buck–boost, and boost converters. The oper-ating modes are dependent on mode selection signals, appliedfrom DSP. The overall configuration of converter and controlleris shown in Fig. 19. G1 and G2 are buck pulse and boost pulse,sequentially. MD_SEL0, MD_SEL1, and MD_SEL2 determineoperation modes: buck, combination mode A; buck–boost, com-bination mode B; and boost, in each control period. Fig. 20presents the experimental setup that is composed of DSP con-troller, gate driver, gating logic, and converter.

Fig. 21 demonstrates the output voltage of the converter, buckpulses, and boost pulses during a direct transition from buckmode to boost mode. Output voltage has a considerable variationduring the transition from buck to boost.

Figs. 21 and 22 present the output voltage of the converter,buck pulses, boost pulses, and buck–boost pulses for an indirecttransition from buck to boost. In this case, transition from buckto boost is carried out through a transition from buck to buck–boost mode, followed by a transition from buck–boost to boostmode.

Fig. 23 presents the input and output voltages of the con-verter in a whole range of operation. Input voltage decreasesfrom 6 to 3.6 V. Output voltage is tightly regulated at 5 V. Inthis experiment, transition from buck to boost is a combina-tion of four different operating modes, as shown in Fig. 12.Fig. 12(a) demonstrates the output voltage variation as well asbuck and boost pulses in transition from buck to combinationmode A. Fig. 12(b) demonstrates the buck and boost pulses



Fig. 20. Experimental setup composed of (a) converter, (b) DSP controller, and (c) gate drivers.

Fig. 21. (a) Output voltage, buck, and boost pulses in a direct transition from buck to boost and (b) zoomed view of the waveforms.



Fig. 22. (a) Output voltage, buck, and buck–boost pulses in transition from buck to buck–boost and (b) zoomed view of the waveforms.

Fig. 23. Input and output voltages for the transition from buck to boost modethrough proposed method.

in combination mode A, where each three buck pulses are as-sociated with one boost pulse. In Fig. 12(c), buck, boost, andbuck–boost pulses in transition from combination mode A tobuck–boost mode are demonstrated. Similarly, Fig. 12(d) shows

Fig. 24. Output voltage, buck pulses, and boost pulses in transition fromcombination mode A to combination mode B.

the buck, boost, and buck–boost pulses in transition from buck–boost mode to combination mode B. In addition, Fig. 12(e)illustrates the buck and boost pulses in combination mode Bbefore transition to boost mode is complete. In combination



Fig. 25. (a) Output voltage, buck–boost, and boost pulses in transition from buck–boost to boost and (b) zoomed view of the waveforms.

mode B, each buck pulse is associated with two boost pulses.Finally, Fig. 12(d) shows the output voltage for a transition fromcombination mode B to boost mode.

In order to improve the efficiency of the converter in additionto improving the output voltage transients, dynamic behavior ofthe system for a direct transition from combination mode A tocombination mode B is investigated. Fig. 24 presents the outputvoltage, buck pulses, and boost pulses in a direct transition fromcombination mode A to combination mode B.

The ripple during direct transition from buck to boost mode isabout 12% (Fig. 20). The voltage ripple is defined as the peak–peak ripple as a percentage of the output voltage of the converter.This ripple can be descended by adding the interface buck–boostmode. Addition of the buck–boost mode improves the voltagetransient to about 4% in transition from buck to buck–boost(Fig. 22) and decreases the ripple transient from buck–boost toboost to about 6% (Fig. 25); however, staying in buck–boostmode for a long time sacrifices the efficiency of the converter.Adding combination modes A and B between buck and boostmodes significantly improves the voltage transients, as shownin Figs. 12, 23, and 25. Table III summarizes the output voltageripple during transients from different modes of operation.

Fig. 26 presents the efficiency of the converter versus the in-put voltage variations for various transition modes for differentloads. Efficiency is a function of load profile. Increasing theload power decreases the efficiency because of increasing theohmic and switching losses. Fig. 26(a) presents the efficiency of

TABLE IIISUMMARY OF THE OUTPUT VOLTAGE TRANSIENTS DURING TRANSITION FROM

DIFFERENT MODES OF OPERATION

the converter in the transition from buck to buck–boost and thenboost mode. Efficiency of the converter in the region where in-put voltage is approximately equal to output voltage plus diodevoltage drop descends to 73% due to the operation of the con-verter in buck–boost mode. Similarly, transition from buck tocombination mode A and then buck–boost mode sacrifices theefficiency when input voltage is about 5.4 V. Eliminating thebuck–boost mode in Fig. 26(c) improves the efficiency by about17% and 20%, respectively, in the cases of R = 5.1 Ω andR = 19.08 Ω.

Comparing the voltage regulation plots of Figs. 12 and23–25 and the efficiency plots of Fig. 26 demonstrates thatthe proposed methodology improves the efficiency and outputvoltage regulation during transition from one operating mode



Fig. 26. Efficiency plot versus the input voltage variations for (a) transitionfrom buck to buck–boost and then boost mode, (b) transition from buck to boostthrough buck–boost and combination modes, and (c) transition from buck toboost without buck–boost mode.

Fig. 27. Control signals in (a) combination mode A and (b) combinationmode B.

to the other. Fig. 27 demonstrates the control signals incombination mode A and combination mode B.

VII. CONCLUSION

A highly efficient control strategy to control a pulsewidthmodulation (PWM) dc/dc positive buck–boost switching con-verter has been illustrated in this paper. The proposed controlscheme can regulate the output voltage for an input voltage,which changes based on the charge status of the battery supply.The technique introduced in this paper is unique of its kind in im-proving the efficiency and the ripple content of the output voltagefor a positive buck–boost converter whenever smooth transitionis needed from the buck mode to the boost mode. In addition, theconcept of DCPC is introduced, which improves the transitionripple by distributing the voltage transients. In this method, thecapability of skipping over higher loss interface stages such asbuck–boost mode in the case of a positive buck–boost convertersignificantly improves the efficiency of the circuit topology. Thepresented simulations and experimental results validate the pro-posed theory and its merits. In this manuscript, the proposedtheory has been utilized to improve the output voltage tran-sients in transition from buck to boost mode. This is an enablingtechnology to improve voltage transients in any applications thatrequire transition between different converter topologies.

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Young-Joo Lee (S’07) received the B.S. degree inelectrical engineering from Korea University of Tech-nology and Education, Cheonan, Korea, in 1996,and the M.S. degree from Gwang-Woon University,Seoul, Korea, in 2003. Currently, he is working to-ward the Ph.D. degree at the Illinois Institute of Tech-nology, Chicago.

His Ph.D. research has been focused on integratedbidirectional converter for plug-in hybrid electric ve-hicles. In 1995, he joined SunStar R&C, Incheon,Korea, which is highly specialized in industrial

sewing machines and controllers, and motors and controllers for industrialsewing machines. Then he joined Genoray Company, Ltd., Seongnam, Korea,which manufactures X-ray fluoroscopy equipment for medical surgery. He hasten years or more of experience in the industrial field and has developed manycommercial system controllers for sewing machines and medical X-ray flu-oroscopy equipment that require control over brushless dc (BLDC) motors,permanent magnet synchronous motors (PMSMs), induction motors, steppermotors, high-frequency full-bridge converters, X-ray electron tubes, and otherelectric–pneumatic actuators.

Alireza Khaligh (S’04–M’06) received the B.S. andthe M.S. degrees (with highest distinction) in electri-cal engineering from the Sharif University of Tech-nology, Tehran, Iran, and the Ph.D. degree from theIllinois Institute of Technology (IIT), Chicago.

He was a Postdoctoral Research Associate in theElectrical and Computer Engineering Department,University of Illinois (UIUC), Urbana-Champaign.He is currently an Assistant Professor of Electricaland Computer Engineering at IIT, where he has es-tablished courses, curriculum, and research in energy

harvesting and renewable energy systems. His major research interests includemodeling, analyses, design, and control of power electronics converters andmotor drives, energy scavenging, biomechanical energy conversion, electricand hybrid electric vehicles, and power management for portable electronics.

Dr. Khaligh is an Associate Editor of the IEEE TRANSACTIONS ON

VEHICULAR TECHNOLOGY. He is a Guest Editor for a special section of IEEETRANSACTIONS ON VEHICULAR TECHNOLOGY on vehicular energy storage sys-tems. He is also a Guest Editor for a special section of IEEE TRANSACTIONS

ON INDUSTRIAL ELECTRONICS on energy harvesting. He is the recipient of theDistinguished Undergraduate Student Award, presented jointly by the Ministerof Science, Research, and Technology and the President of Sharif University,and Exceptional Talents Fellowship from the Sharif University of Technology.He is also the recipient of the Highest Standards of Academic AchievementAward at IIT.

Arindam Chakraborty received the M.S. degreein electrical engineering from Jadavpur University,Kolkata, India, and the Ph.D. degree in electrical en-gineering from the Illinois Institute of Technology(IIT), Chicago.

He was a Research Fellow at the GraingerPower Electronics and Motor Drives Laboratory, IIT,Chicago, for three years, during which he publishedseveral papers in micro-, nano- and power electronicsand digital control of power electronic converters. Heis a Principal Design Engineer in the Electronics and

Controls (R&D) of OSI, a Siemens company, for the past two years in Boston,MA. He holds one U.S. patent issued and three pending to his credit.

Ali Emadi (S’98–M’00–SM’03) received the B.S.and the M.S. degrees in electrical engineering withhighest distinction from the Sharif University ofTechnology, Tehran, Iran, and the Ph.D. degree inelectrical engineering from the Texas A&M Univer-sity, College Station, TX.

He is a Professor of electrical engineering and theDirector of the Electric Power and Power Electron-ics Center (EPPEC), Illinois Institute of Technology(IIT), Chicago, where he has established research andteaching facilities as well as courses in power elec-

tronics, motor drives, and vehicular power systems. He is also the Founder,Director, and Chairman of the Board of the Industry/Multiuniversity Consor-tium on Advanced Automotive Systems (IMCAAS).

Prof. Emadi was the General Chair of the 2005 IEEE Vehicle Power andPropulsion and SAE Future Transportation Technology Joint Conference. Heis the author/coauthor of over 180 journal and conference papers as wellas several books, including Vehicular Electric Power Systems (New York,NY: Marcel Dekker, 2003), Energy Efficient Electric Motors (New York, NY:Marcel Dekker, 2004), Uninterruptible Power Supplies and Active Filters (BocaRaton, FL: CRC Press, 2004), and Modern Electric, Hybrid Electric, and FuelCell Vehicles (Boca Raton, FL: CRC Press, 2004). He is an Associate Editor ofthe IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, IEEE TRANSACTIONS

ON VEHICULAR TECHNOLOGY, and IEEE TRANSACTIONS ON POWER ELECTRON-ICS. He is also the Editor of the Handbook of Automotive Power Electronics andMotor Drives (New York, NY: Marcel Dekker, 2005).

He is the recipient of numerous awards and recognitions. He has been namedthe Eta Kappa Nu Outstanding Young Electrical Engineer of the Year 2003 byvirtue of his outstanding contributions to hybrid electric vehicle conversion. Healso received the 2005 Richard M. Bass Outstanding Young Power ElectronicsEngineer Award from the IEEE Power Electronics Society. In 2005, he was se-lected as the Best Professor of the Year by the students at IIT. He is the recipientof the 2002 University Excellence in Teaching Award from IIT as well as the2004 Sigma Xi/IIT Award for Excellence in University Research. He directeda team of students to design and build a novel motor drive, which won the FirstPlace Overall Award of the 2003 IEEE/DOE/DOD International Future EnergyChallenge for Motor Competition.


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