a comparative study of sepic, cuk and zeta converters - Scientific ...

Scientific Bulletin of the Electrical Engineering Faculty – 2008A COMPARATIVE STUDY OF SEPIC, CUK AND ZETA CONVERTERSFlorian ION 1 , Gabriel PREDUSCA 2Abstract: In this paper a comparative study of DC-DCconverters is presented. The SEPIC, Cuk and ZETAconverters in applications are detailed. Also are presentedthe operating simulation for these converters. The resultsof simulations are compared with the measurements donefor a ZETA converter with an output of 3.3V and differentoutput currents.Keywords: DC-DC converters, SEPIC converter, Cukconverter, ZETA converter2.1. OPERATING PRINCIPLES2.1 Fundamental DC-DC ConvertersFrom the viewpoint of input and output voltages – V 1and V o , the fundamental converters are: (1) step-down orbuck converters, (2) step-up or boost converters, (3) stepdown/up or buck-boost converters. Figure 1 shows theprinciple diagrams of these topologies.1. INTRODUCTIONL1SL2It’s well known that to have the maximum efficiencyof the solar panels, the load must be connected to thesolar panel through a DC-DC converter. The topologiesand the operation of these converters are very welldescribed in the literature. A classification of theseconverters is presented in [1]. The authors of [1]consider the DC-DC converters in six decades: (1)classical/traditional converters, (2) multiple-quadrantconverters, (3) switched component converters, (4) softswitchingconverters, (5) synchronous rectifierconverters, (6) multiple energy-storage elementsresonant converters. The classical/traditional convertersare divided in five categories: (1) fundamentalconverters, (2) transformer-type converters, (3)developed converters, (4) voltage-lift converters, and (5)super-lift converters.The converters studied in this paper are classicaldeveloped converters well known in literature likeconverters in SEPIC (Single Ended Primary InductanceConverter) topology, Cuk and ZETA (Positive OutputLuo Converter). The developed-type converters derivedfrom fundamental converters by addition of a low-passfilter. In [2] these converters are considered like aMASTER converter switched by a PWM signal, and aSLAVE converter achieved with passive components.Because the great usage of converters in thetopologies mentioned above in applications, we foundopportunity for a short presentation of operatingprinciples, simulations and some experimental resultsin this work. The equations of main operatingparameters, advantages and disadvantages of eachtopology are presented in chapter 2 of this paper. Thesimulations and the measurements are presented inchapter 3, and the final conclusions in chapter 4.V1V1V1L1L1C1SC1DSa)b)c)Figure 1. Fundamental DC-DC Converters: a) buckconverter, b) boost converter, c) buck-boost converter.In ideal operating conditions (no voltage loss on theswitch S, the average voltage across inductors L atsteady state zero, no current loss on capacitors C, andno voltage loss on diode at forward conduction) theequations of ratio V o /V 1 are:– for the buck converter (1):V o= D (1)V1where D is the duty cycle of PWM signal of switch S,with the meaning from equation (2),tOND = (2)TDC1L1L2DCoCoCoRsRsRs1 Valahia University/Electronic Department, Targoviste, Romania, e-mail: flion@valahia.ro2 Valahia University/Electronic Department, Targoviste, Romania, e-mail: gpredusca@valahia.ro17

Scientific Bulletin of the Electrical Engineering Faculty – 2008where t ON is the conduction time of switch S and T isthe period of PWM signal.– for the boost converter (3):V o 1= (3)V 1 − D– for the buck-boost converter (4):V o D=V 1 − D11(4)In all these equations the internal resistance ofpower supply V 1 was considered zero [3].V1V1L1L1SSCCa)L2DDL2CoCoRsRs2.2. Developed DC-DC convertersb)Figure 2 shows the topologies of developed DC-DCconverters.These topologies have few similitudes:– The equation of the transfer function of theseconverters is (4), the same with that one of buck-boostconverter, if the conditions of Continuous ConductionMode – CCM are assured.– These converters are used in differentapplications, such as with solar panels, in systemssupplied with electrical energy where the outputvoltage V o of converter can be superior or inferior ofthe input voltage V 1 of converter. The fundamentalconverters don’t accept this situation. These convertersare integrated in the MPPT of solar panels.– The capacitor C assures the galvanic insulationbetween input and output. The short-circuits or othersbreakdown of the load don’t affect the power supply –solar panels.– The output voltage becomes zero if the PWMcontrol signal of switch S is missing.– The diode D can be replaced by a transistorswitched synchronal with the main switch in thesynchronous converters.The differences between these topologies are:– SEPIC and Cuk converters became from the boostconverter, and ZETA converter from the buck-boostconverter.– The ripple current in the load is greater for Cukand ZETA converters than SEPIC, because the SEPICconverter has an inductor L 2 that smooth the currentspikes.– The switch S of SEPIC ad Cuk converters is a Nchannel MOS transistor that needs a Low Side driver,when the ZETA converter has a P channel MOStransistor that needs a High Side driver.Because, these three topologies have manyadvantages mentioned above, these things makeenable their integration in applications with a greatefficiency of using the solar energy in solar panelswith MPP trackers.V1SL1Cc)Figure 2. Developed DC-DC Converters: a) SEPICconverter, b) Cuk converter, c) ZETA converter2.3 Integration of convertors in MPPT systemsThe perturb-and–observe (PAO) method for theMPPT is an iterative approach. The MPP is obtainedby making the derivate of power equal with zero in thefeedback circuit that commands the duty cycle ofswitch S. This is very useful because doesn’t need thedisconnection of panels from the load. Through thismethod can be reached good results if it is compared theinstantaneous conductance of panel with the incrementalconductance of panel – the method is known asIncremental Conductance Technique (ICT) [4].If it is considered the equivalent circuit of the solarpanel like in Fig. 3, with v i the input voltage of paneland r i the equivalent input resistance of panel, P i theinput power, P o the output power (5), the∂P = 0 means (6).2viPi= Po=(5)ri∂viVi=(6)∂ri2RiThe method proposed in [4] resides in theconnection of a SEPIC or Cuk converter between thesolar panel and load. The converter works incontinuous current mode (CCM) through inductor L 1 –Figure 2 a), but with discontinuous voltage (DCV) onthe capacitor C. The duty cycle of PWM signal ofswitch is adjusted in a proper way to achieve the inputresistance of converter equal with the output resistanceof solar panel. Figure 3 shows the equivalent circuit ofsolar panel and converter.DL2CoRs18

Scientific Bulletin of the Electrical Engineering Faculty – 2008Figure 3. Equivalent circuit of a solar panel andconverter [4]The operating equations of SEPIC converter inDCV mode are the next: (7a) – the voltage oncapacitor C, (7b) – the voltage on diode D.vC⎧ I1(1− d)TS⎪ −V⎪C( t)= ⎨−Vo,⎪I⎪1( t − dT −S ) V⎩ Coo,I 2− t,0 < t < d1TSCd T < t < dT1dTSS< t < TSS(7a)vD⎧Vo + vC( t),0 < t < d1TS( t)= ⎨(7b)⎩0,d1TS< t < TSwhere I 1 and I 2 are the inductor currents - assumed tobe constant, dT S is the conduction time of switch, d 1 T Sis the conduction time of diode D, and T S is the period1of PWM signal of switch - TS= , f S – frequency offPWM signal.The three sequences in one switching cycle areshown in Figure 4.Because the voltage of capacitor C at d 1 T SI1is vC( d 1 TS) = −Vo , the duty cycle is d1 = (1 − d).IIn the steady state the voltage on the inductor L 2 iszero. From this moment the output voltage V o is equalwith the average voltage of diode D (8).Vo1 d1TSTS= v ( t)dt I1(1d)d1T ∫ D = − (8)02CSMoreover, the voltage stress on the switch S isgiven by (9).In the same way can be determined the operatingequations of Cuk and ZETA topologies. This isshown in [4].I1v stres = vC( TS) + Vo= (1 − d)TS(9)CS2Figure 4. Operating principle of the SEPIC converter.a) equivalent circuits, b) theoretical waveforms [4]3. SimulationsIn this chapter it will be presented fewrepresentative waveforms of each topology. Thesimulations were done in OrCAD, in the nextconditions:– input voltage – V 1 =12V,– output voltage – V o =3.3V,– load resistance – R S =3.3Ω,– duty cycle – D=0.22,– switching frequency – f S =500kHz,– coupling capacitor C=47μF,– output capacitor C o =100μF,– inductors L 1 =L 2 =6.2μH.3.1 The SEPIC converterTo simulate the operation of SEPIC converters itwas used the diagram from Figure 2 a). In Figure 5 a),b), and c), is shown the output voltage V o , the ripple of19

Scientific Bulletin of the Electrical Engineering Faculty – 2008output voltage ΔV o , and the voltage stress V stress ofswitch S.At steady state – after 1.5ms, these values are:V 0 =3.5V, ΔV o =6mV pp , and V stress =16V.0V-2.0V6.0V-4.0V4.0V2.0V-6.0V0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0msV(R4:2)Timea) V o – output voltage0V-3.60V-2.0V0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0msV(R4:2)Timea) V o – output voltage-3.65V-3.70V3.7200V-3.75V3.7150V3.7100V-3.80V0.8773ms 0.9000ms 0.9500ms 1.0000ms 1.0500ms 1.1000msV(R4:2)Timeb) ΔV o – output ripple20V3.7056V2.960ms 2.962ms 2.964ms 2.966ms 2.968ms 2.970ms 2.972ms 2.974ms 2.976ms 2.978msV(R4:2)Timeb) ΔV o – output ripple15V10V20V5V15V10V0V877.34us 878.00us 879.00us 880.00us 881.00us 882.00us 883.00us 884.00usV(M2:d)Timec) V stress – voltage stress on switch5V0V2.96000ms 2.96400ms 2.96800ms 2.97200ms 2.97600ms 2.97897msV(M2:d)Timec) V stress – voltage stress on switchFigure 5. Simulated waveforms of SEPIC converter: a)output voltage, b) output ripple, c) voltage stress of switch3.2. The Cuk conveterTo simulate the operation of Cuk converter it wasused the diagram from Figure 2 b). Figure 6 a), b) andc) shows the waveforms of output voltage, outputripple and voltage stress of switch in the sameconditions.At steady state – after 0.5ms, these values are:V 0 =3.51V, ΔV o =28mV pp , and V stress =16V.3.3. The ZETA converterTo simulate the operation of ZETA converter it wasused the diagram from Figure 2 c). Figure 7 a), b) and c)shows the waveforms of output voltage, output rippleand voltage stress of switch in the same conditions.At steady state – after 0.5ms, these values are:V 0 =3.41V, ΔV o =26mV pp , and V stress =16V.Figure 6. Simulated waveforms of Cuk converter: a)output voltage, b) output ripple, c) voltage stress of switchConclusions on simulations are in Table 1.Table 1. Simulation ResultsVoltageTopologySEPIC Cuk ZETAV o [V] 3.51 3.51 3.41ΔV o [mV pp ] 6 28 26V stress [V] 16 16 163.4. Experimental verificationsAn experiment has been performed using a ZETAtopology on a LTC1622 – a Current Mode Step-DownDC/DC converter of Linear Technology [5]. Theschematic diagram is the typical application proposedby the producer and is shown in Figure 8 [6].In Figure 9 are the waveforms in the nextconditions: V 1 =8.3V, V o =3.3V, R S =20Ω (I o =165mA).The channels of oscilloscope represent: CH1 – V G –PWM signal on the gate of MOS transistor, CH2 – V D– drain voltage of MOS transistor, CH3 – V C – voltage20

Scientific Bulletin of the Electrical Engineering Faculty – 2008on positive pin of capacitor C, CH4 – ΔV o – outputvoltage ripple.In Figure 10 are the waveforms in these conditions:V 1 =4.9V, V o =3.4V, R S =10Ω (I o =340mA). It can beobserved other values for switching frequency andduty cycle of PWM signal.6.0V4.0V2.0Vsystems where these conditions are reached very often –like solar panels in different levels of solar radiation.The results of measurements are in Table 2.The circuit LTC1622 changes the switchingfrequency – Figure 9, 10, 11, in a wide range – from265 kHz to 1024 kHz. Also, the duty cycle D of PWMsignal in the gate of the MOS transistor is changedaccording with the work conditions – input and outputvoltage, and the output current, to insure a constantvalue of output voltage.The ripple of output voltage measured in realconditions is few times greater than that one obtainedin ideal conditions of simulations.0V0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0msV(R8:2)Timea) V o – output voltage3.62V3.61V3.60V3.59V4.65ms 4.70ms 4.75ms 4.80ms 4.85ms 4.90ms 4.95ms 5.00msV(R8:2)Timeb) ΔV o – output ripple20V15V10VFigure 9. Waveforms of V G , V D , V C , ΔV o in conditions:V 1 =8.3V, V o =3.3V, R S =20Ω5V0V4.650ms 4.652ms 4.654ms 4.656ms 4.658ms 4.660msV(M1:s)- V(M1:d)Timec) V stress – voltage stress on switchFigure 7. Simulated waveforms of ZETA converter: a)output voltage, b) output ripple, c) voltage stress of switchFigure 8. ZETA converter with LTC1622 [9]The experimental waveforms in the conditions:V 1 =7.1V, V o =3.4V, R S =5Ω (I o =680mA) are shown inFigure 11.It can be seen that the ZETA converter build with theLTC1622 integrated circuit works well with inputvoltages less than output voltage and at an input voltageover the output voltage. This advantage can be used inFigure 10. Waveforms of V G , V D , V C , ΔV o in conditions:V 1 =4.9V, V o =3.4V, R S =10Ω21

Scientific Bulletin of the Electrical Engineering Faculty – 2008REFERENCESFigure 11. Waveforms of V G , V D , V C , ΔV o in conditions:V 1 =7,1V, V o =3,4V, R S =5Ω[1] F. L. Luo, H. Ye, “Advanced DC/DC Converters”,CRC Press, 2004.[2] S. Maniktala, “Slave Converters Power AuxiliaryOutputs”, EDN Magazine, Elsevier, 2002.[3] P. Constantin, et al., “Electronică industrială”, EdituraDidactică şi Pedagogică, Bucureşti, 1983.[4] H. S-H. Chung, et al., “A Novel Maximum PowerPoint Tracking Technique for Solar Panels Using aSEPIC or Cuk Converter”, IEEE Transaction on PowerElectronics, Vol. 18, No. 3, May 2003.[5] M. Dobre, F. Ion - supervisor, “Corecţia factorului deputere în convertoarele în comutaţie cu reţele decomutare de ordin zero”, Proiect de diplomă,Universitatea Valahia, Târgovişte, iulie 2007.***, “LTC1622 Low Voltage Input Current Mode Step-Down DC/DC Controller”, Data Sheet, LinearTechnology, 1998.Table 2. Measurement resultsZETA converter with LTC1622with output voltage 3.3VVoltageV 1 =8.3V, V 1 =4.8V, V 1 =3.2V,I o =165mA I o =165mA I o =165mAΔV o [V pp ] 1.37 0.82 0.60f S [kHz] 265 524 504VoltageV 1 =8.3V, V 1 =4.9V, V 1 =8.3V, V 1 =7.1V,I o =340mA I o =340mA I o =680mA I o =680mAΔV o [V pp ] 1.25 0.83 1.45 1.12f S [kHz] 913 759 1024 5624. CONCLUSIONSIn this paper was presented a comparative study ofDC-DC converters in SEPIC, Cuk and ZETAtopologies.It was studied the fundamental converters anddeveloped converters in the topologies mentionedabove. The operation equations of main parameterswere presented.Moreover, it was presented the simulations of theseconverters in the same work conditions.The waveforms that have seen on a ZETAconverter with a constant output voltage and variableinput voltage and load confirmed the simulationsresults of that converter.22