Page - 315 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2016,9, 10 3.2. ControlMethod Unlike the traditionalbuckorboostconverter, thedutycycleDof theq-Zsourcecannotreach50% accordingtoEquation(15). Thevoltagegaincurveof theq-Zsource is showninFigure8,andit clearly showsthat thereare twooperatingregions. Figure8.Voltagegainof theq-Zsource. WhenD isgreater than0.5, theq-Zsourceenters thenegativegainregion,andproducesanegative outputvoltage,which ishardlyused inpractice.WhenD is less than0.5, itproducesapositiveoutput voltage, thusweshould limit thedutycycleD tobelow0.5.All the traditionalcontrol strategies [26] canbeusedtocontrol theq-Zsourceandtheir theoretical input-outputrelationshipsstillhold, theonly difference is that theshoot-throughtimeisadded. Thetraditionalphaseshiftingcontrol iswidelyused toproducethesquare-wavevoltagesandrealize thesoft-switchingconditions.However, itwillnot bediscussedinthispaper,as thiscontrolmethodhasalreadybeenexplainedbefore [36]. It isworth mentioningthat theq-Zsourcehasnoeffectontheoutputwavesof theHBridge in thismode,and onlyactsasakindoffilter. When the chargingpowerneeds tobeboosted, theHBridge enters thenewoperatingmode shown in Figure 9, which supplies the shoot-through state for the q-Zsource to boost the output voltage.Unlike the traditionalphaseshiftingcontrol, anadditional shoot-throughtimeTshoot´through is added into the control sequences, Thedead timeTdead, shifting timeTshifting andshoot-through timeTshoot´through influence theoutputwaves together. Theshoot-throughstate inFigure9 is realized by turningonS3 andS4 simultaneously, or it canbealso realizedbysimultaneously turningonS1 andS2,whichdependsonthepractical situations. The intervalbetween t0 and t7 is thewholecontrol cycle, as it is symmetrical, only theoperatingmodeamong t0~t3 needs to bedemonstrated. S1 is turnedoffat t0,while IH is stillpositive, thus it is forcedtoflowthroughthefree-wheelingdiodeof S2. Before IH changes,S2 shouldbe turnedonat t1,whichcanrealize its soft switching. These two stepsaresimilar to thecontrolofaphase-shift-full-bridge,butnotexactly thesame,as thecascade loads aredifferent. BeforeS3 is turnedoff,S4 is turnedonat t2, and this state is forbidden in the traditional control. However,preciselybecauseof that, the shoot-throughstate isprovided,which allows theq-Zsource toboostvoltage, anddifferentboost factors canbeachievedbyadjusting the intervalbetween t2 and t3. It isnoticeablehere that theequivalentswitchingfrequencyviewedfrom theq-Zsource is twotimes theoperating frequencyof theHBridge,whichgreatlyreduces thevolume andweightof the inductorsandcapacitorsexisted inq-Zsource. Inaddition, the lagging leg (S3,S4) is turnedoffwithsoft switching,but turnedonwithhardswitching,which lowers theefficiencyand needs tobe furtherstudied. 315