Softswitchingarcweldi_省略_torasresonantin

Soft switching arc welding power source using outputinductor as resonant inductorChen Shuj un,W ang J un,L u Zheny ang and Yin Shuyan陈树君,王 军,卢振洋,殷树言Abstract Full -br idge Zer o -V oltage -Sw itch P WM conver ter combines adv antages of the P WM control technique and resonant technique.H owever ,Full ZV S is achiev ed only under lar ge load cur r ent because r esonant tank of this cir cuit is made up of the p arasitic cap acitance of the p ower semiconductors and the leakage inductor of the tr ansf or mer p r imary.I n this p ap er two satur able inductor s as ma gnetic sw itches ar e added to secondary ,so outp ut inductor is alw a y s r ef lected to pr imar y and assists resonant tr ansition.Full ZVS is achieved under lower load curr ent.T he above -mentioned investigated results ar e validated by the comp uteriz ed simulation and har dw are circuit ex periment.Key words arc w elding,inv er ter power source,soft switching ,resonant inductor ,co mputerized simulat ion0 Introduction T he Ful-l Bridge circuit is very suitable for the high -pow er converter.T he FB -ZVS -PWM is the typicalconverter in the Ful-l Bridge soft sw itch transitionconverter [1].T he FB -ZVS -PWM converter combines the PWM control technique and ZVS resonant technique.T he resonance occurs betw een the parasitic capacitance of the pow er sem iconductors and the leakage inductor of the transformer primary.In the case of square w ave pow er transformer,the Full ZVS of the power semiconductor is achieved for reducing the loss of sw itching.T his w ill improve the reliability of the converter.Thoug h the Full ZVS of the pow er semiconductor is achieved in the traditional FB -ZVS -PWM converter,there are many disadvantages w hich depress the reliability of the FB -ZVS -PWM converter,such as duty cy cle loss,circulating current loss,the parasitic resonance of the secondary of the transformer and the narrow load range of the resonance [2].In order to solve those problems,in this paper,two saturable inductors as magnetic sw itches are added to secondary coil,so output inductor is alw ays reflected to prim ary and assists resonant transition.Full ZVS is achieved under low er load current,at the same time theload range of ZVS is ex tended,and the hig h -frequency parasitic resonance of the transform er secondary is w eaken.T he above -m entioned investigated results are validated by the computerized simulation and hardw are circuit experiment.1 Circuit topologyThe resonant transition behavior of the static leg and the shifted leg is different in the traditional FB -ZVS -PWM converter.The resonant transition of the static leg alw ays occurs before the power source transferring energy to the load,however the resonant transition of the shifted leg alw ays occurs after the power source transferring energy to the load.T he actuator of the ZVS of the static leg is only the energy stored by the resonant inductor,how ever the actuator of the ZVS of the shifted leg is mainly the current of output inductor that is reflected to the prim ary of the transform er.The mechanism of the ZVS between the static leg and the shifted leg is different in the FB -ZVS -PWM converter.The actuator of the ZVS of the shifted leg is mainly the resonance between the output inductor and the resonant paring w ith the static leg,the ZVS of the shifted leg is very easy.If theChen Shujun ,Wang Jun ,Lu Zheny ang and Y in Shuyan,College o f M echanical Engineer ing &Applied Electronics T echnology ,Beijing Polytechnic U niversity ,Beijing ,100022. E -mail:sjchen@T his project was supported by Natur al Science Foundation of China (50075003),Natur al Science Foundation of Beijing (3001001)and China Postdoctor al Science Foundation.freewheeling of the output rectifier diode VD 5,VD 6could be prevented in the same time,the output inductor is alw ays reflected to the primary,thus,the transition mechanism of the tw o legs of the converter issame.Fig.1 Soft switching arc welding pow er source usingoutput inductor as resonant inductorT here are two methods to prevent the two rectifier diodes from turning on in the same time.First,we can re -placeVD 5andVD 6w ithhigh -pow ersw itchingsemiconductorthat can be controlled.Not only does it increase the cost of the circuit,but also it increases the complexity of the circuit.Second,as show n in Fig.1,tw o saturable inductors LR 1and LR 2are added in series in the place w here the two rectifier diodes of VD 5and VD 6are.T he two saturable inductors LR 1and LR 2are only used as tw o magnetic switches,how ever,the function of the saturable inductor referred in reference [3]is to promote the transition of the ZVS.When there is current in LR 1and LR 2,or the value of VS on them is g reat enough,inductors saturate,w hich means LR 1or LR 2is short circuit.When there is no current in LR 1and LR 2,inductors are unsaturated,w hich means LR 1or LR 2is open circuit.2 Operating principleThe principle of the FB -ZVS -PWM converter using output filter inductor as resonant inductor is show n in Fig.2.T he controlling method is typical phase -shifted control that is as same as the traditional converter.Only in some interval is there some difference.Fig.2 Operating principle of the converter using output inductor as resonant inductorIn the initial state t 0,the energy is transferred to the secondary from the primary of the transformer w hen the semiconductor VT 1and VT 2turn on.Interval t 0~t 1:T he sw itching semiconductor VT 2turns off during the interval of the shifted leg transition.Because the saturable inductor LR 1acts as the turn on of the magnetic sw itch,the transition process is as same as the traditional FB -ZVS -PWM converter.The reflectionof the output filter inductor does not disappear on the primary,the resonance occurs betw een the output filter inductor,leakage inductor of the transformer and theresonant capacitance.The full ZVS is achieved under low er load current of the semiconductor VT 4of the shifted leg.Interval t 1~t 2:During the interval of the clampingfreewheeling,the saturable inductor LR2does notsaturate,the mag netic sw itch turns off.The current in the output inductor flow s through LR1and VD5.Thus, the output inductor is also reflected to the prim ary,there is little decreasing of the circular current flow ing throug h VD4and VT1.T his is the disadvantage of the circuit.Interval t2~t3:After the sem iconductor VT1turns off during the interval of the shifted leg transition, because there is inductive voltage on the output filter inductor L f and the transformer transfers energy from the secondary to the primary,the resonance occurs betw een the resonant capacitance and the output inductor L f the leakage inductor of the transformer.The capacitance C1 is charged fastly,at the same time the capacitance C3 discharg es.The energy stored in the output inductor is transferred to the pow er source until the turn on of VD3. T he voltage on the saturable inductor LR2increases neg atively,it w ould reach2V s(2V i n/N),w hen the semiconductor VD3turns on.Because the output inductor transfers energy to the primary,the output voltage V rec of the rectifier w ill remain negative value. T he value is V s.Interval t3~t4:During the interval of the pow er transition,because of the turn on of VT3at the tim e of t3,the voltage of the pow er is loaded on the primary, the energy is transferred to the load.It is the prim ary leakage inductor that prevents the prim ary current from transiting during the interval of the primary current transition.T his is as same as the traditional FB-ZVS-PWM converter.T he difference is that the saturable inductor saturates fastly when the primary current begins to transition.At the same time,the inductor LR1 becomes unsaturated,the voltage on it increases until the v alue of the voltage reaches2V s.When the energy stored by the inductor LR1w as released,the diode VD5 turns off,this acts as the turn off of the magnetic sw itch.In the time t4,the semiconductor VT4turns off,the operating process is as same as the above-mentioned investigated process.But the state of the magnetic sw itches are reverse,the magnetic sw itch LR1 turns from turn on to turn off,inversely,the magnetic sw itch LR2turns from turn off to turn on.3 C omputerized simulationIn the softw are of Pspice,the simulation models are built,such as power semiconductor of IGBT model, amorphous core model of transformer,the ferrite core model of the saturable inductor.The circuit of the FB-ZVS-PWM using output filter inductor as resonant inductor is simulated.The main parameters of the circuit are show n as follow s:Pow er semiconductor:BSM75modelOutput rectifier diode:M UR20040CT modelMagnetic core of transformer:amorphous core modelTransformer ratio:N=8(Primary32,secondary 4)Leakag e inductor of the transformer:5 HMagnetic core of the saturable inductor:ferrite R4KB modelInput voltag e:V in=540VDuty:D=0.6Output filter inductor:L=50 HThe sw itching characteristic w aveform of IGBT on the static leg of the converter is show n in Fig.3a. Because of the function of the magnetic sw itch,the output filter inductor is alw ays reflected to the primary. There is enough energy stored by the transformer during the transformer stopping transferring energy to outside, this makes the resonance occur easy.The voltage on the sem iconductor decreases to zero w hen the driving signal occurs,this makes the ZVS realize.T he sw itching characteristic w aveform of IGBT on the shifted leg of the converter is show n in Fig.3b.T he sw itch of the shifted leg achieves ZVS,this is not different from the traditional FB-ZVS-PWM converter.The power transferring w aveform of the high-frequency transformer of the converter is shown in Fig.3c.It is only the leakage inductor of the transformer that prevents the transiting of the current during the interval of the primary current transition.Because of the secondary saturable inductor there is still the loss of the duty during the interval of the current transition of the secondary transformer.Fig.3 Simulated waveform s of converterThe saturable inductor acts only as a magnetic sw itch,so there is little inductor energy stored in it, thus there is no need for high value of VS.As long as the value of VS does not saturate w hen it suffer the voltage of the resonant capacitance during the interval of the clamped freewheeling,the duty cycle loss of the converter is verylow.Fig.4 M easurement wavef orms of the converter4 ExperimentIn order to meet the need of the w elding process, w e design a resonant phase-shifted PWM controlling circuit.The main function of this circuit is to provide PWM sig nal for the m ain circuit to control the turn on and turn off of the pow er semiconductor.T he needed static characteristic and dy namic characteristic of the pow er source is achieved by closed loop controlling of the voltag e or current.A special phase-shifted resonant controller UC3875made by Unitrode is adopted in the controlling circuit of the power source in this paper[4].The current is sampled by LEM,the EXB841is used in the driv ing circuit,w hich is the special driving circuit of IGBT,and the average current mode controltechnology is used in the control circuit.Theexperimental study is done on the FB-ZVS-PWM converter using output filter inductor as resonant inductor.In the end,a sample machine of400A constant current arc welder is developed.T he waveforms of the driving signal,voltage,current of the static leg and shifted leg are show n in Fig.4a and Fig.4b.The w aveform of the voltage,current of the transformer secondary and the w aveform of rectify voltage of the transformer secondary are show n in Fig.4c.The pow er semiconductor of the converter turns on under zero voltage,but the turn off of the pow er semiconductor is hard turn off.The duty loss of the converter is very low, but there is no evidential decrease of the circulating current during the interval of the clam p freew heeling. T he characteristic of the circuit accords w ith the simulation results show n in Fig.3.T wo saturable inductors as magnetic sw itches are added to secondary coil,so the output filter inductor is always reflected to primary.Full ZVS is achieved under low er load current.The load range of the ZVS transition is very w ide.In the actual circuit,the full ZVS is achieved w hen the output current does not reach20A, moreover,there is less loss of duty cy cle and parasitic resonance of the transformer secondary coil.Because there is enough energy of the power semiconductor to realize the ZVS,one capacitance could be paralleled on the output of the power semiconductor,so as to decrease the increasing ratio of the voltag e on the semiconductor and decrease the loss of the turn off.About the loss of the magnetic core,because the saturable inductors LR1and LR2work only in the first quadrant,the loss of the heat in the magnetic core decreased greatly.5 C onclusionIn the FB-ZVS-PWM converter,two saturable inductor are put in the secondary coil of the high frequency transformer w hich acts as magnetic sw itch, the output filter inductor is alw ays reflected to primary, full ZVS is achieved under low er load current of the converter.T his characteristic specially accords w ith the arc welding pow er source that the load situation is variable continuously.Because the saturable inductor could work as magnetic switch,there is no need to store much energy, this makes the loss of the duty of the converter is very low,but the transferring efficiency increases greatly.At the same time,the saturable inductor w orks only in the first quadrant,so,the loss of magnetic lag decreases greatly.References[1] Sabate J A,et al.Hig h-voltage,high-pow er,ZV S,ful-lbridge PWM conv er ter emplo ying an active snubber.I EEEA pplied Power Electronics Confer ence and Ex hibition,1991,158~163[2] Chen Shujun.Soft switching converter and harmonicelimination for arc welding power source:[Doctor thesis].Har bin:Harbin Institute of T echnology,1999.[3] Hua Guichao,et al.A n improved ful-l bridge zero-voltage-switched PW M converter using a saturable inductor.I EEE P ower Electronics Specialists Conference,1993,8(4):530 ~534[4] Bob M ammano.Phase-shifted PW M control-A newintegr ated co ntroller eases the efficient hig h-frequencybridge po wer sw itching.IEEE A pplied Pow er ElectronicsConference and Ex hibition,1991,530~534。