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Unformatted text preview: MITSUBISHI HIGH POWER SEMICONDUCTORSMITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORSFEATURE AND APPLICATION OFGATE TURN-OFF THYRISTORSGate turn-off (GTO) thyristors are able to not only turn on themain current but also turn it off, provided with a gate drive circuit. Unlike conventional thyristors, they have no commutation circuit, downsizing application systems while improvingefficiency. They are the most suitable for high-current, highspeed switching applications, such as inverters and choppercircuits.1. GTO thyristor operation principlesA GTO thyristor consists of four layers, pnpn, as like conventional thyristors. Functions except for turn-off are the sameas those of conventional thyristors, therefore, we mainly describe the turn-off operation here.When a GTO thyristor is in the on-stats, the central base regions are filled with holes supplied from the anode and electrons supplied from the cathode. If reverse bias is applied tomake the gate negative in respect to the cathode, part ofholes in the p-base layer are extracted through the gate, suppressing the injection of electrons from the cathode. In response to this suppression, more hole current is extractedthrough the gate, further suppressing the electron injection.In the course of this process, the cathode emitter junction(J3) is put into a reverse-bias state entirely, GTO thyristor isturned off. Fig. 1 illustrates the turn-off operation, using atwo-transistor model.Gate CathodeIGQIknpJ3Tr1J2nTr2J1pSuppose that a GTO thyristor is divided into npn transistorTr1 on the cathode side and pnp transistor Tr2 on the anodeside, and that they are connected as shown in Fig. 1(b). Inthis figure, the current amplification factor of transistor Tr1 iscalled 1, and that of transistor Tr2, 2. If reverse current IGQflows through the gate, base current IB at transistor Tr1 is reduced when IGQ is increased. This relationship can be expressed by the following equation:IB = 2 IA IGQOn the other hand, electron current IRB, which disappearsdue to the recombination in the Tr1 base layer, can be expressed as follows:IRB = (1 1) IKThe relationship between GTO thyristor anode current (IA)and cathode current (IK) is expressed by the following equation:IA = IK + IGQTo turn off the GTO thyristor, IB must be smaller than IRB. Themagnitude of the reverse-bias current IGQ that satisfies thiscondition can be calculated by the following equation:IGQ = (1 + 2 1) IA/1As can be seen from what has been discussed, it is possiblein theory that a GTO thyristor can carry out the turn-off if anadequate magnitude of reverse bias current is supplied tothe gate. Actually, however sheet resistance exists in the Tr1base region, making it difficult to turn off the on state currentflowing at the emitter junction that is far from the gate. Tominimize the resistance, GTO thyristors for high power applications are finely patterned: unit constructions as illustratedin Fig. 1 are placed in parallel with one another in whole silicon area. (See Fig. 2.)IAAnodeGK(a) Base unitCathode (K)Tr1Gate(G)IkIBnnnnnnn1IGQp1Ikn2TrIApAnode (A)(b) Two-transistor model equivalent circuitFig. 1 GTO Transistor Operation PrincipleAFig. 2 GTO Thyristor Fine PatternAug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS2. GTO thyristor: types and structureThe GTO thyristor unit construction is as shown in Fig. 1.Mitsubishi has two types of GTO thyristors, from which amore suitable one should be selected for a given application.(2) Reverse conducting GTO thyristorThe structure is as shown in Fig. 4 below.(1) Anode short GTO thyristorThe structure is as shown in Fig. 3 below:KGGKJ3nEJ3nEnEPBJ2nEPBJ2nBnBn+J1n+J1PE n+ PE n+ PE n+ PE n+PE n+ PE n+ PE n+ PE n+AADiode partGTO thyristor partFig. 3 Anode short GTO Thyristor StructureFig. 4 Reverse conducting GTO ThyristorAt the J1 junction, the anode is partially shorted due to the n+layers, so that the reverse voltage of the GTO thyristor is assmall as that of the J3 junction (around 15V normally). However, excess carriers are extracted from the gate and fromthe n+ layer during the turn-off, enabling high-speed switching.This type of thyristor is suitable for applications that requirehigh-speed switching but do not need high reverse voltage,such as voltage source inverters.This product consists of a fast recovery diode part and theanode short GTO thyristor part, the former of which is connected in parallel to the latter. The thyristor is the same typeas the one described in (1) above. This product is suitable forapplication to voltage source inverters for example, where aGTO thyristor requires Flywheel diode. No additional diode isnecessary if this GTO thyristor is used, reducing the systemsize and weight.Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS3. GTO thyristor operating waveform and definition ofeach parameterFig. 5 gives the GTO thyristor on-off switching waveforms,along with the definition of the parameters in each waveform.0.9VD0.9ITdv/dtVDITMVDMIT0.1VDVDVDSPAnode:Voltage and 0currentTimetdtstgttgqtGWIGMIG0.9IGMGate:Voltage and 0current0.1IGMVRB Time0.1IGQMdiG/dttwIGQMdiGQ/dtVRG0.5IGQMtAVFig. 5 GTO Thyristor Operation Waveforms and the Definition of Each ParameterThe anode voltage and current waveforms and gate voltageand current waveforms are the same between GTO thyristors and conventional thyristors during the turn-on operation.The gate current first increases up to IGM, and then lowers tothe IG level, on which the current is retained for the time being. The difference between GTO thyristors and conventionalthyristors is that, with the former, the current stays on the IGlevel as long as on state current flows.To carry out the turn-off, the gate drive circuit should be ableto supply gate current whose increasing rate (d iGQ/dt) ishigher than specified. The circuit should have also enoughcapability to increase the current higher than IGQM easily. Onthe other hand, as the GTO thyristor is turned off, anode current is shifted to the snubber circuit, generating spike voltageVDSP. The magnitude of this voltage is dependent on the increasing current rate (di/dt) and snubber circuit inductance.Note that, if this voltage is high, the GTO thyristor fails tocarry out the turn-off. When GTO is turned off, anode voltageincreases at a constant rate of dv/dt (I/C). When the voltagereaches peak voltage VDM, it lowers to the main circuit sup-ply voltage level. As can be seen from the anode currentwaveform, the anode current is abruptly decreased afterstorage period ts. However, after the turn-off period (tgq), tailcurrent flows until excess carriers are completely diminishedin the inside of the silicon (tail period). The gate voltagedrops suddenly after the ts period, and eventually becomesequal to the gate circuit power supply voltage, passingthrough the avalanche period (tAV), which occurs due to thegate circuit inductance.The gate reverse bias time (tgw) is required for the GTO thyristor to turn off anode current. During the tgw period, the gatecircuit impedance should be retained on a low level and reverse bias should be applied between the gate (G) and cathode (K), to extract excess carriers from the inside of thesilicon. Note that, if the gate circuit impedance is not adequately low, gate current generated by excess carriers inthe silicon reduce the reverse gate bias voltage. As a result,if forward bias between G and K occurs, it causes a turn-offfailure and the distruction of the device.Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS4. GTO thyristor instructions(1) Ratings and selection of the device Peak repetitive off state voltage VDRMVoltage must not exceed the VDRM level. Considering thelargest applicable voltage plus an adequate margin, according to the operating conditions, determine off statevoltage to be appropriate, and select an adequate device. Peak repetitive reverse voltage VRRMThe anode short GTO thyristor has a peak repetitive reverse voltage of about 17 19 V. Connect a diode byaniti-parallel connection so that reverse voltage will notbe applied to the GTO thyristor.If the reverse conducting GTO thyristor is used, this instruction shall not be applied because a diode is alreadyconnected by anti-parallel connection and the magnitudeof reverse voltage is dependent on the diode characteristics. Repetitive controllable on state current ITQRMGTO thyristor cannot turn off the current higher than thespecified ITQRM with specified snubber circuit & gate condition.The device may be destroyed if it is tried to turn off current that is higher than ITQRM. Average on state current IT(AV)This current refers to the maximum conductible averageon state current that is determined under the condition of60-Hz single-phase half waves at the specified fin temperature.About GTO thyristor, IT(AV) is usually about one-third ofITQRM. Select the device in consideration with the currentthat is supplied continuously and the peak current that isto be turned off. Surge on state current ITSMSurge on state current ITSM can be flowed at a limitednumber of times by an accident etc.If excessive current flows, the device may be destroyed.Note that it may be destroyed into pieces and they maybe scattered, depending on the conditions.(2) Snubber circuitThe snubber circuit in a GTO thyristor is almost to be equalto the commutation circuit in conventional thyristors. It mustbe capable of absorbing voltage fluctuation that occurs whenthe GTO thyristor turns off the current. Fig. 6 illustrates a typical snubber circuit that may be used in a GTO thyristor.The snubber circuit must satisfy the following requirements: The circuit can conduct a large amount of current, andthe voltage drop in the circuit must be swfficiently low. The circuit is connected by thick and short wires (as indicated with thick lines in Fig. 6) with a low inductance. The snubber capacitor has a capacitance that is higherthan a specified level. The inductance of capacitor mustbe sufficiently low. The snubber diode has a low forward recovery voltageand low reverse recovery charge.DsRsLsCsDs : Snubber diodeCs : Snubber capacitorRs : Snubber resistanceLs : Snubber inductanceFig. 6 GTO Thyristor Snubber Circuit1) Snubber inductance LS can be obtained as shown in Fig.7. Remove the snubber resistance RS from the circuit. Replace the GTO thyristor with switch SW. Fast switching thyristor etc. should be used as the switch. Apply DC voltage tosnubber capacitor CS. When switch SW is closed, a discharge waveform as shown below is obtained. Snubber circuit inductance L S can be obtained using the followingequation, based on the pulse width (tw) of this current waveform.LS = (tw/)2/CSSnubber circuit inductance LS refers to the total inductance ofthe snubber circuit. It includes the inductance of diode DS,capacitor CS, and wiring.Sw+ CstwFig. 7 Snubber Circuit Inductance Measuring MethodWhen LS is large, spike voltage Vdsp is increased. The magnitude of this voltage is dependent on the di/dt rate of the current, which shifts to the snubber circuit during the turn-offoperation. If Vdsp is too large, the turn-off failure occurred andthe device is destroyed. Fig. 8 shows the typical dependencyof the repetitive controllable on state current and snubber inductance of GTO thyristor by FG3000DV-90DA.Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORSFig. 8 Relationship between Repetitive ControllableON State Current and Snubber InductanceREPETITIVE CONTROLLABLE ONSTATE CURRENT ITQRM (A)30002500PW =20001500CONDITIONSVD = 1/2VDRMVDM = 3/4VDRMdiGQ/dt = 40A/msCS = 4.0mF10000. INDUCTANCE LS (mH)Fig. 9 Relationship between Repetitive ControllableON State Current and Snubber Capacitance400035003000250020001500CONDITIONSVD = 2250VVDM = 3375VdiGQ/dt = 40A/sLS = 0.3H10005000CS f [VD2 + (VDM VD)2 ]2468101214According to the obtained value, determine power rating ofRS in actual operation to be sufficiently high value. Table 1shows recommended snubber constants for each GTO thyristor.(3) Diode selectionSelect a snubber diode and flywheel diode for GTO thyristorsto satisfy the following equations:1ITQRM (GTO)10VDRM (GTO) Snubber diode DS2) Snubber circuit capacitance is expressed as CS. Anodevoltage increasing rate dv/dt is proportional to the value ofI/Cs (I: turn off current). Therefore, as CS is decreased, dv/dtis increased, enlarging instantaneous power losses duringthe turn-off, controllable on state current is decreasesd as aresult. To achieve the required controllable on state current,snubber capacitance CS needs to be at least on a specifiedvalue. Fig. 9 shows the maximum data on dependance between snubber capacitance and repetitive controllable ONState current by FG3000DV-90DA.012f: Switching frequency5000.1REPETITIVE CONTROLLABLEON STATE CURRENT ITQRM (A)turn off failure. If snubber resistance R S is small, CS discharge current cannot be suppressed and becomes large, increasing turn-on losses. We recommend the resistance to be5 10.Power losses PW that occur at RS are approximately calculated by the following equation:16SNUBBER CAPACITOR CS (F)3) Snubber resistance is expressed as RS. When R S islarge, snubber capacitor CS discharge time constant (CSRS)becomes high when the GTO thyristor is turned on. It is required to increase the minimum on time (ton mim).We recommend that the minimum on time should be no lessthan 5. This is because capacitor CS discharges electricallycompletely in a period that is five times time constant .If the GTO thyristor is turned off when ton mim is less than 5,capacitor is not discharged completely, so that the remaincharge voltage of capacitor is applied to the GTO thyristor.When this voltage is large, spike voltage is also increased,and in the worst case, the GTO thyristor is destroyed due toIF(AV) Flywheel diode DFVRRM =IF(AV) IT(AV) (GTO)VRRM = VDRM (GTO)where,IF(AV): Diode average forward currentITQRM: GTO thyristor maximum turn-off currentVRRM: Diode peak reverse voltageVDRM: GTO thyristor peak off state voltageIT(AV): GTO thyristor average on state currentTable 1 shows recommended diodes for each GTO thyristor.Fig. 10 shows the relationship between diode forward recoveryvoltage VFP and increasing rate of forward current di/dt.When snubber diode reverse recovery charge Qrr of the snubber diode is large, anode (A)-cathode (K) voltage of the GTOthyristor drops considerably after the VDM period, as shown inFig. 11. It should be made certain that reverse voltage is notapplied between A and K of the GTO thyristor. To ensure this,the snubber diode should have the characteristics of low forward recovery voltage VFP and low reverse recovery chargeQrr.Table 1 also lists a number of recommended snubber diodes.A suitable one should be determined based on the operatingconditions.Flywheel diode DF is often used in a stack, being combinedwith a GTO thyristor. In this case, the flywheel diode needs tobe pressed with the same strength of force as that for the GTOthyristor, therefore, the pressure-mounting force tolerance forflywheel diodes should overlap that for GTO thyristors. Table 1lists diodes with which a GTO thyristor can be pressed together, along with those that cannot be pressed together(marked *).To combine a GTO thyristor and flywheel diode, pressuremounting diameter is different. Construct the stack carefully sothat the difference in the post diameter can be not effected toeach other and that the pressure condition must be uniformedin the whole post of the device.Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORSTable 1. Recommended Snubber Constants and Flywheel Diodes for Each GTO ThyristorSnubber ConstantsGTO constructionTypeFlywheel DiodeCS (F)LS (H)RS ()FG1000BV-90DA0.70.35 ~10FG2000FX-50DA4.00.35 ~10FG2000JV-90DA4.00.25 ~10FG3000GX-90DA3.00.255 ~10FD1000FX-90FG3000DV-90DA6.00.35 ~10FD1000FV-90FG3300AH-50DA6.00.25 ~10FD1000FH-50FG4000BX-90DA3.00.255 ~10FG4000CX-90DA5.00.25 ~10FG4000EX-50DA6.00.25 ~10FG4000FX-90DA3.00.255 ~10FD1000FX-90FG4000GX-90DA4.00.25 ~10FD1000FX-90FG6000AU-120D6.00.25 ~10FD2000DU-120FGR3000FX-90DA6.00.25 ~10FGR3000CV-90DA3.50.25 ~10FD1000FH-50*Anode short TypeReverseconducting TypeFORWARD RECOVERY VOLTAGE VFP (V)Fig. 10 Relationship between Forward RecoveryVoltage and Increasing rate of ForwardCurrent1. FD1000FV-902. FD1500AV-90*1. FD1000FV-902. FD1500AV-901. FD1000FH-50*2. FD1500AV-90Fig. 11 Anode-Cathode Voltage Turn off Waveform inCase of Different Qrr Value of Snubber Diode100Tj = 25C75VAK50VDMFD1000FV-90FD1500AV-90250Qrr : smallVDFD1000FH-56050010001500Qrr : large2000INCREASING RATE OF FORWARD CURRENT di/dt (A /S)Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS OFF gate currentdiGQ/dt: Increasing rate of turn off gate current (10%~50%)tav: Gate avalanche periodBecause of LgIg2 energy by turn-off gate current Ig andgate circuit inductance Lg, gate-cathode of GTO thyristor is put in an avalanche state. This period is called tav.Set this period referring to the following conditions:When tav is extremely short, gate current sharply dropsafter the peak of turn-off gate current as shown in Fig.13. Therefore, tav must be longer than the value shownabove. However, if tav is too long, the period duringwhich avalanche current flows is increased, and alsorms gate current is increased. Therefore, adjust tav mustnot be exceeded higher than 30ms.VGR:Turn-off gate voltageAfter tav in the turn-off period, voltage VGR is applied between gate and cathode in a steady state. About GTOthyristor for turn-off, VGR is needed to be high. However,it should be no more than peak gate reverse voltageVGRM. In consideration with gate voltage fluctuation, setVGR to be the highest possible value but not to exceedVGRM.VRB: Steady-state bias voltageTo retain the GTO thyristor in an off state, reverse biasvoltage of no less than 2V but no more than VGRMshould be applied between gate and cathode.tGW: Gate reverse bias timeDuring the off-gate pulse period (tGW), it is necessary toapply VGR at adequately low impedance, because thetail current must sufficiently flow during the disapperingof excess carriers inside of the GTO thyristor.If tail current is decreased to be a sufficiently low level,by applying VRB between the gate and cathode theGTO thyristor is retained to be in an off state.Fig. 12 Relationship between Gate Trigger Currentand Junction TemperatureGATE TRIGGER CURRENT IGT (mA)(4) GTO thyristor gate drive1) Fig. 5 gives typical gate drive waveforms. Each parameter used in the figure is as defined below: On-gate currentIGM: High on gate currentdig/dt: Increasing rate of on gate current (10% and 90%)tw: High on gate current pulse widthPulse width tw refers the width from 10% of initiation ofgate current to the high gate current lowered to a specified level. It is usually recommended to set tw to be twotimes of the turn-on time.IG: Steady-state on gate currentSteady-state on gate current that is supplied during theGTO thyristor on period should be no less than gate trigger current IGT. Set IGT considering that it is dependenton the junction temperature, as can be seen in Fig. 12.8000VD = 24VRL = 0.1DC METHOD7000600050004000300020001000060202060100140JUNCTION TEMPERATURE Tj (C)Fig. 13 Gate Current and Voltage Waveforms in caseof Different tav (GTO Thyristor Ratings:2 kA, 4.5 kV)vgkigtavWhen tav = 20svgkigtavWhen tav = 2sig : 100A/div.vgk : 5V/div.t : 5s/div.Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORSTable 2 Recommended Gate Drive Conditions for GTO Thyristors (Tj 0C)IGMdig/dttwIG(note)diGQ/dttavVGRVRBNOTEtGWAA/SSAA/SSVVSModelFG1000BV-90DAMINTYPMAX204015203.83060151517217150200FG2000FX-50DAMINTYPMAX305010203.83060201517217150200FG2000JV-90DAMINTYPMAX305010204.53060201517217150200FG3000GX-90DAMINTYPMAX255020123.84060201517217150200FG3000DV-90DAMINTYPMAX4010010206.04060201517217150200FG3300AH-50DAMINTYPMAX4010010206.04060201517217150200FG4000BX-90DAMINTYPMAX255020124.84060201719219150200FG4000CX-90DAMINTYPMAX4010020206.05070201719219150200FG4000EX-50DAMINTYPMAX5010030207.55070201517217150200FG4000FX-90DAMINTYPMAX255020124.84060201719219150200FG4000GX-90DAMINTYPMAX255020123.84060201517217150200FG6000AU-120DMINTYPMAX9015030121380100202022222150200FGR3000FX-90DAMINTYPMAX7510030204.53060201618218150200FGR3000CV-90DAMINTYPMAX4010020204.5406020161821815020050NOTE: Conditions: VD = 1 VDRM, gate circuit conditions are as follows.2These are the standard values.In order to decide more detailed value, the fluctuation of VD etc. must be considered.17VAug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS2) Fig. 14 shows a typical GTO thyristor gate drive circuit(block diagram).The turn-off gate has a number of metal oxide semiconductor field-effect transistors (MOSFETs) having sufficiently lowon resistance. Impedance through thick lines in the figuremust keep to be low value.The turn-off gate current is strongly dependent on the GTOthyristor turn-off characteristics. Current capability of drivecircuit needs to be sufficiently high to completely extract excess carriers in the GTO thyristor. We recommend currentcapability of the gate driver to be achieved as shown below:Fig. 15 Gate Current Waveform Short-circuited GateDriver10%diG/dtIGR90%IGR 1.2 IGQ (IGQ: Peak gate current that is necessary toturn off the maximum current)diG/dt diGQ/dt (diGQ/dt: Specified increasing rate of turn-offgate current)The measurement is determined using the current waveformobtained by connecting the gate lead to the gate drive circuit,and short-circuiting both ends of the gate lead terminal (gateand cathode terminal). (See Fig. 15.)Output circuit+12V++Input signal circuitGTO thyristor+5V+Interface circuitG0Gate0VK CathodeCurrent flow route in an OFF state0 V Cathode Gate FET 17 VFET(Note)++17V(Note) Connect a number of MOSFETs with low ON resistance.Fig. 14 GTO Thyristor Gate Drive Circuit Example (Block Diagram)Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS(5) GTO thyristor power lossFig. 16 shows power loss generated in GTO thyristor operation. Among these losses, the off-state loss is small and negligible.,,,,,,,Anode voltageAnode currentAnode voltageand currentTimeOff-state loss Turn-on lossSteady-state lossTurn-off lossOff-state lossFig. 16 GTO Thyristor Loss Location (the Shaded Portion)When GTO thyristor is used at high frequency, switchinglosses during the turn-on operation and turn-off operationshould be taken into account, along with steady-state lossesthat are determined with on state voltage and on state current. The relationship between the switching loss per pulseand turn on current is shown in this data book for each type.For the turn-on loss, increasing rate of turn on current di/dt isused as the parameter. For the turn-off loss, snubber capacitance Cs is used as the parameter. Switching losses can becalculated by multiplying these value by the switching frequency. Fig. 17 shows the typical data about FG3000DVswitching losses.Fig. 17 FG3000DV Switching Loss (Representative Data)(a) Turn-on switching loss (maximum value)(b) Turn-off switching loss (maximum value)10VD = 2250V7 IGM = 40AdiG/dt = 10A/sCS = 6.0F6R S = 5Tj = 125CSWITCHING LOSS Eoff (J/P)SWITCHING LOSS Eon (J/P)85diT/dt = 500A /s4300A /s3100A /s2109CS = 3.0F8654VD = 2250VVDM = 3375VdiGQ/dt = 40A/sVRG = 17VLS = 0.3HTj = 125C32101000200030004000TURN-ON CURRENT (A)50004.0F6.0F70010002000300040005000TURN-OFF CURRENT (A)Aug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORSFor example, the average of total losses (PT) can be obtained as follows. (See Fig. 18.)ton + toff = 5msFrequency = 200Hzduty =ton= 0.3ton + toffSnubber conditions: CS = 6F, RS = 5, LS = 0.3HMain circuit conditions; IT = 800 Adi/dt = 300A/sVD = 2250VVDM = 3375VAPT = IT VT (I = IT) 0.3 + (Eon + Eoff) f= 800A 2.35V 0.3 + (1.55 + 3.0) 200Hz= 1474WIn actual operation, transient change of junction temperaturemust also be considered. It should be determined by moreprecise calculation in consideration of losses in each on period and transient thermal resistance values.Frequency f = 1/(ton + toff)di /dtITtonVDMVDtoffFig. 18 GTO Thyristor Operation WaveformAug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS(6) Long term DC stability voltage1) OutlineHigh-power semiconductors, such as GTO thyristors, havegenerally been used at a DC voltage that is no more than halfof the blocking voltage rating. With recent advance in application technologies, however, supplied voltage to the semiconductor is increasing. Application with DC voltage that ishigher than half of the blocking voltage rating is becoming increasingly popular.When a DC high voltage is applied continuously over a longtime, however, extremely powerful cosmic rays may enterthe semiconductor device, destroying the device suddenly.This phenomenon is in recent years clarified by the locomotive application in Europe.2) Device destructionA semiconductor device may be destroyed abruptly, withoutcurrent leakage increase either at the moment of device destruction or before and after it. Destruction occurs at randomin the device, melting down the device on the spot.When the destruction possibility is plotted on a Weibull chart,m equals 1, showing that destruction occurs accidentally. Onthe other hand, this destruction phenomenon is known to berelated to voltage, and the failure rate is exponentially dependent on the applied voltage (electric field strength).3) Countermeasures against device destruction due tocosmic rays that occur during high DC stability voltageapplicationThe GTO thyristor is a high-power semiconductor. By increasing the silicon resistivity, electric field strength can bedecreased. Fig. 19 shows outlines of this device construction.4) Mitsubishi semiconductor device made from siliconwith high resistivityMitsubishi GTO thyristors with an device made from siliconwith high resistivity are listed below. They have guaranteedlong-term DC stability voltage VLTDS that is as high as 2/3 ofthe blocking voltage rating.FD1000FX-90 (high-frequency rectifier diode)FG3000GX-90DA (anode short GTO thyristor)FG4000FX-90DA (anode short GTO thyristor)FG4000GX-90DA (anode short GTO thyristor)FGR3000FX-90DA (reverse conducting GTO thyristor)Note: VLTDS refers to Long Term DC Stability voltage.Fig. 20 shows typical dependency between DC voltage VDCand the failure rate (FIT). VDC value at 100 FIT failure rate iscalled VLTDS. Conventional GTO thyristors with 4.5kV blocking voltage have VLTDS of 2500V. For LTDS GTO thyristorswith same blocking voltage, V LTDS can be increased to3,000V.Maximum field strengthE (field strength)NECathodePBNBPEAnodexj (depletion layer)Note1) Dotted line: Field strength of conventional device2) Solid line: Field strength of device made from silicon with high resistivityFig. 19 Outline of an Device with Improved Long-term DC Stability Voltage106FAILURE RATE (fit)CONVENTIONAL DIODE & GTO105104LONG TERMDC STABILITYDIODE & GTOFD1000FX-90FG3000GX-90DAFG4000FX-90DAFG4000GX-90DAFGR3000FX-90DA1031022500300035004000(7) GTO thyristor pressure mountingIn a GTO thyristor, each segment is independent each other.Functions such as turn-off are ensured by press of the wholedevice surface uniformly. Check the uniformity of the pressure condition of the surface by using pressure-sensitive paper.Also pay attention to the cooling fin contact surface flatness,which is recommended to be within 10mm. To press the device of which diameter are different from the GTO thyristordesign the stack structure carefully so that the difference inthe pressure contact diameter will be absorbed when theyare pressed together.4500DC VOLTAGE VDC (V)Fig. 20 Relationship between DC Voltage and Failure RateAug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS5. GTO thyristor applicationsMitsubishi GTO thyristors have the excellent features as thepower switching device. They are suitable for use as a maincontrol device in inverters and choppers. Compared withconventional thyristors, GTO thyristors have the following advantages:(1) With excellent switching characteristics, GTO thyristorshelp improving high frequency characteristics and efficiency.(2) No commutation circuit is necessary, helping to reducethe system size and weight.PWM converter unitR-phaseS-phase(3) All functions become available with semiconductors, enabling maintenance-free operation.(4) Commutation current does not flow, minimizing the generation of noise and electromagnetic waves.GTO thyristors are used in such devices as AC drive (VVVFinvertors), DC drive (DC choppers), AC stabilizing powersupplies (CVCF), and DC circuit breakers. Fig. 21 shows anexample of applying a GTO thyristor to a pulse width modulation converter/inverter system. In this application, the converter/invertor system drives a 3-phase induction motor atvariable speeds with a 3-phase AC power supply.Filter circuitPWM inverter unitU-phaseT-phaseDC reactorGUV-phaseGVW-phaseGW3-phaseinductionmotor3-phaseAC powersupplyMCapacitorGXGYGZFig. 21 Main Circuit of PWM Converter/Inverter System(1) PWM converterRegenerative operation is available by controlling GTOthyristor. High-efficiency operation is available by controlling power factor 1.(2) Filter circuitA filter circuit is constructed from an inverted L-type circuit consisting of a DC reactor and capacitor. It controlssixfold frequency ripples that are generated in the PWMconverter and ripple current that comes from the PWM inverter.The DC reactor is not always used.(3) PWM inverterThree-phase AC with variable voltage and frequency isgenerated by controlling the GTO thyristor, enabling variable speed operation of the three-phase inductive motor.Fig. 22 shows an inverter output voltage waveform for triangular wave modulation, for example.Recently, GTO thyristors are often used in three-levelconverters and three-level inverters.Fig. 23 shows an example of application to a three-levelconverter/inverter system.A three-level inverter can generate voltage whose waveform is stepped on three levels of 0V, 1/2VC, and VC, diminishing noise and torque ripples.Fig. 25 shows a three-level inverter output voltage waveform.Triangular waveU-phasemodulation(Sinusoidal waveformand triangular wavecomparison method)V-phaseW-phaseONGUOFFOFFGXONGVOFFOFFGYONGWONONOFFOFFGZONLine voltage U-VLine voltage V-WLine voltage W-UPhase voltage UPhase voltage VPhase voltage WFig. 22 Inverter Output VoltageAug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORSThree-level converterThree-level inverterThree-phaseinductionmotorThree-phaseAC supplyMVCFig. 23 Main Circuit of Three-level Converter/Inverter System6Phase switching stateGT1GT2GT3GT4Phase voltageONONOFFOFFVC2436ModeOFFONONOFFVC/21OFFOFFONON05Phase voltageVCVC/20121434343312536161611345GT1GT2GT3GT4Fig. 24 Three-level Inverter Operation ModeAug.1998MITSUBISHI HIGH POWER SEMICONDUCTORSFEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS(a) Two-level output voltage waveform(b) Three-level output voltage waveformFig. 25 Two-level & Three-level Inverter Output Voltage WaveformAug.1998...
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