This preview has intentionally blurred parts. Sign up to view the full document

View Full Document

Unformatted Document Excerpt

HIGH MITSUBISHI POWER SEMICONDUCTORS MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS Gate turn-off (GTO) thyristors are able to not only turn on the main current but also turn it off, provided with a gate drive circuit. Unlike conventional thyristors, they have no commutation circuit, downsizing application systems while improving efficiency. They are the most suitable for high-current, highspeed switching applications, such as inverters and chopper circuits. 1. GTO thyristor operation principles A GTO thyristor consists of four layers, pnpn, as like conventional thyristors. Functions except for turn-off are the same as 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 to make the gate negative in respect to the cathode, part of holes 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 extracted through 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 is turned off. Fig. 1 illustrates the turn-off operation, using a two-transistor model. Gate Cathode IGQ Ik n p J3 Tr1 J2 n Tr2 J1 p Suppose that a GTO thyristor is divided into npn transistor Tr1 on the cathode side and pnp transistor Tr2 on the anode side, and that they are connected as shown in Fig. 1(b). In this figure, the current amplification factor of transistor Tr1 is called 1, and that of transistor Tr2, 2. If reverse current IGQ flows 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 IGQ On the other hand, electron current IRB, which disappears due to the recombination in the Tr1 base layer, can be expressed as follows: IRB = (1 1) IK The relationship between GTO thyristor anode current (IA) and cathode current (IK) is expressed by the following equation: IA = IK + IGQ To turn off the GTO thyristor, IB must be smaller than IRB. The magnitude of the reverse-bias current IGQ that satisfies this condition can be calculated by the following equation: IGQ = (1 + 2 1) IA/1 As can be seen from what has been discussed, it is possible in theory that a GTO thyristor can carry out the turn-off if an adequate magnitude of reverse bias current is supplied to the gate. Actually, however sheet resistance exists in the Tr1 base region, making it difficult to turn off the on state current flowing at the emitter junction that is far from the gate. To minimize the resistance, GTO thyristors for high power applications are finely patterned: unit constructions as illustrated in Fig. 1 are placed in parallel with one another in whole silicon area. (See Fig. 2.) IA Anode G K (a) Base unit Cathode (K) Tr1 Gate (G) Ik IB n n n n n n n 1 IGQ p 1Ik n 2 Tr IA p Anode (A) (b) Two-transistor model equivalent circuit Fig. 1 GTO Transistor Operation Principle A Fig. 2 GTO Thyristor Fine Pattern Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS 2. GTO thyristor: types and structure The GTO thyristor unit construction is as shown in Fig. 1. Mitsubishi has two types of GTO thyristors, from which a more suitable one should be selected for a given application. (2) Reverse conducting GTO thyristor The structure is as shown in Fig. 4 below. (1) Anode short GTO thyristor The structure is as shown in Fig. 3 below: K G G K J3 nE J3 nE nE PB J2 nE PB J2 nB nB n+ J1 n+ J1 PE n+ PE n+ PE n+ PE n+ PE n+ PE n+ PE n+ PE n+ A A Diode part GTO thyristor part Fig. 3 Anode short GTO Thyristor Structure Fig. 4 Reverse conducting GTO Thyristor At the J1 junction, the anode is partially shorted due to the n+ layers, so that the reverse voltage of the GTO thyristor is as small as that of the J3 junction (around 15V normally). However, excess carriers are extracted from the gate and from the n+ layer during the turn-off, enabling high-speed switching. This type of thyristor is suitable for applications that require high-speed switching but do not need high reverse voltage, such as voltage source inverters. This product consists of a fast recovery diode part and the anode short GTO thyristor part, the former of which is connected in parallel to the latter. The thyristor is the same type as the one described in (1) above. This product is suitable for application to voltage source inverters for example, where a GTO thyristor requires Flywheel diode. No additional diode is necessary if this GTO thyristor is used, reducing the system size and weight. Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS 3. GTO thyristor operating waveform and definition of each parameter Fig. 5 gives the GTO thyristor on-off switching waveforms, along with the definition of the parameters in each waveform. 0.9VD 0.9IT dv/dt VD ITM VDM IT 0.1VD VD VDSP Anode: Voltage and 0 current Time td ts tgt tgq tGW IGM IG 0.9IGM Gate: Voltage and 0 current 0.1IGM VRB Time 0.1IGQM diG/dt tw IGQM diGQ/dt VRG 0.5IGQM tAV Fig. 5 GTO Thyristor Operation Waveforms and the Definition of Each Parameter The anode voltage and current waveforms and gate voltage and 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 to the IG level, on which the current is retained for the time being. The difference between GTO thyristors and conventional thyristors is that, with the former, the current stays on the IG level as long as on state current flows. To carry out the turn-off, the gate drive circuit should be able to supply gate current whose increasing rate (d iGQ/dt) is higher than specified. The circuit should have also enough capability to increase the current higher than IGQM easily. On the other hand, as the GTO thyristor is turned off, anode current is shifted to the snubber circuit, generating spike voltage VDSP. 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 to carry out the turn-off. When GTO is turned off, anode voltage increases at a constant rate of dv/dt (I/C). When the voltage reaches peak voltage VDM, it lowers to the main circuit sup- ply voltage level. As can be seen from the anode current waveform, the anode current is abruptly decreased after storage period ts. However, after the turn-off period (tgq), tail current flows until excess carriers are completely diminished in the inside of the silicon (tail period). The gate voltage drops suddenly after the ts period, and eventually becomes equal to the gate circuit power supply voltage, passing through the avalanche period (tAV), which occurs due to the gate circuit inductance. The gate reverse bias time (tgw) is required for the GTO thyristor to turn off anode current. During the tgw period, the gate circuit 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 the silicon. Note that, if the gate circuit impedance is not adequately low, gate current generated by excess carriers in the silicon reduce the reverse gate bias voltage. As a result, if forward bias between G and K occurs, it causes a turn-off failure and the distruction of the device. Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS 4. GTO thyristor instructions (1) Ratings and selection of the device Peak repetitive off state voltage VDRM Voltage must not exceed the VDRM level. Considering the largest applicable voltage plus an adequate margin, according to the operating conditions, determine off state voltage to be appropriate, and select an adequate device. Peak repetitive reverse voltage VRRM The anode short GTO thyristor has a peak repetitive reverse voltage of about 17 19 V. Connect a diode by aniti-parallel connection so that reverse voltage will not be applied to the GTO thyristor. If the reverse conducting GTO thyristor is used, this instruction shall not be applied because a diode is already connected by anti-parallel connection and the magnitude of reverse voltage is dependent on the diode characteristics. Repetitive controllable on state current ITQRM GTO thyristor cannot turn off the current higher than the specified 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 average on state current that is determined under the condition of 60-Hz single-phase half waves at the specified fin temperature. About GTO thyristor, IT(AV) is usually about one-third of ITQRM. Select the device in consideration with the current that is supplied continuously and the peak current that is to be turned off. Surge on state current ITSM Surge on state current ITSM can be flowed at a limited number 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 may be scattered, depending on the conditions. (2) Snubber circuit The snubber circuit in a GTO thyristor is almost to be equal to the commutation circuit in conventional thyristors. It must be capable of absorbing voltage fluctuation that occurs when the 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, and the 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 higher than a specified level. The inductance of capacitor must be sufficiently low. The snubber diode has a low forward recovery voltage and low reverse recovery charge. Ds Rs Ls Cs Ds : Snubber diode Cs : Snubber capacitor Rs : Snubber resistance Ls : Snubber inductance Fig. 6 GTO Thyristor Snubber Circuit 1) 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 to snubber 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 following equation, based on the pulse width (tw) of this current waveform. LS = (tw/)2/CS Snubber circuit inductance LS refers to the total inductance of the snubber circuit. It includes the inductance of diode DS, capacitor CS, and wiring. Sw + Cs tw Fig. 7 Snubber Circuit Inductance Measuring Method When 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-off operation. If Vdsp is too large, the turn-off failure occurred and the device is destroyed. Fig. 8 shows the typical dependency of the repetitive controllable on state current and snubber inductance of GTO thyristor by FG3000DV-90DA. Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS Fig. 8 Relationship between Repetitive Controllable ON State Current and Snubber Inductance REPETITIVE CONTROLLABLE ON STATE CURRENT ITQRM (A) 3000 2500 PW = 2000 1500 CONDITIONS VD = 1/2VDRM VDM = 3/4VDRM diGQ/dt = 40A/ms CS = 4.0mF 1000 0.2 0.3 0.4 0.5 0.6 SNUBBER INDUCTANCE LS (mH) Fig. 9 Relationship between Repetitive Controllable ON State Current and Snubber Capacitance 4000 3500 3000 2500 2000 1500 CONDITIONS VD = 2250V VDM = 3375V diGQ/dt = 40A/s LS = 0.3H 1000 500 0 CS f [VD2 + (VDM VD)2 ] 2 4 6 8 10 12 14 According to the obtained value, determine power rating of RS in actual operation to be sufficiently high value. Table 1 shows recommended snubber constants for each GTO thyristor. (3) Diode selection Select a snubber diode and flywheel diode for GTO thyristors to satisfy the following equations: 1 ITQRM (GTO) 10 VDRM (GTO) Snubber diode DS 2) Snubber circuit capacitance is expressed as CS. Anode voltage increasing rate dv/dt is proportional to the value of I/Cs (I: turn off current). Therefore, as CS is decreased, dv/dt is increased, enlarging instantaneous power losses during the turn-off, controllable on state current is decreasesd as a result. To achieve the required controllable on state current, snubber capacitance CS needs to be at least on a specified value. Fig. 9 shows the maximum data on dependance between snubber capacitance and repetitive controllable ON State current by FG3000DV-90DA. 0 1 2 f: Switching frequency 500 0.1 REPETITIVE CONTROLLABLE ON 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 be 5 10. Power losses PW that occur at RS are approximately calculated by the following equation: 16 SNUBBER CAPACITOR CS (F) 3) Snubber resistance is expressed as RS. When R S is large, 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 less than 5. This is because capacitor CS discharges electrically completely 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 remain charge 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 to IF(AV) Flywheel diode DF VRRM = IF(AV) IT(AV) (GTO) VRRM = VDRM (GTO) where, IF(AV): Diode average forward current ITQRM: GTO thyristor maximum turn-off current VRRM: Diode peak reverse voltage VDRM: GTO thyristor peak off state voltage IT(AV): GTO thyristor average on state current Table 1 shows recommended diodes for each GTO thyristor. Fig. 10 shows the relationship between diode forward recovery voltage 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 GTO thyristor drops considerably after the VDM period, as shown in Fig. 11. It should be made certain that reverse voltage is not applied 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 charge Qrr. Table 1 also lists a number of recommended snubber diodes. A suitable one should be determined based on the operating conditions. Flywheel diode DF is often used in a stack, being combined with a GTO thyristor. In this case, the flywheel diode needs to be pressed with the same strength of force as that for the GTO thyristor, therefore, the pressure-mounting force tolerance for flywheel diodes should overlap that for GTO thyristors. Table 1 lists 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 so that the difference in the post diameter can be not effected to each other and that the pressure condition must be uniformed in the whole post of the device. Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS Table 1. Recommended Snubber Constants and Flywheel Diodes for Each GTO Thyristor Snubber Constants GTO construction Type Flywheel Diode CS (F) LS (H) RS () FG1000BV-90DA 0.7 0.3 5 ~10 FG2000FX-50DA 4.0 0.3 5 ~10 FG2000JV-90DA 4.0 0.2 5 ~10 FG3000GX-90DA 3.0 0.25 5 ~10 FD1000FX-90 FG3000DV-90DA 6.0 0.3 5 ~10 FD1000FV-90 FG3300AH-50DA 6.0 0.2 5 ~10 FD1000FH-50 FG4000BX-90DA 3.0 0.25 5 ~10 FG4000CX-90DA 5.0 0.2 5 ~10 FG4000EX-50DA 6.0 0.2 5 ~10 FG4000FX-90DA 3.0 0.25 5 ~10 FD1000FX-90 FG6000AU-120D 6.0 0.2 5 ~10 FD1000FX-90 FG4000GX-90DA 4.0 0.2 5 ~10 FD2000DU-120 FGR3000FX-90DA 6.0 0.2 5 ~10 FGR3000CV-90DA 3.5 0.2 5 ~10 FD1000FH-50* Anode short Type Reverse conducting Type FORWARD RECOVERY VOLTAGE VFP (V) Fig. 10 Relationship between Forward Recovery Voltage and Increasing rate of Forward Current 1. FD1000FV-90 2. FD1500AV-90* 1. FD1000FV-90 2. FD1500AV-90 1. FD1000FH-50* 2. FD1500AV-90 Fig. 11 Anode-Cathode Voltage Turn off Waveform in Case of Different Qrr Value of Snubber Diode 100 Tj = 25C 75 VAK 50 VDM FD1000FV-90 FD1500AV-90 25 0 Qrr : small VD FD1000FH-56 0 500 1000 1500 Qrr : large 2000 INCREASING RATE OF FORWARD CURRENT di/dt (A /S) Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS OFF gate current diGQ/dt: Increasing rate of turn off gate current (10%~50%) tav: Gate avalanche period Because of LgIg2 energy by turn-off gate current Ig and gate 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 drops after the peak of turn-off gate current as shown in Fig. 13. Therefore, tav must be longer than the value shown above. However, if tav is too long, the period during which avalanche current flows is increased, and also rms gate current is increased. Therefore, adjust tav must not be exceeded higher than 30ms. VGR:Turn-off gate voltage After tav in the turn-off period, voltage VGR is applied between gate and cathode in a steady state. About GTO thyristor for turn-off, VGR is needed to be high. However, it should be no more than peak gate reverse voltage VGRM. In consideration with gate voltage fluctuation, set VGR to be the highest possible value but not to exceed VGRM. VRB: Steady-state bias voltage To retain the GTO thyristor in an off state, reverse bias voltage of no less than 2V but no more than VGRM should be applied between gate and cathode. tGW: Gate reverse bias time During the off-gate pulse period (tGW), it is necessary to apply VGR at adequately low impedance, because the tail current must sufficiently flow during the disappering of 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 the GTO thyristor is retained to be in an off state. Fig. 12 Relationship between Gate Trigger Current and Junction Temperature GATE TRIGGER CURRENT IGT (mA) (4) GTO thyristor gate drive 1) Fig. 5 gives typical gate drive waveforms. Each parameter used in the figure is as defined below: On-gate current IGM: High on gate current dig/dt: Increasing rate of on gate current (10% and 90%) tw: High on gate current pulse width Pulse width tw refers the width from 10% of initiation of gate current to the high gate current lowered to a specified level. It is usually recommended to set tw to be two times of the turn-on time. IG: Steady-state on gate current Steady-state on gate current that is supplied during the GTO thyristor on period should be no less than gate trigger current IGT. Set IGT considering that it is dependent on the junction temperature, as can be seen in Fig. 12. 8000 VD = 24V RL = 0.1 DC METHOD 7000 6000 5000 4000 3000 2000 1000 0 60 20 20 60 100 140 JUNCTION TEMPERATURE Tj (C) Fig. 13 Gate Current and Voltage Waveforms in case of Different tav (GTO Thyristor Ratings: 2 kA, 4.5 kV) vgk ig tav When tav = 20s vgk ig tav When tav = 2s ig : 100A/div. vgk : 5V/div. t : 5s/div. Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS Table 2 Recommended Gate Drive Conditions for GTO Thyristors (Tj 0C) IGM dig/dt tw IG (note) diGQ/dt tav VGR VRB NOTE tGW A A/S S A A/S S V V S Model FG1000BV-90DA MIN TYP MAX 20 40 15 20 3.8 30 60 15 15 17 2 17 150 200 FG2000FX-50DA MIN TYP MAX 30 50 10 20 3.8 30 60 20 15 17 2 17 150 200 FG2000JV-90DA MIN TYP MAX 30 50 10 20 4.5 30 60 20 15 17 2 17 150 200 FG3000GX-90DA MIN TYP MAX 25 50 20 12 3.8 40 60 20 15 17 2 17 150 200 FG3000DV-90DA MIN TYP MAX 40 100 10 20 6.0 40 60 20 15 17 2 17 150 200 FG3300AH-50DA MIN TYP MAX 40 100 10 20 6.0 40 60 20 15 17 2 17 150 200 FG4000BX-90DA MIN TYP MAX 25 50 20 12 4.8 40 60 20 17 19 2 19 150 200 FG4000CX-90DA MIN TYP MAX 40 100 20 20 6.0 50 70 20 17 19 2 19 150 200 FG4000EX-50DA MIN TYP MAX 50 100 30 20 7.5 50 70 20 15 17 2 17 150 200 FG4000FX-90DA MIN TYP MAX 25 50 20 12 4.8 40 60 20 17 19 2 19 150 200 FG4000GX-90DA MIN TYP MAX 25 50 20 12 3.8 40 60 20 15 17 2 17 150 200 FG6000AU-120D MIN TYP MAX 90 150 30 12 13 80 100 20 20 22 2 22 150 200 FGR3000FX-90DA MIN TYP MAX 75 100 30 20 4.5 30 60 20 16 18 2 18 150 200 FGR3000CV-90DA MIN TYP MAX 40 100 20 20 4.5 40 60 20 16 18 2 18 150 200 50 NOTE: Conditions: VD = 1 VDRM, gate circuit conditions are as follows. 2 These are the standard values. In order to decide more detailed value, the fluctuation of VD etc. must be considered. 17V Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS 2) 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 low on resistance. Impedance through thick lines in the figure must keep to be low value. The turn-off gate current is strongly dependent on the GTO thyristor turn-off characteristics. Current capability of drive circuit needs to be sufficiently high to completely extract excess carriers in the GTO thyristor. We recommend current capability of the gate driver to be achieved as shown below: Fig. 15 Gate Current Waveform Short-circuited Gate Driver 10% diG/dt IGR 90% IGR 1.2 IGQ (IGQ: Peak gate current that is necessary to turn off the maximum current) diG/dt diGQ/dt (diGQ/dt: Specified increasing rate of turn-off gate current) The measurement is determined using the current waveform obtained by connecting the gate lead to the gate drive circuit, and short-circuiting both ends of the gate lead terminal (gate and cathode terminal). (See Fig. 15.) Output circuit +12V + + Input signal circuit GTO thyristor +5V + Interface circuit G 0 Gate 0V K Cathode Current flow route in an OFF state 0 V Cathode Gate FET 17 V FET(Note) + + 17V (Note) Connect a number of MOSFETs with low ON resistance. Fig. 14 GTO Thyristor Gate Drive Circuit Example (Block Diagram) Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS (5) GTO thyristor power loss Fig. 16 shows power loss generated in GTO thyristor operation. Among these losses, the off-state loss is small and negligible. ,,,,, ,, Anode voltage Anode current Anode voltage and current Time Off-state loss Turn-on loss Steady-state loss Turn-off loss Off-state loss Fig. 16 GTO Thyristor Loss Location (the Shaded Portion) When GTO thyristor is used at high frequency, switching losses during the turn-on operation and turn-off operation should be taken into account, along with steady-state losses that are determined with on state voltage and on state current. The relationship between the switching loss per pulse and 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 is used as the parameter. For the turn-off loss, snubber capacitance Cs is used as the parameter. Switching losses can be calculated by multiplying these value by the switching frequency. Fig. 17 shows the typical data about FG3000DV switching losses. Fig. 17 FG3000DV Switching Loss (Representative Data) (a) Turn-on switching loss (maximum value) (b) Turn-off switching loss (maximum value) 10 VD = 2250V 7 IGM = 40A diG/dt = 10A/s CS = 6.0F 6 R S = 5 Tj = 125C SWITCHING LOSS Eoff (J/P) SWITCHING LOSS Eon (J/P) 8 5 diT/dt = 500A /s 4 300A /s 3 100A /s 2 1 0 9 CS = 3.0F 8 6 5 4 VD = 2250V VDM = 3375V diGQ/dt = 40A/s VRG = 17V LS = 0.3H Tj = 125C 3 2 1 0 1000 2000 3000 4000 TURN-ON CURRENT (A) 5000 4.0F 6.0F 7 0 0 1000 2000 3000 4000 5000 TURN-OFF CURRENT (A) Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS For example, the average of total losses (PT) can be obtained as follows. (See Fig. 18.) ton + toff = 5ms Frequency = 200Hz duty = ton = 0.3 ton + toff Snubber conditions: CS = 6F, RS = 5, LS = 0.3H Main circuit conditions; IT = 800 A di/dt = 300A/s VD = 2250V VDM = 3375V A PT = IT VT (I = IT) 0.3 + (Eon + Eoff) f = 800A 2.35V 0.3 + (1.55 + 3.0) 200Hz = 1474W In actual operation, transient change of junction temperature must also be considered. It should be determined by more precise calculation in consideration of losses in each on period and transient thermal resistance values. Frequency f = 1/(ton + toff) di /dt IT ton VDM VD toff Fig. 18 GTO Thyristor Operation Waveform Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS (6) Long term DC stability voltage 1) Outline High-power semiconductors, such as GTO thyristors, have generally been used at a DC voltage that is no more than half of the blocking voltage rating. With recent advance in application technologies, however, supplied voltage to the semiconductor is increasing. Application with DC voltage that is higher than half of the blocking voltage rating is becoming increasingly popular. When a DC high voltage is applied continuously over a long time, however, extremely powerful cosmic rays may enter the semiconductor device, destroying the device suddenly. This phenomenon is in recent years clarified by the locomotive application in Europe. 2) Device destruction A semiconductor device may be destroyed abruptly, without current leakage increase either at the moment of device destruction or before and after it. Destruction occurs at random in 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. On the other hand, this destruction phenomenon is known to be related to voltage, and the failure rate is exponentially dependent on the applied voltage (electric field strength). 3) Countermeasures against device destruction due to cosmic rays that occur during high DC stability voltage application The GTO thyristor is a high-power semiconductor. By increasing the silicon resistivity, electric field strength can be decreased. Fig. 19 shows outlines of this device construction. 4) Mitsubishi semiconductor device made from silicon with high resistivity Mitsubishi GTO thyristors with an device made from silicon with high resistivity are listed below. They have guaranteed long-term DC stability voltage VLTDS that is as high as 2/3 of the 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 VDC and the failure rate (FIT). VDC value at 100 FIT failure rate is called VLTDS. Conventional GTO thyristors with 4.5kV blocking voltage have VLTDS of 2500V. For LTDS GTO thyristors with same blocking voltage, V LTDS can be increased to 3,000V. Maximum field strength E (field strength) NE Cathode PB NB PE Anode xj (depletion layer) Note 1) Dotted line: Field strength of conventional device 2) Solid line: Field strength of device made from silicon with high resistivity Fig. 19 Outline of an Device with Improved Long-term DC Stability Voltage 106 FAILURE RATE (fit) CONVENTIONAL DIODE & GTO 105 104 LONG TERM DC STABILITY DIODE & GTO FD1000FX-90 FG3000GX-90DA FG4000FX-90DA FG4000GX-90DA FGR3000FX-90DA 103 102 2500 3000 3500 4000 (7) GTO thyristor pressure mounting In a GTO thyristor, each segment is independent each other. Functions such as turn-off are ensured by press of the whole device 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 thyristor design the stack structure carefully so that the difference in the pressure contact diameter will be absorbed when they are pressed together. 4500 DC VOLTAGE VDC (V) Fig. 20 Relationship between DC Voltage and Failure Rate Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS 5. GTO thyristor applications Mitsubishi GTO thyristors have the excellent features as the power switching device. They are suitable for use as a main control device in inverters and choppers. Compared with conventional thyristors, GTO thyristors have the following advantages: (1) With excellent switching characteristics, GTO thyristors help improving high frequency characteristics and efficiency. (2) No commutation circuit is necessary, helping to reduce the system size and weight. PWM converter unit R-phase S-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 (VVVF invertors), DC drive (DC choppers), AC stabilizing power supplies (CVCF), and DC circuit breakers. Fig. 21 shows an example 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 at variable speeds with a 3-phase AC power supply. Filter circuit PWM inverter unit U-phase T-phase DC reactor GU V-phase GV W-phase GW 3-phase induction motor 3-phase AC power supply M Capacitor GX GY GZ Fig. 21 Main Circuit of PWM Converter/Inverter System (1) PWM converter Regenerative operation is available by controlling GTO thyristor. High-efficiency operation is available by controlling power factor 1. (2) Filter circuit A filter circuit is constructed from an inverted L-type circuit consisting of a DC reactor and capacitor. It controls sixfold frequency ripples that are generated in the PWM converter and ripple current that comes from the PWM inverter. The DC reactor is not always used. (3) PWM inverter Three-phase AC with variable voltage and frequency is generated 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-level converters and three-level inverters. Fig. 23 shows an example of application to a three-level converter/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 wave U-phase modulation (Sinusoidal waveform and triangular wave comparison method) V-phase W-phase ON GU OFF OFF GX ON GV OFF OFF GY ON GW ON ON OFF OFF GZ ON Line voltage U-V Line voltage V-W Line voltage W-U Phase voltage U Phase voltage V Phase voltage W Fig. 22 Inverter Output Voltage Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS Three-level converter Three-level inverter Three-phase induction motor Three-phase AC supply M VC Fig. 23 Main Circuit of Three-level Converter/Inverter System 6 Phase switching state GT1 GT2 GT3 GT4 Phase voltage ON ON OFF OFF VC 2 4 3 6 Mode OFF ON ON OFF VC/2 1 OFF OFF ON ON 0 5 Phase voltage VC VC/2 0 1 21 4 3 4 34 3 3 1 2 53 6 1 6 16 1 1 3 4 5 GT1 GT2 GT3 GT4 Fig. 24 Three-level Inverter Operation Mode Aug.1998 MITSUBISHI HIGH POWER SEMICONDUCTORS FEATURE AND APPLICATION OF GATE TURN-OFF THYRISTORS (a) Two-level output voltage waveform (b) Three-level output voltage waveform Fig. 25 Two-level & Three-level Inverter Output Voltage Waveform Aug.1998 ... View Full Document


End of Preview

Sign up now to access the rest of the document