The Gate Turn-Off Thyristors (GTO) Part 2

Transcription

1 The Gate Turn-Off Thyristors (GTO) Part 2 Static Characteristics On-state Characteristics: In the on-state the GTO operates in a similar manner to the thyristor. If the anode current remains above the holding current level then positive gate drive may be reduced to zero and the GTO will remain in conduction. However, as a result of the turn-off ability of the GTO, it does possess a higher holding current level than the standard thyristor, and in addition, the cathode of the GTO thyristor is subdivided into small finger elements to assist turn-off. Thus, if the GTO thyristor anode current transiently dips below the holding current level, localized regions of the device may turn-off, thus forcing a high anode current back into the GTO at a high rate of rise of anode current after this partial turn-off. This situation could be potentially destructive. It is recommended, therefore, that the positive gate drive is not removed during conduction but is held at a value I G(ON ), where I G(ON ) is greater than the maximum critical trigger current (I GT ) over the expected operating temperature range of the GTO thyristor. Figure 7.5 shows the typical on-state V I characteristics for a 4000 A, 4500 V GTO from Dynex range of GTOs at junction temperatures of 25 C and 125 C. The curves can be approximated to a straight line of the form

2 where V 0 = voltage intercept, models the voltage across the cathode and anode forward biased junctions and R 0 = on state resistance. When average and RMS values of on-state current (I TAV, I TRMS ) are known, then the on-state power dissipation P ON can be determined using V 0 and R 0. That is, Off-state Characteristics: Unlike the standard thyristor, the GTO does not include cathode emitter shorts to prevent non gated turn-on effects due to dv/dt induced forward biased leakage current. In the off-state of the GTO, steps should, therefore, be taken to prevent such potentially dangerous triggering. This can be accomplished by either connecting the recommended value of resistance between the gate and the cathode (R GK ) or by maintaining a small reverse bias on the gate contact (V RG = 2 V). This will prevent the cathode emitter becoming forward biased and therefore sustain the GTO thyristor in the off state. The peak off-state voltage is a function of resistance R GK. This is shown in Fig Under ordinary operating conditions, GTOs are biased with a negative gate voltage of around 15 V supplied from the gate drive unit during the off-state interval. Nevertheless, provision of R GK may be desirable design practice in the event of the gate-drive failure for any reason (R GK 1.5 Ω is recommended for a large GTO). R GK dissipates energy and hence adds to the system losses. Rate of Rise of Off-state Voltage (dv T /dt): The rate of rise of off-state voltage (dv T /dt) depends on the resistance R GK connected between the gate and the cathode and the reverse bias applied between the gate and the cathode. This relationship is shown in Fig. 7.7.

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. 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 I GM, and then lowers to the I G 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 I G 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 (di GQ /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 V DSP. 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 V DM, it lowers to the main circuit supply voltage level. As can be seen from the anode current waveform, the anode current is abruptly decreased after storage period t s. However, after the turn-off period (t gq ), tail current flows until excess carriers are completely diminished in the inside of the silicon (tail period). The gate voltage drops suddenly after the t s period, and eventually becomes equal to the gate circuit power supply voltage, passing through the avalanche period (t AV ), which occurs due to the gate circuit inductance. The gate reverse bias time (t gw ) is required for the GTO thyristor to turn off anode current. During the t gw 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 destruction of the device.

4 GTO thyristor instructions Ratings and selection of the device 1. Peak repetitive off state voltage V DRM Voltage must not exceed the V DRM 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. 2. Peak repetitive reverse voltage V RRM The anode short GTO thyristor has a peak repetitive re-verse voltage of about V. Connect a diode by anti-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. 3. Repetitive controllable on state current I TQRM GTO thyristor cannot turn off the current higher than the specified I TQRM with specified snubber circuit & gate condition. The device may be destroyed if it is tried to turn off current that is higher than I TQRM. 4. Average on state current I T(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, I T(AV) is usually about one-third of I TQRM. Select the device in consideration with the current that is supplied continuously and the peak current that is to be turned off. 5. 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.

5 Applications GTO thyristors find many applications such as in motor drives, induction heating, distribution lines, pulsed power, and Flexible AC transmission systems.

6 Assign # 3 Finals: 1. Explain the reason why it is recommended that the positive gate drive of a GTO is not removed during conduction but is held at a value I G(ON)? 2. Based on figure 7.5 of the handout (GTO part 2), explain the relationship between instantaneous onstate voltage and instantaneous on state current. What happens to the slope of the curve if the temperature of the junction increases? 3. What are the two methods used to prevent undesired turn-on of a GTO due to dv/dt induced forward bias leakage current during its turn-off state? 4. Base on figure 7.8 of the handout (GTO part 2), explain the relationship between gate trigger voltage and junction temperature, and, between gate trigger current and junction temperature. 5. What is the main difference of a GTO and a conventional thyristor in terms of the turn-on characteristic of the gate current?

7 Ans: 1. As a result of the turn-off ability of the GTO, it possess a higher holding current level than a standard thyristor, and in addition, the cathode of the GTO is subdivided into small finger elements to assist turnoff, thus forcing a high anode current back into the GTO at a high rate of rise of anode current after its partial turn-off. This situation could be potentially destructive. It is recommended, therefore, that the positive gate drive is not remove during conduction but is held at a value I G(ON). 2.The relationship between the two quantities is directly proportional. When instantaneous on-state voltage increases, instantaneous on-state current also increases. If one quantity decreases the other also decreases. When the junction temperature increases, the slope decreases. 3. -connecting the recommended value of resistance between the gate and the cathode (R GK ) -by maintaining a small reverse bias on the gate contact (V RG = -2 V) 4. Gate trigger voltage and gate trigger current are both inversely proportional to junction temperature. 5. For conventional thyristor, the gate current stays on a constant level when turned on, while for the conventional thyristor, the gate current first increases up to I GM, and then lowers to the I G level.