Exercise 3-3 Basic Operations of GTO Thyristors EXERCISE OBJECTIVES At the completion of this exercise, you will be able to switch on and off the power GTO thyristor using the 0 to 10 V positive power source and the 0 to -10 V negative power source. DISCUSSION As seen previously, the GTO thyristor is the controllable switch that can be used with the highest power level. Considering this fact, it is important to clearly understand its operation. 3-23
To turn on a GTO thyristor, a positive current pulse should circulate from the gate to the cathode (I G ). A current will then begin to flow from the anode to the cathode (I A ), the same as for the diode and the thyristor. To turn off the GTO thyristor, a negative current pulse must be applied to the gate. It is not necessary to maintain the flow of gate current for the GTO thyristor to remain on. However, if the current I A flowing from the anode to the cathode decreases below a certain threshold (holding current), the GTO will turn off. To avoid this situation, which can result in severe complications in a power circuit, gate current must be maintained during the whole period the GTO thyristor must stay on. 3-24
In most power electronics circuits, large reverse voltages cannot be applied to the electronic switch. Numerous transistors can be damaged by a reverse voltage greater than 15 V. The GTO thyristor is the only controllable electronic switch that can support a reverse voltage as large as the forward voltage. For example, some GTO thyristors withstanding a forward voltage of 1000 V can also support a reverse voltage of 1000 V. PROCEDURE * 1. Connect the POWER INPUT terminals of the circuit board to the power supply. Do not turn on the power supply at this time. 3-25
* 2. Set up the circuit shown in the figure. To do so, place a jumper to connect R1, and use a second jumper to short inductor L1. In the DRIVER (DR) circuit block, place a jumper between the positive power source terminal and output A. Then, connect terminal A of the DRIVER (DR) circuit block through an ammeter to terminal A of the GTO THYRISTOR circuit block. In the GTO THYRISTOR circuit block, place a jumper to connect resistor R2 and another one to connect terminal B to the +15 V. Finally, connect terminals B and C of the LOAD (Z) circuit block to terminals B and C of the GTO THYRISTOR circuit block. Note: The oscilloscope must be isolated from ground to allow correct signal observation. 3-26
* 3. On the base unit, turn the positive supply control fully CCW in order to obtain a voltage of 0 V. Then, turn on the power supply. CAUTION! The load resistors will get very hot. Avoid touching them to prevent burn injury. * 4. On the oscilloscope, channel 1 should indicate 0 V, which is the voltage applied to the GTO thyristor gate. What is the voltage between the anode and the cathode of the GTO thyristor as measured on channel 2? V AK = V * 5. Considering the result of step 4, can you say that the GTO thyristor is off and prevents the current (I A ) from flowing? * Yes * No * 6. Turn the positive supply control CW in order to increase the GTO thyristor gate voltage to 10 V. * 7. What is the voltage between the anode and the cathode of the GTO thyristor? V AK(ON) = V * 8. Considering the result of the preceding step, can you say that the GTO thyristor is turned on and allows the flow of current I A? * Yes * No * 9. Using the ammeter, measure the gate current. I G = ma * 10. Using the positive supply control, vary the supply voltage between 0 and 10 V several times, while observing the signals on the scope. 3-27
* 11. Does the GTO thyristor act as a controllable switch that is on when a gate current sufficiently large is applied, and off when no current is applied to the gate? * Yes * No * 12. Turning the positive and negative supply controls fully CW, set the positive and the negative output to +10 V and -10 V respectively. Remove the jumper connecting the positive output (+10 V) to terminal A and connect the negative output (-10 V) to terminal A to apply a negative pulse on the GTO thyristor gate. * 13. What is the voltage on the GTO thyristor terminals as measured on the scope? V D = V * 14. Using the jumper, vary the voltage from +10 V to -10 V several times, in order to alternately apply some positive and negative pulses to the GTO thyristor gate while observing the signals on the scope. * 15. Does the GTO thyristor act as a controllable switch that is on when it receives a positive gate current pulse and off when it receives a negative gate current pulse? * Yes * No 3-28
* 16. Turn off the power supply (don't turn off the base unit) and set up this new circuit. To do so, you should remove the jumper connecting terminal B to the +15 V in the GTO THYRISTOR circuit block and place it to connect terminal B to the 0-10 V supply output. Then, connect channel 1 of the scope to resistor R4 terminals to measure the GTO thyristor anode current. Remove the channel 2 connections. * 17. Make sure the positive supply control is turned fully CW to provide a voltage of +10 V to the positive supply outputs of the DRIVER (DR) and the GTO THYRISTOR circuit blocks. Remove the jumper in the DRIVER (DR) circuit block. Turn on the power supply. Note: In order to protect components, a current limiting device has been included in the circuit board. It is recommended that you complete without delay the rest of the exercise. If the device is activated (current interruption in the circuit), turn off the base unit for at least 30 seconds to allow the protection circuit to reset. * 18. In the DRIVER (DR) circuit block, use the jumper to connect terminal A to the positive supply output; it should cause the GTO thyristor to turn on. A voltage of approximately 0.7 V on channel 1 reveals that current is flowing in the GTO thyristor. Since the value of R4 is 1 6, this voltage indicates that a current of 0.7 A is flowing through the GTO thyristor. 3-29
* 19. In the DRIVER (DR) circuit block, remove the jumper to interrupt the gate current. The voltage on channel 1 should remain constant, indicating that a current is still flowing through the GTO thyristor. * 20. Decrease the voltage of the positive supply until the GTO thyristor turns off. Repeat steps 18 to 20 several times and record the value of current that causes the GTO thyristor to turn off. I A = A * 21. Turn off the power supply and remove all the connecting wires. CONCLUSION & & The GTO thyristor turns on when a positive current pulse is applied to its gate. It turns off when a negative current pulse is applied to its gate. The GTO thyristor can remain on, even when there is no current on its gate. However, when there is no gate current to maintain conduction, it can turn off if the current (I A ) flowing from its anode to its cathode decreases below a given threshold. 3-30
REVIEW QUESTIONS 1. In which situation is the GTO thyristor required? a. For low power applications. b. For applications requiring high switching frequency. c. For very high power applications where other power semiconductors cannot be used. d. None of the above. 2. How is a GTO thyristor turned on? a. By applying a negative voltage to the gate. b. By applying a positive current pulse to the gate. c. By applying a negative current pulse to the gate. d. None of the above. 3. When there is no gate current, a GTO thyristor can stay conductive if a. the current flowing from the anode to the cathode does not decrease below a given threshold. b. the current flowing from the anode to the cathode does not become alternating. c. the voltage across its terminals remains positive. d. None of the above. 4. To control the interruption of the anode current flow in a GTO thyristor a. this current should decrease below the holding current threshold. b. the gate current must be interrupted. c. a negative current pulse must be applied at the gate. d. None of the above. 5. The GTO thyristor is a. the only self-commutated electronic switch that can support a large reverse voltage across its terminals. b. the self-commutated electronic switch that can switch the largest currents and support the highest voltages. c. an electronic switch turned on by a positive current pulse and turned off by a negative current pulse. d. All of the above. 3-31