Exercise 6-2 The IGBT EXERCISE OBJECTIVES At the completion of this exercise, you will know the behaviour of the IGBT during switching operation. You will be able to explain how IGBT switching can be improved. You will also be able to visualise the difference between MOSFET and IGBT switching, principally at turn-off. DISCUSSION The IGBT, Insulated Gate Bipolar Transistor, is a switching transistor with a device operation and structure similar to that of an Insulated Gate Field Effect Transistor, more commonly known as a MOSFET. As MOSFETs, IGBTs are voltage controlled devices, they only require voltage on the gate to maintain conduction through the device. IGBTs have higher current densities than comparable bipolar transistors, while at the same time having simpler gate-drive requirements than the familiar power MOSFET. IGBTs are manufactured in voltage and current ratings extending well beyond what is normally available in power MOSFETs. For exemple, at the high power end, devices with a voltage rating of 1200 V and current rating of 600 A are available. 6-27
In general, the IGBT offers clear advantages in high voltage (>300 V), high current (1-3 A/mm 2 of active area), and medium speed (to 10-20 KHz). The circuit symbol generally used for the IGBT is shown in Figure a). It is similar to that of an npn bipolar transistor with an insulated gate terminal in place of the base. It is also possible to encounter the other symbol presented in Figure b). The equivalent circuit of the IGBT can be depicted quite accurately by a pnp bipolar transistor, where the base current is controlled by a MOSFET and limited by a variable base resistor. The conductivity of the base resistor is increased (modulated) when the IGBT is turned on. 6-28
It is possible to enhance the IGBT model by a more complex equivalent circuit. The IGBT consists of a pnp bipolar transistor driven by an n-channel MOSFET in a pseudo-darlington configuration. The JFET supports most of the voltage and allows the MOSFET to be a low voltage type, and consequently have a low R DS(on). As it is apparent from the equivalent circuit, the voltage drop V CE(on) across the IGBT is the sum of two components: a diode drop across the p-n junction and a voltage drop across the driving MOSFET. 6-29
Since the IGBT is a MOS gate device, it has three characteristic capacitances C ies, C oes, and C res. These capacitances are specified in the data sheet because they are most readily measured. They can be used to determine the IGBT junction capacitances C CG, C GE and C CE. C ies = C CG + C GE (in parallel) C oes = C CG + C CE (in parallel) C res = C CG The switching speeds of IGBTs are higher than those of bipolar power transistors. The switching performance at turn-on is very similar to that of power MOSFETs, but turn-off times are longer. Therefore, the maximum switching frequencies with IGBTs fall between those of bipolar power transistors and power MOSFETs. 6-30
During turn-off, the initial fall in current is steep, similar to that of the power MOSFET. But this is followed by a long "tail" during which the decay takes place relatively slowly. Typically, the tail starts around 25% of the on-state current. During the tail, the IGBT supports the load voltage while the tail current is flowing. This causes increased switching power loss, and therefore limits the switching frequency. 6-31
Like for a MOSFET, selecting the proper series gate resistor R G for IGBT gate drive is very important. The value of the gate resistor has a significant impact on the dynamic performance of the IGBT. The IGBT is turned on and off by charging and discharging the gate capacitance. A smaller gate resistor will charge/discharge the gate capacitance faster, thus reducing, principally at turn-on, the switching times and switching losses. 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. 6-32
* 2. Set up the circuit shown in the figure. Note: The oscilloscope must be isolated from ground to allow correct signal observation. * 3. Turn on the power supply and the square wave generator. Adjust the generator frequency to 20 khz. CAUTION! The load resistors will get very hot. Avoid touching them to prevent burn injury. 6-33
* 4. Adjust the oscilloscope to observe the gate-emitter voltage V GE and the collector-emitter voltage V CE during IGBT turn-on. Set the time base to 500 ns/div or less. * 5. You should now observe signals similar to these ones. They are similar to those obtained with the power MOSFET. * 6. In the DRIVER (DR) circuit block, move the jumper so that the switching signal passes from unipolar to bipolar then to unipolar. Repeat this manipulation while observing the oscilloscope. * 7. Do you observe a significant change in the turn-on signal when the switching is controlled with a bipolar signal rather than with a unipolar signal? * Yes * No Make sure the driver circuit is set to provide a bipolar switching signal. * 8. A CM (circuit modification) will now be introduced into the circuit. Observe carefully the signals on the oscilloscope in order to see the modification resulting from this change. Introduce the CM 12 into the circuit. It place a resistor of 47 6 in parallel with the 220 6 resistor R1. This significantly reduces the resistance value and thus, allows an increase in the current charging the IGBT input capacitance. (To again observe the signal variation resulting from CM 12 activation, go back and redo the step.) 6-34
* 9. Do you observe a large decrease in the charging time of the IGBT input capacitance? * Yes * No * 10. Remove the CM 12 from the circuit. In the DRIVER (DR) circuit block, move the jumper so that the switching signal is changed from bipolar to unipolar. * 11. Adjust the oscilloscope to observe the gate-emitter voltage V GE and the collector-emitter voltage V CE during IGBT turn-off. * 12. You should now observe signals similar to these ones. * 13. In the DRIVER (DR) circuit block, move the jumper so that the switching signal passes from unipolar to bipolar then to unipolar. Repeat this manipulation while observing the oscilloscope. * 14. Do you observe a significant change in the discharging time when switching is controlled with a bipolar signal rather than with a unipolar signal? * Yes * No Make sure the driver circuit is set to provide a bipolar switching signal. 6-35
* 15. A CM (circuit modification) will now be introduced into the circuit. Observe carefully the signals on the oscilloscope in order to see the modification resulting from this change. Introduce the CM 12 again into the circuit. This CM significantly reduces the resistance value of R1 and thus, allows an increase in the current discharging the IGBT input capacitance. (To again observe the signal variation resulting from CM 12 activation, go back and redo the step.) * 16. Do you observe a decrease in the discharging time of the IGBT input capacitance? * Yes * No * 17. Remove the CM 12 from the circuit. Connect the oscilloscope as shown in the figure. Set channel 2 to reverse mode in order to correctly read the IGBT voltage V CE. Adjust the oscilloscope so you can observe the current I C (measured from the voltage across the resistor R2) and the voltage V CE during IGBT turn-on. Keep the same time base setting. 6-36
* 18. You should now observe two signals similar to these ones. Observe ripples on the current curve. This is noise, generated by the reverse recovery of the free-wheeling diode, and this interferes with the measuring instrument. * 19. Referring to the previous figure, evaluate the turn-on rise time t r of the IGBT. t r = nsec * 20. Adjust the oscilloscope so you can observe the current I C and the voltage V CE during IGBT turn-off. Change the time base setting to 1µsec/div. 6-37
* 21. You should now observe two signals similar to these ones. It is easy to identifythe slow current tail characteristic of IGBT turn-off. * 22. Referring to the previous figure, evaluate the turn-off fall time t f of IGBT. t f = µsec * 23. Remove the oscilloscope from the circuit. * 24. In the LOAD (Z) circuit block, decrease the load by moving the jumper to connect R2 in series with R1. 6-38
* 25. In the DRIVER (DR) circuit block, move the jumper so that the switching signal is changed from bipolar to the dc variable positive source. At the BASE UNIT, turn the POSITIVE SUPPLY knob fully CW (clockwise). * 26. Using the dc voltmeter function of the multimeters, measure the collectoremitter voltage V CE(on) during IGBT conduction. V CE(on) = V * 27. Turn off the power supply and the square wave generator and remove all the connecting wires. CONCLUSION & & & The IGBT input capacitance charging and discharging speed can be modified by increasing or decreasing the value of the driving circuit resistor R1. The IGBT requires a simple driving circuit delivering very low power. This transistor is characterized by a very high power gain. The current tail presence at turn-off limits the IGBT switching frequency. The switching frequencies obtained with IGBTs are generally lower than those obtained with power MOSFETs. 6-39
REVIEW QUESTIONS 1. In general, IGBTs offer clear advantages in a. high voltage, high current, and high speed. b. high voltage, low current, and medium speed. c. low voltage, high current, and medium speed. d. high voltage, high current, and medium speed. 2. The equivalent circuit of the IGBT can be depicted quite accurately by a. a MOSFET and a Darlington transistor. b. a pnp bipolar transistor, a MOSFET, and a variable resistor. c. a npn bipolar transistor, a MOSFET, and a variable resistor. d. None of the above. 3. From the equivalent circuit, the voltage drop V CE(on) across the IGBT is the sum of two components: a. a diode drop across the p-n junction and a voltage drop across the driving MOSFET. b. a diode drop across the p-n junction and a voltage drop across the variable resistance. c. the voltage drop across the variable resistance and the driving MOSFET. d. the voltage drop across the pnp bipolar transistor and the voltage drop across the driving MOSFET. 4. The presence of a slow current tail at turn-off a. increases the falling time t f. b. causes increased switching power loss. c. limits the switching frequency. d. All of the above. 5. Class the following switches in ascending order according to their maximum switching frequencies. a. Power MOSFETs, bipolar power transistor and IGBTs. b. IGBTs, bipolar power transistors and power MOSFETs. c. Bipolar power transistors, IGBTs and power MOSFETs. d. IGBTs, power MOSFETs and bipolar power transistors. 6-40