Switching Time and Conduction Voltage Drop

Transcription

1 Exercise 4-1 EXERCISE OBJECTIVES At the completion of this exercise, you will be able to describe the behavior of a bipolar transistor when it turns on or turns off, and during conduction. DISCUSSION To visualize what is happening in a commutating electronic device, we will study a circuit formed by a bipolar transistor, a load resistor and a fixed dc power supply. 4-5

2 To simplify the study, the resistive load, the power supply and the connecting cable won't be considered as inductive. However, it is important to note that, in practice, these components are inductive and influence circuit behavior. The electronic switches used in power electronics do not exhibit ideal behaviour. 4-6

3 The response time after the application of a turn-on or turn-off pulse is not instantaneous. When a base current is applied to turn on a transistor and there is no current flowing in the transistor and load resistor, the transistor doesn't respond before a certain time. This time interval is called turn-on delay time (t d(on) ). The transistor will then progressively begin to conduct for a few microseconds. The current will increase in the load and the transistor up to a value equal to V CC /R. The time interval during which the current increases to maximum amplitude is called the current rise time t r. 4-7

4 While this current I C increases in the transistor collector, the voltage (V CE ) across the transistor will decrease from V CC to a weak on-state voltage V CE(ON), as shown in the figure. The voltage across the transistor can be approximated from the following equation V CE = V CC - RI C If the base current is interrupted, the transistor does not respond before a certain time called the turn-off delay time (t d(off) ). It will then cease to conduct and the current I C flowing through it progressively decreases for a few microseconds until it stops completely. The figure also shows that voltage V CE increases progressively during the current fall time (t f ). 4-8

5 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. * 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. Using the oscilloscope, adjust the generator frequency to 20 khz. 4-9

6 CAUTION! The load resistors will get very hot. Avoid touching them to prevent burn injury. * 4. Connect the oscilloscope as shown in the figure, making sure that the probe commons are both connected to the collector terminal of the bipolar transistor. Set the oscilloscope controls so that both channels can be observed at the same time. Set channel 2 in the reverse mode so you can correctly read the transistor voltage V CE. Set the time base to 10 µsec/div. * 5. You should now observe the current I C (measured from the voltage across the resistor R4) on channel 1 and the voltage V CE on channel 2. Both signals are being switched at the frequency of the square wave. * 6. Observe the signals when the transistor turns on. Does the current increase in the transistor at the same time the voltage across its terminals decreases from 15 to 0 Volts? * Yes * No * 7. Set the oscilloscope to observe the signals when the transistor is turning on and adjust the time base to observe the rising edge of the signal on channel 1 (less than 1µsec/div). 4-10

7 * 8. When the transistor turns on, can you say that switching is instantaneous, that is, the current rise time t r = 0 sec? * Yes * No * 9. How much time does it take for the current I C to rise to its maximum amplitude when the transistor turns on? Current t r = nsec * 10. Set the oscilloscope so you can observe the signals when the transistor is turning off (V CE increases from 0 to 15 V). * 11. When the transistor turns off, can you say that switching is instantaneous, that is, the current fall time t f = 0 sec? * Yes * No * 12. When the transistor turns off, how much time does it take for the current I C to change from its maximum amplitude to its minimum amplitude? Current t f = nsec * 13. Set the time base to 10 µsec/div again in order to observe transistor turn-on and turn-off. Set channel 2 gain so you can observe the voltage across the transistor when it is conducting. Measure the on-state voltage of the transistor. V CE(ON) = mv 4-11

8 * 14. Place the oscilloscope probes so that you can observe the control signal and the transistor voltage V CE simultaneously. To do so, place the probe commons on the circuit common, place probe 1 on terminal A of the BIPOLAR TRANSISTOR circuit block and probe 2 on the bipolar transistor collector. Restore the non-reverse mode on channel 2. * 15. Set the oscilloscope controls so you can observe the transistor turning on. * 16. Can you say that the transistor responds rapidly (t d(on) < 300 nsec) to the turn-on control signal? * Yes * No * 17. Set the oscilloscope controls so you can observe the transistor turning off. * 18. Measure the time interval between the application of the turn-off control signal and the moment the voltage starts to respond (voltage V CE begins to rise). T d(off) = nsec * 19. Referring to these results, can you say that the bipolar transistor behaves like an ideal switch? * Yes * No 4-12

9 * 20. Turn off the power supply and remove all the connecting wires. CONCLUSION & & & While the bipolar transistor can turn on or turn off rapidly, it doesn't switch instantaneously (t r > 0 sec. and t f > 0 sec.). One can observe a voltage across a bipolar transistor that is in conduction (V CE(ON) > 0 V). There is a delay (t d(off) ) between the application of a turn-off control signal and the moment where the current flow starts to decrease in the bipolar transistor. This delay is smaller when the transistor turns on (t d(on) ). REVIEW QUESTIONS 1. In a resistive circuit, when the bipolar transistor turns on, a. the current I C increases while the voltage V CE decreases. b. the current I C decreases while the voltage V CE increases. c. the current I C increases after which, the voltage V CE decreases. d. the voltage V CE increases after which, the current I C decreases. 2. In a resistive circuit, when the bipolar transistor turns off, a. the current I C increases while the voltage V CE decreases. b. the current I C decreases while the voltage V CE increases. c. the current I C increases after which, the voltage V CE decreases. d. the voltage V CE increases after which, the current I C decreases. 3. Changing the duty cycle a. decreases the current rise time. b. changes the output voltage of a buck chopper. c. changes the input voltage of a buck chopper. d. All of the above. 4. If the base current of a bipolar transistor is interrupted, a. collector current is immediately stopped. b. voltage V CE increases immediately. c. collector current decreases after a delay t d(on). d. collector current decreases after a delay t d(off). 4-13

10 5. The bipolar transistor is not an ideal self-commutated switch because a. it cannot switch instantaneously. b. a voltage remains across its terminals when it is in conduction. c. it doesn't respond immediately when the current is interrupted at its base. d. All of the above. 4-14