Exercise 5-1. The Bipolar Power Transistor EXERCISE OBJECTIVES

Transcription

1 Exercise 5-1 EXERCISE OBJECTIVES At the completion of this exercise, you will be able to identify different ways of improving bipolar transistor switching. You will know the role of the various circuit components that aid in switching. You will be able to describe the relationship between the base current I B intensity and the overload capacity of the bipolar transistor. DISCUSSION As seen in previous exercises, the bipolar transistor can be turned-on or turned-off by applying or interrupting the current at its base. This change of state doesn't occur instantaneously and mainly depends on the intensity and the form of the base current (I B ). 5-7

2 The addition of an RC circuit at the bipolar transistor base allows the creation of positive and negative current peaks that will help accelerate turn-on and turn-off without causing large power dissipation during the conduction phase. With a bipolar, rather than a unipolar control source, higher negative current pulses will be obtained. This increase of negative current peaks will result in faster turn-off switching times. You have seen, in power electronics, that a bipolar transistor is held in conduction by applying a base current sufficiently large to obtain saturation. However, if the base current is increased too much, it will require a bigger power supply for the accelerating circuit and high losses will result in the transistor base. 5-8

3 It is desirable to have a base current as weak as possible, but one which will hold the transistor in saturation all across the load current I C range. In order to allow the transistor to resist overload, the current I C range can fluctuate from 0 A up to about two to five times the nominal current. Therefore, it is necessary to provide an accelerating circuit able to deliver a base current I B of at least two to five times that required to obtain the nominal load current. 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 that the accelerating circuit is not connected to the base of the bipolar transistor. 5-9

4 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. * 4. Adjust oscilloscope channel 1 so you can observe the current I B (measured from the voltage across the resistor R3) at the base of the transistor. Set the time base to 10 µsec/div. 5-10

5 * 5. You should now observe a signal on channel 1 similar to this one. This is the current I B applied at the base to turn-on and turn-off the transistor. * 6. Connect the oscilloscope as shown in the figure. Set channel 2 in reverse mode in order to correctly read the transistor voltage V CE. Adjust the oscilloscope so you can observe the current I C (measured from the voltage across the resistor R4) and the voltage V CE. The time base is still set to 10 µsec/div. 5-11

6 * 7. You should now observe two signals similar to those shown in the figure. * 8. The previous figure allows you to see simultaneously the control signal (current I B ) applied to the transistor and its response (current I C and voltage V CE ). Considering this figure, can you say that the turn-off delay time of the bipolar transistor is large (t d(off) > 0.5 µsec)? * 9. Set the oscilloscope time base to 5 µsec/div. Observe the current I C signal. Are the switching times weak (t r < 0.5 µsec and t f < 1 µsec)? 5-12

7 * 10. Connect again the oscilloscope as shown in the figure and set channel 1 to 2 V/div. In order to improve switching, modify the previous circuit by connecting an RC snubber circuit as shown in the figure. 5-13

8 * 11. You should now observe a signal on channel 1 similar to this one. You can observe that current I B now shows current peaks during its rise and fall. * 12. Connect the oscilloscope as shown in the figure and set channel 1 to 500 mv/div. Observe the current I C and the voltage V CE (channel 2 is still set in reverse mode). 5-14

9 * 13. You should now observe two signals similar to those shown in the figure. * 14. The figure of step 13 allows you to see simultaneously the control signal (current I B ) and its response (current I C and voltage V CE ). Considering this figure, can you say that the presence of the current peaks allowed us to reduce significantly the turn-off delay time of the bipolar transistor? * 15. While observing the oscilloscope, remove the jumper connecting the RC snubber to the base of the transistor then, reconnect it. Repeat this manipulation if necessary. Does the use of the accelerating circuit help to reduce the switching times of the bipolar transistor? 5-15

10 * 16. Connect the oscilloscope as shown in the figure and set channel 1 to 2V/div. * 17. In order to further improve the switching time, modify the DRIVER (DR) circuit block to obtain a bipolar switching signal. * 18. You should now observe a signal on channel 1 similar to this one. * 19. In the DRIVER (DR) circuit block, move the jumper so that the switching signal passes from bipolar to unipolar then to bipolar. Repeat this manipulation while observing the oscilloscope. 5-16

11 * 20. Referring to observations made in step 19, can you say that the use of a bipolar supply allows the creation of higher negative current peaks? * 21. Connect oscilloscope channels 1 and 2 so you can observe the current I C and the voltage V CE, and set channel 1 to 500 mv/div. * 22. You should now observe the signals for current I C and voltage V CE. * 23. In the DRIVER (DR) circuit block, move again the jumper so the switching signal passes from bipolar to unipolar then to bipolar, as you did in step 19. Does a bipolar supply allow a reduction in the turn-off switching time of the transistor? * 24. Make sure the DRIVER (DR) is set to provide a bipolar signal. Set the oscilloscope time base to 500 nsec/div or less, and adjust it to observe the signals when the transistor turns off. Measure the current I C fall time (t f ). t f = nsec 5-17

12 * 25. Adjust the oscilloscope to observe the signals when the transistor turns on. Measure the current I C rise time (t r ). t r = nsec * 26. Connect again the oscilloscope to observe current I B and set the time base to 5 µsec/div. Set channel 1 to 2 V/div and centre its reference in the screen. * 27. A CM (circuit modification) will now be introduced into the circuit. Observe carefully the signal I B on the oscilloscope in order to see the modification that will result from this change. Introduce the CM 1 into the circuit. It reduces by half the value of the RC snubber resistor R1. (To again observe the signal variation resulting from CM 1 activation, go back and redo the step.) Are the current peaks higher? * 28. Remove the CM 1. * 29. A new CM will be introduced into the circuit, observe carefully the response of signal I B. Introduce the CM 3 into the circuit. It doubles the value of the RC snubber capacitor C1. (To again observe the signal variation resulting from CM 3 activation, go back and redo the step.) Are the current peaks wider? * 30. Remove the CM 3. * 31. A new CM will be introduced into the circuit, observe carefully the response of signal I B. Introduce the CM 4 into the circuit. It adds a 1 k6 resistor in series with resistor R

13 (To again observe the signal variation resulting from CM 4 activation, go back to the previous page and redo the step.) (To again observe the signal variation resulting from CM 4 activation, go back and redo the step.) (To again observe the signal variation resulting from CM 4 activation, go back and redo the step.) Does the increase in value of resistor R2 cause an increase of the current I B? * 32. Connect the oscilloscope so you can observe the current I C and the voltage V CE. Set the channel 1 to 500 mv/div and the time base to 10 µsec/div. Note that CM 4 is still activated. * 33. You should now observe signals similar to these. * 34. In the LOAD (Z) circuit block, increase the load by adding a jumper to place resistor R2 in parallel with R1, and another jumper to short-circuit inductor L

14 Note: In order to protect components, a current limiting device has been included in the circuit board. Since you increased the load by reducing the resistance in the LOAD (Z) circuit block, this device may become activated. 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 temporarily the +15 V power supply to allow the protection circuit to reset. * 35. You should observe that there is an alteration of current I C and voltage V CE waveforms caused by the application of a signal I B at the bipolar transistor base that is too weak. The transistor passes from its saturation region to its linear region. * 36. Remove the CM 4 from the circuit to increase the base current I B. Is the transistor now conducting normally? * 37. Turn off the power supply and the square wave generator and remove all the connecting wires. CONCLUSION & & & In power electronics, the use of a bipolar transistor requires a complex accelerating circuit that is able to deliver large power. Large current peaks must be delivered to the base during switching and small current peaks during conduction. Switching (especially the turn-off switching time) of a bipolar transistor is improved by the use of a bipolar control signal. The bipolar transistor has a weak capacity for supporting current overload. & The bipolar transistor has a weak on-state voltage (V CE(ON) ). REVIEW QUESTIONS 1. To improve bipolar transistor switching time a. the base current I B must be decreased. b. the collector current I C must be increased. c. the base current I B must be increased. d. None of the above. 5-20

15 2. The RC snubber circuit is used a. to deliver large current peaks at the base of the bipolar transistor during switching. b. to avoid the presence of a large current I B during conduction. c. to avoid large power dissipations during conduction. d. All of the above. 3. The current peak delivered by the RC snubber circuit will be wider a. if the resistor value of the RC circuit is increased. b. if the resistor value of the RC circuit is decreased. c. if the capacitor value of the RC circuit is decreased. d. None of the above. 4. In order to increase the amplitude of the negative current peaks applied at the base of the bipolar transistor a. the resistor value of the RC circuit must be increased. b. a bipolar control supply must be used. c. the RC snubber circuit must be removed from the circuit. d. None of the above. 5. The accelerating circuit must provide a base current of at least two to five times that required for nominal load current a. to ensure bipolar transistor saturation. b. to turn off the bipolar transistor. c. to avoid bipolar transistor saturation. d. None of the above. 5-21