Exercise 4-2. Switching Power in an Inductive Load EXERCISE OBJECTIVES

Transcription

1 Exercise 4-2 Switching Power in an Inductive Load EXERCISE OBJECTIVES At the completion of this exercise, you will be able to switch the current in an inductive load and you will understand the purpose of the free-wheeling diode. DISCUSSION You have seen in the previous exercise how to switch the current in a pure resistive load. However, for different reasons, pure resistive loads are rarely used in power electronics. 4-15

2 High power resistors are generally inductive. They are usually located far from the command circuit and so, require long connecting cables that are also inductive. Moreover, the abrupt current interruptions in a load create large undesirable electromagnetic emissions. It is desirable to filter the current with a smoothing inductor placed in series with the load resistor. Knowing that it is not possible to interrupt the current in an inductor without creating a large voltage surge, it is necessary to understand how to switch an inductive load while avoiding this problem. When a transistor switching an inductive load begins to conduct, current I C increases exponentially, as shown in the figure, and the voltage V CE decreases in a few microseconds. 4-16

3 The current rise is slower than in the case of a resistive load since the inductor opposes fast current variations. Notice that a weak voltage can still be observed across the transistor when it is in conduction (V CE(ON) ). If the transistor ceases to conduct, current I C decreases and this fast current drop will induce a voltage surge across the transistor. This surge is caused by the energy stored in the inductor that has to be released when the current is stopped. The inductor value and the current flowing through it before transistor turn-off determines the quantity of energy stored in the inductor. The higher the energy, the longer the voltage surge. The amplitude of this surge is a function of the transistor current cut-off rate. The faster the transistor interrupts the current, the higher the surge. This voltage surge can cause destruction of the transistor. On the Power Transistors and GTO Thyristor circuit board, protective circuits limiting the surges were placed in order to avoid transistor damage. 4-17

4 To switch an inductive load while avoiding voltage surges, a diode has simply to be placed in parallel with the load. In this new circuit configuration, when the transistor turns off and interrupts the load current (I L ), it can now flow through diode (I D ), called the free-wheeling diode. There is therefore no current cut-off in the inductor and, thus, no voltage surge. Observe the behavior of the currents in the transistor (I C ), in the load (I L ) and in the diode (I D ), and also voltage V CE when the transistor turns off. The voltage V CE increases rapidly to a voltage of V CC V. When this voltage is reached, the diode starts to conduct. The current flow is then transferred from the transistor to the diode. One then observes zero current in the transistor (I C ) and full load current in the diode (I D ). 4-18

5 It is important to note that the voltage across the load is approximately V CC when the transistor conducts. The current I L is then increasing in the load. When the transistor is off, the voltage across the load is equal to the diode on-state voltage, and so, the current I L is decreasing in the load. One can observe that the load current oscillates between I MAX and I MIN. As explained previously, it is desirable to smooth this current. To decrease the amplitude of the current oscillation, either the switching frequency, or the inductor value can be increased. However, if the inductor value is increased, so will be the cost of the circuit. You will also see in the next exercises that, if the switching frequency of the transistor is increased, it can then overheat. 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. 4-19

6 * 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, set the generator frequency to approximately 1 khz. CAUTION! The load resistors will get very hot. Avoid touching them to prevent burn injury. 4-20

7 * 4. Set oscilloscope channel 2 in reverse mode so you can correctly measure the transistor voltage V CE. Adjust the time base (200 µsec/div.) to observe two complete switching cycles of the current I C and the voltage V CE. * 5. When the transistor turns on, does the current I C increase exponentially, indicating the presence of an inductor which resists fast current variations? * Yes * No * 6. When the transistor turns off, does the current I C decrease rapidly? * Yes * No * 7. Does the current interruption in the inductive load, resulting from the transistor turn-off, create a voltage surge? * Yes * No * 8. Measure the amplitude of this voltage surge. V Note: Zener diodes of 51 V have been placed across inductor L1 of the LOAD (Z) circuit block to prevent voltage-surge damage the transistors. The diode presence modifies the current and voltage waveform when the transistor turns off. * 9. Place a jumper in the LOAD (Z) circuit block to connect diode CR3 into the circuit. Observe the signals on the oscilloscope. The introduction of the free-wheeling diode into the circuit allows one a. to increase the voltage surge amplitude. b. to decrease the voltage surge amplitude. c. to avoid the voltage surges. d. None of the above 4-21

8 * 10. Connect the oscilloscope probes so you can measure the load current I L and the current I C in the transistor, as shown in the figure. Keep the reverse mode on channel 2. * 11. The load current signal (I L ), displayed on channel 1 show abrupt current interruptions. * True * False * 12. Considering the result of the preceding step, when the transistor turns off, can you say that diode CR3 starts to conduct and allows continued load current flow? * Yes * No Note: The voltage is measured across a resistor of 1 6. Thus, it can be considered that a voltage of 1 V measured on the oscilloscope is equivalent to a current of 1 A in the circuit. * 13. On channel 1, measure the maximum amplitude of the load current when the circuit is switching at 1 khz. A * 14. Gradually increase the square wave frequency up to 20 khz while adjusting the oscilloscope to observe the behavior of the load current and the current in the transistor. 4-22

9 * 15. Measure the maximum amplitude of the load current when the circuit is switching at 20 khz. ma * 16. Place a jumper to short-circuit the inductor while observing the waveforms of the load current I L and the collector current I C. Can you say that the inductor is efficient in smoothing the load current? * Yes * No * 17. Turn off the power supply and the square wave generator and remove all the connecting wires. CONCLUSION & & The free-wheeling diode is used to prevent voltage surges in an inductive circuit. The smoothing inductor allows load current with less ripple to be obtained. REVIEW QUESTIONS 1. If we use a transistor to stop the current in an inductive circuit a. a voltage surge will inevitably occur across the transistor. b. overheating of the transistor could occur. c. the inductor can be damaged. d. None of the above. 2. In an inductive circuit, the free-wheeling diode allows a. the circuit to operate at higher frequencies. b. the load current to decrease. c. voltage surges to be avoided by ensuring continued load current flow when the transistor turns off. d. None of the above. 3. The smoothing inductor allows a. losses in the transistor to be decreased. b. a load current with a fewer ripples to be obtained. c. operation at higher frequencies. d. None of the above. 4-23

10 4. In this circuit, where does the load current circulate immediately after transistor turn-off? a. In the free-wheeling diode. b. In the load resistor. c. In the smoothing inductor. d. All of the above. 5. In the same circuit, if the smoothing inductor is short-circuited, a. the transistor can be damaged. b. a pulsed load current is obtained. c. the free-wheeling diode must absolutely be removed from the circuit. d. None of the above. 4-24