Introduction to High-Speed Power Switching

Transcription

1 Exercise 3 Introduction to High-Speed Power Switching EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the concept of voltage-type and current-type circuits. You will also be familiar with the use of free-wheeling diodes. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: High-speed power switching circuits Voltage-type circuit Current-type circuit Free-wheeling diodes Power efficiency Interconnecting voltage-type and current-type circuits DISCUSSION High-speed power switching circuits In high-speed power switching circuits, electronic switches are opened and closed rapidly to transfer electric power from one circuit to another with minimal power dissipation. In many cases, some characteristics of the electric power, such as the voltage, current, and frequency are modified. A solid-state converter is a converter whose operation depends on the control of electrical or magnetic phenomena in solids, such as a transistor, crystal diode, or ferrite device. Basically, electronic switches open, close, short-circuit, or interconnect circuits to modify the voltages and currents related to these circuits. In power electronics, however, the characteristics of a circuit must be taken into account before opening or short-circuiting a circuit, or before interconnecting a circuit to another, to avoid damaging the circuits. Two types of circuits are distinguished in solidstate converters: voltage-type and current-type circuits. Voltage-type circuit A voltage-type circuit opposes voltage variations but not current variations. The voltage across a voltage-type circuit tends to remain constant. Batteries, capacitors, and dc voltage sources are examples of voltage-type circuits. A very high current flows through a voltage-type circuit when it is short-circuited. Therefore, a voltage-type circuit should never be short-circuited because the high current which results could damage the circuit or trigger an overcurrent protection circuit. Because capacitors oppose voltage variations, they are widely used in electronic circuits to smooth the output voltage of power supplies, this is why they are often called smoothing or filtering capacitors. The ability of a capacitor to oppose voltage variations is called capacitance, and is measured in units of farad. The larger the capacitor, the larger the capacitance and the opposition to voltage variations. Festo Didactic

2 Exercise 3 Introduction to High-Speed Power Switching Discussion Electrical symbol of a capacitor In electrical circuit schematics, capacitors are identified with the letter and represented by the symbol shown in the left margin. Figure 26 shows various types of capacitors. The plus (+) sign that is sometimes included in the symbol indicates that the capacitor is polarized. Polarized capacitors must be used in such a way that their positive terminal is connected to the positive side (high side) of a circuit. Figure 26. Various types of capacitors. Current-type circuit A current-type circuit opposes current variations but not voltage variations. The current flowing in a current-type circuit tends to remain constant. Inductors, motors, and current sources are examples of current-type circuits. A very high voltage develops across a current-type circuit when it is opened while a current is flowing through the circuit. Therefore, a current-type circuit should never be opened when current is flowing through the circuit, because the high voltage which results could damage the circuit or trigger an overvoltage protection circuit. Because inductors oppose current variations, they are widely used in electronic circuits to smooth the current, this is why they are often called smoothing or filtering inductors. The ability of an inductor to oppose current variations is called inductance, and is measured in units of Henry. The larger the inductor, the larger the inductance and the opposition to current variations. Electrical symbol of an inductor. In electrical circuit schematics, inductors are identified with the letter and represented by the symbol shown in the left margin. Figure 27 shows various types of inductors. 40 Festo Didactic

3 Exercise 3 Introduction to High-Speed Power Switching Discussion Figure 27. Various types of inductors. Free-wheeling diodes In order to prevent fast current variations in a current-type circuit, and thereby prevent high voltages from being induced across it, a diode can be connected in parallel with the current-type circuit as shown in Figure 28. This diode is usually referred to as a free-wheeling diode. In the circuit of Figure 28, an electronic switch is used to connect a voltage source (a voltage-type circuit) to a current-type circuit consisting of inductor. If diode were removed, a high voltage would develop across inductor when the switch opens, that is, when electronic switch turns off. Note that the circuit shown in Figure 28 is in fact a buck chopper. The free-wheeling diode is considered as part of the buck chopper. Voltage-type circuit Current-type circuit Free-wheeling diode Figure 28. Free-wheeling diode connected in parallel with a current-type circuit. Such a high voltage could destroy electronic switch. However, no high voltage is allowed to develop across inductor because of the presence of diode in parallel with inductor, as is explained below. Festo Didactic

4 Exercise 3 Introduction to High-Speed Power Switching Discussion When electronic switch turns on, voltage is applied to inductor and the current in it gradually builds up. Since diode is reverse-biased, it is blocked and no current flows through it. When electronic switch turns off, voltage source is disconnected from inductor and the current begins to decrease. The inductor reacts to the current variation and this causes the voltage across the inductor to decrease extremely rapidly until its polarity reverses and its value reaches approximately -0.7 V (consequently, the voltage across electronic switch increases very rapidly until it reaches approximately voltage V). As a result, a forward-bias voltage of approximately 0.7 V appears across diode. Consequently, diode turns on and the current continues to flow through inductor, returning now through diode, while gradually decreasing. The rate of decrease depends upon the constant of inductor, where is the internal resistance of inductor. When electronic switch turns on again, the current in inductor builds up to its previous value, and the cycle repeats. Power efficiency The power which a chopper (e.g., a buck chopper) delivers at its output is equal to the power it receives at its input minus the power dissipated in the electronic switch. The power dissipated in the electronic switch is small compared to the output power. The power efficiency of choppers, thus, often exceeds 90% and can even approach 100%. Notice that the power efficiency is the ratio of the output power on the input power times 100%, as stated in Equation (5). Power efficiency = (5) where is the power the chopper delivers. is the power the chopper receives. Interconnecting voltage-type and current-type circuits When two batteries having the same polarity but different voltages are connected in parallel, the battery having the higher voltage discharges into the other until the voltages of the two batteries are equal. The current flowing in the batteries reaches a very high value since it is only limited by the internal resistance of the batteries. Obviously, this could damage the batteries. A similar phenomenon occurs when two charged capacitors are connected in parallel. One capacitor discharges into the other until the voltage across the capacitors is the same. Since the internal resistance of capacitors is very low, a high current flows momentarily in the capacitors. If an electronic switch is used to connect these two capacitors, it could be damaged by the high current peak which flows every time the electronic switch closes. These two examples show that it is not recommended to directly connect two voltage-type circuits in parallel, unless their voltages and polarities are identical. 42 Festo Didactic

5 Exercise 3 Introduction to High-Speed Power Switching Procedure Outline A similar reasoning applies to current-type circuits. They should not be directly connected together unless the current in both circuits is equal and flows in the same direction. In effect, when two current-type circuits are connected together, a high voltage develops across each circuit when the current in these circuits is not the same. If the circuit contains electronic switches, these could be damaged by the high voltages that are produced. However, a voltage-type circuit can be connected to a current-type circuit without creating any problems. For example, a voltage-type source can be connected to an inductor by means of an electronic switch which opens and closes at a high rate as long as a free-wheeling diode is connected in parallel with the inductor. This corresponds to the circuit shown in Figure 28. When electronic switch turns on, the current drawn from voltage-type source passes rapidly from zero to the value of the current which was flowing in inductor just before electronic switch turned off. Simultaneously, voltage across inductor passes rapidly from nearly zero (i.e., approximately -0.7 V) to voltage. This causes no problems because voltage-type source does not oppose current variations and inductor does not oppose voltage variations. When electronic switch turns off, the current drawn from the voltage-type source decreases rapidly to zero. At the same time, the voltage across inductor passes rapidly from voltage to nearly zero (more precisely -0.7 V), thereby applying a forward-bias voltage across diode which enters into conduction. Again, this causes no problem because the voltage-type source does not oppose current variations and inductor does not oppose voltage variations. PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Voltage-type circuit Connecting a voltage-type circuit to a current-type circuit via an electronic switch without a free-wheeling diode Connecting a voltage-type circuit to a current-type circuit via an electronic switch with a free-wheeling diode Power efficiency Interconnecting two voltage-type circuits via an electronic switch PROCEDURE High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified. Setup and connections In this part of the exercise, you will set up and connect the equipment. 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise. Install the required equipment in the Workstation. Festo Didactic

6 Exercise 3 Introduction to High-Speed Power Switching Procedure 2. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply. Connect the Low Power Input of the IGBT Chopper/Inverter to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on. 3. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer. Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer. 4. Make sure that the main power switch of the Four-Quadrant Dynamometer/ Power Supply is set to O (off), then connect the Power Input to an ac power outlet. Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power Supply to Power Supply. Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main power switch to I (on). 5. Connect the Digital Outputs of the Data Acquisition and Control Interface (DACI) to the Switching Control Inputs of the IGBT Chopper/Inverter using a DB9 connector cable. Connect Switching Control Input 1 of the IGBT Chopper/Inverter to Analog Input 1 of the Data Acquisition and Control Interface using a miniature banana plug lead. Connect the common (white) terminal of the Switching Control Inputs on the IGBT Chopper/Inverter to one of the two analog common (white) terminals on the Data Acquisition and Control Interface using a miniature banana plug lead. 6. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition and Control Interface and the Four-Quadrant Dynamometer/Power Supply are detected. Make sure that the Computer-Based Instrumentation and Chopper/Inverter Control functions for the Data Acquisition and Control Interface are available, as well as the Standard Functions (C.B. control) for the Four-Quadrant Dynamometer/Power Supply. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window. 44 Festo Didactic

7 Exercise 3 Introduction to High-Speed Power Switching Procedure 7. Set up the circuit shown in Figure 29. Use the 210 F capacitor in the Filtering Inductors/Capacitors module to implement. Notice that for this part of the exercise, capacitor replaces capacitor of the Chopper/Inverter. In this circuit, the voltage source and capacitor form a voltage-type circuit. Voltage-type circuit Chopper/Inverter 25 V 210 µf 86 Switching control signals from digital outputs on DACI Figure 29. Voltage-type circuit connected to a resistive load via an electronic switch. 8. Make the necessary connections and switch settings on the Resistive Load in order to obtain the resistance value required. Voltage-type circuit In this part of the exercise, you will observe the behavior of a voltage-type circuit supplying a resistive load via an electronic switch. 9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window and make the following settings: Select the Voltage Source (+) function. Set the voltage to 25 V. Start the voltage source. Festo Didactic

8 Exercise 3 Introduction to High-Speed Power Switching Procedure 10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings: Make sure that the Buck Chopper (high-side switching) function is selected. Set the switching frequency to 500 Hz. Set the Duty Cycle Control parameter to Knob. Set the duty cycle to 50%. Make sure that the acceleration time is set to 0.0 s. Make sure that the deceleration time is set to 0.0 s. Make sure that the parameter is set to PWM. Start the buck chopper. 11. In LVDAC-EMS, open the Oscilloscope window and display the voltage and current (inputs E1 and I1) at the buck chopper input, the switching control signal (AI-1), the output voltage across resistor (input E2), and the output current flowing through resistor (input I2). Select the Continuous Refresh mode, set the time base to display at least two complete cycles, and set the trigger controls so that the Oscilloscope triggers when the rising edge of the switching control signal (AI-1) reaches 2 V. Figure 30 shows an example of what the Oscilloscope should display. Oscilloscope settings Channel-1 Input... E1 Channel-1 Scale V/div Channel-1 Coupling... DC Channel-2 Input... I-1 Channel-2 Scale A/div Channel-2 Coupling... DC Channel-3 Input... AI-1 Channel-3 Scale... 5 V/div Channel-3 Coupling... DC Channel-4 Input... E-2 Channel-4 Scale V/div Channel-4 Coupling... DC Channel-5 Input... I-2 Channel-5 Scale A/div Channel-5 Coupling... DC Time Base ms/div Trigger Source... Ch3 Trigger Level... 2 V Trigger Slope... Rising Figure 30. Waveforms at the input and output of a buck chopper connected to a resistive load. 46 Festo Didactic

9 Exercise 3 Introduction to High-Speed Power Switching Procedure 12. Observe the waveforms displayed on the Oscilloscope. Are the voltage and current (inputs E1 and I1) waveforms at the buck chopper input in accordance with the characteristics of a voltage-type circuit? Explain. 13. Print or save the waveforms displayed on the Oscilloscope for future reference. It is suggested that you include these waveforms in your lab report. 14. Stop the buck chopper and the voltage source. Connecting a voltage-type circuit to a current-type circuit via an electronic switch without a free-wheeling diode In this part of the exercise, you will modify your circuit by adding inductor in series with resistor as shown in Figure 31 to observe what happens when a voltage-type circuit is connected to a current-type circuit via an electronic switch without a free-wheeling diode. 15. Modify your circuit to connect inductor in series with resistor as shown in Figure 31. Use the 50 mh inductor in the Filtering Inductors/Capacitors module to implement. In this circuit, resistor limits the current in inductor and prevents excessively high voltages from developing in the circuit. Resistor and inductor form a current-type circuit. Do not modify the values of resistor (86 ) and capacitor (210 F). Voltage-type circuit Chopper/Inverter Current-type circuit 50 mh 25 V 210 µf 86 Switching control signals from digital outputs on DACI Figure 31. Voltage-type circuit connected to a current-type circuit via an electronic switch without a free-wheeling diode. Festo Didactic

10 Exercise 3 Introduction to High-Speed Power Switching Procedure 16. Start the buck chopper and the voltage source. Wait two minutes for the voltage at the buck chopper output to stabilize, then select convenient vertical scale and position settings in the Oscilloscope to facilitate observation. Describe what happens to the voltage across the current-type circuit (input E2) at the instant when electronic switch turns off (when the current stops flowing). 17. Determine the value of the voltage which appears across the current-type circuit at the instant when electronic switch turns off. 18. What can be done to prevent a large voltage from being induced across the current-type circuit when electronic switch turns off? 19. Print or save the waveforms displayed on the Oscilloscope for future reference. It is suggested that you include these waveforms in your lab report. 20. Stop the buck chopper and the voltage source. Do not modify the other settings in LVDAC-EMS. Connecting a voltage-type circuit to a current-type circuit via an electronic switch with a free-wheeling diode In this part of the exercise, you will add a free-wheeling diode in parallel with the current-type circuit and observe its effect on the voltage induced across the current-type circuit when electronic switch turns off. 48 Festo Didactic

11 Exercise 3 Introduction to High-Speed Power Switching Procedure 21. Modify your circuit to add diode in parallel with the current-type circuit as shown in Figure 32. Notice that capacitor has been removed from the previous circuit (Figure 31). It is replaced by capacitor, which is part of the IGBT Chopper/Inverter module. Capacitor is automatically inserted in the circuit when diode is added to the circuit, and is used to smooth the dc bus voltage at the input of the buck chopper. Voltage-type circuit Chopper/Inverter Current-type circuit 25 V + 50 mh 86 Switching control signals from digital outputs on DACI Figure 32. Voltage-type circuit connected to a current-type circuit via an electronic switch with a free-wheeling diode (i.e., a buck chopper). 22. Start the buck chopper and the voltage source. 23. On the Oscilloscope, display the voltage (input E1) at the buck chopper input, the switching control signal (AI-1), the voltage across the current-type circuit (input E2), and the current flowing through the voltage-type circuit (input I2). Make sure that the time base is set to display at least two complete cycles. Also make sure that the trigger controls are set so that the Oscilloscope triggers when the rising edge of the switching control signal (AI-1) reaches 2 V. Select convenient vertical scale and position settings in the Oscilloscope to facilitate observation. 24. Observe the waveforms displayed on the Oscilloscope. Describe the effect which adding a free-wheeling diode has on the voltage waveform at the buck chopper output. Festo Didactic

12 Exercise 3 Introduction to High-Speed Power Switching Procedure 25. Describe the effect which adding a free-wheeling diode has on the current waveform at the buck chopper output. a Notice that the opposition to current variation produced by inductor causes the current to flow without interruptions in the circuit. The time the electronic switch is turned off is too short to allow the current to decrease down to zero. 26. Print or save the waveforms displayed on the Oscilloscope for future reference. It is suggested that you include these waveforms in your lab report. Power efficiency In this part of the exercise, you will measure the power at the input and output of the buck chopper to determine the power efficiency. 27. In the Chopper/Inverter Control window, set the duty cycle to 50%, and the switching frequency to 1000 Hz. 28. In the Four-Quadrant Dynamometer/Power Supply window, set the source voltage to 50 V. 29. In the Oscilloscope window, momentarily disable the Continuous Refresh mode by clicking on the Continuous Refresh button. 50 Festo Didactic

13 Exercise 3 Introduction to High-Speed Power Switching Procedure 30. Open the Metering window and enable the power meters PQS1 (E1,I1) and PQS2 (E2,I2) to display the input power and output power respectively. Make sure that power meters PQS1 (E1,I1) and PQS2 (E2,I2) show active power [the letter W (watts) should appear at the bottom of each meter]. Make sure that meters E1, E2, I1, and I2 are enabled, then select the DC mode for each of these meters by clicking on the AC/DC button at the bottom of each meter. Measure the dc voltage, dc current, and power at the input and output of the buck chopper by clicking on the Single Refresh button in the Metering window, and record the values in Table 5. Table 5. Power at the input and output of the buck chopper. Buck chopper input Buck chopper output DC voltage (V) DC current (A) Power (W) DC voltage (V) DC current (A) Power (W) Duty cycle = 50%, switching frequency = 1000 Hz 31. Determine the power efficiency of the buck chopper using the values in Table 5 and the following equation. Power efficiency = = 32. Determine the power dissipated in the electronic switch by subtracting the output power from the input power. Power dissipated in the electronic switch: 33. Do your results confirm that the power dissipated in the electronic switch is small compared to the output power? Yes No Festo Didactic

14 Exercise 3 Introduction to High-Speed Power Switching Procedure 34. Using the values in Table 5, determine the current ratio and voltage ratio for a 50% duty cycle. Current ratio Voltage ratio You should observe that the voltage ratio is virtually equal to the duty cycle whereas the current ratio is virtually equal to the reciprocal of the duty cycle (1/ ). 35. Stop the buck chopper and the voltage source. Close the Metering window. Interconnecting two voltage-type circuits via an electronic switch In this part of the exercise, you will interconnect two voltage-type circuits via a buck chopper, and observe the current flowing between the two circuits. 36. Set up the circuit shown in Figure 33. Use the 5 F capacitor in the Filtering Inductors/Capacitors module to implement. In this circuit, resistor is used as a current limiter. Resistor and capacitor form a voltage-type circuit. Voltage-type circuit Chopper/Inverter V 5 µf 171 Voltage-type circuit Switching control signals from digital outputs on DACI Figure 33. Two voltage-type circuits connected via a buck chopper. 37. Make the necessary connections and switch settings on the Resistive Load in order to obtain the resistance values required. 52 Festo Didactic

15 Exercise 3 Introduction to High-Speed Power Switching Conclusion 38. In the Four-Quadrant Dynamometer/Power Supply window, set the source voltage to 25 V, then start the voltage source. 39. In the Chopper/Inverter Control window, make sure that the duty cycle is set to 50%, then set the switching frequency to 500 Hz. Start the buck chopper. 40. In the Oscilloscope, enable the Continuous Refresh mode by clicking on the Continuous Refresh button. Make sure that the voltage (input E1) at the buck chopper input, the switching control signal (AI-1), the voltage across resistor (input E2), and the current (input I2) flowing through the two voltage-type circuits are displayed. Select convenient vertical scale and position settings in the Oscilloscope to facilitate observation. Describe what happens to the voltage waveform across resistor (i.e., the voltage across the voltage-type circuit at the buck chopper output) when the switching control signal turns on and off. Explain why. 41. Observe the waveform of the current (input I2) flowing through the two voltage-type circuit. You should observe a sharp increase in current followed by a gradual decrease to a lower value at the instant when the electronic switch closes. Explain why. 42. Print or save the waveforms displayed on the Oscilloscope for future reference. It is suggested that you include these waveforms in your lab report. 43. Stop the buck chopper and the voltage source. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables. CONCLUSION In this exercise, you learned that a voltage-type circuit opposes voltage variations, and that a current-type circuit opposes current variations. You learned that a free-wheeling diode connected in parallel with a current-type circuit supplied by a chopper prevents high voltages from being induced across the current-type circuit. Festo Didactic

16 Exercise 3 Introduction to High-Speed Power Switching Review Questions You learned that a voltage-type circuit can be connected to a current-type circuit without creating any problems, but that two voltage-type circuits or two currenttype circuits, should not be connected together using an electronic switch unless precautions are taken. You also learned that the power efficiency of a buck chopper is high and that the power dissipated in the electronic switch is small compared to the output power. REVIEW QUESTIONS 1. What is the main characteristic of a voltage-type circuit? 2. Explain why a voltage-type circuit should never be short circuited. 3. Give examples of voltage-type circuits. 4. Explain the role of the free-wheeling diode in a buck chopper. 5. Explain why two circuits of the same type should not be interconnected directly using an electronic switch. 54 Festo Didactic