Exercise 7 The Buck/Boost Chopper EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the operation of the buck/boost chopper. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: The Buck/Boost Chopper DISCUSSION The Buck/Boost Chopper As discussed in the previous exercises of this manual, the buck chopper converts a dc into a dc having a lower value and the boost chopper converts a dc into a dc having a higher value. In these choppers, the current always flows in the same direction, that is, from the input to the output of the chopper as shown in Figure 49a and Figure 49b. In a buck/boost chopper, the current can flow in either direction. When the current flows from the low side to the high side, power also flows from the low side to the high side, and the buck/boost chopper operates as a boost chopper. On the other hand, when the current flows from the high side to the low side, power also flows from the high side to the low side, and the buck/boost chopper operates as a buck chopper. This is summarized in Figure 49c. Festo Didactic 86356-00 105
Exercise 7 The Buck/Boost Chopper Discussion Current (power) flow High Input Buck chopper Output Low (a) Current (power) flow Low Input Boost chopper Output High (b) Current (power) flow (Buck) (Boost) High Buck/Boost chopper Low (c) Figure 49. Current (power) flow in various types of choppers. A buck/boost chopper built with two electronic switches and two diodes (freewheeling diodes) is shown in Figure 50. This figure shows that the buck/boost chopper consists of a buck chopper and a boost chopper connected together. The buck chopper operates when the current flows from the high- side to the low- side. In this case, the components of the boost chopper could be removed without disturbing the operation of the buck chopper. Conversely, the boost chopper operates when the current flows from the low- side to the high- side. In this case, the components of the buck chopper could be removed without disturbing the operation of the boost chopper. 106 Festo Didactic 86356-00
Exercise 7 The Buck/Boost Chopper Discussion Buck/Boost chopper Boost chopper Buck chopper High Low Switching control signal of the buck chopper Amplitude (V) 5 0 Time Switching control signal of the boost chopper Amplitude (V) 5 0 Time Figure 50. A buck/boost chopper built with two electronic switches and two diodes. Figure 50 also shows the waveforms of the switching control signals applied to the electronic switches. These signals are pulse trains whose duty cycles are complementary. For instance, when the duty cycle of one signal is 60%, the duty cycle of the other signal is 40%. Therefore, when one electronic switch is switched on, the other electronic switch is switched off and vice versa. The equation relating s (high ) and (low ) in the buck chopper and the equation relating s (low ) and (high ) in the boost chopper also apply for the buck/boost chopper. The duty cycle of the switching control signal applied to the buck chopper electronic switch must be used when using the buck chopper equation. Similarly, the duty cycle of the switching control signal applied to the boost chopper electronic switch must be used when using the boost chopper equation. Table 12 shows that both the equations relating s and in a buck chopper and a boost chopper can be used to determine the relationship between and when the appropriate duty cycle is used. In this example the duty cycle of the buck chopper is 40%, consequently the duty cycle of the boost chopper is 60%. Festo Didactic 86356-00 107
Exercise 7 The Buck/Boost Chopper Procedure Outline Table 12. Relationship between s and determined using the equation for a buck chopper and a boost chopper. Buck chopper Boost chopper PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Switching control signals of a buck/boost chopper Operation of a buck/boost chopper PROCEDURE High s 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. Make sure that the batteries in the Lead-Acid Battery Pack are fully charged by measuring the open-circuit with a voltmeter. If the open-circuit is lower than 51 V, ask your instructor for assistance as the battery pack is probably not fully-charged. Appendix C of this manual indicates how to prepare (charge) the Lead-Acid Battery Pack before each laboratory period. Install the required equipment in the Workstation. 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. 108 Festo Didactic 86356-00
Exercise 7 The Buck/Boost Chopper Procedure 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 Inputs 1 and 4 of the IGBT Chopper/Inverter to Analog Inputs 1 and 2 of the Data Acquisition and Control Interface using miniature banana plug leads. 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 and frequency that correspond to the and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window. 7. Set up the circuit shown in Figure 51. Use the 50 mh inductor of the Filtering Inductors/Capacitors module to implement. Use the Lead-Acid Battery Pack as low source. Festo Didactic 86356-00 109
Exercise 7 The Buck/Boost Chopper Procedure Chopper/Inverter 50 mh 100 V High source + 50 V Low source Switching control signals from digital outputs on DACI Figure 51. Buck/Boost chopper circuit. Switching control signals of a buck/boost chopper In this part of the exercise, you will observe that the switching control signals of the electronic switches in a buck/boost chopper are complementary. 8. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window and make the following settings: Select the Voltage Source (+) function. Set the to 100 V. Do not start the source now. 110 Festo Didactic 86356-00
Exercise 7 The Buck/Boost Chopper Procedure 9. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings: Select the Buck/Boost Chopper function. Set the switching frequency to 1000 Hz. Set the Duty Cycle Control 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 and parameters are set to PWM. Start the buck/boost chopper now. 10. In LVDAC-EMS, open the Oscilloscope window and display the and the current (inputs E1 and I1) at the high- side, the two switching control signals (AI-1 and AI-2), as well as the and current (inputs E2 and I2) at the low- side. 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) of electronic switch reaches 2 V. Select convenient vertical scale and position settings in the Oscilloscope to facilitate observation of the waveforms. 11. Referring to the waveform of the switching control signals (AI-1 and AI-2), describe the switching sequence of electronic switches and. Festo Didactic 86356-00 111
Exercise 7 The Buck/Boost Chopper Procedure 12. In the Chopper/Inverter Control window, set the duty cycle to 60%. Referring to the waveform of the switching control signals (AI-1 and AI-2), does the duty cycle setting that you have made in the Chopper/Inverter Control window correspond to the duty cycle of the switching control signal of electronic switch (AI-1) or to the duty cycle of the switching control signal of electronic switch (AI-2)? 13. From what you observed in the previous steps, what will the duty cycles of the switching control signals of the electronic switches be if the duty cycle in the Chopper/Inverter Control window is set to 20%? Duty cycle of the switching control signal of : Duty cycle of the switching control signal of : Operation of a buck/boost chopper In this part of the exercise, you will vary the duty cycle to observe the buck/boost chopper operation in the buck and boost chopper modes. 14. In the Chopper/Inverter Control window, set the duty cycle to 50%. In the Four-Quadrant Dynamometer/Power Supply window, start the source, then using the arrow buttons below the control knob, adjust the so that the current at the high- side and low- side, measured using inputs I1 and I2, are almost null. This setting is required to balance the circuit taking account of the state of charge of the batteries in the Lead-Acid Battery Pack. 15. Successively set the duty cycle to each of the values shown in Table 13. For each value, record the average (dc) and current at the high side (inputs E1 and I1) and low- side (inputs E2 and I2) in the corresponding cells of Table 13. Table 13. Average dc Voltages and currents in a buck/boost chopper. High- side DC (V) DC current (A) Duty cycle (%) 45 50 55 Low- side DC (V) DC current (A) 112 Festo Didactic 86356-00
Exercise 7 The Buck/Boost Chopper Procedure 16. According to the polarity of the currents measured at the high- side and low- side when the duty cycle is 45%, in which direction does the power flow? 17. Does the circuit operate as a buck chopper or boost chopper when the duty cycle is 45%? Explain why. 18. According to the currents measured at the high- side and low- side, does the circuit operate as a buck chopper or boost chopper when the duty cycle is set to 50%? Explain why. 19. According to the polarity of the currents measured at the high- side and low- side, does the circuit operate as a buck chopper or boost chopper when the duty cycle is set to 55%? Explain why. 20. Using the arrow buttons below the Duty Cycle control knob in the Buck/Boost Chopper window, slowly decrease the duty cycle from 55% (current value) to 45% while observing the currents using the Oscilloscope. Describe what happens to the currents when the duty cycle passes through 50%. What does this mean? Festo Didactic 86356-00 113
Exercise 7 The Buck/Boost Chopper Conclusion 21. From the observations you carried out in this exercise, which feature distinguishes the buck/boost chopper from the buck chopper and boost chopper? 22. Stop the buck/boost chopper and the source. 23. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables. CONCLUSION In this exercise, you verified that a buck/boost chopper can operate either as a buck chopper or a boost chopper. You observed that the duty cycles of the two switching control signals in a buck/boost chopper are complementary, that is, when the duty cycle of one switching control signal is 70% for example, the duty cycle of the other switching control signal is 30%. You found that the current can flow in either direction in a buck/boost chopper, and that the direction of the current determines whether the circuit operates as a buck chopper or a boost chopper. REVIEW QUESTIONS 1. Briefly describe the operation of a buck/boost chopper. 2. Refer to the circuit shown in Figure 52. Knowing that the duty cycles of the switching control signals applied to electronic switches and are 25% and 75%, respectively, is the 12 V battery supplying or receiving power? 114 Festo Didactic 86356-00
Exercise 7 The Buck/Boost Chopper Review Questions 40 V 12 V Figure 52. A buck/boost chopper circuit. 3. Under the conditions stated in review question 2, would it be possible to remove electronic switch from the circuit without disturbing the operation of the buck/boost chopper? Explain. 4. Refer to the circuit shown in Figure 52. Knowing that the duty cycles of the switching control signals applied to electronic switches and are 60% and 40% respectively, is the circuit operating as a buck chopper or as a boost chopper? 5. What is the main feature which distinguishes the buck/boost chopper from the buck chopper and the boost chopper? Festo Didactic 86356-00 115