Exercise 6. The Boost Chopper EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION. The boost chopper

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1 Exercise 6 The Boost Chopper EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the operation of the boost chopper. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: The boost chopper Power efficiency DISCUSSION The boost chopper You learned in the previous exercises that buck choppers are used to convert a dc voltage into a dc voltage having a lower value. You will learn in this exercise, that a dc voltage can also be converted to a dc voltage having a higher value using a boost chopper. Figure 46 shows a boost chopper built with inductor, electronic switch, diode, and capacitor, and some waveforms related to this circuit. When electronic switch switches on, the voltage across its terminals becomes almost null, the dc power supply voltage is applied to inductor, and current flowing in inductor starts to increase. Simultaneously, diode switches off since it becomes reverse-biased. At this moment, capacitor starts to discharge into the resistive load and both the boost chopper output current and voltage start to decrease. Notice that without the opposition to current variations caused by inductor, the dc power source would be short-circuited when electronic switch is switched on. Festo Didactic

2 Exercise 6 The Boost Chopper Discussion Switching control signal Inductor current ( ) Output Current ( ) Output voltage ( ) Amplitude (V) Amplitude (A) Amplitude (A) Amplitude (V) Time Time Time Time Figure 46.Operation of a boost chopper. When electronic switch switches off, the voltage across its terminals increases very rapidly until it reaches approximately V. This applies a forward-bias voltage of approximately 0.7 V to diode, which therefore switches on. At this moment, a current equal to - starts to charge up capacitor, and both and start to increase. 94 Festo Didactic

3 Exercise 6 The Boost Chopper Discussion The dc voltage at the boost chopper output is proportional to the dc voltage at the boost chopper input and the time the electronic switch is on during each cycle. This time, which is referred to as the on-time, is in turn proportional to the duty cycle of the switching control signal of electronic switch. The equation relating voltages and is given by Equation (6). (6) where is the dc voltage at the boost chopper output. is the dc voltage at the boost chopper input. is the duty cycle expressed as a decimal (e.g., 50% = 0.5). Thus, voltage can be varied by varying the duty cycle. This equation indicates that voltage can range between voltage and an infinite voltage when the duty cycle varies between 0% and 100%. In practice, however, the duty cycle only approaches 0% and 100%. Therefore, voltage can vary between a voltage slightly higher than voltage and many times voltage. In most circuits, the maximum value of duty cycle must be limited to limit the maximum voltage the boost chopper can produce. Varying the switching frequency while maintaining the duty cycle constant does not vary the output voltage and current at the boost chopper output. However, the ripple magnitude decreases as the switching frequency is increased. Power efficiency The power which the boost chopper delivers at its output is equal to the power it receives at its input minus the power dissipated in the electronic switch and the inductor. The power dissipated in the electronic switch and the inductor is usually small compared to the output power. The power efficiency of boost choppers, thus, often exceeds 80%. Notice that the power efficiency is the ratio of the output power to the input power times 100%, as stated in Equation (7). Power efficiency = (7) where is the power the boost chopper delivers. is the power the boost chopper receives. Festo Didactic

4 Exercise 6 The Boost Chopper Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Output voltage versus duty cycle Output voltage versus switching frequency Power efficiency 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. 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). 96 Festo Didactic

5 Exercise 6 The Boost Chopper Procedure 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 4 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. 7. Set up the circuit shown in Figure 47. Use the 50 mh inductor in the Filtering Inductors/Capacitors module to implement. Chopper/Inverter 50 mh 20 V Switching control signals from digital outputs on DACI Figure 47. Boost chopper circuit with resistive load. 8. Make the necessary connections and switch settings on the Resistive Load in order to obtain the resistance value required. Festo Didactic

6 Exercise 6 The Boost Chopper Procedure Output voltage versus duty cycle In this part of the exercise, you will vary the duty cycle of the switching control signal while observing the voltage conversion performed by the boost chopper. 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 20 V. Start the voltage source. 10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings: Select the Boost Chopper function. Set the switching frequency to 400 Hz. Set the Duty Cycle Control to Knob. Set the duty cycle to 20%. 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 boost chopper. 98 Festo Didactic

7 Exercise 6 The Boost Chopper Procedure 11. For each duty cycle shown in Table 10, calculate the theoretical dc output voltage when the dc voltage at the boost chopper input is 20 V. Use equation. Record your results in the column "Calculated" of Table 10. Table 10. Output voltage versus duty cycle. DC voltage at the boost chopper input (V) DC voltage across electronic switch (V) Duty cycle (%) DC output voltage (V) Measured Calculated Switching frequency = 400 Hz 12. In LVDAC-EMS, open the Oscilloscope window and display the voltage and current (inputs E1 and I1) at the boost chopper input, the switching control signal (AI-1), the dc voltage across electronic switch Q (input E3), the dc output voltage (input E2), and the dc output current (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. Select convenient vertical scale and position settings in the Oscilloscope to facilitate observation of the waveforms. Figure 48 shows an example of what the Oscilloscope should display. a To facilitate the readings on the Oscilloscope, momentarily select the Single Refresh mode by pressing the Single Refresh button. This will automatically disable the Continuous Refresh mode. Do not forget to refresh the display when a new setting is made. Festo Didactic

8 Exercise 6 The Boost Chopper Procedure Oscilloscope Setting 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-3 Channel-4 Scale V/div Channel-4 Coupling... DC Channel-5 Input... E-2 Channel-5 Scale V/div Channel-5 Coupling... DC Channel-6 Input... I-2 Channel-6 Scale A/div Channel-6 Coupling... DC Time Base ms/div Trigger Source... Ch3 Trigger Level... 2 V Trigger Slope... Rising Figure 48. Voltage and current waveforms in a boost chopper operating at 400 Hz. 13. Successively set the duty cycle to each of the values in Table 10. For each value, measure the dc voltage (input E1) at the boost chopper input, the dc voltage across electronic switch (input E3), and the dc output voltage (input E2) across the resistive load. The average (dc) voltage values are indicated in the column "AVG" located under the Oscilloscope display. Record your results in the column "Measured" of Table Do your measurement results confirm that the voltage at the output of the boost chopper increases when the duty cycle is increased? Yes No a Note that the measured and calculated voltages differ slightly. The difference, which increases with the duty cycle, is caused by the losses in inductor as well as the voltage drops across electronic switch and diode that are not taken into account in the equation ( ). These are significant and must be considered for precise estimations. 100 Festo Didactic

9 Exercise 6 The Boost Chopper Procedure Output voltage versus switching frequency In this part of the exercise, you will vary the frequency of the switching control signal while observing the voltage conversion performed by the boost chopper. 15. In the Chopper/Inverter Control window, set the duty cycle to 50%. Make sure that the source voltage is still set to 20 V in the Four-Quadrant Dynamometer/Power Supply. In the Chopper/Inverter Control window, successively set the switching frequency to each of the values in Table 11. For each value, measure the dc voltage (input E2) at the output of the boost chopper using the Oscilloscope. Record your results in the corresponding cells of Table 11. a To facilitate the reading of the voltage, you may select the Single Refresh mode by pressing the Single Refresh button. This will automatically disable the Continuous Refresh mode. Do not forget to return to the Continuous Refresh mode after each reading. Table 11. Output voltage versus switching frequency. DC input voltage (V) Switching frequency (Hz) DC output voltage (V) (1) (1) (1) (1) (1) (1) Change the time base setting in the Oscilloscope as required to display two complete cycles (2) Duty cycle = 50% 16. Do your results confirm that the switching frequency has no effect on the output voltage of a boost chopper? Yes No Power efficiency In this part of the exercise, you will determine the power at the input and output of the boost chopper to determine the power efficiency of the boost chopper. You will also observe the direction of power flow. 17. In the Chopper/Inverter Control window, set the switching frequency to 500 Hz and the duty cycle to 80%. 18. In Four-Quadrant Dynamometer/Power Supply window, make sure that the source voltage is still equal to 20 V. Festo Didactic

10 Exercise 6 The Boost Chopper Conclusion 19. In the Oscilloscope window, display the value of power P1 and power P2. Power P1 is the power in watts determined from the voltage and current measured using inputs E1 and I1, and power P2 is the power in watts determined from the voltage and current measured using inputs E2 and I2. Since inputs E1 and I1 measure the input voltage and current of the boost chopper, power P1 corresponds to the input power. Similarly, since inputs E2 and I2 measure the output voltage and current of the boost chopper, power P2 corresponds to the output power of the boost chopper. Record the input power and the output power. Input power : Output power : 20. Determine the power efficiency of the boost chopper using your measured powers and the following equation. Power efficiency = = 21. Do your results confirm that the power efficiency is high (exceeding 80%)? Yes No 22. Slowly vary the duty cycle between 20% and 80% while observing the load current (input I2). Notice that the load current polarity is positive and remains positive as the duty cycle of the switching control signal is varied. This indicates that the power flow is always from the power source to the load. 23. Stop the boost chopper and the voltage source. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables. CONCLUSION In this exercise, you verified that the voltage at the output of a boost chopper increases as the duty cycle of the switching control signal is increased. You observed that the frequency of the switching control signal has no effect on the output voltage of a boost chopper. Nevertheless, you observed that as the switching frequency is increased, the current ripple decreases. You verified that the power efficiency of a boost chopper exceeds 80%. You saw that power always flows in the same direction in a boost chopper. REVIEW QUESTIONS 1. Describe the effect the switching frequency has on the output voltage of a boost chopper. 102 Festo Didactic

11 Exercise 6 The Boost Chopper Review Questions 2. A boost chopper is powered by a 12-V battery. What is the output voltage range of this chopper if the duty cycle can vary between 20% and 95%? 3. Explain why the dc power source shown in Figure 47, does not become shorted when electronic switch is switched on. 4. Explain why the maximum value of the duty cycle must be limited in certain boost choppers. 5. Is the diode in the Figure 47 forward-biased or reverse-biased when electronic switch is switched off? Festo Didactic