Exercise 2 The Buck Chopper EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the operation of the buck chopper. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: The buck chopper Operation of a buck chopper with a resistive load High-side versus low-side switching DISCUSSION The buck chopper Many electric devices that are powered by batteries contain sub-circuits that require a voltage level different than that supplied by the battery or the external supply (sometimes higher or lower than the supply voltage, and possibly even of opposite polarity). The chopper, also called dc-to-dc converter, is a power electronic circuit which converts the voltage of a dc source from one value to another. When the initial voltage is converted to a higher voltage, the chopper is called a boost chopper; conversely, when the initial voltage is converted to a lower voltage, it is called a buck chopper. Choppers use electronic switches to convert dc voltages and currents from high to low levels and vice versa. The electronic switches can be designed with IGBTs (insulated gate bipolar transistors), MOSFETs (metal-oxide semiconductor field-effect transistors), etc. Figure 20 shows a buck chopper built with electronic switch, and some waveforms related to this circuit. Festo Didactic 86356-00 27
Exercise 2 The Buck Chopper Discussion Switching control signal generator Resistive load on off on Switching control signal Amplitude on Time Output voltage ( ) Amplitude Time Output current ( ) Amplitude Time Figure 20. Operation of a buck chopper with a resistive load. Operation of a buck chopper with a resistive load When the electronic switch shown in Figure 20 switches on, the dc power supply voltage is applied to the resistive load and current flows through the load. When the electronic switch switches off, the dc power supply voltage is no longer applied to the load, and the current stops flowing through the load. The dc voltage (i.e., the average voltage) at the buck chopper output is proportional to the voltage at the buck chopper input and the time that 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 applied to the control signal input of electronic switch. 28 Festo Didactic 86356-00
Exercise 2 The Buck Chopper Discussion The equation relating voltages and is given by the expression: (4) where is the dc (average) voltage at the buck chopper output. is the dc (average) voltage applied at the buck chopper input. is the duty cycle expressed as a decimal (50% = 0.5). Thus, voltage can be varied by varying the duty cycle. Since the duty cycle can vary between 0% and 100%, voltage can vary between 0 and. A low duty cycle corresponds to a low output voltage, because the voltage at the buck chopper output is zero most of the time. Pulse-width modulation (PWM) of the switching control signals is a very efficient way of providing intermediate amounts of electrical power between 0 and maximum. Note that varying the frequency of the switching control signal while maintaining the duty cycle constant does not vary the average value of the output voltage and current at the buck chopper output. High-side versus low-side switching Figure 21 shows two buck choppers implemented with electronic switch placed at two different locations relative to the power supply and the load. When the electronic switch is placed on the high-voltage side of the power supply as shown in Figure 21a, the configuration is called high-side switching. Conversely, when the electronic switch is placed on the low-voltage side of the power supply as shown in Figure 21b, the configuration is called low-side switching. Load Load (a) High-side switching (b) Low-side switching Figure 21. High-side and low-side switching configurations. Although both configurations produce the same voltage and same current in the load, the low-side switching configuration is often preferred to the high-side switching configuration because it is easier to implement and requires fewer electronic components. In fact, for current to flow through the switching transistor, a low voltage (typically 5 V) must be applied between the gate and emitter of the switching transistor. The voltage at the gate must be higher than the voltage at the emitter of the switching transistor for current to flow through the transistor. Figure 22b shows a typical case of a switching transistor in the low-side switching configuration using an NPN type transistor. Festo Didactic 86356-00 29
Exercise 2 The Buck Chopper Procedure Outline In order to maintain a positive voltage of 5 V across the gate and emitter of a switching transistor in a high-side switching configuration, the gate voltage must be equal to the voltage (47 V) across the load when the transistor is on plus about 5 V. This corresponds to a voltage of approximately 52 V in the example shown in Figure 22a. In comparison, a gate voltage of only 5 V is required to turn on the transistor in a low-side switching configuration as shown in Figure 22b. G 5 V + 47 V C 47 V 48 V E 47 V 48 V G 5 V C E (a) High-side switching (b) Low-side switching Figure 22. Voltage required at the gate in high-side and low-side switching configurations. The high-side switching configuration is generally used in applications where one terminal of the load must be at the same voltage level as the common of the power supply (negative terminal of the power supply). Since the load is not connected to the common of the power supply in the low-side switching configuration, the load is said to be floating. PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Output voltage versus duty cycle Output voltage versus switching frequency Low-side and high-side switching 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. 30 Festo Didactic 86356-00
Exercise 2 The Buck Chopper 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. This connects the internal power supply of the module to the Power Supply terminals on the front panel. 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. Festo Didactic 86356-00 31
Exercise 2 The Buck Chopper Procedure 7. Set up the circuit shown in Figure 23. Notice the presence of capacitor and diode in the Chopper/Inverter. Capacitor is used to filter the dc voltage at the buck chopper input. The use of capacitors and diodes in chopper circuits is explained in more detail in the next exercise. Notice that many components included in the Chopper/Inverter are not shown in Figure 23 because they are not used in the circuit. Chopper/Inverter 50 V + 57 Switching control signals from digital outputs on DACI Figure 23. Buck chopper circuit with resistive load (high-side switching). 8. Make the necessary connections and switch settings on the Resistive Load in order to obtain the resistance value required. Output voltage versus duty cycle In this part of the exercise, you will vary the duty cycle of the switching control signal to vary the time the electronic switch is on while observing the voltage at the buck chopper output. This will allow you to verify the relationship between the duty cycle and the voltage conversion performed by the buck 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 50 V. Start the voltage source. 10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings: Make sure that the Function parameter is set to Buck Chopper (highside switching). This setting allows the Data Acquisition and Control 32 Festo Didactic 86356-00
Exercise 2 The Buck Chopper Procedure Interface to generate the switching control signal required by a buck chopper implemented with an electronic switch located on the highvoltage side of the power supply. Set the Switching Frequency parameter to 2000 Hz. This sets the frequency of the switching control signal to 2000 Hz. Set the Duty Cycle Control parameter to Knob. Set the Duty Cycle parameter to 50%. This sets the duty cycle of the switching control signal to 50 %. Set the Acceleration Time parameter to 0 s. Set the Deceleration Time parameter to 0 s. Make sure that the parameter is set to PWM. Notice that the Status parameter is set to Stopped. This indicates that the buck chopper is disabled. 11. For each duty cycle shown in Table 2, calculate the average output voltage of the buck chopper using an input voltage equal to 50 V and the equation. Record your results in the column "Calculated" of Table 2. Table 2. Average output voltage versus duty cycle (high-side switching). DC input voltage (V) 50 Duty cycle (%) 0 20 40 60 80 100 Average output voltage (V) Calculated Measured Note: switching frequency = 2000 Hz 12. In the Chopper/Inverter Control window, start the buck chopper by clicking on the Start/Stop button (or by setting the Status parameter to Started). 13. In LVDAC-EMS, open the Oscilloscope window and display the following parameters: voltage (input E1) at the buck chopper input, switching control signal (AI-1), average output voltage measured across resistor (input E2), and the output current flowing through resistor (input I2). Festo Didactic 86356-00 33
Exercise 2 The Buck Chopper Procedure Oscilloscope Settings Channel-1 Input... E1 Channel-1 Scale... 50 V/div Channel-1 Coupling... DC Channel-2 Input... AI-1 Channel-2 Scale... 5 V/div Channel-2 Coupling... DC Channel-3 Input... E-2 Channel-3 Scale... 20 V/div Channel-3 Coupling... DC Channel-4 Input... I-2 Channel-4 Scale... 0.5 A/div Channel-4 Coupling... DC Time Base... 0.1 ms/div Trigger Source... Ch2 Trigger Level... 2 V Trigger Slope... Rising Select the Continuous Refresh mode, then 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. Figure 24 shows an example of what the Oscilloscope should display. Figure 24. Waveforms at the input and output of the buck chopper (high-side configuration). 14. For each duty cycle value shown in Table 2, measure the average output voltage across resistor using input E2 of the Data Acquisition and Control Interface. The average values of the observed waveforms are indicated in the column "AVG" located under the Oscilloscope display. Record your results in the column "Measured" of Table 2. 15. Compare the calculated voltages with the measured voltages. Are they approximately the same? Notice that a slight difference is normal due to the voltage drop across the switching transistor. Yes No 16. Do your measurement results confirm that the average output voltage is proportional to the duty cycle of the switching control signal ( )? Yes No 34 Festo Didactic 86356-00
Exercise 2 The Buck Chopper Procedure Output voltage versus switching frequency In this part of the exercise, you will vary the switching frequency while observing the voltage at the buck chopper output. This will allow you to verify the relationship between the switching frequency and the voltage conversion performed by the buck chopper. 17. In the Chopper/Inverter Control window, set the duty cycle to 50%. This sets the average output voltage of the buck chopper to about 25 V. For each switching frequency shown in Table 3, measure the average output voltage across resistor using input E2. The average output voltage value is indicated in the column "AVG" located under the Oscilloscope display. Record your results in Table 3. Table 3. Average output voltage versus switching frequency (high-side switching). DC input voltage (V) Switching frequency (Hz) Average output voltage (V) 50 500 (1) 1000 (1) 5000 (1) 10 000 (1) (1) Change the time base setting in the Oscilloscope as required to display at least two complete cycles (2) Duty cycle = 50% 18. Does the buck chopper output voltage vary with the switching frequency? Yes No 19. Do your results confirm that the switching frequency has no effect on the output voltage of the buck chopper? Yes No 20. Stop the buck chopper and the voltage source. Festo Didactic 86356-00 35
Exercise 2 The Buck Chopper Procedure Low-side and high-side switching In this part of the exercise, you will change the position of the electronic switch in the buck chopper to obtain a low-side switching configuration. Then you will verify if the buck chopper still converts the voltage in the same way as with the high-side configuration. 21. Modify your circuit as shown in Figure 25 to set up a low-side switching configuration. Disconnect Switching Control Input 1 of the Chopper/Inverter from Analog Input 1 of the Data Acquisition and Control Interface. Connect Switching Control Input 4 of the Chopper/Inverter to Analog Input 1 of the Data Acquisition and Control Interface. Chopper/Inverter 50 V + 57 Switching control signals from digital outputs on DACI Figure 25. Buck chopper circuit with resistive load (low-side switching). 22. In the Chopper/Inverter Control window, make the following settings: Set the function to Buck Chopper (low-side switching). This setting allows the Data Acquisition and Control Interface to generate the switching control signal required by a buck chopper implemented with an electronic switch located on the low-voltage side of the power supply. Set the switching frequency to 2000 Hz. Set the duty cycle to 0%. Make sure that the acceleration time is set to 0 s. Make sure that the deceleration time is set to 0 s. Set the parameter to PWM. Start the buck chopper. 36 Festo Didactic 86356-00
Exercise 2 The Buck Chopper Conclusion 23. In the Four-Quadrant Dynamometer/Power Supply window, start the voltage source. 24. For each duty cycle value shown in Table 4, measure the average output voltage across resistor using input E2 of the of the Data Acquisition and Control Interface. Record your results in Table 4. Table 4. Average output voltage versus duty cycle (low-side switching). DC input voltage (V) 50 Duty cycle (%) 0 20 40 60 80 100 Average output voltage (V) 25. Compare the average output voltages measured in the low-side configuration (Table 4) with those measured in the high-side configuration (Table 2). Are they approximately the same? Yes No 26. Do your results confirm that the switching transistor can be located on either side of the voltage source without affecting the voltage conversion performed by the buck chopper? Yes No 27. 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 the average voltage at the output of a buck chopper is proportional to the duty cycle. You learned that the maximum average voltage that can be obtained at the buck chopper output is slightly lower than the voltage at its input. You observed that the switching frequency has no effect on the voltage conversion performed by the buck chopper. You also observed that the electronic switch can be located on the low-voltage side or the high-voltage side of the power supply without affecting the voltage conversion taking place in the buck chopper. Festo Didactic 86356-00 37
Exercise 2 The Buck Chopper Review Questions REVIEW QUESTIONS 1. A buck chopper is powered by a 300 V dc power supply. What is the output voltage range of this chopper if the duty cycle can vary between 5% and 95%? 2. Give a brief description of a chopper. 3. What differentiates a buck chopper from a boost chopper? 4. How does the frequency of the switching control signal affect the average output voltage provided by a buck chopper? 5. Explain why the low-side switching configuration is often preferred to the high-side switching configuration. 38 Festo Didactic 86356-00