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Electrical and Electronics Engineering Department Electrical & Electronics Laboratory 2 (BAEL1101) Experiment 11 Buck Converter Design Name: Due Date: 12 July 2016 Submission Date: Instructor: Tan Teck Siang KOLEJ UNIVERSITI SELATAN Southern University College 1 Course: BAEL1101 Date: ________________ Time: ________________ Name:________________ Lab Report Score:(1) Pre -lab: On time.(10%) _____ (2) Lab Report: Format, Purpose, Approach, Results (40%) _____ (3) Lab Report: Analysis and Conclusion (40%) _____ (4) Teamwork and Oral Communication (10%) _____ (4) Late Penalties _____ Total _____ 2 Objectives To analyse the operation principle of a dc-dc converter To characterize the different operation modes of switching-mode converter To design basic switching-mode converter circuit INTRODUCTION Fig. 1 Simple Buck Converter Most buck converters are designed for continuous-current operation (CCM). The choice of switching frequency and inductance to give continuous current is given by equation Eq-14, and the output ripple is described by equation Eq-17. Note that as the switching frequency increases, the minimum size of the inductor to produce continuous current and the minimum size of the capacitor to limit output ripple both decrease. Therefore, high switching frequencies are desirable to reduce the size of both the inductor and the capacitor. The trade-off for high switching frequencies is increased power loss in the switches. Increased power loss for the switches, decreases the converter’s efficiency, and the large heat sink required for the transistor switch offsets the reduction in size of the inductor and capacitor. Typical switching frequencies are in the 200-1000 kHz range. As switching devices improve, switching frequencies will increase. The inductor wire must be rated at the rms current, and the core should not saturate for peak inductor 3 current. The capacitor must be selected to limit the output ripple to the design specification, to withstand peak output voltage, and to carry the required rms current. The switch and diode must withstand maximum voltage stress when off and maximum current when on. The temperature ratings must not be exceeded, possibly requiring a heat sink.. EQUIPMENT REQUIRED a) DC Power Supply b) Function Generator c) Breadboard d) Resistors e) 0.1uF and 10uF Capacitors f) 22uH Inductor g) SR150 Schottky Diode i) PBSS303 PNP Power Transistor PROCEDURE Section A: Effect of frequency and size of inductor on the output voltage 1. Construct the circuit shown in Fig. 2 below. Use the following value for each component. Inductor 100uH R 1kohm Cin 470uF, 10V Cout 470uF, 10V Sch. Diode 30V, 1A Vin 3.5Vdc 4 Fig. 2 2. Connect the LED load to the output. 3. Set the function generator to the following settings and record down the required data in Table 1 Inductor 100uH Frequency 400Khz Duty Cycle 50% Output Voltage Output Current Output Power Input Power Efficiency Table 1 4. Change the frequency and duty cycle as shown below and record down the data in Table 2. Inductor 100uH Frequency 800Khz Duty Cycle 50% Output Voltage Output Power Input Power Efficiency Table 2 5 5. Change the inductor to 22uH and repeat step 3 and 4. Record down and fill in Table 3 ad 4. Inductor 22uH Frequency 400Khz Duty Cycle 50% Output Voltage Output Power Input Power Efficiency Table 3 Inductor 22uH Frequency 800Khz Duty Cycle 50% Output Voltage Output Power Input Power Efficiency Table 4 6. The equation for calculating the inductor value is: L = (Vin-Vo) x D / (f x Δi) where D is the duty cycle, f is the frequency and Δi is the ripple current. 7. Calculate the best inductor value and verify its efficiency. 6 Section B: Effect of capacitor value on the efficiency of the converter 1. The equation for calculating the capacitor value is: C = Δi / (f x 8 x ΔV), where Δi = ripple current f= frequency and Δi = output ripple voltage. 2. Select the appropriate capacitor value and verify if it provides better efficiency. Support your answer with some experimental data. CONCLUSION Based on your experiment, make an overall conclusion for step down converters. 7