Download 1 - Southern University College

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
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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
_____
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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
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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
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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
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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.
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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.
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