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FACULTY OF ENGINEERING
LAB SHEET
EEE 3076 POWER ELECTRONICS
TRIMESTER 1, 2016-2017
PE 1 – Power Semiconductor Switches
PE 2 –DC-DC Buck Converter

Note: On-the-spot evaluation will be carried out during or at the end of the
experiment. Students are advised to read through this lab sheet before doing
experiment. Your performance, teamwork effort, and learning attitude will count
towards the marks.
Technical part of this lab sheet has been revised, there may be some grammatical errors. We
will correct them in next version. Apologies for any inconveniences caused.
EEE3076 Power Electronics: PE1 & PE2
2016/2017
Experiment PE1
POWER SEMICONDUCTOR SWITCHES
A. OBJECTIVES:
1) To demonstrate a practical go/no-go method of testing an SCR with a multimeter
2) To study the turn-on/turn-off states of the SCR
3) To study the effects of gate current on SCR and determine the minimum holding current
to keep the SCR conducting
4) To study the switching parameters of an npn BJT
B. LEARNING OUTCOME OF SUBJECTS:
This experiment will help student to achieve one of the learning of outcomes of the subject
which is
LO1 - Select an appropriate power semiconductor switch for converters based on given input and
output specifications. (cognitive – applying, level 3)
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C. MATERIALS REQUIRED:
a) Equipment
1. DC Power Supply (variable 0 to 15V)
1
2. Digital multimeter
1
3. Dual channel oscilloscope
1
4. Function generator
1
5. Breadboard
1
b) Electronic components
1. SCR: C106D (could be different model)
2. npn BJT: BC548 or BC547 or equivalent
3. Voltage regulator IC 7805 (+5V, 1A)
4. Signal diode: 1N4148 or equivalent
5. Single turn potentiometer (linear): 100k or 2 x 50k
6. Resistor: 10/0.25W
7. Resistor: 100/0.25W
8. Resistor: 1k/0.25W
9. Resistor: 2k/0.25W
10. Resistor: 10k/0.25W
11. Resistor: 22k/0.25W
12. Resistor: 1M/0.25W
13. Resistor: 100/2W
14. Ceramic disc capacitor: 1nF
15. Electrolytic capacitor: 47F/(16V or above)
16. Electrolytic capacitor: 100F/(16V or above)
17. Inductor: 100H/0.29, 0.79A
1
1
1
2
1
1
1
2
1
1
1
2
1
2
2
1
1
D. INTRODUCTION
1. Introduction of SCR
The silicon-controlled rectifier (SCR) is a four-layer pnpn bipolar semiconductor device
with three terminals, as shown in Fig-1. The SCR belongs to the thyristor family of
electronic devices, which operates on the principle of current conduction when the break
over voltage is reached. An SCR has an anode, a cathode and a gate terminal. A gate
terminal can also trigger the device into conduction below the break over voltage level. It
operates similar to a normal diode, where current flows only in the forward-biased condition
but must be triggered into conduction by the gate terminal. Once the SCR is triggered into
conduction, it acts like a latched switch, and the gate no longer has control of the current
flow through the SCR.
Anode (A)
Gate (G)
Anode (A)
Gate (G)
P
N
P
N
+A
Metal
case
Cathode (C)
device
structure
Cathode (C)
circuit
symbol
G
-C
Cathode
Anode
Gate
Plastic
case
Gate (G)
Plastic
Anode (A)
case
Cathode (C)
Practical packages
Fig-1: Device structure, circuit symbol and practical packages
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2. Operation of SCR
Fig-2 shows schematically the basic operation of a SCR. The anode is connected through a
series-limiting resistor RL to a positive voltage. The cathode is connected to ground via
switch S2 and the gate is connected to switch S1, which is connected to ground. Under this
configuration as in Fig-2(a), junction 1 and 3 (i.e. J1 and J3) are forward-biased but junction
2 (J2) is reverse biased, which prevents any appreciable current from flowing through the
SCR. When S1 is moved up to the bottom side of RA as in Fig-2(b), a small gate current
flows into the gate (electrons flow out from the gate). This introduces holes into the p-type
gate region, which induce electron-injection across J3 into the p-type gate layer. The
electrons will diffuse across the p-type layer and be swept across J2 by the localized field at
J2 into the upper n-layer. These electrons in the n-layer will induce hole-injection across J1
into the upper n-layer. The holes will diffuse across the n-layer and be swept across J2 into
the p-type gate layer. A new cycle of induced process will begin but the holes are generated
internally, not by the gate current. This cyclic process is called regenerative process, which
speeds up the SCR into conduction state without the help of the gate current anymore. The
SCR is in heavy minority carrier injection and brings J2 to forward-bias (saturation
condition). Now, the gate can be set back to ground via S1 and RG as in Fig-2(c), the large
current flowing through the SCR is on or latched. The SCR can only be turned off if this
main current flowing from the anode to the cathode is reduced below its minimum holding
current (IH). This can be accomplished by momentarily opening switch S2 in the cathode
lead of the circuit. The SCR can be considered reset or off. The SCR can be turned on again
by the gate current triggering.
VAA=+12V
(a)
(b)
RL=100
RA=22k
Anode (A)
RA=22k
--P
+++
IG
J1 fwd
-N- biased
+
J2 rev
Y Gate
-P
-+- +
Y Gate (G)
(G)
+- -+-+ J3 fwd
X
X
S1
N
S1
+ + + biased
Cathode (C)
RG=10k
RG=10k
Y S2
X
VAA=+12V
RL=100
(c)
VAA=+12V
RL=100
RA=22k
Anode (A)
Anode
(A)
--P
P
+ + + J1 fwd
+
+
+
-N- - - - J1 fwd
- - - J2 fwd
-N- - biased
J2 fwd
Gate
Y
+P+ +
+P+ +
(G)
+- -+-+
+- -+-+
J3 fwd
J3 fwd
X
N
S1
N
+++
+ + + biased
Cathode (C)
Cathode (C)
RG=10k
Y S2
Y S2
X
X
Fig-2: Operation of an SCR: (a) off condition, (b) triggering on; (c) on condition without triggering
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3. Current-Voltage Characteristics of an SCR
The SCR operates similar to a normal diode when in the reverse biased condition, as shown
in Fig-3(a). The SCR exhibits very high internal impedance, with perhaps a slight reverse
blocking current. However, if the reverse breakdown voltage is exceeded, the reverse current
rapidly increases to a large value and may destroy the SCR. In the forward bias condition
(gate is grounded), the internal impedance of the SCR is very high with a small current
flowing called the forward blocking current. When the forward voltage (+VF) is increased
beyond the forward break over voltage point, an avalanche breakdown occurs and the
current from the cathode to anode increases rapidly. A regenerative action occurs with the
conduction of p-n junctions and the internal impedance of the SCR decreases. This results in
a decrease in voltage across the anode and the cathode as verified by Ohm's law where V =
IR. When R is small, so is the voltage drop across it. The forward current flowing through
the SCR is limited primarily by the impedance of the external circuit, and the SCR will
remain on as long as this current does not fall below the holding current. If the gate current
is allowed to flow as shown in Fig-3(b), the forward break over point will be smaller. The
larger the gate current flows, the lower the point at which forward break over will occur, as
illustrated in Fig-3(c). Normally, SCRs operated with applied voltage lower than the forward
break over voltage point (with no gate current flowing) and the gate triggering current is
made sufficiently large to ensure complete turn on.
IF
+V
High current
(on condition)
RL
Holding Current
+A
Reverse break over voltage
G
Regenerative action
-V
-VR
RL
-C
RG
VF
+A
G
Reverse blocking current
(off condition)
-C
RG
(a)
Forward break over voltage
+V (variable)
+V
IF
RL
(b)
IA
A +
RG
-IR
Forward blocking current (off condition)
G
IG
+
VG
-
IA
IG2 > IG1 >
IG0
VAC
C
VF
VAK
(c)
Fig-3: I-V characteristics for an SCR, (a) I-V curves, (b) test for gate current, (c) gate current curves.
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4. Switching parameters of BJT
Bipolar junction transistors (BJT) are moderate speed switches in among the power
semiconductor switches. It is because carriers (electrons and holes) are collected at the BE
junction during on state. During switching off, these carriers have to be removed before the
depletion layer at the BE junction starts to develop and turn off the BJT. During switching
on, carriers have also to be collected at the junction before the BJT starts to turn on. Finite
times are required for the BJT to fully turn on and fully turn off. Below are four defined
switching parameters, which can be used to characterize the BJT switching characteristics
for a given test circuit with conditions. td is the turn-on delay time, tf the fall time of vCE, ts
the storage time and tr the rise time of vCE. The switching-on time is tsw-on = td + tf and
switching-off time is tsw-off = ts + tr. tPW is the negative-going pulse-width of vCE.
VI (max)
+VCC
vI
vCE
0.1VI (max)
0V
VCE(max)
0.9VCE(max)
RB
0.5VCE(max)
VI (max)
RC
DUT
0.1VCE(max)
0V
0V
tP
at DC, tr & tf
(b)
tPW
td
tf
tsw-on
ts
(a)
tr
tsw-off
Fig-4: (a) Typical vI and vCE waveforms of npn BJT. (b) A simple test circuit for npn BJT.
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E. EXPERIMENT
Experimental effort evaluation
Student is expected to use multimeter and oscilloscope for all measurement
and monitoring during this experiments.
Part I: SCR switch
Know your SCR before starting the SCR experiments (refer to Appendices)
Section 1: Testing an SCR with a digital multimeter (diode test mode)
Procedures:
Connect the circuit as shown in Fig-7.
Set the multimeter in DIODE TEST MODE.
Set the position of the switches S1 and S2 as indicated in sequence no.1 in Table 1.
Record the meter reading for each sequential setting of the switches as shown in Table 1.
Meter reading (in diode test mode): A 3- or 4-digit number means the device is
conducting current with voltage drops in V or mV (depended on meter used). A “1”
displayed at the left means the device is not conducting current.
4) Complete table 1 by repeating step (3). Follow the sequence from No 1 to 4.
1)
2)
3)
Results:
S2
S1
Table 1
No.
S1
S2
1
2
3
4
Close
Close
Close
Open
Open
Close
Open
Open
Multimeter
Reading
A
State
(On/Off)
+
Multimeter
-
G
K
Fig-7: Go/no-go testing of SCR
Questions:
i)
Compare the measured readings in Table 1 and briefly explain how the observations of
these readings relate to the conduction states of the SCR.
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Section 2: Basic operation of an SCR
VAA=12V
Results:
RA=22k
Table 2
Sequence No.
1
2
3
4
5
S1
X
Y
X
X
X
S2
close
close
close
open
close
VAK/V
VGK/V
RL=100/2W
A
State
Y
S1
X
RG=10k
RG=1k
+
VGK
-
G
K
S2
+
VAK
-
Fig-8: Test circuit of an SCR
Procedures:
Connect the circuit shown in Fig-8.
Set the position of the switches S1 and S2 as indicated in sequence no.1 in Table 2 and
then apply power to the circuit. Record the readings of VGK and VAK and indicate the
states of the SCR for each sequential setting as in Table 2.
3) Complete table 2 by repeating step (2). Follow the
experimental sequence from 1 to 5.
1)
2)
Questions:
i)
ii)
Before firing (triggering), what is the VAK? Give reason to support your answer.
What is the VAK when the SCR is conducting? Give reason to support your answer.
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Section 3: Current control of an SCR
VAA=12V
RB=
22k
RA=
2 x 1M
RL=100/2W
A
Y
S1
RG=1k G
K
+ IG
VGK
-
X
+
VAK
-
(a)
VAA=12V
RH =2 x 50k
or 100k
RL=2k
RB=
22k
IA
A
G
S1
K
+
VAK
-
+
+
VRS
-
+
Multimeter
-
RG=1k
RS=100/
0.25W
Multimeter
-
(b)
Fig-9: Current control of an SCR, (a) gate current control, (b) holding current control
Section 3.1 Gate Current Control
Procedures:
1) Remove power supply and modify the circuit in Fig-8 as shown in Fig-9(a).
2) Set the switch S1 at position X and then apply power to the circuit. Record the voltage
VGK and VAK.
3) Move Switch S1 to position Y. Record VGK and VAK.
4) Turn-off the SCR and repeat step 2) and 3) to confirm your results.
Results:
Step 2) VGK = ________ , VAK = ________
Step 3) VGK = ________ , VAK = ________
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Questions:
i) What is the current flowing through the gate (IG) in step 2)? Is the SCR on or off? Why?
ii) What is the current flowing through the gate (IG) in step 3)? Is the SCR on or off? Why?
Section 3.2 Holding Current Control
Procedures:
1)
2)
3)
4)
5)
6)
Remove the power supply and modify the circuit in Fig-9(a) as shown in Fig-9(b).
Set the wipers of the potentiometers RH so that the resistance is 0.
Ensure that the switch is opened.
Make firm connections to the multimeters as shown in Fig-9(b).
Set the multimeter in DC 2V range.
Apply power supply to the circuit and close the switch S1 and then open it again. Record
the voltage VAK and VRS in Table 3.
7) Slowly adjust RH and record VAK and VRS in Table 3 with VAK change (VAK) at
approximately 0.02V (Note: VAK will decrease and then increase again). The record ends
when the reading in VAK suddenly jumps to ~12V.
Results:
Table 3
VAK/V
VRS/V
(VAK ~ 0.02V)
Questions:
VRS
. Comment on your graph.
RS
ii) From the graph, determine the holding current.
i) Plot a graph IA versus VAK, where I A 
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Part II: BJT switch
Before starting these experiments:
1. Test your BJT and diodes.
2. Check your voltage probes, oscilloscope and function generator.
Fixed +5V power supply
Construct a fixed +5V voltage source as shown in Fig-10. This output will be the VS in the
circuits in Fig-11.
+9V to +10V
IN
7805
+
From adjustable output
DC power supply
+5V
OUT
+
COM
47F
47F
Constant output voltage
as VS in Fig-11.
Fig-10: Voltage regulator IC 7805 circuit for fixed +5V output.
Section 4: Temporal switching behaviour of BJT and effect of inductor
Note: Waveforms must be drawn on a common time axis as shown in the figure in each
section. Each waveform has its own vertical scale with its ground level (channel position) at
one of the vertical major grid position, e.g. at +2 division means at 2 divisions above
centre of the vertical axis. Use a single page graph paper to draw all the waveforms.
P4
VS = +5V RS=100/2W
CI = +
100F
P1
RB1=1k P2
CB=1nF
(a)
P3 B
RB2=
100
C
Q (npn)
E
DB=1N4148
Fig-11: A simple BJT switching circuit
Section 4.1: BJT temporal switching behavior


Caution when using the electrolytic capacitor: The polarity of the
capacitor must be connected correctly, otherwise, explosion may occur.
Caution when using the function generator: Never short-circuit the
output, which may burn the output stage of the function generator.
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Procedures:
1) Construct the circuit as shown in Fig-11.
2) Oscilloscope settings: You must use VOLTS/DIV and TIME/DIV values as
mentioned in each part, if any. Channel POSITION must be put at one of the vertical
major grid position. Set AC/GND/DC input coupling switches at DC. Make sure the
VARIABLE knobs for CH1, CH2 and time base at the CAL positions.
3) Function generator settings: Select square-wave mode and set frequency at 40kHz.
Connect the function generator output to CH1. Set the output voltage amplitude to 5V
(or peak-to-peak to 10V). Set the Oscilloscope settings (volt/div, time/div
accordingly to get a stable waveform)
4) Skill to draw voltage waveforms: The HORIZONTAL position knob should not be
moved before all the waveforms, which share a common time axis, are drawn. Draw the
waveform by using one-to-one scale, i.e. 1 cm on the graph paper is equivalent to 1
division on the oscilloscope screen. Draw the ground level and locate some of the
important points, e.g. maximum and minimum points, turning points, points where the
waveform cuts through the ground level and the major grids. Connect the points together
by a smooth curve.
5) Using graph paper (Note: this section and next section use the same time axis. Start
P1 waveform at top-left corner of the graph paper), draw the voltage waveforms at
P2, P3 and P4 with respect to the reference voltage waveform at P1 to show the
detailed v(t) and t relationships among them. To do this:
i) Connect CH1 (5V/div, ground level: +2 division, trigger edge: +) to P1 and draw
the waveform (keep this connected all the time),
vP1
ii) Connect CH2 (2V/div) to P2 and draw the waveform,
iii) Connect CH2 (2V/div) to P3 and draw the waveform,
vP2
iv) Connect CH2 (2V/div) to P4 and draw the waveform.
With
vP3
CB
v) Now, remove capacitor CB from the circuit, Connect CH2 (2V/div)
to P2 and draw the waveform. (w/o CB)
vP4
Note: You must draw the waveforms as shown at the left and
indicate the ground level of each waveform. State the volt/div and
time/div used on the graph paper.
w/o
CB
t
t
t
t
vP2
Questions:
i) vP2 waveform with CB: Explain why there are positive spike and negative spike?
ii) vP3 waveform with CB: Comment and explain on the waveform.
iii) vP4 waveform with CB: Comment and explain on the waveform.
iv) vP2 waveform without CB: Comment on this waveform to the vP2 waveform with CB.
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Lab report format, evaluation and submission
 The report should consists of
a). Results and answers for all the questions (Please write down the corresponding step or
procedure number as the identification of your answer in appropriate order),
b). Discussion and Conclusion.
c). Lab report to be submitted to the lab technician within one week from the experiment
date.
End of Lab sheet
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Tables Sheet (To be included in lab report)
Section 1: Testing an SCR with a digital multimeter in diode test mode
Table 1
No.
S1
S2
1
2
3
4
Close
Close
Close
Open
Open
Close
Open
Open
Multimeter
Reading
State
(On/Off)
Section 2: Basic operation of an SCR
Table 2
Sequence No.
1
2
3
4
5
S1
X
Y
X
X
X
S2
close
close
close
open
close
VAK/V
VGK/V
State
Section 3.1 Gate Current Control
Step 2) VGK = ________ , VAK = ________
Step 3) VGK = ________ , VAK = ________
Section 3.2 Holding Current Control
Table 3
VAK/V
VRS/V
Section 4.1: BJT temporal switching behavior
Please sketch the following on a graph paper.
To be verified by Lab Supervisor (Stamp and
Signature)
vP1
t
vP2
With
CB
t
vP3
t
vP4
w/o
CB
t
vP2
………………………………………………
t
1s/div
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Experiment PE2
DC-DC BUCK CONVERTER
A. OBJECTIVES:
1) To study the operation principle of a dc-dc converter
2) To provide hand-on design experience on a basic converter circuit
B. LEARNING OUTCOME OF SUBJECTS:
This experiment will help student to achieve three of the learning of outcomes of the subject
which are:
LO3 - Design a Buck/Boost converter for resistive-inductive loads. (cognitive – creating, level 6)
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C. MATERIAL REQUIRED:
a) Equipment
1. DC power supply (variable 0 – 15V, fixed +5V)
2. Digital multimeter
3. Dual channel oscilloscope
4. Function generator
5. Breadboard
b) Electronic components
1. npn BJT: BC548 or BC547 or equivalent
2. pnp BJT: 2N2905 or 2N4403 or BFY64 or equivalent
3. Voltage regulator IC 7805 (+5V, 1A)
4. Schottky diode: 1N5817/18/19 or equivalent
5. Signal diode: 1N4148 or equivalent
6. Resistor: 100/0.25W
7. Resistor : 100/02W
8. Resistor: 1k/0.25W
9. Resistor: 47/1W
10. Resistor: 22/2W
11. Resistor: 33/2W
12. Ceramic disc capacitor: 1nF
13. Electrolytic capacitors: 47F/(16V or above)
14. Electrolytic capacitors: 100F/(16V or above)
15. Inductor: 100H/0.29, 0.79A
16. LED
17. DC Motor Fan (12V)
1
1
1
1
2
1
1
1
1
1
3
1
1
1
1
1
1
2
2
1
1
1
+
D. INTRODUCTION
+
VO
VS
1. Basic Switching Converter
RL
V
In a switching converter circuit, the transistor operates as an electronic switch by beingO
completely on or off. This circuit is known as a dc chopper circuit. Assuming
the switch is
ton
ideal in Fig-1, the output is the same as the input when the switch is closed, and the output is
zero when the switch is open.
VS
+
+
VS
VO
VS
RL
-
VO
VO-
-
RL
T
DT
-
ton
+
+
VS
VO
-
RL(a)
VS
VO
(b)
ton
+
VS
VS
-
+
-
VO
RL
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closed
RL
VO
closed
+
VS
+
-
open
(c) open
Fig-1 : (a) basic dc-dc switching
t
(b) switching
equivalent,
(c) output voltage
converter,
closed
T
closed
t
DT
(1-D)T
T
DT
open
closed
+
+
(1-D)T
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The periodic openings and closings of the switch results in the pulsed output shown in Fig1(c). The average or dc component (using capital letter) of the output is
1 T
1 DT
Vo   v o ( t )dt   v s ( t )dt  Vs D
(Eq-1)
T 0
T 0
The dc component of the output is controlled by adjusting the duty cycle ratio D, which is
the fraction of the period that the switch is closed, i.e.
t on
t
D
 on  t on f
(Eq-2)
t on  t off
T
where f is the switching frequency in hertz. The dc component of the output will be less than
or equal to the input for this circuit.
The power absorbed by the idea switch is zero. When the switch is open, there is no current
in it; when the switch is closed, there is no voltage across it. Therefore, all power is
absorbed by the load, and the energy efficiency is 100%. Losses will occur in a real switch
because the voltage across will not be zero when it is on, and the switch must pass through
the linear region when making from one state to the other.
-
VL
+
In practice, switching is done using various method depending
on theL type of switches used,
i
Switch
i
e.g. MOSFET or BJT. In this experiment, BJT is used hence, the switching pulse
+(as base
i
+
+ pulse is produced
current) is given to the base terminal of BJT. The
separately by a pulse
VS
VX
RL
C
VO
circuit (Fig-8)
R
L
C
-
-
2. The Buck Converter
Low Pass Filter
L
Switch
VS
VX
C
+
+
-
VS
RL
VO
L
Switch
iR
iC
-
-
+
iL
+
+
VL = VS - VO
-
VL
+
VX = VS
-
-
-
+
+
C
VO
-
Low Pass Filter
(a)
(b)
VL = VS - VO
+VL = - VO+L
-
Switch
Switch
+
+
VS
+
L
VX+= VS
VS-
VX = 0
-
-
-
+
+
C
C
VO
VO
-
RL
RL
(c)
Fig-2: (a) Buck dc-dc converter, (b) Equivalent circuit for switch closed, (c) Equivalent
circuit for switch open.
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Controlling the dc component of a pulsed output of the type in Fig-1(c) may be sufficient for
some applications, but often the objective is to produce an output that is purely dc. One way
of obtaining a dc output from the circuit of Fig-1(a) is to insert a low-pass filter after the
switch. Fig-2(a) shows an inductor-capacitor (L-C) low-pass filter added to the basic
converter. The diode provides path for the inductor current when the switch is opened and is
reverse biased when the switch is closed. The circuit is called a buck converter or a down
converter because the output voltage is less than the input.
a) Voltage and Current Relationships
If the low-pass filter is ideal, the output voltage is the average of the input voltage to the
filter. The input to the filter, vx in Fig-2(a), is Vs when the switch is closed and is zero when
the switch is open, provided that the inductor current remains positive, keeping the diode on.
If the switch is closed periodically at a duty ratio D, the average voltage at the filter input is
Vs D, as seen by Eq-1.
This analysis assumes that the diode remains forward biased for the entire time that the
switch is open, implying that the inductor current remains positive. An inductor current that
remains positive throughout the switching period is known as continuous current.
Conversely, discontinuous current is characterized by the inductor current returning to zero
during each period.
The buck converter (and dc-dc converters in general) has the following properties when
operating in the steady state:
1. the inductor current is periodic:
iL ( t  T )  iL ( t )
(Eq-3)
2. The average inductor voltage is zero:
VL 
1 t T
v (  )d  0
T t L
(Eq-4)
3. The average capacitor current is zero:
1 t T
iC (  )d  0
(Eq-5)
T t
4. The power supplied by the source is the same as the power delivered to the load. For
non-ideal components, the source also supplies the losses:
IC 
Ps = Po (ideal)
and Ps = Po + losses
(non-ideal)
(Eq-6)
Analysis of the buck converter of Fig-2(a) begins by making these assumptions:
1. The circuit is operating in the steady state.
2. The inductor current is continuous (always positive)
3. The capacitor is very large, and the output voltage is held constant at voltage Vo. This
restriction will be relaxed later to show the effects of finite capacitance.
4. The switching period is T; the switch is closed for the time DT & open for time (1- D)T
5. The components are ideal.
The key to the analysis for determining the output Vo is to examine the inductor current and
inductor voltage first for the switch closed and then for the switch open. The net change in
inductor current over one period must be zero for steady-state operation. The average
inductor voltage is zero.
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b) Analysis for the switch closed
When the switch is closed in the buck converter circuit of Fig-2(a), the diode is reversebiased and Fig-2(b) is an equivalent circuit. The voltage across the inductor is
v L  Vs  Vo  L
diL
dt
Rearranging,
diL Vs  Vo
with switch closed

dt
L
Since the derivative of the current is a positive constant, the current increases linearly, as
shown in Fig-3(b). The change in current while the switch is closed is computed by
modifying the preceding equation:
diL iL iL Vs  Vo



dt
t
DT
L
 ( iL )closed  (
Vs  Vo
) DT
L
(Eq-7)
c) Analysis for the switch open
When the switch is open, the diode becomes forward biased to carry the inductor current,
and the equivalent circuit of Fig-2(c) applies. The voltage across the inductor when the
switch is open is
di
v L  Vo  L L
dt
Rearranging,
diL
V
 o
with switch open
dt
L
The derivative of current in the inductor is a negative constant value, and the current
decreases linearly, as shown in Fig-3(b). The change in inductor current when the switch is
open is
iL
iL
V

 o
t (1  D)T
L
V
( iL )open  ( o )(1  D )T
(Eq-8)
L
Steady-state operation requires that the inductor current at the end of the switching cycle be
the same as that at the beginning, meaning that the net change in inductor current over one
period is zero. This requires
( iL )open  ( iL )closed  0
Using equation Eq-7 and Eq-8,
(
Solving for Vo,
Vs  Vo
V 
) DT   o  (1  D )T  0
 L
L
Vo  Vs D
(Eq-9)
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This gives the same result as equation Eq-1. The bulk converter produces an output, which
is less than or equal to the input.
An alternative derivation of the output voltage is based on the inductor voltage, as shown in
Fig-3(a). Since the average inductor voltage is zero for periodic operation,
VL  (Vs  Vo ) DT  ( Vo )(1  D )T  0
Solving the preceding equation for V0 yields the same result as equation Eq-9, Vo= VsD.
vL
Vs-Vo
-Vo
iL
Imax
Imin
(a)
t
DT
Fig-3: (a) Inductor voltage, vL,
(b) Inductor current, iL,
(c) Capacitor current, iC.
T
iL
IR
(b)
t
iC
t
iL
(c)
Note that the output voltage depends only on the input and the duty ratio D. If the input
voltage fluctuates, the output voltage can be regulated by adjusting the duty ratio
appropriately. A feedback loop is required to sample the output voltage, compare it to a
reference and set the duty cycle of the switch accordingly.
The average inductor current must be the same as the average current in the load resistor,
since the average capacitor current must be zero for steady-state operation (IL = IC + IR):
IL  IR 
Vo
R
(Eq-10)
Since the change in inductor current is known from equations Eq-7 and Eq-8, the maximum
and minimum values of the inductor current are computed as
i L
2
 1 (1  D ) 
V
1 V

 o   o (1  D )T   Vo  
R 2 L
2 Lf 

R
i
I min  I L  L
2
 1 (1  D ) 
V
1 V

 o   o (1  D )T   Vo  
R 2 L
2 Lf 

R
I max  I L 
(Eq-11)
(Eq-12)
where f = 1/T is the switching frequency in hertz.
Equation Eq-12 can be used to determine the combination of L and f that will result in
continuous current. Since I min= 0 is the boundary between continuous and discontinuous
current.
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 1 (1  D ) 
I min  0  Vo  
2 Lf 
R
(1  D ) R
( Lf ) min 
2
Vs – Vo
0
vL
(Eq-14)
iL
1T
DT
Vo
(Eq-13)
2T
T
Fig-4: Discontinuous current
d) Discontinuous current operation
Fig-4 shows the regions for iL > 0 (during DT and 1T) and iL = 0 (2T). During the interval
2T, the power to the load is supplied by the filter capacitor alone. Again, equating the
integral of the inductor voltage over one T to zero (equation Eq-4) yields
(Vs – Vo)DT + (-Vo)1T = 0
Vo
D

Vs D  1
(Eq-15)
e) Output voltage Ripple
In the preceding analysis, the capacitor was assumed to be very large to keep the output
voltage constant. In practice, the output voltage cannot be kept perfectly constant with a
finite capacitance. The variation in output voltage, or ripple, is computed from the voltage
current relationship of the capacitor. The current in the capacitor is iC  iL  iR , as shown in
Fig-5(a), where I C  I L  I R . Assuming I L  I R  Vo / R , then I C  I L .
While the capacitor current is positive, the capacitor is charging. From the definition of
Q
Q  CVo
Vo 
capacitance, Q  CVo
C
iC
Q
iL/2
t
T/2
Fig-5: (a) Capacitor current, iC,
(b) capacitor ripple voltage
(a)
vo
Vo
Vo
t
(b)
The change in charge, Q is the area of the triangle above the time axis;
Q 
1  T   iL  TiL

2  2   2 
8
Thus,
T iL
8C
Using equation Eq-8 for iL,
Vo 
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TVo
Vo
(Eq-16)
(1  D)T 
(1  D)
8CL
8CLf 2
In this equation, V0 is the peak-to-peak ripple voltage at the output, as shown in Fig-5(b). it
is also useful to express the ripple as a fraction of the output voltage:
Vo 
Vo (1  D)
f 
2
(1  D) c 
=

2
Vo
2
8CLf
 f 
where f c 
2
(Eq-17)
1
is the corner frequency of the low pass LC filter.
2 LC
If the ripple is not large, the assumption of a constant output is reasonable and the preceding
analysis is essentially valid.
3. Design Considerations
Most buck converters are designed for continuous-current operation. 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 20-50kHz 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 current. The capacitor must be selected to limit the output ripple to the design
specifications, 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.
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E. Experiment: Buck Converter
Student must show multimeter reading, oscilloscope display, etc to lab
experiment supervisor before proceeding to the next section. The
experimental effort evaluation is performed within 3-hour lab session
only.
Before starting experiments:
1. Test your BJTs and diodes.
2. Check your voltage probes, oscilloscope and function generator.
Procedures
1) Construct a fixed +5V voltage source as shown in Fig-6. This output (terminal X1 and
Y1) will be the VS in the circuits in Fig-7 and 8.
+9 V to +10 V
IN
OUT
7805
from DC
power supply
X1
47µF
47µF
Constant 5 V
output
Y1
Fig-6: Voltage regulator IC 7805 circuit for fixed +5V output.
2) Measure voltage across X1-Y1 and make sure voltage value is 5 V.
3) Construct the Pulse Circuit (Fig 7) on a breadboard.
X1
R1
100 
B1
R2
100 
P1
R4
1 k
R3
100 
C
B
From Function
Generator
C
1 nF
D1
IN4148
Q2
E
Y1
Fig-7: Pulse circuit
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4) Function generator settings: Select square-wave mode and set frequency at 62.5 kHz.
DO NOT RELY ON READING AT THE FUNCTION GENERATOR; YOU NEED TO
CHECK THE PERIOD (T) AT THE OSCILOSCOPE (EXAMPLE, FOR FREQUENCY
62.5 kHz, PERIOD IS 1/62.5kHz = 16 s). Make sure the duty cycle is 50%
(RAMP/PULSE knob is pushed in). Connect the function generator output to CH1,
check if the square wave pulse is correct. Set output voltage amplitude to 5V or peakto-peak 10V
5) Connect terminal P1 to channel 1 (DO NOT REMOVE THIS CONNECTION
THORUGHOUT THIS EXPERIMENT). Connect terminal B1 to Channel 2. Verify the
results. You may check with the lab supervisor.
6) Construct the Buck converter on a separate breadboard. Circuit diagram for the circuit is
shown in Fig-8. Caution when using the electrolytic capacitor for buck converter circuit,
the polarity of the capacitor must be connected correctly, otherwise, explosion may occur
Q1
E
X1
L
C
A1
100µH
B
+
B1
100µF
DF
C
Schottky
diode
Y1
VO
RL
22  /2 W
A2
Fig-8: Buck converter circuit
7) Connect terminals B1 (from the pulse circuit) to B1 (at the buck converter circuit).
8) Connect terminal A1 to channel 2. This is to capture the output voltage signal. Verify the
results. You may check with the lab supervisor.
9) Plot both the signals from terminals P1 and A1 on a graph paper. Make sure the time and
volt division information are stated on the graph paper.
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SECTION 1: Study of Effect of Switching Frequency on the output current ripple
In this section, effect of switching frequency on the output current ripple will be
studied. At the end of this section, students shall be able to relate the effect of switching
frequencies on average output current and output current ripple.
1) Measure the Vmin and Vmax from oscilloscope, and Vaverage using Multimeter. Record in
table 1. [Since the output load is resistor, hence output current can be calculated by
equation V/RL].
Table 1
Switching
Frequency
(kHz)
Measurement
Vmin
Vmax
Vaverage
(V)
(V)
(V)
*Use
multimeter
Iout max
Iout min
(Vmax/RL)
(A)
(Vmin/RL)
(A)
Calculation
Iaverage
 Iout
(Iout max Iout min)
(Vaverage/
RL)
(A)
(A)
Output current
ripple
( Iout/
Iaverage)
(%)
62.5
40
20
2) Set switching frequencies to other value as indicated in Table 1 and repeat step 1.
3) Plot graph switching frequencies versus output current ripple. Analyze the graph.
SECTION 2: Effect of duty cycle on output voltage V0 and its ripple V0
This section is to observe the effect of the duty cycle on the output voltage and the
output voltage ripple. The experimental results will be compared to the theoretical
values. From these comparisons, you may reveal some of the non-ideal components
that affect the efficiency of the Buck converter.
Procedures:
1) Set Function generator: 100kHz (T=10s) and 5V amplitude. Connect CH1 to P1,
CH2 to A1 and multimeter test leads across RL (Across Terminals A1 and A2).
2) By changing ton (adjust the function generator RAMP/PULSE knob with knob pulled
out), measure the corresponding V0 and V0 values as shown in table below. Here, ton is
the Q1 on-state time duration (seen from terminal P1), V0 the average output
voltage (read from multimeter) and V0 the peak-to-peak voltage at A1 (read from
CH2).
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Table 2
3
ton /s**
4
5
6
7
8
V0 /V
V0 /mV
**The frequency may be changed when ton is changed. Adjust the
frequency adjustment knob to return to 100kHz if necessary.
SECTION 3: Application of Buck Converter as a LED Dimmer and DC Motor Speed
Controller.
This section is to observe the dimming effect of a LED, when variable DC voltage is
supplied from the Buck converter. The variable voltage is achieved by varying the duty
cycle. The same concept it used to control speed of a dc motor.
Procedures:
1) Function generator settings : 100 kHz (T=10s) and 5 V amplitude.
2) Connect terminal A1 and A2 to a LED in series to a resistor, R as shown in Fig-9.
3) Vary the duty cycle to 20%, 50% and 80% and observe changes in light intensity from
the LED.
Q1
E
X1
L
C
A1
100µH
B
+
B1
100µF
DF
Schottky
diode
Y1
C
R
100 /0.5 W
VO
LED
A2
Fig-9: Buck converter circuit with LED load
4) Now, remove the LED and resistor from terminal A1 and A2.
5) Connect a DC motor fan to the terminals (A1 and A2)
6) Repeat step 3 and observe changes in fan speed.
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Tables Sheet (To be included in lab report)
Table 1
Switching
Frequency
(kHz)
Vmin
(V)
Measurement
Vmax
Vaverage
(V)
(V)
*Use
multimeter
Iout max
Iout min
(Vmax/RL)
(A)
(Vmin/RL)
(A)
Calculation
Iaverage
 Iout
(Iout max Iout min)
(Vaverage/
RL)
(A)
(A)
Output current
ripple
( Iout/
Iaverage)
(%)
62.5
40
20
Table 2
ton /s**
3
4
5
6
7
8
V0 /V
V0 /mV
-
Write comments based on your observation on the LED with the Buck converter.
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
…………………………………………………………………………………………………………....
-
What happen to speed of the fan when duty cycle is varied?
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
…………………………………………………………………………………………………………....
To be verified by Lab Supervisor (Stamp and
Signature)
………………………………………………
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Lab report format, evaluation and submission
 The report should consists of the following (Do not attached any other documents other
the following)
1. Faculty Lab Sheet front page
2. The following results and answer in order
Section 1, Step 5 : Graph Paper and Write-up based on the graph paper
Section 1, Table 1 : Fully filled up (using the Table Sheet), With stamp and signature of Lab
Supervisor
Section 1, Step 8 : Graph Paper and Write-up based on analysis
Section 2, Table 2 : Fully filled up (using the Table Sheet), With stamp and signature of Lab
Supervisor
Section 3 Step 3, Write-up based on observation
Section 3 Step 6, Write-up based on observation
3. Discussion and Conclusion.
4. Lab report to be submitted to the lab technician within one week from the date of the
experiment
End of Lab sheet
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APPENDICES
Component pin layout
Pin layout
7805
IN COM OUT
The Resistor color code
chart
Capacitance
ABC
.abc
AB x 10C pF 0.abc F
Potentiomete
r
Breadboard internal connection
25 holes connected horizontally
25 holes connected
horizontally
5 holes connected vertically
5 holes connected vertically
Diodes
1. The packaging of 1N4007 and 1N5819 is the same, check diode code written on the diode.
2. The packaging of 1N4148 and 1N5231 is the same, check diode code written on the diode.
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COMPONENT AND EQUIPMENT CHECKS
The go/no-go method of testing is used. Always do these checks before start your
experiment.
Diode and Zener diode checks
Use multimeter in “diode test” mode. A good diode will give a reading for forward-biased
test and no reading for reverse-biased test.
“Diode test” mode: “COM” terminal is negative “–“. “V, , mA” terminal is positive “+”.
Forward bias displays reading in mV, V or other unit. Reversed bias displays a symbol.
All these depend on the multimeter used. The test current is in mA (said 0.1mA, 1.5mA,
etc).
“Ohm mode” test: This mode cannot be used for most of the multimeter.
SCR check
Use multimeter in “diode test” mode. A good SCR will only give forward-biased readings for
GK (gate-to-cathode) test (gate is connected to “+” and cathode is connected to “–“) and
AK (anode-to-cathode) test after gate triggering (connect anode to gate momentarily).
Some SCRs (non-sensitive gate type) give forward-biased readings for KG test due to the
internal resistor connected in between the gate and the cathode (cathode short).
BJT check
1) Use multimeter in “diode test” mode. A good npn BJT will only give forward-biased readings
for BE test and BC test. A good pnp BJT will only give forward-biased readings for EB test and
CB test. Some BJTs (for inductive loads) give forward-biased readings for npn EC test or pnp
CE test due to the internal diode connected in between the collector and the emitter.
2) Use multimeter in “hFE test” mode. A good npn or pnp BJT will give an hFE reading within the
specification range of the BJT.
“hFE test” mode: An hFE test socket labeled with pnp and npn is provided in some multimeter panel.
Plug in the BJT being tested into the corresponding holes of the socket. The reading is the DC
current gain IC/IB. The test current for IB is in A (e.g 10A).
Oscilloscope voltage probe check
Use oscilloscope calibration (CAL) terminal. A good probe will give a waveform of positive
square wave with 2V peak-to-peak and about 1 kHz.
Oscilloscope channel check
Use oscilloscope calibration (CAL) terminal and a good voltage probe. A good input
channel will give the corresponding waveform of the CAL terminal.
Function generator check
Check the output waveform by oscilloscope. A good function generator will give a stable
waveform on the oscilloscope screen. Caution: Never short-circuit the output to
ground, this can burn the output stage of the function generator.
Resistors, capacitors and inductors
Resistors are hard to fail.
Capacitors are hard to fail except over-voltage or wrong connections of polar capacitors.
Inductors are hard to fail except coil burned by over-current.
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OSCILLOSCOPE INFORMATION
Below are the functions of switches/knobs/buttons:
INTENSITY knob: control brightness of displayed waveforms. Make sure the intensity is
not too high.
FOCUS knob: adjust for clearest line of displayed waveforms.
TRIG LEVEL knob: adjust for voltage level where triggering occur (push down to be
positive slope trigger and pull up to be negative slope trigger).
Trigger COUPLING switch: Select trigger mode. Use either AUTO or NORM.
Trigger SOURCE switch: Select the trigger source. Use either CH1 or CH2.
HOLDOFF knob: seldom be used. Stabilize trigger. Pull out the knob is CHOP operation.
This operation is used for displaying two low frequency waveforms at the same time.
X-Y button: seldom be used. Make sure this button is not pushed in.
POSITION (Horizontal) knob: control horizontal position of displayed waveforms. Make
sure that it is pushed in (pulled up to be ten times sweep magnification).
POSITION (vertical) knobs: control vertical positions of displayed waveforms. Pulled out
CH1 POSITION knob leads to alternately trigger of CH1 and CH2. Pulled out CH2
POSITION knob leads to inversion of CH2 waveform.
Time base:
TIME DIV: provide step selection of sweep rate in 1-2-5 step.
VARIABLE (for time div) knob: Provides continuously variable sweep rate by a factor of 5.
Make sure that it is in full clockwise (at the CAL position, i.e. calibrated sweep rate as
indicated at the time div knob).
Vertical deflection:
VOLTS DIV: provide step selection of deflection in 1-2-5 step.
VARIABLE (for volts div) knob: A smaller knob located at the center of VOLTS DIV
knob. Fine adjustment of sensitivity, with a factor of 1/3 or lower of the panel-indicated
value. Make sure that it is in full clockwise (at the CAL position). Pulled out knob leads
to increase the sensitivity of the panel-indicated value by a factor of 5 (x 5 MAG state).
Make sure that it is pushed down.
AC/GND/DC switches: select input coupling options for CH1 and CH2. AC: display AC
component of input signal on oscilloscope screen. DC: display AC + DC components of
input signal on oscilloscope screen. GND: display ground level on screen, incorporate
with AUTO trigger COUPLING selection).
CH1/CH2/DUAL/ADD switch: select the operation mode of the vertical deflection. CH1:
CH1 operates alone. CH2: CH2 operates alone. DUAL: Dual-channel operates with CH1
and CH2 swept alternately. This operation is used for displaying two high frequency
waveforms at the same time.
Note: Keep the oscilloscope ON. The oscilloscope needs an amount of warm up time for
stabilization.
CAUTION: Never allow the INTENSITY of the displayed waveforms too bright. This can
burn the screen material of the oscilloscope.
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