YEDITEPE UNIVERSITY
ENGINEERING & ARCHITECTURE FACULTY
INDUSTRIAL ELECTRONICS LABORATORY
EE 432 – INDUSTRIAL ELECTRONICS
Deniz
Yildirim
Digitally signed by Deniz
Yildirim
DN: CN = Deniz Yildirim, C
= TR, O = Istanbul
Technical University, OU =
Electrical Engineering
Reason: I am the author of
this document
Date: 2011.10.29 22:27:23
+03'00'
EXPERIMENT 4
SWITCHED–MODE DC/DC CONVERSION
USING BUCK CONVERTER
Introduction:
In this experiment, characteristics of DC/DC switched-mode converters will be
observed. An AC/DC converter using a linear regulator was constructed in experiment
1 and important characteristic values such as efficiency were measured. It was
observed that the efficiency of linear regulator was very low. A similar AC/DC
conversion using a switched–mode converter will be performed in this experiment and
it will be seen that how employment of solid state switching devices affect the
efficiency of such a converter.
Equipments:
537 34
726 80
726 86
734 02
735 01
735 02
735 09
735 046
735 095
735 261
735 341
Table 1. List of equipments
Variable Load Resistor (0-100Ω)
(two sets of resistors are needed)
Transformer 45/90, 3N
DC Power Supply
Reference Variable Generator
Bridge rectifier
Diode
Load Power Electronics
IGBT
2 x 1000µF Capacitors
Isolation Amplifier (x 2)
Control Unit PWM, PFM
DW-6060
Metra Hit 25S
AC Power Wattmeter
Multimeter (x 2)
Oscilloscope
General Information:
Electric and electronic devices and systems need to be supplied with power. If the
power is supplied from the mains network, adaptation of the voltage amplitude and
electrical isolation are usually required. These functions are performed by power
supplies which are available in several topologies.
Most of the linear power supplies consist of a transformer, rectifier, a charging
capacitor and a linear regulator (as you have done in the second experiment). A stable
current/voltage supply is achieved by the means of stabilization units. The
EE432 Industrial Electronics, Fall 2011
Experiment 4, page 1/7
Last updated October 29, 2011 10:25 PM by D. Yildirim
conventional stabilizer circuits consisting of linear regulators usually cause significant
power loss (as was observed in the second experiment). On the other hand, the use of
a proper switching circuitry instead of the linear regulator will be a much better
solution with consideration to power loss.
Before we proceed to the construction of the high efficiency AC/DC converter, let us
examine how a switched-mode DC/DC Converter works.
DC/DC Converters:
The DC/DC converters are also known as DC choppers where a fixed DC voltage
source is converted in to a variable – voltage DC source. A chopper can be considered
as DC equivalent to an AC transformer with continuously variable turns ratio. Like
transformer, it can be used to step-down or step-up a DC voltage source as well as
inverting (negative) applied voltage.
Operation Principles of Step-Down Converters (Buck Converters):
The principle of operation can be simply explained by Figure 1. When switch is in
position 1 for a time DTs, the input voltage Vg appears across the load. If the switch is
moved in to position 2 for a time (1-D)Ts, the voltage across the load will be zero. The
output voltage waveform vs(t) for a resistively loaded DC chopper is shown in Figure
2. The chopper switch can be implemented by a power semiconductor switching
device such as a MosFET, an IGBT, or a BJT.
S
1
+
+
2
Vg
L
iL(t)
vL(t)
-
+
RL
ic(t)
V
vs(t)
C
-
-
(a)
vs(t)
Vg
Vs = DVg
0
DTs
(1–D) Ts
t
1
2
1
switch
position:
(b)
Figure 1. (a) Circuit diagram of a resistive loaded buck converter and (b) output
voltage waveform [4].
The average output voltage is given by,
Vavg =
EE432 Industrial Electronics, Fall 2011
1
Ts
Ts
∫
0
vs (t ) ⋅ dt =
ton
Vg ⇒ Vavg = DVg
Ts
Experiment 4, page 2/7
Last updated October 29, 2011 10:25 PM by D. Yildirim
and the average load current can be found by I o,avg =
Vo,avg
, where D is the duty
R
cycle. The duty cycle D can be varied from 0 to 1 allowing us to change output
voltage V from 0 to Vg. By controlling D the power delivered to load can be
controlled.
The switching frequency fs (or chopping period Ts) is kept constant and on time DTs is
varied in which the width of the pulse is varied and this type of control is known as
pulse - width - modulation (PWM) control. We can notice from Figure 1b that the
output of the DC chopper with resistive load is discontinuous and contains harmonics.
The ripple content is normally reduced by an LC filter and power semiconductor
switch implementation is illustrated in Figure 2.
+ vds
L
iL(t)
-
+
-
+
V
D
DTs
RL
ic(t)
+
−
M
Vg
vL(t)
C
Ts
-
Figure 2. Buck converter employing power semiconductor switches.
Operation Principles of Step-Up Converters (Boost Converters):
If an output voltage higher than the input voltage is required, a boost converter
can be employed as depicted in Figure 3. Operation principle is same as the Buck
converter except that the location of switch, diode and inductor is changed. The
average output voltage of Boost converter can be computed by averaging the inductor
voltage waveform over one switching period and is given by,
Vg
Vavg =
1− D
L
iL(t)
+ vL(t) -
id
Vg
M
DTs
Ts
+
−
D
+
vds
-
ic(t)
+
V
RL
C
-
Figure 3. Boost converter.
Switched–Mode Regulators:
DC choppers can be used as switching – mode regulators to convert a DC voltage to a
variable DC voltage − normally unregulated to a regulated DC output voltage. The
regulation is normally achieved by pulse – width – modulation at a fixed frequency.
EE432 Industrial Electronics, Fall 2011
Experiment 4, page 3/7
Last updated October 29, 2011 10:25 PM by D. Yildirim
The basic elements of the switched–mode regulators are control blocks in a typical
closed-loop system to regulate the output voltage as shown in Figure 4.
L
+
Vg
C
V
H(s)
RL
-
gate driver
compensator
pulse-width
modulator
D(t)
DTs
Gc(s)
vc
ve
Vm
Ts
+Vref
controller
Figure 4. Main blocks of switched–mode regulators.
Procedure of Experiment:
Note: When capturing oscilloscope screen and include in your report, you have to
specify the time base (… ms/div) and scale of voltages/currents (…V/div - …A/div).
1. Full-Wave Bridge Rectifier
Circuit Set-up: Assemble the circuit shown in Figure 5.
A
2U3
2U1
Cf
+
Vsec
oscilloscope
Iout
+
2U2
537 34
Ch1 Ch2
Vdc
V
-
Rload
transformer 726 80
-
0 I V
Deniz Yildirim
Oct 31, 2010
e4_1.eps
735 01
735 095
735 261
Figure 5. Full-Wave Bridge Rectifier
• Load resistor should be selected such that output current is 1A.
• Actually you are making an uncontrolled unregulated rectifier. You have already
done this in your first experiment.
• Make the output voltage ripple as low as possible by changing the capacitor
values.
• Obtain the time waveforms of the input and output voltages with and without
capacitors and capture the oscilloscope screen.
EE432 Industrial Electronics, Fall 2011
Experiment 4, page 4/7
Last updated October 29, 2011 10:25 PM by D. Yildirim
• Write down the values of output voltage, output current, and voltage ripple ∆V
along with capacitor value in Table 2.
2. Characteristics of PWM Control Unit
Circuit Set-up: Set the circuit shown in Figure 6.
+15V
+15V
734 02
0V
0V
0
726 86
-15V
DC power supply
10
-15V
oscilloscope
control
unit
PWM
pulse
generator
Ch1 Ch2
Deniz Yildirim
Oct 31, 2010
e4_1c.eps
735 341
reference variable
generator
Figure 6. Control Unit PWM
• Set switching frequency to 5kHz.
• Observe the change in the square-wave for different reference voltage values
adjusted from the reference variable generator.
• Obtain the characteristic of duty cycle values as a function of reference voltage
(D-Vref plot). Use at least ten reference voltage values and write down values in
Table 3.
• Plot the reference voltage versus duty cycle in your report.
3. Regulated Power Supply Using Buck (Step-Down) Converter
Circuit Set-up: Set the circuit shown in Figure 7.
• Set the duty cycle from the reference variable generator such that the output
voltage will be 15 Volts. Load resistor should be selected such that output current
is 1A.
• Obtain the time waveforms of the voltage across load resistor, diode and the
current passing through the inductor. Capture the oscilloscope screen.
• Write down the values of input power (Pac), output voltage, output current,
voltage ripple (∆V), and current ripple (∆I) in Table 4 (also write down switching
frequency, capacitor and inductor values).
• Place a parallel capacitor from the load module to the resistor (4µF, 8µF and
16µF). What are the effects of these capacitors to the voltage ripples?
• Find the efficiency of the converter (Pout/Pac) at full load (15V, 1A).
EE432 Industrial Electronics, Fall 2011
Experiment 4, page 5/7
Last updated October 29, 2011 10:25 PM by D. Yildirim
+15V
+15V
734 02
0V
0V
0
726 86
10
-15V
DC power supply
control
unit
PWM
pulse
generator
735 341
-15V
reference variable
generator
735 09
A
537 34
735 046
Iout
+
2U2
2U3
2U1
Cf
+
Vsec
Vdc
Co
V
-
transformer 726 80
-
Deniz Yildirim
Oct 31, 2010
e4_buck.eps
Pac
735 01
735 095
735 02
735 09
Pout
Figure 7. Regulated power supply using buck converter
Conclusion:
We have investigated the principles of operation for a switched-mode DC chopper and
used a Buck converter as a voltage regulator. As a conclusion, it is ask for the students
to answer the following questions and submit results as a report.
•
Simulate and analyze the boost converter shown in Figure 3 by the help of a
computer design tool such as Pspice, PSIM or Proteus. It is required for you to
plot the output voltage – time (V - t), output voltage – duty cycle (V - D) and
efficiency – duty cycle (ηeff - D) at full load condition.
where:
Vg=12 V
L=0.35 mH
RL=30 Ω
C=33 µF
Switching frequency = 20 kHz
Your goal is to make the output voltage V equal to 24 Volts. Find the
appropriate duty cycle that fits these specifications. If you are not capable of
reaching 24 volts, you may try to increase the value of the inductor, but it
should not be any greater than 1 mH.
References:
[1] M. H. Rashid, “Power Electronics; Circuits, Devices and Applications”, 3rd
edition, Prentice Hall.
[2] D. W. Hart, Introduction to Power Electronics, Prentice Hall, 1997.
[3] B. K. Bose, “Modern Power Electronics and AC Drives”, Prentice Hall 2002.
[4] R. W. Erickson and D. Maksimovic, Fundamentals of Power Electronics, 2nd
ed., Kluwer Academic Publishers, 2000.
EE432 Industrial Electronics, Fall 2011
Experiment 4, page 6/7
Last updated October 29, 2011 10:25 PM by D. Yildirim
EXPERIMENT RESULT SHEET
This form must be filled in using a PEN. Use of PENCIL IS NOT ALLOWED
EXPERIMENT 3: SWITCHED–MODE DC/DC CONVERSION
USING BUCK CONVERTER
STUDENT NO
STUDENT NAME
SIGNATURE
DATE
1
2
3
4
INSTRUCTOR APPROVAL
Table 2: Bridge rectifier
output voltage
(V)
output current
(A)
filter capacitor, Cf
(µF)
output voltage ripple, ∆V
(V)
Table 3: Variation of duty cycle with control voltage
Variable Reference
Voltage (V)
Duty Cycle
(%)
Table 4: Buck converter
output voltage
(V)
output current
(V)
switching frequency
(kHz)
EE432 Industrial Electronics, Fall 2011
input power
(W)
output current
ripple, ∆I (A)
inductor value
(mH)
Experiment 4, page 7/7
output voltage
ripple, ∆V (V)
capacitor value
(µF)
Last updated October 29, 2011 10:25 PM by D. Yildirim