DC-DC CONVERTERS ( Switched Mode Power Supplies )
Introduction
This experiment forms an integral part of the short course emphasizing the DC-DC converters of
the switched mode power supplies.
In this laboratory session you will experiment with some of the BOOST, BUCK, and FLYBACK
converters. First a brief explanation of the circuit equations will be given, and a simulatoin
package will be used to analyse the circuits and produce some graphical displays of the various
waveforms. Finally the practical circuits are put together and measurements would be made.
1 Boost Converter ( step-up )
(i) circuit diagram and eqations
Vs
Vo=1-D
where
D = duty cycle and,
Io=(1-D)*Is
f = frequency of the
∆IL=IL(max)-IL(min)
Io*D
∆Vc= f*C
Vs*D
∆IL= f*L
input pulse
∆Vc=Vc(max)-Vc(min)
(ii) simulation
The circuit shown is used for simulation here.
The netlist for this circuit is already written to save time, and can be loaded by using the
mouse and the command :
File -> Open -> boost.cmp
Load the netlist and study it with respect to the above diagram.
Note specifically the line:Vi +output:3 -output:gnd v=5.000 Ex=Pulse Fr=25.000k Df=300.00m
which indicates a pulse generator with 5 volts amplitude, frquency of 25 KHz and duty ratio of
0.3, meaning the pulse is ON for 30% of the total peroid.
Run the simulation by using the following command :-
Analysis -> Transient
Now expand the Transient window by clicking on button on the top right hand corner and you
should get a diagram like the one shown below.
boost converter D=30%
Study the inductor and diode traces with respect to input Pulse. Clear this window by first
reducing it, with clicking the mouse TWICE on the button on the top-left-hand corner. Now change
the duty cycle of the input pulse to 70% by changing Df of the Vi line to 700m.
Run analysis and you should get :-
boost converter D=70%
Note all the waveforms and their values.
Because the above currents can NOT be delivered by our equipment, therefore different values of
supply voltage, load and the duty ratio are used. Load the circuit for the actual experiment,
which is called BOOSTEXP.CMP, and run the simulation and draw all the waveforms in your book.
Change the duty ratio to 50% and repeat the above.
(iii) practical experiment
Procedure :
1 ) Set the pulse generator to 25 KHz using the oscilloscope to measure a duty cycle of 25%.
2 ) Set the power supply to 4 volts and switch the output OFF.
3 ) Connect the circuit as shown in simulation section 1(ii) with BOOSTEXP.CMP values.
4 ) Switch power supply output ON and make measurements and complete a table in your book.
Note : P indicates Practical measurement, and T indicates the theoretical value
(calculated).
measure output voltage with voltmeter and supply current with power supply.
D
Vo
25%
50%
P
T
Is
P
T
Io
P
T
5 ) switch OFF the power supply and set the duty cycle to 50% and repeat 4.
And finally comment on the results.
2 Buck Converter (step-down)
(i)
circuit diagram and equations
Vo=DVs
Is
Io= D
(ii) simulation
The circuit shown is used for simulation
which its NETLIST is called
BUCK.CMP
VsD(1-D)
∆IL=
fL
VsD(1-D)
∆Vc= 8fLC2
Load the net list and study it with respect to the diagram above and take note of Vi line. As
before run analysis to get:
Buck Converter D=30%
As before study the waveforms and then change the duty cycle to 70% to get :
Buck Converter D=70%
Look at the waveforms and load the BUCKEXP.CMP for analysis of the actual experiment, run it
and draw all the waveforms in your book. Change the duty cycle to 50% and then to 75% and copy
the waveforms.
(iii) practical experiment
Procedure :
1 ) Set the pulse generator to 25 KHz using the oscilloscope to measure a duty cycle of 25%.
2 ) Set the power supply to 6 volts and switch the output OFF.
3 ) Connect the circuit as shown in simulation section 2(ii) with BUCKEXP.CMP values.
4 ) Switch power supply output ON and make measurements and complete a table in your book.
5 ) Switch OFF the power supply and set the duty cycle to 50% and repeat 4.
6 ) Switch OFF the power supply and set the duty cycle to 75% and repeat 4.
And finally comment on the results.
3 Flyback Converter
(i) circuit diagram and equations
-VsD
Vo= 1-D
Is(1-D)
Io=
D
VsD
∆IL= fL
IoD
∆Vc= fC
(ii) simulation
For simulation load FLYBACK.CMP and run the Analysis and then change the duty cycle to 70%
and study the waveforms. Load the FLYBKEXP.CMP for the experimental analysis and run the
analysis for 25%, 50% and 75% duty cycles.
(iii) practical experiment
Procedure :
1 ) Set the pulse generator to 25 KHz using the oscilloscope to measure a duty cycle of 25%.
2 ) Set the power supply to 6 volts and switch the output OFF.
3 ) Connect the circuit as shown in simulation section 3(ii).
4 ) Switch power supply output ON and make measurements and complete a table in your book.
5 ) Switch OFF the power supply and set the duty cycle to 50% and repeat 4.
And finally comment on the results.
G.H.Parchizadeh sept 1994