EXPERIMENT No.
STUDY OF STEP DOWN CONVERTER
Name ID No.
Sec.No Batch No. Marks obtained
Date Instructor’s signature
OBJECTIVES
a. To Study the PWM generation Circuit for step down converter.
b. Study the performance of the down converter for various loads with (i) MOSFET (ii) IGBT and
(ii) Thyristor as the switch.
EQUIPMENTS NEEDED
1. Oscilloscope
2. Digital Multimeter
3. BNC to Test Probe
4. ST2724 Step Down Chopper and power supply.
5. Patch Cords & Operating manual
CONNECTION DIAGRAM:
Figure 4.1: PWM generation circuit
PROCEDURE FOR STUDYING THE PWM CIRCUIT.
Make sure that there is no connection on the board initially.
1. Make the connections according to Figure 4.1.
2. Rotate the frequency control potentiometer gradually in the anticlockwise direction.
3. Similarly, rotate the PWM control potentiometer also in the anticlockwise direction.
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4. Switch ‘On’ the power supply.
5. Observe the square wave output between Test Points TP1 with respect to TP2.
6. Observe the triangular wave output at Test Points TP3 with respect to the ground.
7. Vary the frequency control potentiometer and observe the variation in frequency.
8. Observe the reference voltage (Vref) at Test Points TP4 with respect to ground.
9. Vary the PWM control potentiometer & observe the change in voltage either on the multimeter or on
the oscilloscope by putting it into the DC Mode.
10. Observe the PWM signal at PWM pulse with respect to ground.
11. Set the frequency control potentiometer to some value, then vary the PWM control
potentiometer & observe the PWM pulse.
12. Record the maximum & minimum frequency and pulse width in the observation table 1.1.
OBSERVATION TABLE 4.1:
Minimum Maximum Minimum Pulse Maximum Pulse
Frequency (Hz) Frequency (Hz) Width (%) Width (%)
Figure 4.2 : Step Down converter
PROCEDURE FOR STUDYING THE DOWN CONVERTER WITH VARIOUS LOADS.
Make sure that there is no connection on the board initially.
1. Switch on the power supply.
2. Set the frequency of PWM pulse by frequency control potentiometer at 4KHz.
3. Switch off the power supply.
4. Connect the power supply +24V & ground at their indicated positions.
5. Connect the MOSFET at chopper section in step down chopper configuration.
6. Connect the PWM pulse from the PWM circuit to the Gate (G) of the MOSFET. Connect the Gnd from
the PWM circuit to the Source (S) of the MOSFET.
7. Connect the load (R load without L, RL load without L and motor load without L) at its indicated
position.
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8. Switch ‘On’ the power supply. Connect the oscilloscope and vary the PWM control potentiometer
& observe switch voltage, diode voltage and output voltage waveform across the load.
9. Connect the multimeter in DC mode. Vary the PWM control potentiometer and
observe the output voltage across the load, switch and diode. Verify the output DC
voltage with the theoretical value.
10. Switch ‘Off’ the power supply.
11. Disconnect PWM pulse and input voltage from the power circuit. Set the frequency of PWM
pulse by frequency control potentiometer at 500Hz & repeat the experiment.
12. Repeat the experiment for different switches (IGBT and thyristor) at different duty ratios.
OBSERVATION TABLE 4.2
Input Voltage = 𝑉i𝑛
Output Voltage = 𝑉0
Duty ratio = 𝐷
𝑉0 = 𝐷 𝑉i𝑛
R- Load
Measured Measured
Theoretical Measured
S. Frequency PWM Output voltage across
Device Output voltage across
No. (Hz) (%) Voltage the device (V)
Voltage (V) the diode (V)
(V)
1 MOSFET 4kHz 80%
2 MOSFET 500Hz 40%
3 IGBT 4kHz 80%
4 IGBT 500Hz 40%
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RL- Load
Measured Measured
Theoretical Measured
S. Frequency PWM Output voltage across
Device Output voltage across
No. (Hz) (%) Voltage the device (V)
Voltage (V) the diode (V)
(V)
1 MOSFET 4kHz 80%
2 MOSFET 500Hz 40%
3 IGBT 4kHz 80%
4 IGBT 500Hz 40%
Motor Load
Measured Measured
Theoretical Measured
S. Frequency PWM Output voltage across
Device Output voltage across
No. (Hz) (%) Voltage the device (V)
Voltage (V) the diode (V)
(V)
1 MOSFET 4kHz 80%
2 MOSFET 500Hz 40%
3 IGBT 4kHz 80%
4 IGBT 500Hz 40%
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GRAPHS
1. Draw the graphs of PWM output, output voltage and diode voltage at switching frequency 500Hz
for duty ratios 0.4 for RL load with filter inductor when MOSFET is used as a switch.
2. Draw the graphs of PWM output, output voltage and diode voltage at switching frequency 4 k Hz
for duty ratios 0.8 for motor load with filter inductor when MOSFET is used as a switch.
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Specification for the ST2724 Step down converter
On Board PWM Circuit : Triangular Comparator Circuit
Frequency Variation : 27 Hz to 5 KHz (approximately) PWM
Variation : 0-90%
Inductor : 68 mH
R load : 1kΩ
RL Load : 1kΩ, 10mH
DC Geared Motor : 24V/ 0.5A, 100 RPM
Interconnections : 2 mm sockets
MOSFET Assembly : MOSFET IRFZ44N, 55V, 49A IGBT
Assembly : IGBT G4BC20S, 600V, 10A Transistor
Assembly : Transistor TIP122, 100V, 5A SCR Assembly
: SCR TYN 616, 600 V, 16A Fuse : 1A
DC Power Supply (SMPS) : 12V; 0.5A, 24V; 1A
Power Supply : 230 V 10%; 50 Hz
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