Power Electronics Lab

0

LAB MANUAL

Power Electronics Lab

EE-397

Department of Electrical Engineering

Zakir Husain College of Engineering and Technology Aligarh Muslim University

Aligarh

1

Course Title Power Electronics Lab

Course number EE-397

Credit Value 2.0

Course Category DC

Pre-requisite Power Electronics

Contact Hours (L-T-P) 0-0-3

Type of Course Lab

Course

Objectives To familiarize the different types of characteristics of various types Power Electronic Devices and realize various power electronic converters and triggering circuits for specific applications.

Course

Outcomes At the end of the course the students will be able to:

1. Interpret different characteristics of an SCR.

2. Implement the phase controlled switching using DIAC and TRIAC.

3. Realize different type of triggering circuits for particular application.

4. Use UJT as a relaxation oscillator and for triggering circuits.

5. Implement different types of converters for various applications like speed control of DC motor.

Syllabus List of Experiments

1. Static Characteristics of SCR 2. TRIAC and AC phase control

3. UJT based relaxation oscillator and trigger circuit.

4. R, RC trigger circuits and speed control of Universal motor.

5. Uncontrolled AC-DC Converter.

6. Monostable based trigger circuits.

7. Speed control of DC motor by a phase controlled converter.

8. MOSFET based flyback DC-DC converter.

9. Study of DC-DC boost converter.

Books*/Refe

rences 1. *G.K.Dubey, et al, Thyristorised Power Controllers; New Age International, New Delhi.

2. *M.H. Rashid, Power Electronics; PHI Learning, New Delhi.

3. V.Subramanyam, Power Electronics, New Age International, New Delhi.

4. Jai P Agarwal, Power Electronics Systems, Addison Wesely.

Course Assessment/

Evaluation/

Grading Policy

Sessional

Evaluation of report 40 Marks

Viva 20 Marks

Sessional total 60

End Semester Examination (3 Hours) 40 Marks Total 100 Marks

2

PROGRAM OUTCOMES:

a. Students will demonstrate knowledge of mathematics, science and Electrical Engineering.

b. Students will demonstrate an ability to identify, formulate and solve Electrical engineering problems.

c. Students will demonstrate an ability to design electrical and electronic circuits and conduct experiments with electrical systems, analyze and interpret data.

d. Students will demonstrate an ability to design a system, component or process as per needs and specification within realistic constraints.

e. Students will demonstrate an ability to visualize and work on laboratory and multidisciplinary tasks.

f. Students will demonstrate skills to use modern engineering tools, software and equipment to analyze problems.

g. Students will demonstrate knowledge of professional and ethical responsibilities.

h. Students will be able to communicate effectively.

i. Students will show the understanding of impact of engineering solutions on the society and also will be aware of contemporary issues.

j. Students will develop confidence for self-education and ability to engage in life-long learning.

k. Students who can participate and succeed in competitive examinations.

Mapping with Programme Outcomes (POs) POs

a b c d e f g h i j k

x x x x x x x

3

General instructions for the lab

1. They should come prepared with the procedure and related theory.

[Get the observations and other details signed by a teacher after performing the experiment]

2. Report is to be submitted with calculations and answers to the related questions on the consecutive next turn.

3. No student will be allowed to proceed to next experiment, unless he/she submits the report of previous experiment. In such case no attendance will be marked for the defaulter student.

4. The lab report will be checked and returned on the same day of submission.

5. A grand viva-voce will be held in the last two turns of the lab course.

6. The Instructors and lab staff are available to assist the students in their work.

7. It is prohibited to smoke, eat or drink in the Laboratory.

4 Object: Static characteristics of an SCR.

Experiment:

( i ) To study the forward conducting characteristics of a given SCR ( ii ) To determine latching current and holding current.

Brief Theory: An SCR turn & on, when a positive gate signal is applied and when it is in forward biased condition.

Simple V-I method can be used to find the forward conducting characteristics of the SCR. Anode current can be varied with the help of load resistance to find out the static i-v characteristics of the SCR. Similarly by slowly increasing and decreasing the load resistance the values of the holding current and latching current, respectively can also be found.

Procedure:

1. Make the connection as shown in Fig 1.

2. Put the ‘toggle switch’ at dc position which provides a dc voltage to the load circuit.

3. Press the push button switch (S) to bring the SCR into conduction mode by providing gate current.

4. Adjust the anode current about 0.5 Amp. with the help of load resistance RL. Increase RL gradually. Note the anode current and voltage (vAk) across the SCR.

Take about 10 readings.

5. Latching and holding current can be found with the help of variation of the load resistance. Increase slowly the load resistance. Observe the critical anode current when the SCR returns to blocking state. Note down this value as holding current of SCR.

6. Now decrease RL slowly and apply the gate pulse (by pressing the press-switch to the gate of SCR. While removing the gate pulse, if the SCR conducts and stays ON, then that value of the anode current is the latching current.

7. Put the toggle switch to AC mode. Now a half – wave voltage is applied to the load circuit. Adjust the load resistance and press the press-switch. The conduction of the SCR will continue only for a period the switch was pressed.

5 Sample Questions:

1. Why SCR conducts so long as gate signal is present in ac test?

2. Why SCR fails to maintain conduction below holding current?

3. What are the specifications of SCR used in the experiment?

4. How much experimental data deviate from specified data?

6 Object: Triac and a.c phase control.

Experiment:

( i ) To study a diac based trigger circuit.

( ii ) To study ac phase controlled switching using a diac and triac.

Brief Theory: A triac can be triggered ON by the application of either a positive or a negative gate signal in both positive and negative of the ac supply voltage. Therefore a triac has four modes of triggering. A diac conductors at a particular positive and negative voltage. Therefore it supplies trigger signals to a triac in both positive and negative half-cycles of ac voltage.

When gate trigger signal is delayed, a thyristor triggers at an instant other than the zero cross over of the ac supply voltage. Thus the load voltage becomes non-sinusoidal and only a part of the supply (ac) voltage appears across the load. Therefore by controlling the switching angle, the output ac voltage is controlled effectively.

Procedure:

1. Complete the circuit connection as shown in Figure 1.

2. Connect MT1 to K and A to MT2.

3. Apply 230V ac supply to P and N terminals from an autotransformer.

4. Observe the phase angle control by the variation of potentiometer RP.

5. For a particular setting of RP, measure the load voltage by a multimeter and trace the voltage waveforms available on CRO for three settings of potentiometer.

Report:

1. What are the advantages and disadvantages of a triac over SCR?

2. Discus the characteristics of a Diac.

3. What are maximum V, I,

dt di

and

dt

dv

ratings of triac used?

4. Draw load voltage (theoretical) and load voltage (experimental) versus switching angle.

7 Circuit Diagram:

Figure 1.

RL

MT2

MT1

680 

220k pot

10k 10w

0.1μF

P

N 230 V

AC Triac

G

AC Phase Control by triac

8

Object: To study UJT relation Oscillator for SCR triggering.

Experiment:

( i ) To study UJT relaxation oscillator.

( ii ) To study UJT based trigger circuit.

( iii ) To study phase-controlled switching of SCR.

Theory: UJT (Uni Junction Transistor) Oscillator is widely used in the triggering circuits. The output frequency of the oscillator depends upon the RC circuit. For triggering of an SCR at a particular switching angle, the UJT relaxation oscillator should be synchronized with the voltage of ac mains. Therefore, the charging of capacitor starts only after the zero-crossing point of the ac voltage signal and not arbitrarily. The switching angle of the thyristor depends upon the arrival of the first trigger pulse of UJT oscillator, which depends upon the frequency of the oscillator. Hence for the higher frequency setting of the oscillator the switching angle will be smaller, while for lower frequency setting, the switching angle will be larger. In this way the switching angle of the SCR is controlled by the resistance (pot) of the RC branch of the Oscillator.

Procedure:

(a) UJT Oscillator:

1. Observe the UJT (2N2646) Oscillator (Fig.1).

2. Connect a 12 volts regulated dc power supply to UJT oscillator circuit, at correct terminals and with correct polarity (Fig.2).

3. Observe with the help of CRO, charging and discharging of capacitor C1, for different settings of pot. Also observe the high frequency pulses across RB1. 4. Trace the wave shapes (capacitor voltage and output pulse) at minimum

frequency condition. Note the time setting of the sweep frequency of CRO and distance (in division) between two triggering pulses, to find out the time period or frequency of pulse.

5. Add more capacitor (C2 = 0.33  F) to the existing one (C2 = 0.01 F) in the relaxation oscillator. Adjust the pot for minimum frequency. Trace the waveform as in procedure 4.

6. Remove D.C. power supply.

9 (b) Trigger Circuit:

1. Connect load resistance, SCR voltmeter and ammeter as shown in Fig.3.

2. With the help of an autotransformer, keep the input ac voltage constant (about 12 volts) before each observation.

3. Connect the oscillator circuit with the gate of the SCR. Take three observations, including the minimum and maximum switching angles, for various settings of pot. Note down output (load)voltage and load current for each case.

4. Observe and trace voltages waveforms across the load and THEN voltage across the SCR.

Reports & Sample Questions:

1. Find the frequency of the output signal of the UJT oscillator.

2. Plot graph for output voltage (observed) v/s switching angle.

3. Propose a suitable UJT trigger circuit for a full wave control applications.

4. How can you use a UJT trigger circuit for feed back application.

5. Why steep rising pulses are suitable for thyristor triggering.

N P

T

E B2

B1

V A

V B2

B1

E

2N 2646 UJT

Top View

2.7 K

100 K Pot . 0.047 C2

0.33 K C1

2N 2646 By 127

1.1 K

220  RB1

To Gate G K A

RL=25  P

230 V, AC

Fig. 3.

2.7 K

100 K Pot . 0.047 MFD C2

0.33 MFD C1

2N 2646 1.1 K

220  RB1

12 V D.C.

Fig. 2.

Fig. 1.

Auto-Transformer X

Y

_

+ X

Y

10 Object: Phase controlled switching of SCR.

Experiment:

( i ) To study resistance trigger circuits.

( ii ) To study RC trigger circuits.

Theory:

Ac phase controlled switching is done to control the output voltage from the fixed input ac voltage. Thus delay in switching angle of a thyristor in each half cycle controls (reduces) the output voltage. The output voltage may be ac or dc which depends upon the circuit configuration/connection of the thyristors.

The beginning of conduction (switching) of SCR depends upon the magnitude of anode to cathode voltage (VAK) and the magnitude of the gate current. When thyristor is forward biased the magnitude of gate current can be controlled by a variable resistance.

It changes the switching instant on the voltage wave. Similarly the triggering signal on the gate can also be controlled by a simple RC phase shifter circuit. Thus positive half rectified voltage of this signal controls the switching angle.

Procedure:

Resistance Trigger Circuit:

1. Complete the circuit for resistance triggering as shown in Fig. 1. (Do not connect the capacitor).

2. Connect the fixed terminals of the rheostat as load resistance.

3. Observe the waveforms of the output voltage for different settings of the pot (for three setting).

4. Note the load current input voltage and output voltage for each case.

RC Trigger Circuit:

5. Connect the capacitor and repeat the procedure (1- to - 4) for RC trigger

circuit.

11

Observation Format:

(i) R- Trigger circuit:

S.No. Supply Voltage

(V) Load current

(mA) Output voltage

(V) Output Power

(W) (Calculated)

(ii) RC – trigger circuit S.No. Supply Voltage

(V)

Load current (mA)

Output voltage (V)

Output Power (W) (Calculated)





12

Object: Speed control of Universal Motor (UM) by SCR.

Experiment:

( i ) To study an RC triggering circuit for SCR.

( ii ) To study the Speed variation of UM using above triggering circuit for half wave control of ac supply voltage.

Theory: A Universal Motor is a dc series motor, which is designed to be operated with ac supply also. This can be named as ac series motor.

In the present experiment a RC triggering circuit is used for the switching of the SCR.

The SCR operates as a phase controlled switch as shown in Fig. When the SCR is ON the supply voltage is applied across the UM. When the SCR is OFF the current through the UM is zero. By this simple operation (ON and OFF) of the SCR, the average voltage applied to the UM can be varied to change its speed.

Procedure:

1. Connect the circuit as shown in the circuit diagram.

2. Take the readings of the voltmeter, ammeter and the tachometer for different values of the variable resistance of the trigger circuit.

3. Also trace the waveforms of the load voltage across UM (or across the potential divider for CRO probe). With the help of these waveform calculate the delay angle

.

Observation Format:

S.No Voltage Across the

Motor in Volts Speed in RPM Switching angle in degrees (calculated)

Report:

1. Plot speed versus switching angle and comment on it.

2. Plot speed versus voltage and comment on it.

3. For an inductive circuit, explain how the load voltage changes even if switching angle did not change.

13 Circuit Diagram:

P

N 230 V AC Supply



0.1 F

680

G K

10 K, 10 W

220 K

10 K

U.M .

A



To CRO

14 Object: Study of ac to dc uncontrolled converters.

Experiment:

(i) Study of half-wave converter with RL load.

(ii) Study of full-wave mid-point converter with RL load.

(iii) Study of freewheeling action of a diode.

Brief Theory: Thyristor based naturally commutated converters are normally employed for ac to dc controlled conversion. A fixed ac input voltage supply is used to obtain an adjustable dc output voltage. When only diodes are used in place of SCR, the converter circuit, is called uncontrolled converter. When an RL load is connected at the output, the conduction of diode continues for some portion of the negative half-cycle too (up to

, beyond t = ). Because at t = , diode has sufficient current which prevents the formation of depletion layer to bring back the diode in reverse biased.

Condition, until the diode current reduces to zero (ideally). Thus negative supply voltage (during some portion of the negative half cycle) also appears across the load.

The value of  can be found with the help of total current response equation of an RL circuit whose power factor angle is

ɸ

.

 ^{sin( ) sin exp( / tan )} 

)

(  V m  t    t 

t

i 

Z

  

When a free-wheeling diode (Df) is used, it prevents this negative excursion of the load voltage. When the load voltage becomes negative, Df conducts (as its cathode becomes negative), thus load voltage due to the conduction of Df1 becomes zero during this period.

In a half-wave or single-pulse uncontrolled converter, the rectification is done with only one diode in the circuit. This gives only one pulse in the output dc voltage per cycle of the ac supply voltage.

When diode bridge is used to make a converter bridge or a diode pair is used along with a centre- tapped transformer, there are two output dc voltage pulses per cycle. This converter is called full- wave or two-pulse converter. In this case line current is symmetrical about time-axis with zero average value per cycle.

Procedure:

1. Connect the resistance and inductor in the load circuit as shown in the figure.

Half-Wave Mode:

15

2. Complete the connection with only ONE diode (D1) connected between terminals 1 and 3 as shown in the Figure.

3. Observe the current waveform (across the load resistor or between terminals 5 & 0) and output voltage waveform (between terminals 4 and 0).

Full-Wave Mode

7. Disconnect the cathode of diode D3, connected to terminal # 3.

8. Connect the diode, D2 (terminals # 6 and # 3) as shown in Figure.

9. Repeat steps no. (3) through (5).

Observations:

Output voltage of transformer (between terminal # 1 and 0) = ………V R (in Ohms) =…

L (in millihenry) =…

Output Voltage

Output

Current Extinction Angle (₎

(dc) (ac) Experimental

Computed Half Wave

i) Without free-wheeling action

ii) With free-wheeling action Full Wave

(i) Without free-wheeling action

(ii) With free -wheeling action

Circuit Diagram:

16

Report:

a) Why diode conducts even for negative input voltage at its anode?

b) Find and compare the extinction angle from the values of the load parameter as well as from the experimental values.

c) Compare the performances of 1-pulse & 2- pulse converter circuit.

17

Object: An advance mono-stable monostable based trigger circuit.

Experiment:

(i)To study operation of a monostable based triggering circuit for a fully controlled converter.

(ii) To observe the waveforms at different stages of a buffer and driver circuit.

(iii) To observe phase controlled switching operation by this trigger circuit.

Theory: For accurate switching angle control and to incorporate feedback signal advance trigger circuits are used. Different analog and digital trigger circuits are used for this purpose.

When linear and digital ICs are used they require separate power supply for biasing. Normally driver and buffer circuits are needed to amplify the weak output signals of CMOS and TTL IC, and to isolate the control circuit from the power circuit.

In this circuit, zero crossing detectors (ZCD) and a retriggerable monostable with logic gates are used to generate trigger signals in the positive half cycle between  to  and in the negative half cycle between ( + ) to 2. A positive pulse is generated at positive and negative zero cross- over instants by zero crossing detector circuit. It reaches to a monostable (IC74122). An RC delay network controls the ON period of output (Q) of monostable from 0 to . Thus the ON - period of the output (

Q

) varies between  to  and  +  to 2. A 555 timer based stable multivibrator generates carrier signal (at 10 KHz) for buffer and driver circuits.

Q

with logic combinations, provides independent trigger signals, in positive and negative half cycles. The output ultimately reaches to the gate of a thyristor through driver and buffer circuit. This is used to switch thyristor, and hence the output voltage is controlled.

Procedure:

1. Adjust the voltage level of the dual dc regulated power supply to 5V. Put the switch in tracking mode of d.c. power supply.

2. Connect +5V and –5V dc voltage supply to the monostable based trigger circuit as shown in Fig. 1.

3. Connect 230V ac signal to transformer (230V/ 3V) of this circuit. Observe the waveforms at each stage of the trigger circuit.

4. Observe the variation of  and the duration of trigger signal (Q) generated at the output of pulse transformer at G1 K1 and G2 K2 by varying the pot.

5. Connect the power circuit (Fig.2). Vary the pot setting and measure the current and the load voltage of or different settings.

6. Trace the wave form and calculate the triggering angle.

18 Observation Format:

S. No. Supply Voltage Load current (Experimental)

Load Voltage (Experimental)

Load current (Theoretical)

Report:

1. Find theoretically the load current.

2. Draw voltage v/s switching angle characteristics curve.

3. How is the performance of this trigger circuit different from R and RC trigger circuit.

4. Draw the graph of (a) power factor (PF) versus α, and (b) total harmonic distortion (THD) versus α; for α = 0, 30, ……… 180 for an ac regulator with resistive load.

Circuit Diagram:

7

230 V A.C.

Control Circuit Transformer

3 V A.C.

1 K + 5 V

3

2 6 4

B 0.1F OA

81

R C

MS 74122

- 5 V

220 

B

AND

7408 7408 56 K

56  5 W

K1

+ 12 V G1

1 : 1

SL 100

Carrier Signal

ZCD1 PFC1 MS DBC1

Fig. 1. TRIGGERING CIRCUIT

ZCD2 PFC2 DBC2

7408 7408

2 B

3

G2

K2

Q

19

20

Object: Speed control of dc motor by a phase-controlled converter.

Experiment: To study speed control of separately excited dc motor by half-wave phase controlled converter.

Theory: A full converter can be used to control the speed of a dc motor in the first quadrant, as well as in fourth quadrants of torque-speed characteristics. In case of a half-controlled converters the average output dc voltage can be controlled for  between 0^o to 180^o.

Here half-wave converter (single-pulse with only one thyristor) is used to control the speed of a separately excited dc motor. With the help of this converter armature- winding voltage is controlled. Field winding is energized by a constant dc voltage supplied by a diode bridge. An RC trigger circuit is used to supply the trigger pulse to the SCR between 0⁰ and 180⁰. The controlled output voltage of the converter circuit is applied to the armature winding of the dc motor. Thus by adjusting the switching angle , armature voltage hence the speed of dc motor is controlled.

Procedure

1. Connect input of the rectifier bridge to the output of an autotransformer and complete the power circuit as shown in Fig. 1.

2. Complete the connection of the armature circuit with an RC trigger circuit as shown in Fig.2.

3. Connect the input of the autotransformer with ac mains and keep the output voltage constant (230V).

4. By changing the settings of the pot of the RC trigger circuit control .

5. Note down the speed, armature current. Trace the waveforms of supply (input) voltage and armature current (vR) for different setting of .

6. Calculate the triggering angle with the help of traces of waveforms.

Observations Format:

S. No. Input voltage (ac)

Armature Current

(A)

Speed (r/min)

Triggering Angle ()

21

Report:

Draw the graph of speed v/s switching angle and comment.

Circuit Diagram:

P1

P2

230 V

AC v

F

FF

To CRO CH1 P

N

Com

Fig. 1. Power Circuit for field winding of dc motor.

22 Object: Study of dc –dc fly back converters.

Experiment:

(i) Study of a 555-timer based frequency oscillator.

(ii) Study of switching of a power BJT.

(iii) Study of switching of a power MOSFET.

(iv) Study of a BJT based driver circuit for a power MOSFET.

(v) Study of an RC Snubber Circuit used with power MOSFET.

(vi) Study of the performance of an isolated output voltage, flyback converter.

(vii) Study of the performance of an isolated Buck Boost, flyback converter

Theory: The linear ac-dc converters are bulky, less efficient and costly. The switched mode dc-dc converters are compact, cheap and efficient (as high as 90-99%). There are large number of circuit topologies of these dc to dc converters. The flyback converter was invented by NASA in 1960 for space applications. The switch, when ON, makes the primary winding at a high frequency transformer to operate as an inductor to store the energy (with open circuited secondary winding). When the switch turns off, interrupts a highly inductive current, generates a high voltage spike



^vL ^Ldi /^dt



across the primary winding of the high frequency transformer. This voltage also appears across the secondary winding which in turn charge the filter capacitor and supplies power to the load. Thus, during the OFF period of the switch, the transformer release energy to the load. The controlling the frequency and the duty ratio of the switch, the desired (adjustable) output voltage is obtained.

Procedure:

(i) Study of 555 timer and switching of power BJT:

1. Connect the circuit with a 12 volts dc supply a from regulated dc power supply (linear dc voltage stabilizer).

2. Observe the waveforms of the output voltage of 555 timer and collector terminal voltage of the power BJT (2N3055).

3. Trace the waveform for the lower and higher settings of frequency.

(ii) MOSFET based fly back converter:

1. Connect the gate of the MOSFET with the collector terminal of the power BJT i.e. link terminals (1) and (2). Now, BJT becomes driver of MOSFET.

2. Observe the voltage waveforms at Drain (across drain and source) and at the load (output voltage).

23

3. For FOUR different frequency of the oscillator (555 timer), take SIX observations (Three in Buck Mode and Three in Boost Mode).

4. Note dc voltage and frequency of oscillator (555 timer) in each case).

(iii) Snubber Circuits:

1. Observe the voltage waveform at Drain (across Drain and Source).

2. Vary the magnitude of resistance of the snubber by the pot and observe the variation of the peak voltage (spike) that appear across the MOSFET.

Observation Table:

S.

No

DC output voltage

ON period of oscillator

signal

Time period of Oscillator signal

Switching frequency (calculated)

Duty ratio (Calculator)

Report:

1. What is the lowest and the highest frequency of oscillator.

2. Plot dc output voltage verses switching frequency curve.

3. Plot dc output voltage verses duty ratio curve 4. What is the use of the transformer in the circuit?

5. Why special design of the transformer is needed?

6. How this circuit can be modified to a forward dc to dc converter?

7. Observe the effects of variation of different parameters of different dc to dc converter.

Reference:

(i) M.S.J. Asghar, “Power Electronics”, PHI.

(ii) M.H. Rashid, “Power Electronics”, PHI.

24 Circuit diagram:

25

Object: Study of dc-to-dc boost converter.

Experiment

a. To study the effect of duty ratio on the output of the converter b. To study the effect of inductance on inductor current ripple c. To study effect of filter capacitance on output voltage ripple

Theory

Boost converter is used to enhance the voltage level available from dc supply such as battery or solar PV panel to a level suitable for various systems. For example, electric vehicles running on batteries require much higher voltage then 12 volts available from a single battery to drive the motor. dc-dc boost converter is an ideal solution for such system requirement. Figure 1 shows the circuit of a basic boost converter.

 

^D

V

in

Vout

Figure 1

The switching device (S) in the circuit could be power MOSFET or insulated gate bipolar transistor (IGBT). The choice of the switching device depends on application requirement of voltage, current and frequency along with cost.

Operation of boost converter:

The converter switch S is closed for on period Ton and opened for a time interval Toff. The time period of the operating cycle T = Ton + Toff. The ratio of on-period and time-period is defined as duty ratio

𝑑 = (1)

The on and off periods of the switch can be written as

𝑇 = 𝑑 ∙ 𝑇 (2)

26

𝑇 = (1 − 𝑑) ∙ 𝑇 (3)

When switch is closed for duty interval 𝑑 ∙ 𝑇, the diode D is reverse biased and load is disconnected from the source as shown in Figure 2. The load receives energy from the capacitor.

 

^D

iL

V

in

Figure 2

During this interval, voltage across inductor is given by

𝑣 = 𝑉 (4)

The inductor current rises from minimum value Imin to maximum value Imax during this interval.

The inductor current ripple can be written as

∆𝐼 = 𝐼 − 𝐼 = × 𝑑 ∙ 𝑇 (5)

When the switch (S) is opened for interval (1 − 𝑑) ∙ 𝑇 the diode starts conducting and the circuit becomes as shown in Fig. 3. The voltage across inductor during this period can be written as

𝑣 = 𝑉 − 𝑉 (6)

 

^D

V

in

Figure 3

Since the average voltage across L is zero over one complete cycle

𝑉 = 𝑑𝑇 ∗ 𝑉 + (1 − 𝑑)𝑇 ∗ (𝑉 − 𝑉 ) = 0 (7)

Equation (7) can be simplified as

27

𝑉 = (8)

It can be observed from Equation (8) that the output voltage is more than input voltage and the output voltage decreases as the duty ratio d is increased.

During off period of the switch, as the voltage across inductor Vin - Vout is negative, the inductor current decreases linearly from Imax to Imin as shown in Figure 4.

Output voltage increases during off-period of the switch and decreases during on-period.

The ripple in output voltage as ratio of output voltage is given by

∆𝑣

𝑣 = 𝑑 ∙ 𝑇 𝑅 ∙ 𝐶 Where R is the load resistance.

Figure 4

Procedure

1. Open the MATLAB/ SIMULINK model of the boost converter BOOST.mdl. Window as shown in Figure 5 will appear.

2. In the MATLAB Command window set the following parameters of the converter Duty ratio d = 0.2

Inductance L = 0.001 H Capacitance C = 0.0004 F

Run the Simulation model. Note the Average output voltage. Study the nature of inductor voltage and inductor current along with capacitor voltage. Also observe the output

voltage, switch current, switch voltage and diode current.

3. Repeat Step 2 for d= 0.4, 0.6 and 0.8.

4. For d = 0.8, note the ripples in output voltage (𝑣 ) and inductor current (𝑖 ) from the waveforms seen on the scope of the model.

28

5. Keeping L constant at 0.001 H change C to 0.008 F. Run the simulation and observe the ripple in output voltage.

6. Set L = 0.002 H with C= 0.0004 F. Run the simulation and observe the ripple in inductor current.

Observation:

1. Input Voltage= ………

S. No. Duty Ratio (d) Average output Voltage

2. Inductor Current Ripple

S. No. Inductance (L) Current ripple 1

2

3. Output Voltage Ripple

S. No. Capacitance (C) Voltage ripple

Report

1. Plot Average output Voltage Vs duty ratio.

2. Comment on the effect of the inductance on inductor current and capacitance on output voltage.

3. What will be the value of the average output voltage for d=1?

29

Figure 5: Simulation model of boost converter