INTRODUCTION OF POWER ELECTRONICS

INTRODUCTION OF POWER ELECTRONICS

SALAMIYAH BT MOHAMAD YUSRIZA BINTI YUSOFF @ AWANG JUSOH NURUL AZIYANA BINTI KAMARUDDIN

POLITEKNIK SULTAN HAJI AHMAD SHAH

INTRODUCTION OF POWER ELECTRONICS EDITION 2022 SALAMIYAH BT MOHAMAD YUSRIZA BINTI YUSOFF @ AWANG JUSOH NURUL AZIYANA BINTI KAMARUDDIN

Published by POLITEKNIK SULTAN HAJI AHMAD SHAH SEMAMBU 25350 KUANTAN

Copyright ©2022, by Politeknik Sultan Haji Ahmad Shah Materials published in this book under the copyright of Politeknik Sultan Haji Ahmad Shah. All rights reserved. No part of thispublication may be reproducedor distributed in any form or bymeans, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers.

PREFACE

Electric energy is an imperative need in our lives. It is commonly generated, transmitted and distributed to the consumers in AC form. The invention of modern appliances urges electric energy conversion from the AC main source to the desired form in order to meet the various power demand of the circuits. This book is written to provide basic knowledge in power electronics emphasizing the operational principle of power converter circuits and the output voltage waveforms.

Chapter 1 focusses on the power electronic devices for switching that will be used in the following chapters. Chapter 2 discusses on DC to DC converter (DC Chopper), how to step up and step down the DC voltage. Next, DC to AC converter (Inverter) circuits and its operation are elaborated for Chapter 3.

Although this book is intended mainly for the students, we hope it will benefit as well to all who are interested to gain knowledge in electrical and electronic engineering specifically in power electronics.

our tEAM SALAMIYAH BT MOHAMAD LECTURE DEPARMENT OF ELECTRICAL ENGINEERING salamiyah@polisas.edu.my

YUSRIZA BINTI YUSOFF @ AWANG JUSOH LECTURE DEPARMENT OF ELECTRICAL ENGINEERING yusriza@polisas.edu.my Yellow

Greenery

Mustard

NURUL AZIYANA BINTI KAMARUDDIN LECTURE DEPARMENT OF ELECTRICAL ENGINEERING aziyana@polisas.edu.my

INTRODUCTION OF POWER ELECTRONICS 1.0 OVERVIEW OF POWER ELECTRONIC DEVICES

Introduction Switch Mode Power Supply Power Devices Silicon-Controlled Rectifiers (SCR) Gate Turn-Off SCR (GTO-SCRs) TRIAC Insulated-Gate Bipolar Transistor (IGBT)

1 5 11 13 23 31 38

2.0 DC TO DC CONVERTER Definition Chopper Operation Method of Control in Dc to Dc Control Step Down Converter (Buck Chopper) Step Up Converter (Boost Chopper)

3.0 DC TO AC CONVERTER (INVERTER) Definition Classification of Inverter Bridge Inverter Single Phase Half Bridge Inverter Three Phase Bridge Inverter

46 51 54 56 68

75 80 84 85 105

OVERVIEW OF POWER ELECTRONICS #1.0 Introduction #1.1 Switch Mode Power Supply #1.2 Power Devices #1.3

Silicon-Controlled Rectifiers (SCR)

#1.4

Gate Turn-Off SCR (GTOSCRs)

#1.5 TRIAC #1.6

Insulated-Gate Bipolar Transistor (IGBT) 1

1.0 THE APPLICTION OF POWER ELECTRONICS What is the Power Electronics ?

A field of Electrical Engineering that deals with the application of power semiconductor devices for control and conversion of electrical power.

Figure 1.0 : Power electronics

2

1.01 SCOPE OF POWER ELECTRONICS Power Level (watts)

System

0.1-10

Battery-operated equipment Flashes/ strobes

10-100

Satellite power system Typical offline fly back supply

100-1KW

Computer power supply Blender

1-10KW

Hot tub

10-100KW

Electrical car Eddy current breaking

100KW-1MW

Bus Micro-Superconducting magnetic energy storage (SMES)

1MW-10MW

Superconducting magnetic energy storage (SMES)

10MW-100MW

100MW-1GW

>1GW

Magnetic aircraft launch Big locomotives

Power Plant

Sandy Pond substation (2.2GW)

3

1.02 CONVERTER CLASSIFICATION The objective of a power electronics circuits is to match the voltage and current requirements of the load to those of the source. Power electronics circuit convert one type or level of a voltage or current waveform to another and are hence called converters.

Converters are classified by the relationship between input and output.

Figure 1.1 : Block diagram of Converter

4

Power supply is an electronic circuit that is used for providing the electrical power to appliances or loads such as computers and machines. These electrical and electronic loads require various forms of power at different ranges and with different characteristics. So, for this reason the power is converted into the required forms (with desired qualities) by using some power electronic converters or power converters.

Table1.1 : Power Supply

5

What is Switch-Mode Power Supply?

The electronic power supply integrated with the switching regulator for converting the electrical power efficiently from one form to another form with desired characteristics is called as Switch-mode power supply. It is used to obtain regulated DC output voltage from unregulated AC or DC input voltage.

Figure 1.2 : Block diagram of SPMS

6

1.12 Block Diagram of SMPS

Figure 1.3 : Block diagram of an SMPS

7

1.13 Concept of AC to DC Converter in SMPS Input Rectifier Stage If the SMPS has an AC input, then the first stage is to convert the input to DC. Inverter Stage The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by running it through a power oscillator.

Output Transformer If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a highfrequency transformer and this converts the voltage up or down to the required output level on its secondary winding Output Rectifier And Filter Id a DC output is required, the AC output from the transformer is rectified Chopper Controller Feedback circuit monitors the output compares it with a reference voltage

voltage

and 8

1.14 HIGH VOLTAGE DIRECT CURRENT ( HVDC)

High voltage direct current (HVDC) power systems use D.C. for transmission of bulk power over long distances (>600KM). For long-distance power transmission, HVDC lines are less expensive, and losses are less as compared to AC transmission. It interconnects the networks that have different frequencies and characteristics.

HVDC lines increase the efficiency of transmission lines due to which power is rapidly transferred.

9

1.15 HVDC TRANSMISSION SYSTEM WORKING PRINCIPLE In generating substation, AC power is generated which can be converted into DC by using a rectifier. In HVDC substation or converter substation rectifiers and inverters are placed at both the ends of a line. The rectifier terminal changes the AC to DC, while the inverter terminal converts DC to AC. The DC is flowing with the overhead lines and at the user end again DC is converted into AC by using inverters, which are placed in converter substation. The power remains the same at the sending and receiving ends of the line. DC is transmitted over long distances because it decreases the losses and improves the efficiency.

Figure 1.4: HVDC Substation Layout

10

1.2 These devices are classified as follows

POWER DIODE Power Diode is uncontrolled device. When anode (A) is positive with respect to cathode (K), diode start conducting (forward biased). The diode does not conduct when anode to cathode voltage is negative.

Figure 1.5 : Diode

11

Structure of Power Transistor Power transistors are devices that have controlled turn-on and turn-off characteristic. These devices are used a switching devices and are operated in the saturation region.

The switching speed of modern transistors is much higher than that of thyristors and are used extensively in dc-dc and dc-ac converters. Suitable used in low to medium power applications.

THYRISTOR Thyristor is a general name given to a family of power semiconductor switching devices. They are operated as bistable switches, operating from non-conducting state to conducting state. Many applications assume thyristor as ideal switches but practically they exibit certain characteristic. SCR and TRIAC are widely used in high power control circuits.

Thyristor Family The other members of thyristor family are : SCR - Silicon controlled rectifier TRIAC - Bidirectional triode thyristor GTO - Gate turn-off thyristor DIAC - Bidirectional diode thyristor PUT - Programmable unijunction transistor 12

SILICON CONTROL RECTIFIER (SCR)

13

1.3 Silicon Controlled Rectifiers (SCR) Silicon Controlled Rectifier (SCR) is a four layer PNPN silicon-switching device. The SCR has three terminals the anode (A), cathode (K) and gate (G). There are correspondingly three junctions J1, J2 and J3 respectively.

The symbol and structure of SCR are shown below:

Figure 1.6(a) :Schematic Symbol

Figure 1.6(b): Block Construction

14

SCR is a unidirectional device conducting only from anode to cathode (forward bias).

It blocks the current flow from anode to cathode until it is triggered in to conduction by the application of suitable gate signal between gate and cathode.

SCR is so called because Silicon is used for its construction and its operation as a Rectifier can be controlled.

Once the SCR start conducting, it behaves like a conducting diode and there is no control over the device although the gate voltage has stopped.

Turn-on characteristics of SCR Two condition must be met to turn-on an SCR : i Anode voltage should be positive with respect to the cathode. ii. Gate voltage should be positive with respect to the cathode.

15

Turn-off characteristics of SCR The gate has no control over the SCR once it goes into conduction. Turn-off must be achieved in the anode-tocathode circuit such as : i. Reversing anode-to-cathode terminal ii. Forcing current in the anode circuit in the reverse direction. iii. Decrease forward current (IF) lower than holding current (IH) (Gate-controlled effect)

I-V characteristics of SCR

Figure 1.7: I-V characteristic SCR 16

Figure 6 is V-I characteristic of SCR where the forward current, IF is plotted with respect forward voltage, VF. ‘VBO’ = forward breakover voltage ‘VBR’ = reverse breakover voltage.

at gate current IG = 0, if forward voltage is applied on the device there will be a leakage current.

If gate signal is applied, the voltage exceeds its critical limit or equal to forward breakover voltage, (VBO) then SCR is ON-STATE. SCR will turn on successfully if a minimum anode current, called a latching current (IL), is maintained.

During ON-STATE, if the IG =0 and the anode current (IA) falls below a critical limit, called the holding current (IH), SCR reach its forward blocking state.

As the reverse voltage (VR ) across the SCR increases from zero, only a small reverse current (IR) will flow through the device due to leakage.

This current will remain small until VR becomes large enough to cause the SCR to breakdown.

Then IR will increase rapidly if VR increases even slightly above the breakdown point (the curve is almost vertical and straight).

If too much reverse current is allowed to flow through the SCR after breakdown occurs, the device could be permanently damaged.

17

Gate-controlled effect for Forward Berakover Voltage (VBO)

If gate current (IG) is supplied sufficiently, the forward breakover voltage (VBRF) will start early. Means, the higher rate of gate current (IG), the lower VBRF needed for SCR to conduct.

For example in figure above, VBRF0 is at the instance of IG0 = 0. If the IG0 is increased to IG1, VBRF1 will happen earlier than VBRF0. The rate of VBRF can be lowered by increasing IG. If it is adjusted to a value that is higher enough, SCR will act as a diode.

When supply voltage < forward breakover voltage (VBO), SCR is in ‘OFF’ condition.

If supply voltage ≥ VBO , SCR is in ‘ON’ condition.

When SCR is in ‘ON’ condition, the rate of current flow should not lower than the rate of holding current of the SCR.

18

Forward-breakover voltage,VBO This is the voltage at which the thyristor enters the forward-conduction region.

Reverse-breakover voltage,VBR It is the maximum reverse voltage value to stay in blocking state (off). If the reverse voltage through thyristor is more than VRBO. This will damage the P-N junction and causes reverse current unexpectedly flow.

Latching Current (IL) The minimum anode current required to maintain the thyristor in the on-state immediately after switching from the off-state to the on-state has occurred and the triggering signal has been removed.

Holding current ,IH The minimum anode current necessary to keep the device in forward-conduction after it has been operating at a high anode current value.

Or The minimum anode current required to maintain the thyristor in the on-state.

19

Alternatively, SCR also can be generated using twotransistor model Gate requires small positive pulse for short duration to turn SCR on. Once the device is on, the gate signal serves no useful purpose and can be removed.

Figure 1.8: Symbol SCR using BJT

20

The Regenerative Action Using Two-Transistor Model

Figure 1.9: Circuit the Regenerative action When the gate current, IG, is zero, both transistors in the

off state.

When a positive pulse of current (trigger) is applied to the gate, the value is enough to “on” Q2 (VBE2=VG).

Current collector Q2 will increase and will “on” Q1 (IB1=IC2).

When Q1 “on”, IC1 will increase and also increase IB2.

IB2 at Q2 increase and it also increase IC2. This process is called “Regenerative Action” where the current collector for each transistor will increase and this will make the process continuous for each transistor.

In this condition, we can assume that the SCR will “on”.

The collector current of Q1 provides additional base current for Q2, so that SCR stays in conduction after the trigger pulse is removed from the gate.

21

Concept and Principle of SCR as a switch.

Thyristors are high-speed solid-state devices which can be used to control motors, heaters and lamps

Figure 1.11: DC Thyristor Switching Circuit

22

GATE-TURN-OFF (GTO-SCR)

23

1.4 GATE-TURN-OFF (GTO-SCR) A gate-turn-off (GTO) is a member of the thyristor family. It has three terminals : anode (A), cathode (C) and gate (G).

The basic structure of a GTO is similar to a thyristor (a four-layer pnpn SCR).

A GTO is turned on by forward biasing the gate-cathode junction and turned off by reverse biasing the same junction (contrary to SCR).

Observe that there is double arrow on the gate. This indicates that bidirectional current flows through the gate.

The rest of the symbol is similar to SCR.

Figure 1.12: GTO-SCR Structure diagram

Figure 1.13: GTO-SCR Symbol

24

Basic Operation of (GTO-SCR) In terms of operation, GTO has same function as SCR. But in terms of method to turn-on and turn-off GTO, it is easier than SCR as long as certain pulse is supplied between the gate and the cathode.

GTO-SCR do not need external circuit for turn-off. Hence turn-off of GTO can be achived by negative current from gate.

Turn-On Characteristic of (GTO-SCR) The GTO can be turned on by the application of the gate signal, and can also be turned-off by a gate signal of negative polarity. Thus gate has full control over the operation of GTO. Turn on is accomplished by a "positive current" pulse between the gate and cathode terminals. As the gatecathode behaves like PN junction, there will be some relatively small voltage between the terminals.

25

On- State Condition of (GTO-SCR) The turn on phenomenon in GTO is however, not as reliable as an SCR and small positive gate current must be maintained even after turn on to improve reliability.

The on-state voltage drop for the GTO is about 2V to 3V which is high as compared to the conventional thyristor (1 to 1.5V).

The switching speeds are in the range of the few microsec to 30 m sec, means that a short duration gate pulse is enough to drive them in the on-state.

Turn-off Characteristic of (GTO-SCR) Turn off is accomplished by a "negative voltage" pulse between the gate and cathode terminals.

Some of the forward current (about one-third to one-fifth) is "stolen" and used to induce a cathode-gate voltage which in turn induces the forward current to fall and the GTO will switch off (transitioning to the 'blocking' state.)

26

I-V Characteristic of (GTO-SCR)

Figure 1.14: I-V Characteristics

In this figure 11 observe that the V-I characteristics of GTO in forward direction are similar to that of SCR. However, they have relatively larger holding current and gate trigger current But in reverse direction GTO has virtually no blocking capability. Observe that GTO starts conducting in reverse direction after very small reverse (20 to 30V) voltage. This is because of the anode short structure.

27

Latching current, IL – the minimum forward current that flows through the GTO to keep it in forward conduction mode (i.e ON state) at the time of triggering. If forward current is less than latching current, GTO does not turnon.

Holding current, IH – the minimum forward current that flows through the GTO to keep it in forward conduction mode. When forward current reduces below holding current, GTO turn-off.

VBO – Forward Breakover Voltage is the value forward biased voltage that turns GTO from blocking state to conduction state without positive voltage at gate.

VBR – Reversed Breakover Voltage is the maximum value of reverse biased voltage to be in blocking state. If the reverse voltage is exceeds VBR, the GTO is damaged.

28

Concept and Principle of (GTO-SCR) as a switch

Figure 1.15: GTO-SCR as a swtitch

The basic operation of GTO is the same as that of the conventional SCR.

29

Merit of GTO-SCR i. Gate has full control over the operation of GTO. ii. Low on-state loss. iii. High ratio of peak surge current to average current. iv. High on-state gain.

Limitations of GTO-SCR i. GTO’s require large negative gate currents for turn-off.

Hence they are suitable for low power applications. ii. Very small reverse voltage blocking capability. iii. Switching frequencies are very small.

30

TRIAC (Bidirectional Triode Thyristor)

31

1.5 TRIAC Triac is also called bi-directional device. It conducts in both the direction.

It contains 3 terminals : Main Terminal 1 (MT1), Main Terminal 2 (MT2) and Gate (G).

The control terminal, gate is near to terminal MT1. This device is turned-on by +ve gate current that the gate signal being applied between the gate and MT1 terminals.

Figure 1.16(a):Symbol

Figure 1.16(b):Structure Diagram

Figure 1.16(c):Equivalent Circuit Using SCR

32

Turn On characteristic of TRIAC This device is turned-on by +ve gate current that the gate signal being applied between the gate and MT1 terminals.

Gate has no control over the conduction once triac is turned on.

Turn Off characteristic of TRIAC Triac turns-off when voltage is reversed.

33

I-V Characteristic of TRIAC

Figure 1.17:I-V Characteristic

The supply voltage at which the Triac is turned ON depends upon the gate current.

The Triac operates in quadrant 1, where the terminal MT2 is positive with respect to terminal MT1. Now the Triac is positively biased.

The Triac operates in quadrant 3 where the terminals MT2 is negative with respect to terminal MT1. Now the Triac is negatively biased.

34

The static switching characteristic in quadrant 1 is identical to that of the SCR and the characteristic in quadrant 3 is symmetrical to that in quadrant 1. Thus the Triac is effectively equivalent to an inverse-parallel arrangement of two SCRs. Other characteristic that same as SCR curve is that the higher rate of gate current (IG), the lower VBRF needed for Triac to conduct.

Concept and Principle of TRIAC as a switch Simple triac switching circuit is require an additional positive or negative gate supply to trigger the triac into conduction.

Figure 1.18:DC Triggered Triac Power Switching Circuit

35

Concept and Principle of TRIAC as a switch

Figure 1.19:Simple Static AC Power Switch Circuit

But can also trigger the triac using the actual AC supply voltage itself as the gate triggering voltage

36

Merits of TRIAC i. Triacs is a bidirectional device, i.e. it conducts in both directions. ii. Triacs turns-off when voltage is reversed. iii. Single gate controls conduction is both directions. iv.Triacs with high voltage and current ratings are available.

Demerits of TRIAC i. Triacs are latching devices like SCR. Hence, they are not suitable for DC power applications. ii. Gate has no control over the conduction once triac is turned on. iii. Triacs have very small switching frequencies.

37

IGBT (Insulated Gate Bipolar Transistor)

38

IGBT (Insulated Gate Bipolar Transistor) The Insulated Gate Bipolar Transistor (IGBT) is the latest device in power electronics.

This device combines the advantages of MOSFET and BJT. Like MOSFET, it is not having the secondary breakdown problem. It is having high input impedance.

Like BJT, it is having the low-on state power loss.

IGBT has three terminals : Gate (G), collector (C) and emitter (E).

Sometime the collector is also called drain and emitter is also called source.

Figure 1.20(a):n-Channel IGBT Figure 1.20(b):p-Channel IGBT 39

Structure of IGBT

Figure 1.21:Structure Diagram

The IGBT structure is very close to the n-channel MOSFET. The major difference between the structure of the n-channel IGBT and MOSFET is that a highly doped P+ type substrate is provided with IGBT. From the basic structure, n+ P body region, n-forms the power MOSFET. The n- drift region forms the drain.

The next part will constitute the three layers p+ n- p, that forms a bipolar junction transistor between the drain and source terminals p n- p+ regions will behave as collector, base and emitter (E) of pnp transistor respectively.

40

Turn On Characteristics of IGBT Current flows from collector to emitter whenever a voltage between gate and emitter is applied. The IGBT is said to have turned ‘on’.

When the gate to emitter voltage is applied, very small (negligible current flow. This is similar to the gate circuit of MOSFET. The on-state collector to emitter drop is very small like BJT.

Turn Off Characteristics of IGBT The IGBT can be turned-off by the removal of gate voltage signal.

When gate emitter voltage is removed, IGBT turns-off.

Thus gate has full control over the conduction of IGBT.

41

I-V Characteristic of IGBT

Figure 1.22:I-V Characteristics

The figure shows the V-I characteristics of n-channel IGBT. Sometime the collector is also called drain and emitter is also called source.

The characteristics are plotted for drain (collector) current iD with respect to drain source (collector emitter) voltage VDS. The characteristics are plotted for different values of gate to source (VGS) voltages.

42

When the gate to source voltage is greater than the threshold voltage VGS(th), then IGBT turns-on.

The IGBT is off when VGS is less than vGS(th).

The figure shows the ‘on’ and ‘off’ regions of IGBT. The BVDSS is the breakdown drain to source voltage when gate is open circuited.

Characteristic of IGBT IGBT have turn-on and turn-off times of order of 1µsec and are available in large ratings as large as 1700V and 1200A. Voltage ratings up to 2.3 kV are projected.

The switching frequency of IGBT is in the range of 50kHz. It can be used for the speed control of AC and DC motor drives.

43

Concept and Principle of IGBT as a switch

Figure 1.23:Switching Circuit for IGBT

The IGBT is a Voltage controlled device, hence it only requires a small voltage to the gate to stay in the conduction state. And since these are unidirectional devices, they can only switch current in the forward direction which is from collector to emitter. A typical switching circuit of IGBT is shown below, the gate volt VG is applied to the gate pin to switch a motor (M) from a supply voltage V+. The resistor Rs is roughly used to limit the current through the motor.

44

Merits of IGBT i. Voltage controlled device. Hence drive circuit is very simple. ii. On-state losses are reduced. iii. Switching frequencies are higher than thyristor. iv. No commutation circuits are required. v. Gate have full control over the operation of IGBT. vi. IGBTs have approximately flat temperature coefficient.

Demerits of IGBT

i. IGBTs have static charge problems. ii. IGBTs are costlier than BJTs and MOSFETs.

45

DC TO DC CONVERTER #2.0 Definition. #2.1 Chopper Operation

Method of Control in #2.2 Dc to Dc Control Step Down Converter #2.3 (Buck Chopper).

Step Up Converter #2.4 (Boost Chopper).

46

DC TO DC CONVERTER

47

2.0 Definition The dc to dc converter or also known as dc chopper convert the input dc voltage into fixed or variable dc output. Figure 3.0 shows the basic block diagram of the chopper :

Figure 2.0 (i):Basic Block diagram DC to Dc Converter

They are several types of DC to Dc Chopper are : 1. Buck Chopper (Step - Down Converter) - The output Voltage is less than input voltage.

48

2. Boost Chopper (Step-Up Converter) - The output voltage higher than input voltage.

3. Buck - Boost Chopper (Step-up-down Converter) - The output voltage either higher or lower than the input voltage.

49

DC/ DC Application Chopper i. Renewable Energy ii. Medical Devices iii. Vehicles iv. Smart Lighting

Figure 2.0(ii) :Application High Gain Dc-Dc Converter

50

Advantages of Chopper A chopper circuit has several advantages as listed below: i. Offers high efficiency (reduced losses) ii. Faster response to control signals iii. Requires less maintenance iv. Smaller in size v. Cost is less

2.1 Chopper Operation A chopper is high speed on/off semiconductor switch. it is connects sources to load and disconnects the load from source at a fast speed. The basic dc chopper circuit as shown in Figure

Figure 2.1(a) :Basic DC Chopper

51

Figure 2.1(b) shows the output voltage and current of a basic Dc Chopper. Its shows that Ton is the time interval when the switch conducts and Toff is the time interval for when the switch is OFF.

Figure 2.1(b) output voltage and current basic Dc Chopper Therefore, the switching frequency or chopping period ,Ts is the total of conduction and blocking time period (Ton + Toff). The duty cycle D defines the amount of the signal is in high state 'ON' as a percentage of the total time it takes to complete one cycle.

Figure 2.1(c) output voltage basic Dc Chopper 52

Duty cycle can be express as;

Switching Frequency can be express as;

Period can be express as;

Conduction Times can be express as;

Blocking Times can be express as;

53

2.2 Method of Control in Dc to Dc Converter The output dc voltage can be varied by the following methods: i. Pulse width modulation control (PWM) or constant frequency operation

Figure 2.2(a):PWM Control

In pulse width modulation the pulse width (tON) of the output waveform is varied keeping chopping frequency ‘f’ and hence chopping period ‘T’ constant. Therefore output voltage is varied by varying the ON time, tON. Figure 2.2(a) shows the output voltage waveforms for different ON times.

54

ii. Variable Frequency Control •In this method of control, chopping frequency, f is varied keeping either tON or tOFF constant. This method is also known as frequency modulation.

•Figure 3.2(b) shows the output voltage waveforms for a constant tON and variable chopping period T. •In frequency modulation to obtain full output voltage, range frequency has to be varied over a wide range. This method produces harmonics in the output and for large tOFF load current may become discontinuous.

Figure 2.2(b):Output voltage waveform for time ratio control

55

2.3 STEP DOWN CONVERTER (BUCK CHOPPER) Buck converter is the output voltage is less than input voltage.

A buck converter can be remarkably efficient and selfregulating, making it useful for tasks such as converting the 12-24V typical battery voltage in a laptop down to the several volts needed by processor.

i. Circuit Diagram of Buck chopper.

Figure 2.3:Circuit Diagram Buck Converter

56

ii. The Operation of Buck chopper. (a) When the switch is closed the diode D becomes reversebiased and the effective circuit formed is shown in Figure 2.3(a).

Figure 2.3(a):Switch is Closed of Buck Converter

Inductor will save energy from Imin to Imax thus its polarity will make D(diode) in reverse bias.

The supply will reach to the load through close switch.

This result in positive inductor voltage, VL = Vs-Vo

57

(b) When the switch is reopened the diode D becomes a short-circuit and the effective circuit formed is shown in Figure 2.3(b).

Figure 2.3(b):Switch is reopened of Buck Converter

Inductor will release energy from Imax to Imin thus its polarity will make D(diode) in forward bias.

The supply cannot reach to the load but due to the effect there will be an output at the load.

This inductor voltage, VL = -Vo

58

ii. Derive the Equation of Buck chopper. (a):When the switch is closed, the equation are, VL = Vs-Vo

the voltage VL across the inductor is related to the change in current flowing through it according to the relation,

Equating VL in both of the above equations, we obtain

Rearranging the equation to bring diL/dt to the LHS, we obtain

59

Since diL/dt is constant, we can rewrite as follows:

we can write ΔtON as;

can now be rewritten as;

Hence, we arrive at the required result,

60

(a):When the switch is reopened, the equation are, VL = -Vo

the voltage VL across the inductor is related to the change in current flowing through it according to the relation,

Equating we obtain when switch is reopened

Rearranging the equation to bring diL/dt to the LHS, we obtain

61

we can write ΔtOff as;

Hence, we arrive at the required result,

From equation (i) and Equation (ii) into the equation below;

Hence;

62

The formula Output Voltage, Vo for Buck Chopper are

Figure 2.3(c) show the graph current inductor;

Figure 2.3(c):Graph the maximum and minimum current inductor

63

The Minimum current inductor,

The Maximum current inductor,

64

Continuous Current Mode (CCM) Operation of Buck chopper.

Figure 2.3(d):Continous Current Mode (CCM) The inductor current flows continuously, which turns On and OFF at the same frequency as the switching frequency

Lmin is the minimum inductor value to ensure CCM Normally Lpractical is chosen to be »Lmin.

Usually, value of inductor must be 25% larger than the minimum inductor to ensure that inductor current is continuous. ( L=1.25Lmin)

65

Output Voltage Ripple;

If the converter components are assumed to be ideal, then the power supplied by the source must be the same as the power absorbed by the load resistor:

66

Output waveform Buck Chopper;

67

2.4 STEP UP CONVERTER (BOOST CHOPPER)

Boost converter is the output voltage is higher than input voltage.

Boost converters can increase the voltage and reduce the number of cells. Two battery-powered applications that use boost converters are hybrid electric vehicles (HEV) and lighting systems.

i. Circuit Diagram of Boost chopper.

Figure 2.4:Boost Chopper

68

ii. The Operation of Boost chopper. (a) When the switch is closed circuit formed is shown in Figure 2.4(a).

Figure 2.4(a):Switch Closed (Boost Chopper )

Inductor will save energy from Imin to Imax thus its polarity will make D(diode) in reverse bias.

This result in positive inductor voltage, VL = Vs

69

(b) When the switch is reopened the diode D becomes a short-circuit and the effective circuit formed is shown in Figure 2.4(b).

Figure 2.4(b):Switch Reopened (Boost Chopper )

the current will be reduced as the impedance is higher, Therefore, change or reduction in current will be opposed by the inductor. Thus the polarity will be reversed, so that the current inductor Imax to Imin. the voltage become, VL = VS-Vo

70

ii. Derive the Equation of Boost chopper. (a):When the switch is closed, the equation are, VL = Vs

the voltage VL across the inductor is related to the change in current flowing through it according to the relation,

Equating VL in both of the above equations, we obtain

Rearranging the equation to bring diL/dt to the LHS, we obtain

we can write ΔtON as;

Hence, we arrive at the required result,

Rearranging the equation to bring LHS, we obtain

71

(a):When the switch is reopened, the equation are, VL = Vs-Vo

the voltage VL across the inductor is related to the change in current flowing through it according to the relation,

Equating VL in both of the above equations, we obtain

Rearranging the equation to bring diL/dt to the LHS, we obtain

we can write ΔtOff as;

Hence, we arrive at the required result,

72

From equation (i) and Equation (ii) into the equation below;

Input power=output power

Average inductor current:

73

Maximum ripple inductor current:

Minimum ripple inductor current:

For continuous operation, I min≥0

Ripple factor:

74

DC TO AC CONVERTER (INVERTER) #3.0 Definition. #3.1 Classification of Inverter #3.2 Bridge Inverter

Single Phase Half #3.3 Bridge Inverter Three Phase Bridge #3.4 Inverter 75

DC TO AC CONVERTER (INVERTER)

76

3.0 Definition Inverter is a device which converts DC power to AC power at a certain output voltage and frequency or at a certain current level and frequency.

Figure 3.0:Basic Block diagram DC to AC Converter

This system produces variable or fixed ac voltage from a fixed or variable dc source.

The output voltage waveform of the inverter can be square-wave , quasi-square wave or low distorted sine wave.

The switching devices used in DC to AC converter such as thyristor, power BJT, MOSFET, IGBT, GTO, SIT etc. thyristor based inverter are used only in high-power applications.

For low and medium power inverters, gate-controlled turn-off devices (gate-commutation devices), such as power BJT, MOSFET, IGBT, GTO, SIT, etc., are used.

In addition to being fully controlled, these have highswitching frequencies.

77

Application in industry

78

Application in industry

Figure 3.0(a):Example DC to AC converter

Figure 3.0(b):Example DC to AC converter

79

3.1 Classification of Inverter Inverters are mainly classified based on the nature of input power source : Voltage Source Inverter (VSI) Current Source Inverter (CSI)

a) Voltage Source Inverter (VSI) The DC source has small or negligible impedance. In order word, a voltage source inverter has stiff DC voltage source at its input terminal.

Figure 3.1(a):Configuration diagram for VSI types

b) Current Source Inverter (CSI) Fed with adjustable current from a DC source of high impedance. In a CSI fed with stiff current source, output current waves are not affected by the load.

80

Figure 3.1(b):Configuration diagram for CSI types

Comparison between VSI and CSI Table 3.1(a):Comparison between VSI and CSI

81

Basic concept of switching scheme in VSI The inverters can be classified according to the nature of output voltage waveform as : i) Pulse-width Modulation (PWM) inverter ii) Square Wave inverter

i) Pulse-width Modulation (PWM) Inverter The pulse-width modulation (PWM) inverter techniques are most commonly used to control the output voltage of inverters. It uses a switching scheme within the inverter to modify the shape of the output voltage waveform.

The output voltage of inverter contain harmonics whenever it is non-sinusoidal. The harmonics can be reduced by using proper control schemes.

82

Figure 3.1(c):Switching using PWM

ii) Pulse-width Modulation (PWM) Inverter Square-wave inverter produces a square-wave ac voltage of a constant magnitude. The output voltage of this type of inverter can only be varied by controlling the input dc voltage. It is synthesized from a dc input by closing and opening the switches in an appropriate sequence.

Square-wave ac-output voltage of an inverter is adequate for low and medium power applications.

83

Figure 3.1(d):Switching using Square Wave

3.2 Bridge Inverter Two types of bridge inverter : i) Single-phase half bridge inverter with resistive load with inductive load. ii) Single-phase full bridge inverter with resistive load with inductive load.

84

3.3 Single Phase Half Bridge Inverter

Single Phase Half Bridge Inverter with Resistive Load i) Circuit Diagram

Figure 3.2(a):Single Phase Half Bridge inverter with R load

Switches S1 & S2 are the gate-commutated devices such as power BJT, MOSFET, GTO, SCR, IGBT, etc.

Gating circuit should be designed such that switch S1 & S2 should not turn-on at the same time.

The diodes D1 & D2 do not play any role.

85

ii) Operation with Resistive Load

Figure 3.2(a)(i):Equivalent Circuit During Period -I During Period-I at (0 ≤ t ≤ T/2), Switch S1 is closed for half-time period from 0 to T/2.

Current flow from the DC supply to S1, and resistive load from point A to B. Hence output voltage at load become positive, Vo = Vs/2.

Figure 3.2(a)(ii):Output Voltage waveform During Period -I

86

During Period-I at (T/2 ≤ t ≤ T),

Figure 3.2(b)(i):Equivalent Circuit During Period -II Switch S2 is closed from T/2 to T and switch S1 is open.

Current flows from the DC supply to S2, and resistive load from point B to A. Hence output voltage at load become negative, Vo = –Vs/2.

Figure 3.2(b)(ii):Output Voltage Waveform During Period -II 87

iii) Output Waveform

Figure 3.2(b)(iii):Output Voltage and output Current iv) Derivation for RMS value

88

Single Phase Half Bridge Inverter with Resistive and Inductive Load i) Circuit Diagram

Figure 3.2(c):Single Phase Half Bridge inverter with RL load

Diodes D1 & D2 are connected across the switches. These diodes conduct for inductive load.

Switches S1 & S2 are the gate-commutated devices such as power BJT, MOSFET, GTO, SCR, IGBT, etc.

Gating circuit should be designed such that switch S1 & S2 should not turn-on at the same time.

89

ii) Operation with Resistive and Inductive Load a) During Mode-I at (t1< t < t2)

Figure 3.2(c)(i):Equivalent Circuit During Mode-I

Switch S1 is turned-on at instant t1.

Current flow from DC supply to S1, then to RL load (A to B) makes the load voltage become Vo = +Vs/2.

The +ve load current increases gradually due to inductive load (charging) the energy is stored by the inductive load.

At instant t2, the +ve load current reaches the peak value.

Switch S1 is turned-off at this instant.

Figure 3.2(c)(ii):Output Waveform During Mode-I

90

b) During Mode-II at (t2< t < t3)

Figure 4.2(c)(iii):Equivalent Circuit During Mode-II

At instant t2, switch S1 is turned-off and switch S2 is not conduct.

The load current is tried to maintained in the same direction by the load inductance, hence generates a large voltage Ldio/dt (discharge). This voltage polarity forward biases diode D2.

In this mode, the stored energy in load is feedback to the DC supply and the load voltage become Vo = -Vs/2.

This current goes on decreasing and become zero at t3.

Figure 3.2(c)(iv):Output Waveform During Mode-I 91

c) During Mode-III at (t3< t < t4)

Figure 3.2(c)(v):Equivalent Circuit During Mode-III

At instant t3, switch S2 starts turned-on from t3 to t4.

Current flow from DC supply to RL load from point B to A. This will produce a negative load voltage, Vo = -Vs/2 and negative load current.

The -ve load current increases gradually due to inductive load (charging) the energy is stored by the load.

Load current reaches negative peak at instant t4 and switch S2 is turned off

Figure 3.2(c)(vi):Output Waveform During Mode-I 92

d) During Mode-IV at (to< t < t1)

Figure 3.2(c)(vii):Equivalent Circuit During Mode-III

Switch S2 is turned-off at instant t4.

The self induced voltage in the induced load will maintain the load current. The load inductance generates a large voltage Ldio/dt (discharge). This voltage polarity forward biases diode D1.

The load voltages changes its polarity to become positive Vo=VDC/2, load current remains negative.

The output current decreases from negative maximum towards zero at t1 (or t5). At t1 (or t5), the load current goes to 0 and S1 can be turned-on again. This cycle of operation repeats.

93

Figure 3.2(c)(viii):Output Waveform During Mode-IV

iii) Output Waveforms

Figure 3.2(c)(ix):Voltage and current waveforms with RL load

94

iv) Derivation for RMS value

95

Single Phase Full Bridge Inverter with Resistive Load i) Circuit Diagram

Figure 3.2(d):Single Phase Full Bridge inverter with R load ii) Operation with Resistive Load a) During Mode-I at (0< t <T/2 )

Figure 3.2(d)(i):Equivalent Circuit-I

Transistor T1 and T2 conduct from 0 to T/2.

Equivalent circuit - I in figure shows the current path when T1 & T2 conduct.

The output voltage and current are positive.

Note that the amplitude of load voltage is Vs.

96

b) During Mode-I at (T/2< t <T)

Figure 3.2(d)(ii):Equivalent Circuit-II

T1 and T2 are turned-off. Transistors T3 and T4 conduct from T/2 to T. Note that the output current is negative. The voltage is also negative. Vo=-VS iii) Output Voltage and Current Waveforms

97

iv) Derivation for RMS value

98

Single Phase Full Bridge Inverter with Resistive and Inductive Load i) Circuit Diagram

Figure 3.2(e):Single Phase Full Bridge inverter with RL load ii) Operation with RL Load a) During Mode-I at (t1< t < t2)

Figure 3.2(e)(i):Equivalent Circuit-1 99

T1 and T2 are applied the drive at t = 0. But they does not conduct till t1. Diodes D1 and D2 conduct from 0 to t1.

Hence T1 & T2 are reversed biased and they do not conduct.

From t1 to t2, T1 and T2 conduct. It circuit is shown in equivalent circuit-I The load current is positive and increases from zero to +Imax. The output voltage is also positive. b) During Mode-I at (t2< t < t3)

Figure 3.2(e)(i):Equivalent Circuit-II

At T/2. transistors T1 and T2 are turned-off and T3 and T4 are applied drives.

The load inductance generates the large voltage L dio/dt with polarities shown in equivalent circuit-II. The diodes D3 and D4 are forward biased due to inductance voltage. These diodes conduct and output current flow through DC supply. The energy stored in the load inductance is supplied to the DC supply. This operation is called feedback operation.

100

Due to conduction of D3 and D4, transistor T3 and T4 are reversed biased. Hence they do not conduct, even though base drives are applied .

At t2, the load current becomes zero. Hence transistors T3 and T4 start conducting.

c) During Mode-I at (t3< t < t4)

Figure 3.2(e)(i):Equivalent Circuit-III

At t2 the transistors T3 and T4 start conducting.

Figure shows the equivalent circuit-III for this operation.

The output current is negative and increases towards – Imax.

The supply current is is positive. The output voltage is negative during this period.

101

d) During Mode-I at (to< t < t1)

Figure 3.2(e)(i):Equivalent Circuit-IV At T, transistor T3 and T4 are turned off and T1, T2 are

applied the drive. The output current is at –Imax.

Hence load inductance generates the large voltage L dio/dt with polarities as shown equivalent circuit-IV. Due to this voltage the diodes D1 and D2 are forward biased. Hence they start conducting.

The output current flows through the DC supply and it goes on decreasing.

The energy stored in the load inductance is supplied to the DC supply during this period. This is called feedback operation.

Transistor T1 and T2 are reversed biased due to conduction of D1 and D2. } Hence T1 and T2 do not conduct even if their base drive is applied.

102

iii) Output Voltage and Current Waveforms

103

iv) Derivation for RMS value

104

3.4 Three Phase Bridge Interter Single-phase inverters are used for low power applications.

For higher powers and three-phase induction motor drives, three-phase inverters are used.

An inverter generates three-phase output R, Y and B. The load can be connected to the inverter in star or delta mode.

Two types of three-phase bridge inverter : i) Three-phase bridge inverter with 120° firing with resistive load. ii) Three-phase bridge inverter with 180° firing with resistive load. a) Basic Circuit Diagram

Figure 3.3 (a):Basic Circuit diagram

105

i) Three-phase bridge inverter with 120° conduction with resistive load.

In 180° conduction, the base drives are applied for 180° duration. Similarly in 120° mode of conduction, the base drives are applied for 120° duration.

The circuit diagram and method of analysis remains same as in 180° mode.

The waveforms are shown in Figure 15. there are six intervals : I, II, III, IV, V and VI. In each interval two transistors conduct. In interval-I, T1 and T6 conducts.

106

a) Circuit Diagram

Figure 3.3(b):Basic Circuit diagram

107

b) Conducting Switching Each switch conducts for 120 degree.

Only 2 switches ON at any instant of time.

Conduction sequence of switches is 61, 12, 23, 34, 45 and 56. Figure 3.3(c) show to produced the conducting switching.

In the base drives of Figure 3.3 (c), observe that the successive base drives are delayed by 60°.

Figure 3.3 (c):Conducting Switching 120 Degree

108

c) Triggering sequence :

d) Equivalent circuit: During interval 0 to 60, switching S1 and S6 is ON VRN = Vs/2 VYN = -Vs/2 VBN =

0

Figure 3.3 (c)(i):Equivalent Circuit S1S6

109

During interval 60 to 120, switching S1 and S2 is ON VRN = Vs/2 VYN =

0

VBN = -Vs/2

Figure 3.3 (c)(ii):Equivalent Circuit S1S2

During interval 120 to 180, switching S2 and S3 is ON VRN = VYN =

0 Vs/2

VBN = -Vs/2

Figure 3.3 (c)(iii):Equivalent Circuit S2S3

110

During interval 180 to 240, switching S3 and S4 is ON VRN =

-Vs/2

VYN =

Vs/2

VBN = 0

Figure 3.3 (c)(iv):Equivalent Circuit S3S4

During interval 240 to 300, switching S4 and S5 is ON

VRN =

-Vs/2

VYN =

0

VBN =

Vs/2

Figure 3.3 (c)(v):Equivalent Circuit S4S5

111

During interval 300 to 360, switching S5 and S6 is ON VRN =

0

VYN =

-Vs/2

VBN =

Vs/2

Figure 3.3 (c)(vi):Equivalent Circuit S5S6

112

Table 3.1 : Three-phase bridge inverter with 120° conduction with resistive load.

113

e) Output Waveform for Phase Voltage

114

e) Output Waveform for Line Voltage

115

ii) Three-phase bridge inverter with 180° conduction with resistive load.

Figure 3.3 (d) : Circuit diagram of Three Phase Inverter with Star Load a) Operation Principe

The base drives of all the six BJTs are shown in output waveform of Figure 3.3(b).

The base drive of T1 is applied for 180° and it is off for remaining 180°.

Base drives of T2 is applied with 60° delay with respect to T1.

Similarly base drives of other transistors are also delayed by 60° with respect to previous one.

116

One cycle of 360° is divided into six intervals of each 60° each. These intervals are named as I, II, III, IV, V and VI. In each interval three transistor conduct. For example in interval I, T1, T5 and T6 are applied the base drive. Thus in interval I, T1, T5 and T6 are conducting. b) Conducting Switching

Figure 3.3 (d)(i):Conducting Switching 120 Degree

117

c) Triggering sequence :

d) Equivalent circuit: During interval 0 to 60, switching S1, S5 and S6 is ON

Figure 3.3 (d)(ii): Equivalent Circuit S1S5S6

118

During interval 60 to 120, switching S1, S6 and S2 is ON

VRN = 2Vs/3 VYN = -Vs/3 VBN = -Vs/3

Figure 3.3 (d)(iii):Equivalent Circuit S1S6S2

During interval 120 to 180, switching S1, S2 and S3 is ON

VRN = Vs/3 VYN = Vs/3 VBN = -2Vs/3

Figure 3.3 (d)(iv):Equivalent Circuit S1S2S3 119

During interval 180 to 240, switching S2, S3 and S4 is ON

VRN = -Vs/3 VYN = 2Vs/3 VBN = -Vs/3

Figure 3.3 (d)(v):Equivalent Circuit S2S3S4

During interval 240 to 300, switching S3, S4 and S5 is ON

VRN = -2Vs/3 VYN = Vs/3 VBN = Vs/3

Figure 3.3 (d)(vi):Equivalent Circuit S3S4S5 120

During interval 300 to 360, switching S4, S5 and S6 is ON

VRN = -Vs/3 VYN = -Vs/3 VBN = 2Vs/3

Figure 3.3 (d)(vii):Equivalent Circuit S4S5S6

121

Table 3.2 : Three-phase bridge inverter with 180° conduction with resistive load.

Example to calculated line voltage (0 to 60) VRY = VRN- VYN= Vs/3-(-2Vs/3) =Vs VYB = VYN- VBN= -2Vs/3-(Vs/3) =-Vs VBR = VBN- VRN= Vs/3-(Vs/3) = 0 122

e) Output Waveform for Phase Voltage

123

e) Output Waveform for Line Voltage

124

REFERENCES

Hamidah A.Hamid and Pimpa Soowan (2020), Power Electronics, Oxford Fajar Chitode, J. S. (2014). Power Electronics Devices & Circuits.

Technical Publications.

El-Sharkawi, M. A. (2013). Electric Energy : An Introduction, Third Edition. Bosa Roca, United States: Taylor & Francis Inc. Rashid, M. H. (2013). Power Electronics : Circuits, Devices & Applications. Boston, United States: Pearson Education (US).

125