EE4532, POWER ELECTRONICS & DRIVES
Weeks 1-7: Power Electronic Systems & Devices




Section 1: Power Electronic Systems
Section 2: Power Devices
Section 3: Uncontrolled & Controlled Rectifiers
Section 4: Advanced PWM in Rectifiers
Total Lecture Hours: 11+ CA + Review
Tutorial Hours: 5+1 Review
Instructor:
Dr. Ali I. Maswood
Associate Professor, EAMASWOOD@NTU.EDU.SG
1
TYPES OF POWER ELECTRONIC CIRCUITS
 The conversion of electric power from one form to another is
necessary and the switching characteristics of the power devices
permit these conversions.
 The static power converters perform these functions of power
conversions.
 A converter may be considered as a switching matrix. The power
electronics circuits can be classified into 5 types:
1. Diode rectifiers
2. Ac-dc converters (controlled rectifiers)
3. Ac-ac converters (ac voltage controllers)
4. Dc-dc converters (choppers)
5. Dc-ac converters (inverters)
Diode Rectifiers:
 Converts ac voltage into a fixed dc voltage.
 The input voltage to the rectifier vS could be either single or three
phase.
RMS
Average
2
Thyristor (controlled) rectifier:
 A single-phase converter with two natural commutated thyristors is
shown.
 The average value of the output voltage vo can be controlled by
varying the conduction time of thyristors or delay angle α.
Lower
Average
AC-AC converters:
 Used to obtain a variable ac output voltage vo from a fixed ac source
and a single-phase converter with a TRIAC.
 The output voltage is controlled by varying the conduction time of
TRIAC.
Lower RMS
3
DC-DC converters:
 Also known as a chopper.
 The average output vo voltage is controlled by varying the
conduction time t1, of transistor Q1. If T is the chopping period,
then t1 = δT. Where; δ- duty cycle of the chopper.
Average
DC-AC converters:
 Also known as an inverter. If transistors M1, and M2 conduct for
positive half (in Red) of a period and M3 and M4 conduct for the
Negative half, the output voltage is of alternating form.
 The RMS output voltage can be controlled by varying the
conduction time of transistors.
RMS
4
DESIGN CONSIDERATIONS OF POWER ELECTRONIC EQUIPMENT
The design of a Power electronic equipment can be divided into four
parts:
1. Design of power circuits
3. Control strategy.
2. Protection of power devices
4. Logic and gating circuits
PERIPHERAL EFFECTS
 The operations of the power converters are based mainly on the
switching of power semiconductor devices;
 Converters introduce current and voltage harmonics into the supply
system and on the output of the converters.
 Problems of distortion of the output voltage, harmonic generation
into the supply system.
 It is normally necessary to introduce filters on the input and output
of a converter system to reduce the harmonic.
5
MULTIDISCIPLINARY NATURE OF POWER ELECTRONICS
2
POWER MODULES
 Power devices are available as a single unit or in a module. A power
converter often requires two, four, or six devices.
 Power modules with dual (in half-bridge configuration) or quad (in
full bridge) or six (in three phase) are available for almost all types
of power devices.
SWITCHING IN POWER ELECTRONICS & LOSSES
3
VA RANGES OF POWER DEVICES & APPLICATIONS
High
Power
Your Notes:
High
Frequency
4
CLASSIFICATION OF POWER DEVICES (L2)
1.
Uncontrolled turn on and off (e.g., diode);
2.
Controlled turn on and uncontrolled turn off (e.g., SCR);
3. Controlled turn-on and -off characteristics (e.g., BJT, MOSFET,
GTO);
4.
Continuous gate signal requirement (BJT, MOSFET, IGBT, SIT);
5.
Pulse gate requirement (e.g., SCR, GTO, MCT);
6.
Bipolar voltage-withstanding capability (SCR, GTO);
7. Unipolar voltage-withstanding capability (BJT, MOSFET, GTO,
IGBT);
8.
Bidirectional current capability (TRIAC);
9.
Unidirectional current capability (SCR, BJT, MOSFET,IGBT,
diode)
Figure below shows the applications and frequency range of power
devices. A super (ideal) power device should:
(1) have a zero on-state voltage,
(2) withstand an infinite off-state voltage,
(3) handle an infinite current, and
(4) turn on and off in zero time, thereby having infinite switching
speed.
Your Notes:
5
Thyristor
SOA
IGBT
SOA
MOSFET
SOA
Fig. V-I Characteristics (SOA) of various power Devices
CONTROL CHARACTERISTICS OF POWER DEVICES
 Power semiconductor devices can be operated as switches by applying
control signals to the gate terminal of thyristors, and to the base of bipolar
transistors.
 The required output is obtained by varying the conduction time of these
switching devices.
 Figure below shows the output voltages and control characteristics of
commonly used power switching devices
.
6
Fig. Control Characteristics of Power devices
7
High. IB—High Ic
Current Controlled
High. VGS—High Ic
Voltage Controlled
Fig. Device Symbols & their V/I Characteristics
8
POWER SEMICONDUCTOR DIODES AND CIRCUITS
A diode acts as a switch to perform various functions such as:
 switches in rectifiers,
 freewheeling in switching regulators,
 charge reversal of capacitor and energy transfer between components,
Power diodes can be assumed as ideal switches for most applications, but
practical diodes differ from the ideal characteristics and have certain limitations.
The power diodes are similar to pn-junction signal diodes. However, the power
diodes have larger power, voltage, and current-handling capabilities than those of
ordinary signal diodes. The frequency response (or switching speed) is low
compared with that of signal diodes.
Semiconductor basics
Power semiconductor devices are based on high-purity, single-crystal silicon.
A pure silicon material is known as an intrinsic semiconductor with resistivity
that is too low to be an insulator and too high to be a conductor. It has high
resistivity and very high dielectric strength (over 200 kV/cm).
n-Type silicon material: If pure silicon is doped with a small amount of a Group
V element, such as phosphorus, arsenic, or antimony, each atom of the dopant
forms a covalent bond within the silicon lattice, leaving a loose electron. These
loose electrons greatly increase the conductivity of the material.
p-Type material: If pure silicon is doped with a small amount of a Group III
element, such as boron, gallium, or indium, a vacant location called a hole is
introduced into the silicon lattice. These holes greatly increase the conductivity
of the material.
In a p-type material the holes are called the majority carriers and electrons are
called the minority carriers.
In the n-type material, the electrons are called the majority carriers, and holes are
called the minority carriers.
9
DIODE CHARACTERISTICS
A power diode is a two-terminal pn-junction device and a pn-junction is
normally formed by alloying, diffusion.
 When the anode potential is positive with respect to the cathode, the diode
is said to be forward biased and the diode conducts.
 When the cathode potential is positive with respect to the anode, the diode
is said to be reverse biased.
Under reverse-biased conditions, a small reverse current (also known as leakage
current) in the range of micro or milliampere flows.
The v-i characteristics shown in following figure can be expressed by an equation
known as Schockley diode equation, and it is given under dc steady-state
operation by
Fig. Diode Practical & Ideal V/I Characteristics
VD >0…Forward biased, VD <0…Reverse biased, VD <-VBR…Breakdown
10
REVERSE RECOVERY CHARACTERISTICS
Once a diode is in a forward conduction mode and then its forward current is
reduced to zero due to the natural behavior of the diode circuit or application of
a reverse voltage, the diode continues to conduct due to charges that remain stored
in the pn-junction and the bulk semiconductor material.
The charges require a certain time to recombine with opposite charges and to be
neutralized. This time is called the reverse recovery time of the diode.
Fig. Reverse Recovery (RR) Characteristic
The reverse recovery time is denoted as trr and is measured from the initial zero
crossing of the diode current to 25% of maximum (or peak) reverse current IRR.
t rr  t a  t b
trr consists of two components, ta and tb.
The ratio tb/ta, is known as the softness factor (SF).
The peak reverse current can be expressed in reverse di/dt as
di
I rr  t a
dt
Reverse recovery charge QRR, is the amount of charge carriers that flows across
the diode in the reverse direction due to changeover from forward conduction to
reverse blocking condition.
11
RR charge:
Qrr 
1
1
1
i rr t a  i rr t b  i rr t rr
2
2
2
If tb is negligible compared ta as usually is the case, then the following eqns. Are
valid:
2Qrr

RR time, t rr
di / dt
, RR current
I rr
 2Qrr
di
dt
Example: Reverse Recovery Current
Your Notes:
FIN
12
POWER DIODE TYPES (L3)
Depending on the recovery characteristics and manufacturing techniques, the
power diodes can be classified into the following three categories:
1. Standard or general-purpose diodes
2. Fast-recovery diodes
3. Schottky diodes
General-Purpose Diodes
 The general-purpose rectifier diodes have relatively high reverse recovery time,
typically 25 μs; and are used in low-speed applications, where recovery time is
not critical, e.g., diode rectifiers and converters for a low-input frequency up to
1-kHz applications and line-commutated converters.
 current ratings from less than 1 A to several thousands of amperes, with voltage
ratings from 50 V to around 5 kv
Following Figure shows various configurations of general-purpose diodes, which
basically fall into two types.
o
o
One is called a stud, or stud-mounted type;
Other one is called a disk, or hockey-puck type.
Fast-Recovery Diodes
 The fast-recovery diodes have low
recovery time, normally less than 5
μs.
 They are used in dc-dc and dc-ac
converter circuits, where the speed
of recovery is often of critical
importance.
 Ratings of voltage from 50 V to
around 3 kV, and current from less
than 1 A to hundreds of amperes.
Fig. Stud & Hockey Puck
configuration of power diodes.
13
Schottky Diodes
 The charge storage problem of a pn-junction can be eliminated (or
minimized) in a Schottky diode.
 A Schottky diode has a relatively low forward voltage drop.
 The leakage current of a Schottky diode is higher than that of a pn-junction
diode. The maximum allowable voltage of this diode is generally limited to 100
V The current ratings of Schottky diodes vary from 1 to 400 A.
 The Schottky diodes are ideal for high-current and low-voltage dc power
supplies. However, these diodes are also used in low-current power supplies for
increased efficiency.
The zener diode
 The Zener diode or reference diode, whose symbol is shown in following
Fig., finds primary usage voltage regulator or reference.
 The forward conduction characteristic of a Zener diode is much the as that
of a rectifier diode;
Sharp
Reverse V
Breakdown
1.
The reverse voltage breakdown is rather sharp. The breakdown voltage
can be controlled through the manufacturing process so it has a reasonably
predictable value.
2.
When a Zener diode is in reverse breakdown, its voltage remains
extremely close to the breakdown value while the current varies from rated
current to 10 percent or less of rated current.
Rated
Zener regulator is designed so that iz

O.l*IZ to ensure the constancy of vz.
18
Example:
Find the voltage vz across the Zener diode of above Fig. if iz = 10 mA and it is
known that Vz(Rated) = 5.6 V, Iz(rated) = 25 mA, and Rz = 10 Ω.
Since 0.1*Iz< iz< Iz, operation is along the safe and predictable region of Zener
operation.
vz≈ Vz + iz* Rz =5.6+(25x10-3)(10)=5.7V
Rz is frequently neglected in the design of Zener regulators.
SERIES-CONNECTED DIODES
In many high-voltage applications, e.g., high-voltage direct current HVDC
transmission lines, one commercially available diode cannot meet the required
voltage rating, and diodes are connected in series to increase the reverse blocking
capabilities.
In the following Fig., iD and VD are the current and voltage, respectively.
Different Voltages
Same
Current
Fig. Series connection of diodes
19
In the forward direction; vD1, and vD2 are the sharing reverse voltages of diodes
D1, and D2, respectively. In practice, the v-i characteristics for the same type of
diodes differ due to tolerances in their production process.
In the forward-biased condition, both diodes conduct the same amount of
current, and the forward voltage drop of each diode would be almost equal.
However, in the reverse blocking condition, each diode has to carry the same
leakage current, and as a result the blocking voltages may differ significantly.
From the Fig. above:
A simple solution to this problem is shown in following Figure:
IS
 I S 1  I R1  I S 2  I R 2
However, IR1 = VD1/R1, and IR2 = VD2/R2= VD1/R2. For equal voltage sharing:
I S1 
v D1
 IS2 
R1
v D1
R2
If R1=R2, two diode voltages can be slightly different because of their V-I
characteristic.
Same Voltage
Different CurrentsDifferent R1, R2
Is1
Is2
Fig. Equal voltage sharing among series diodes

Force equal voltage sharing by connecting a resistor across each diode.
 Due to equal voltage sharing the leakage current of each diode would be
different. The total leakage current must be shared by a diode and its resistor,
Values of VD1 and VD2 can be determined from:
20
I S1 
v D1
R
 I S2 
v D1
;
R
assuming R1=R2
VD1+ VD2 = VS
Voltage sharing under transient condition
PARALLEL-CONNECTED DIODES

In high-power applications, diodes are connected in parallel to increase
the current carrying capability

The current sharings of diodes would be in accord with their respective
forward voltage drops.

Uniform current sharing can be achieved by providing equal inductances
(e.g., in the leads) or by connecting current-sharing resistors (not practical due
to power losses);
21

It is possible to minimize this problem by selecting diodes with equal
forward voltage drops or diodes of the same type. Because the diodes are
connected in parallel, the reverse blocking voltages of each diode would be the
same.

The resistors R1, R2 help current sharing under steady-state conditions.

Current sharing under dynamic conditions can be accomplished by
connecting coupled inductors as shown in Figure b. If the current through
D1, rises, the L*di/dt across L1, increases, and a corresponding voltage of
opposite polarity is induced across inductor L2. The result is a low-impedance
path through diode D2 and the, current is shifted to D2.
Fig. Parallel Diode Connection (current Sharing)
Your Notes:
22
DIODES WITH RC AND RL LOADS
Following figure shows a diode circuit with an RC load. For the sake of
simplicity the diodes are considered to be ideal. By ideal we mean that the reverse
recovery time and the forward voltage drop VD are negligible. The source
voltage Vs is a dc constant voltage. When the switch S1, is closed at t=0, the
charging current i that flows through the capacitor can be found from:
Observation:

In a RC circuit, voltage rises exponentially

In a RC circuit, current falls exponentially
On the other hand:

In a RL circuit, ??? rises exponentially

In a RL circuit, ??? falls exponentially
23
FREEWHEELING DIODE

If switch S1, in following Fig. is closed for time t1, a current is established
through the load, and then if the switch is opened, a path must be provided for
the current in the inductive load.

This is normally done by connecting a diode Dm, as shown in Fig. a, and
this diode is usually called a freewheeling diode.
Operation in two modes:

Mode 1 begins when the switch is closed at t = 0, and mode 2 begins when
the switch is then opened.

The equivalent circuits for the modes are shown in b. i1 and if are defined
as the instantaneous currents for mode 1 and mode 2, respectively. t1 and t2 are
the corresponding durations of these modes.
No Current
FIN
24
UNCONTROLLED DIODE RECTIFIERS (L4)
 A rectifier is a circuit that converts an ac signal into a unidirectional (DC)
signal. the rectifiers are classified into two types:
 single phase and
 three phase.
 Diodes are considered to be ideal. By ideal we mean that the reverse
recovery time trr and the forward voltage drop VD are negligible. That is,
trr, = 0 and VD = 0.
Single-phase half-wave rectifiers
 During the positive half-cycle of the input voltage, diode D, conducts and
the input voltage appears across the load.
 During the negative half-cycle of the input voltage, the diode is in a
blocking condition and the output voltage is zero.
Peak
Value
rms value
DC value
Fig. Rectifier with resistive load
25
PERFORMANCE PARAMETERS
 The output voltage is discontinuous and contains harmonics.
 A rectifier is a power processor that should give a dc output voltage with
a minimum amount of harmonic contents.
 At the same time, it should maintain the input current as sinusoidal as
possible and in phase with the input voltage so that the power factor is
near unity.
 The power-processing quality of a rectifier requires the determination of
harmonic contents of the input current, the output voltage and current.
 We can use Fourier series expansions to find the harmonic contents of
voltages and currents,
The average value of the output (load) voltage Vdc. The average value of the
output (load) current, Idc.
The output dc power,
Pdc =Vdc*Idc
Output rms power (sometimes referred as Pac):
Pd(rms) = Vd(rms)* Id(rms)
Rectifier Efficiency:

P dc
P D ( rms )
Ripple factor (RF) measuring the DC ripple content (for DC side only):

RF   V
 V

 1

2
D ( rms )
DC
Transformer utilization factor (TUF):
TUF 
P DC
VS IS
Where VS and IS are the transformer secondary (input ac) rms voltage & current
respectively.
26
Displacement factor (DF) is defined as:
DF  Cos( );
where:  is the phase angle between voltage and fundamental current.
Total harmonic distortion (THD) on the AC side:
  2 
THD   I S   1
 I 1 



1/ 2
Where: I 1 is the rms fundamental input current & I S is the
net (fundamental+harmonics) rms input current.
Rectifier input power factor (PF) is defined as:
PF 
V S I S1
Cos ( ) 
VS IS
PF 
I S1
IS
Cos ( ) (Using FOURIER Series)
Pdrms (output _ rms )
Input _ volt _ ampere(input _ rms )
(No Fourier)
 THD is a measure of the distortion of an AC waveform.
 If the input current is purely sinusoidal, IS = IS1, and the power factor
(PF) equals the displacement factor (DF) '
 Displacement factor DF is often known as displacement power factor
(DPF).
 An ideal rectifier should have:
 = 100%,
RF = 0,
Possible?
TUF = 1,
27
THD = 0,
PF = DPF = 1.
WAVEFORMS AND THEIR HARMONICS
Waveforms
Harmonics (FFT)
Pure ac
Fundamental
Hz
50
Square wave ac
3rd Harmonic
50
50
*3
Pure dc
DC Component
0
Hz
Pulsating dc
2*
50
28
Rms Current
Fundamental Current
Always Sinusoidal
Input (ac) voltage, current and its fundamental component (IS1) of a 1phsae full rectifier under highly inductive load.
Example: Performance Parameters of a 1-P half-wave rectifier
Note: In examples, reference to certain equations, pls see Text book
by M. H. Rashid, Power Electronics Circuits Devices & Aplications
A Half Wave Rectifier
Rectifier
=0.318Vm
29
DC Side
RMS
VALUES
DC side AV.
value
RMS output Power, Pd rms
Efficiency=PDC/PAC=40%,

RF   V
 V
Pdc
Pdrms
30

 1

= 1.21
2
D ( rms )
DC
Half Wave Rectifier with R-L load and Battery
Without
FHD, Dm
With FHD
Dm
31
Battery
Net Voltage Pushed Down
Fig. Half wave rectifier with a battery at the load.
Diode Conduction from:
1
V m Sin   E , Hence,   Sin (
E
)
Vm

Average charging current:
1 VmSin(t )  E

d (t )
I O(av.) 2 
R

32
FIN
SINGLE-PHASE FULL-WAVE RECTIFIERS (L5)

Each half of the transformer with its associated diode acts as a half-wave
rectifier and the output of a full-wave rectifier is shown in Figure b.

Because there is no dc current flowing through the transformer, there is
no dc saturation problem of transformer core. The average output voltage is:
2
V dc  T
T /2
V
m
sin(t )dt 
0
2v m

 double _ of _ halfwave 
Where: Vm – Input peak voltage
PIV
 Instead of using a center-tapped transformer, we could use four diodes
33
 This is known as BRIDGE TOPOLOGY
FOURIER
Next
As shown in the following Fig.
THD
following Fig.
34
Peak Value
RMS Fundamental Value
Rectifier input Power
Factor (PF)=
35
THREE-PHASE BRIDGE RECTIFIERS

Used in high-power applications

Can operate with or without a transformer and gives six-pulse ripples on the
output voltage

The diodes are numbered in order of conduction sequences and each one
conducts for 1200

The pair of diodes which are connected between that pair of supply lines
having the highest amount of instantaneous line-to-line voltage will
conduct
The average output voltage (DC):
V
12

dc
2
 /6

3V m cos( t ) d  t 
0
Where: Vm
V
 /6

0

2
2
3v m cos ( t ) d  t 

1/2
3 9 3


= 2
4



V
m
36

m
– Input line–neutral peak voltage
The rms output voltage:
 12

rms
 2
3 3V
1/2

Current for Highly
Inductive Load
Rms diode current:
 4
 /6

1/ 2
Ir  
 I 2m cos ( t ) d t 
 2 0

2
 0.5518 I m
Where: Im- peak value of the input/load/diode current
Rms input (transformer secondary) current=
 8

2

 I 2m cos ( t ) d  t 
 2 0

 /6
IS
1/ 2
2  1
2  
 I m    sin


6
2
6

 
37
1/ 2
 0.78 I m
VDC
VDRMS
RF = SQRT(VDRMS/VD)2-1=4%
TUF= PDC
/3VS*IS
38
Diode/Thyristor Protection (snubber)
below
`
VAK
Step Voltage
T charging < T discharging
Lower Ireverse
39
Load in Series with snubber
Above.
d Above.
EXAMPLE
R=5 Ohm
40
Your Notes:
FIN
41
RECTIFIER CIRCUIT DESIGN (L6)
Design of a rectifier involves:

Determining the ratings of semiconductor diodes.

Ratings of diodes are normally in terms of:
 average current, rms current, peak current, peak inverse voltage.
Other parameters involve:

Supply transformer ratings (kVA),

Input/output filters

Switch protection circuit (snubbers)
dv/dt

Thyristor gating signal generator
Gate Drivers
Diode/Thyristor Converter Topology & Their Key Ratings
42
Input Line-Line Peak voltage
FILTERS
Input (top) & Output (bottom) filter Topologies
43
As shown in the following
Fig
Rectifier
Harmonic
Source
R-L Load
RF-10%
Your Notes:
44
Voltage aft.
Filtering
Output
Output Voltage
bef. Filtering
From the above eqn.
n=2
2nd harmonic, 2X2=4. For 3 phase . 2X6=12
45
EFFECTS OF SOURCE AND LOAD INDUCTANCES
 In a practical circuit due to transformer leakage and supply inductances
(source) inductances are always present and the performances of rectifiers
are changed
 Consider point ωt = π where voltages vac and vbc are equal.
 Due to L1, the current cannot fall to zero immediately and the transfer of
current cannot be instantaneous
 Current through D1 id1 decreases resulting in an induced voltage across L1
 Current through D3, id3 increases from zero, inducing an equal negative
voltage across L2
Actual
Voltage
Notch
µ- Commutation angle
Voltage across L2 is:
V L 2  L2
di
V L 2 t
dt , with a linear rise of current,
46
 L 2 i
Average DC voltage drop:
VX 
1
2v L1  v L 2  v L 3 i  6 f L S I dc
T
VL-L-- VL-N
Theoretical,
Your Notes:
FIN
47
THYRISTORS (L7)
 It’s a bistable switch, operating from non-conducting state to conducting state
 Compared to transistors, thyristors have lower on-state conduction losses and
higher power handling capability.
 On the other hand, transistors generally have superior switching performances
in terms of faster switching speed and lower switching losses.
A thyristor is a four-layer semiconductor device of pn-pn structure.
 When the anode voltage is made positive with respect to the cathode, the
junctions J1, and J3 are forward biased.
 The junction J2 is reverse biased, and only a small leakage current flows from
anode to cathode.

The thyristor is then said to be in the forward blocking or off-state condition
 If the anode-to-cathode voltage VAK is increased to a sufficiently large value,
the reverse-biased junction J2 breaks. This is known as avalanche breakdown and
the corresponding voltage is called forward breakdown voltage VBO. Because the
other junctions J1, and J3 are already forward biased, there is free movement of
carriers across all three junctions, resulting in a large forward anode current. The
device is then in a conducting state, or on-state.
 The voltage drop would be due to the Ohmic drop in the four layers and it is
small, typically 1 V In the on-state, the anode current is limited by an external
impedance.
 The anode current must be more than a value known as latching current IL to
maintain the required amount of carrier flow across the junction; otherwise, the
device reverts to the blocking.
48
 Once a thyristor conducts, it behaves like a conducting diode and there is no
control over the device.
 If the forward anode current is reduced below a level known as the holding
current IH, a depletion region develops around junction J2 due to the reduced
number of carriers and the thyristor is in the blocking state.
 The holding current is on the order of milli amperes and is less than the latching
current IL. That is, IL > IH.
 Holding current IH is the minimum anode current to maintain the thyristor in
the on-state.
 In practice, the forward voltage is maintained below VBO and the thyristor is
turned on by applying a positive voltage between its gate and cathode
G
Switching Characteristics
 Thyristor latches into conduction when its anode is positive with respect to the
cathode and only when a voltage pulse is applied to its gate terminal.
 The forward anode current of a thyristor must be more than its latching
current to latch into the conduction state
49
TWO-TRANSISTOR MODEL OF THYRISTOR
The latching action can be demonstrated by using a two-transistor model of thyristor.
A thyristor can be considered as two complementary transistors, one pnp-transistor
Q1, and other npn-transistor Q2, as shown. Equivalent circuit model is shown in
Figure b.
The thyristor load (anode current) is linked
to the gate current, the gains of the two
transistors (α1, α2) and the leakage currents
of the collector-base junctions of
respective transistors ICBO, as:
IA 
α -Current Gain, α=α1+α2 <1
 2 I G  I CBO 1  I CBO 2
1  (1   2 )
IE, mA
If (α1+α2) ≈1, anode current can be very
large, thyristor can be turned on with a
small gate current.
50
THYRISTOR TURN-ON
 During the turn-on process of a thyristor, there is a regenerative or positive
feedback effect. As a result, a thyristor can turn on with a small gate current and
latch into conduction carrying a large value of anode current.
 In the blocking mode, the applied dv/dt must be less than the rated voltage
A thyristor is turned on by increasing the anode current. This can be accomplished in
one of the following ways:
 Light: The light-activated thyristors are turned on by allowing light to strike
the silicon wafers.
 dv/dt: The manufacturers specify the maximum allowable dv/dt of thyristors.
 Gate current: If a thyristor is forward biased, the injection of gate current by
applying positive gate voltage between the gate and cathode terminals turns on
the thyristor.
As the gate current is increased, the forward blocking voltage is decreased, as shown
following figure.
51
Designing gate circuit:
1. The gate signal should be removed after the thyristor is turned on. A continuous
gating signal would increase the power loss in the gate junction.
2. A gating signal must be wide enough so that IT > IL.
Where: td – delay time
tr – Rise time
ton
On time,
tON = td + tr
 Turn-off time tq is the minimum value of time interval between the instant when
the on-state current has decreased to zero and the instant when the thyristor is
capable of withstanding forward voltage without turning on.
 Reverse recovered charge QRR is the amount of charge that has to be recovered
during the turn-off process.
A thyristor that is in the on-state can be turned off by reducing the forward current to
a level below the holding current IH.
52
NATURAL
COMMUTATION
FORCED
COMMUTATION
Charged Separately
Turn off time,
53
tq = trr + tr
THYRISTOR TYPES
 Manufacturers use various gate structures to control the dildt, turn-on time, and
turn-off time.

Thyristors can easily be turned on with a short pulse
 For turning off, they require special drive circuitry or special internal structures
to aid in the turning-off process.

There are several versions of thyristors with turn-off capability
Depending on the physical construction, and turn-on and turn-off behavior,
thyristors can be broadly classified into many categories:

Phase-controlled thyristors (or SCRs)
Commonly Used

Fast switching thyristors (or SCRs) ,
Fast Switching

Light-activated silicon-controlled rectifiers (LASCRs)

Bidirectional triode thyristors (TRIACs)

Gate turn-off thyristors (GTOs)
Your Notes: FIN
54
Noisy Signal
AC-AC application
High Power appl.
CONTROLLED RECTIFIERS (L8)
 Diode rectifiers provide a fixed output voltage only
 For controlled output voltages, thyristors are used instead of diodes.
 The output voltage of thyristor rectifiersis varied by controlling the delay or
firing angle of thyristors.
Used in variable-speed drives, ranging from fractional horsepower to megawatt
power level.
Phase-control converters can be classified into two types:
(1) single-phase converters, and
(2) three-phase converters
Each type can be a:
(a) semiconverter, (b) full converter, and
(c) dual converter.
 A semiconverter is a one-quadrant converter and it has one polarity of output
voltage and current.
 A full converter is a two-quadrant converter
 A dual converter can operate in four quadrants; and both the output voltage
and current can be either positive or negative
 In the converter of the following Fig., during the positive half-cycle of input
voltage, the thyristor anode is positive with respect to its cathode and the
thyristor is said to be forward biased
 When thyristor T1 is fired at ωt = α, T1, conducts and the input voltage appears
across the load.
 When the input voltage starts to be negative at ωt =Π, the thyristor anode is
negative with respect to its cathode and thyristor T, is said to be reverse
biased; and it is turned off.
55
SINGLE-PHASE SEMICONVERTER
RMS
DC
One
Quadrant
The average output voltage (Vm is the peak input voltage):
V dc

1 
Vm
 cos  t 

 v m sin  t * d ( t ) 
2 
2
Vm
2
(1  cos  )
Rms output voltage:
V rms
 1  2

2

 v m sin  t * d ( t ) 
 2 

sin 2






2   
2
Vm 1 



1/ 2
1/ 2
56
V m 2 


 (1  cos 2 t ) d ( t ) 
 4 

1/ 2
SINGLE-PHASE FULL CONVERTERS
 Converter is shown in Figure with a highly inductive load so that the load current
is continuous and ripple free
 During the positive half-cycle, thyristors T1, and T2 are forward biased
 Due to the inductive load, thyristors T1, and T2 continue to conduct beyond ωt=Π,
even though the input voltage is already negative.
 During the negative half-cycle of the input voltage, thyristors T3 and T4 are
forward biased; and firing of thyristors T3 and T4 applies the supply voltage across
thyristors T1, and T2 as reverse voltage
Two
Quadrant
Output
Current
Positive
Current Phase
Shifted by α
 Provides two quadrant operation, vo= + or –
 However, io is always positive
 With a resistive load, vo is positive only
57
The average output voltage (Vm is the peak input voltage):
V dc

2  
2V m

 cos t  
 v m sin  t * d ( t ) 
2 
2
2V m

cos 
Rms output voltage:
2

V rms 2

v

 V m
sin t *d(t)  
  2
1/2
 
2
m
2  
2
58


1/2

(1cos(t)d(t) V m V S
2

In Previous Fig.
Previous Fig.
Not Zero anymore
Because of phase shifting
59
RMS
Current
THD for any α
Not 1 anymore
Because of ‘α’
0.9 for alpha=0
60
For α=60 deg.
Same as α=0
Same waveshape
THD
PF=0.9, for α=0
DUAL CONVERTER
 If two full converters are connected back to back, as shown, both the output
voltage and the load current flow can be reversed
 The system provides a four-quadrant operation and is called a dual
converter. Dual converters are normally used in high-power variable-speed
drives
 If α1 and α2 are the delay angles of converters 1 and 2, respectively, the
corresponding average output voltages are Vdc1 and Vdc2
 The delay angles are controlled such that one converter operates as a
rectifier and the other converter operates as an inverter (Negative output
voltage), but both converters produce the same average output voltage.
V dc1 
2V m
cos( 1 )

V dc 2 
Where: α2 = π- α1
61
2V m
cos( 2 )

Four
Quadrant
62
3-phase half wave Controlled Rectifiers (L9)
 Higher average output voltage compared to a 1-phase
 Higher DC side pulses resulting in higher harmonic orders
 Lower filtering requirement
Two
Quadrant
DC Input
Current
63
For Continuous V
If the input L-N peak voltage is Vm, then the:
3 5 / 6
3 3Vm

sin

t
*
d
(

t
)

cos
Vdc
 vm
Average output voltage:
2  / 6
2
Rms output voltage;
1/ 2
1 3

 3 5 / 6 2 2

cos2 
V drms  
 vm sin t * d(t)  3V m  
 2  / 6

 6 8

1/ 2
For Discontinuous Voltage
DC output voltage:
3

V dc 2
 FIXED
3V m 1  cos    
sin

t
*
d
(

t
)



vm

2

6




 / 6 
Rms output voltage:
V drms
 3

 2

 v m 2 sin  t * d ( t ) 
 / 6 


1/ 2
2
5

1


3V m  

sin   2  
3

 24 4 8

1/ 2
Disadvantages:
 Input currents are pulsating DC, can saturate the transformer
 Only limited to low power applications
64
THREE-PHASE FULL CONVERTERS
 Extensively used in industrial applications up to the 120-kW level,
where a two-quadrant operation is required.
 The thyristors are fired at an interval of 600
 The frequency of output ripple voltage is 6f, and the filtering
requirement is less than that of half-wave converters
 When thyristors T1 and T2 conduct, the line-to-line voltage va
appears across the load
 If the thyristors are numbered, as shown in Figure, the firing
sequence is 12, 23, 34, 45, 56, and 61
If the line-to-neutral voltages are defined as:
Corresponding line-to-line voltages are:
65
T1 Ready
To Fire
T1 actually
Fired
RMS
DC
Thyristor Current
Input Current
66
Average output voltage:
6  / 2
3  / 2
 
*
d
(

t
)

3
sin
V dc 
v
v
t  * d (t)
 ab

m
2  / 6
  / 6
 6

3 3V m

cos
Rms output voltage:
 6
V drms  
 2


2
2
 3v m sin  t   * d (t ) 
 / 6 
6


 / 2 
1/ 2
1/ 2
1 3 3


 3V m 
cos 2 
 2 4

Finding the Performances of a three-phase Full-wave Converter (R load)
A three-phase full-wave converter is operated from a three-phase Y-connected,
(Vab(rms))208-V, 60-Hz supply and the load resistance is R=10Ω. If it is required to
obtain an output voltage of 50% of the maximum possible output voltage,
calculate:
(a) the delay (α), (b) the rms and average output currents, (c) the average and rms
thyristor currents. (d) rectification efficiency, (e) the TUF, and (f) the input PF
Solution:
From Current
Waveform
67
Highly Inductive load
an= -4Ia/πn* Sin(πn/3)*Sin(nα)
68
0.9 Ia for 1Phase Rectifier
THD
48% for 1Phase Rectifier
PF Lower Compared to
R load for same α
Previous example
The load type, i.e. R or L.
For Highly Inductive Load and for the same delay angle, PF is Lower. FIN
69
Twelve-pulse Rectifiers (L10)
AФ
12-pulses
Vo
Voy, VoΔ
6-pulses
70
LOWEST
HARMONIC
12
 One of the bridges is supplied through a Y-Y connected transformer,
 The other is supplied through a Y-Δ (or Δ-Y) transformer
 The purpose of the Y-Δ transformer connection is to introduce a 300
phase shift between the source and the bridge.
 This results in input voltage-current phase shift, which are 300 apart.
 The two bridge outputs are similar, but also shifted by 300.
 The dc output is the sum of the dc output of each bridge:
Net output voltage:
V 0  V oy  V o 
3V m ( L  L )

cos 1 
3V m ( L  L )

cos  2 
6V m ( L  L )

cos 
 Since a transition between conducting SCRs occurs every300, there are a
total of 12 such transitions for each period of the ac source.
 The output has harmonic frequencies which are multiples of 12 times the
source frequency (12k, where: k = 1, 2,3 ... ).
 Filtering to produce a relatively pure dc output is less costly than that
required for the six-pulse rectifier.
 Another advantage of using a twelve-pulse converter rather than a six-pulse
converter is the reduced harmonics that occur in the ac system.
The current in the ac lines supplying the Y- Y transformer in Fourier series:
iY (t ) 
2 3 
1
1
1
1

I o  cos o t  cos 5o t  cos 7o t  cos11o t  cos13o t...

5
7
11
13


The current in the ac lines supplying the Y-Δ transformer is in Fourier series:
i (t ) 
2 3 
1
1
1
1

cos13o t...
I o  cos o t  cos 5o t  cos 7o t  cos11o t 

5
7
11
13


71
The ac system current, which is the sum of 2 transformer currents in Fourier
series:
i ac (t )  iY (t )  i (t ) 
4 3 
1
1

cos13o t...
I o  cos  o t  cos11 o t 

11
13


Iac
(No 5th and 7th harmonics. Lowest harmonic 11)
Some of the harmonics (5th & 7th) on the ac side are canceled by using the twelvepulse scheme.
The harmonics that remain in the ac system are of order l2k±1.
This principle can be expanded to an-arrangements of higher pulse numbers by
incorporating increased pulse Nos. of six/pulse converters with transformers with
appropriate phase shifts.
lower-voltage industrial systems commonly have converters with up to 48 pulses.
POWER FACTOR IMPROVEMENTS
 The PF of phase-controlled converters depends on delay angle α, and is in
general low, especially at the low output voltage range. These converters
generate harmonics into the supply.
 Forced commutations can improve the input PF and reduce the harmonics
levels.
1. Extinction angle control
2. Symmetric angle control
3. Pulse-width modulation (PWM)
4. Single-phase sinusoidal PWM
5. Three-phase PWM control
Extinction Angle Control
72
 The switching actions of S1, and S2 can be performed by GTO or IGBT.
An IGBT remains on as long as a gate voltage is applied to its gate terminal.
 The output voltage is controlled by varying the extinction angle β.
 The fundamental component of input current leads the input voltage, and the
displacement factor (and PF) is leading.
 In some applications, this feature may be desirable to simulate a capacitive
load and to compensate for line voltage drops.
Forced
Commutated
Switch
Average output voltage:
73
2  
Vm
1  cos  
V dc 
 v m sin t * d (t ) 
2 0

Vdc can be varied from 2Vm /π to 0 by varying β from 0 to π.
The rms output voltage:
V d ( rms ) 
 2

2
2
sin

t
*
d
(

t
)
v

m
 2 0



 
1/ 2
sin 2 
vm  1 







2
2  



1/ 2
Where; β= non-Conduction Angle
Symmetric Angle control (Circuit same as the Previous)
Provides an unity Displacement factor
Average and rms output voltage can be controlled by non-conduction angle β.
The average output voltage:
2 (  ) / 2

2V m

sin

t
*
d
(

t
)

sin
V dc
 vm
(



2 ) / 2

2
The rms output voltage:
 2
V d ( rms )  
 2

2
v 2m sin  t * d ( t ) 

(   ) / 2

(   ) / 2
Where; β= Conduction Angle
74
1/ 2

vm  1




sin



2  

1/ 2
iS3
S3
75
76
From Integration of Is
THD
THD
Thyristor .
Rect. @α =600
PF=0.45
Fin
77
PWM CONTROL (L11)
 In PWM control, the converter switches are turned on and off several times
during a half-cycle and the output voltage is controlled by varying the width of
pulses.
Fundamental
Current
 The gate signals are generated by
comparing a triangular wave with a dc
signal
 PWM Can reduce lower order
harmonics. Reduce THD, Increase PF
78
The average output voltage due to p number of pulses per half cycle:
 2
V dc   
m 1  2
p
 m  m

m
 vm p
 cos  m  cos( m   m )
v m sin t * d (t )  
m
1


PWM shifts
Harmonics to High
Order. For Exam.
Lower 3rd or 5th
Input Current
78
Single-Phase Sinusoidal PWM
 It is possible to choose the widths of pulses in such a way that certain
harmonics could be eliminated
 Most common one is the sinusoidal pulse-width modulation (SPWM)
 Pulse widths are generated by comparing a triangular reference vr with a
carrier half-sinusoidal voltage vc of variable amplitude and fixed
frequency 2fs
 The widths of the pulses (and the average/rms output voltage) are varied
by changing the amplitude of vc, or the modulation index M from 0 to 1.
The modulation index is defined as:
M 
AC
Ar
79
 In a sinusoidal PWM control, the DF is unity and the PF is improved
 The lower order harmonics are eliminated or reduced
 For example, with four pulses per half-cycle the lowest order
harmonic is the fifth; and
 with six pulses per half-cycle, the lowest order harmonic is the
seventh.
Your Notes:
80
PF IMPROVEMENT BY FILETRS
Sharply Tuned
Eliminates 5th
harmonic
Broadly Tuned
Eliminates 5th &
7th harmonic
AC
5*50 Hz
DC
81
Choose C
L chosen
based on C
XL
XC
82
Both Fundamental &
Harmonic V/I Contents
need to be considered
Your Notes:
83