PE4MSc_Ch1_SC_devices.pptx

Symbol
Power diode: is a two-terminal pn-junction device. The equivalent circuit and i-v curve are displayed as
follows, where the diode conducts when a battery is connected across its terminals .
P-N junction P-N junction
𝑰𝑫 = 𝑰𝒔 (𝒆
𝑽𝑫
𝒏𝑽𝒕 − 𝟏)
where,
VD = voltage across the diode
Is = the reverse bias saturation current (or scale current),
n=quality factor or emission coefficient
Vt=KT/q=thermal voltage
where,
K =Boltzmann constant
T= absolute temperature
q=charged electron

• P-N junction is formed by alloying diffusion and epitaxial growth.
• Modern techniques in diffusion and epitaxial processes permit
desired device characteristics.
• The diodes have the following advantages including:
high mechanical and thermal reliability,
high peak inverse voltage,
low reverse current,
low forward voltage drop,
high efficiency and
compactness.
Power Diode:

Fig. static V- I characteristics of diode

Ideal diode characteristics Real diode characteristics
vs

Ideal diode circuit symbol: Real diode circuit symbol:
vs

Fig. Switching characteristics of diode
OFF state
ON state

Fig. Turn-Off Characteristics of Power Diode: a) Variation of Forward Current if ; b) Variation of Forward Voltage
Drop vf ; c) Variation of Power Loss
Significant power loss

Very common diode parameters are:
 Forward recovery time tFR is the time required for the diode voltage
to drop to a particular value after the forward current starts to flow.
Reverse recovery time trr is the time interval between the
application of reverse voltage and the reverse current dropped to a
particular value.
• Parameter ta is the interval between the zero crossing of the diode
current and when it becomes IRR. On the other hand, tb is the time
interval from the maximum reverse recovery current to 0.25 of IRR .
 ta-due to storage
charge in the
depletion region
 tb- due to storage
charge in the
bulk resistance.

• The ratio of the two parameters ta and tb is known as the softness
factor SF.
• Diodes with abrupt recovery characteristics are used for high-
frequency switching.
• In practice, a design engineer frequently needs to calculate
reverse recovery time in order to evaluate the possibility of high-
frequency switching.
• As a rule of thumb, the lower trr is, the faster the diode can be
switched.
𝑡𝑟𝑟 = 𝑡𝑎 + 𝑡𝑏
If tb is negligible compared to ta (which commonly occurs), then the
following expression is valid:
𝑡𝑟𝑟 =
2𝑄𝑟𝑟
𝑑𝑖/𝑑𝑡
how? Assignment

from which the reverse recovery current is
𝐼𝑟𝑟 =
𝑑𝑖
𝑑𝑡
2𝑄𝑟𝑟 how? Assignment
where Qrr is the storage charge, and can be calculated from the
area enclosed by the path of the recovery current.

During “Turn On” the instantaneous forward voltage drop across a
diode may reach a level considerably higher than its steady state
voltage drop for the given forward current. This is called forward
recovery voltage.
During “Turn Off” the diode current goes negative first before
reducing to zero. This is called reverse recovery of a diode.
The peak negative current flowing through a diode during Turn Off is
called the “reverse recovery current” of the diode.
The total time for which the diode current remains negative during
Turn Off is called “the reverse recovery time” of the diode.
In general

A diode can not block reverse voltage till the reverse current
through the diode reaches its peak value.
Both the “reverse recovery current” and the “reverse recovery
time” of a diode depends on the forward current during Turn Off,
rate of decrease of the forward current (di/dt) and the type of the
diode.
Normal or slow recovery diodes have smaller reverse recovery
current but longer reverse recovery time. They are suitable for line
frequency rectifier operation.
 Fast recovery diodes have faster switching times but
comparatively lower break down voltages. They are suitable for
high frequency rectifier or inverter free- wheeling operation.
Fast recovery diodes need to be protected against voltage
transients during Turn Off” using R-C snubber circuit.

Snubber circuit
Snubber circuits are essential for diodes
used in switching circuits. It can save a
diode from overvoltage spikes, which may
arise during the reverse recovery process.
High power dissipation in the device
A very common snubber circuit for a
power diode consists of a capacitor and a
resistor connected in parallel with the diode
as shown in the fig. And the typical
formulas Fig. A typical snubber
circuit
𝑑𝑣
𝑑𝑡
=
0.632 ∗ 𝑉
𝑠
𝑅𝑠𝐶𝑠
𝑅𝑠 =
𝑉
𝑠
𝐼𝑅𝑅
and

Diodes Classification : Depending on the recovery characteristics,
and manufacturing.
1. General - purpose diodes: they have high trr=25 µS and with
frequency < 1kHz, applied in AC to DC circuits. The current rating
reaches up to 1000A, and up to 5kV.
2. Fast recovery diodes: they have small trr=5 µS and with frequency
< 5kHz, applied in DC to DC and DC to AC circuits. The current
rating reaches up to 100A, and up to3kV.
3. Schottkey diode: they have approximately zero reverse recovery
time with high frequency up to 10kHz, and applied in high current
low voltage applications. The current rating reaches up to 300A and
100V circuit voltage.

Series –Connected Diodes :
 In high voltage DC applications (mainly), the diodes are connected in series with the
purpose to increase the reverse blocking capabilities.
 The difference in the i-v curve in the reverse blocking condition occurs due to
manufacturing errors and tolerances, therefore each diode should carry different voltage
, while the leakage current is the same.
 The solution is to enforce equal voltage sharing across the diodes by connecting a sharing
resistances as well shown below:

Rs 1 Rs 2 Rs 1 Rs 2
 (Is 2  Is1)  Rs1 

V D 1

VD 2 VD 1 . Rs 2
Rs 1 Rs 2 VD 2  Rs 2 .(Is 2  Is1)
c a se : V D 1  V D 2  Vs 2
Is1 
VD 1
 Is 2 
VD 2
;  
VD 1

VD 2
 (Is 2  Is 1 );
Mathematical Modeling:
Is1  Id 1  Is 2  Id 2
Vs  VD 1  VD 2  VD 2  Vs  VD 1

Parallel –Connected Diodes :
 In high power applications, diodes are connected in parallel with the purpose to increase the current carrying
capability. Due to some differences in the Bulk resistances of both diodes, there is a different current will
flow through the diodes. Therefore by connecting resistances in series with the diodes the diode voltage is
shared equally as well shown below:
The function of both Ls1 and Ls2 is to equally sharing the current under dynamic behaviors.
When ID1 rises, the inductor voltage Ls1.d(Id1)/dt increases, and a corresponding voltage of opposite polarity
is induced across inductor Ls2. This resulting low impedance in the circuit of D2, therefore shifting the current
to flow through D2 path.

Mathematical Modeling: The following equations are derived under steady-state conditions
VD1  V3  VD2  V4;
VD1  Rs3.ID1  VD2  Rs4.ID2
Let Rs1  Rs2  R R(ID 2  ID1)  VD1  VD2
 R 
VD1  VD2
ID2  ID1

 High forward current
 High reverse voltage
 Fast switching
The three most desired characteristics of diode?

Example (R Load):-PSCAD software
100.0
[ohm]
Vsource
240 kV, 50 Hz
AC Source
R=0
V
HALF-WAVE
RECTIFIER
V
v(t) = V*sin(theta)
T
LOAD
V
A
Diode

Simulated results (half-wave rectifier):

A Simple Circuit (R-L Load)
V
A
T
v(t) = V*sin(theta)
V HALF-WAVE
RECTIFIER
R=0
V
240 kV, 50 Hz
AC Source
Vsource
LOAD
100
[ohm]
Vd
Ia
Vdiode
VL
1 [H]

• Current continues to flow for a while even after the input voltage has gone negative
Vo,Is
dependi
ng on
R,L
Illustrated by
PSCAD
example!
t1
t2 t3
t0

• Current continues to flow for a while even after the input voltage has gone negative
For
L=2H
t1
t2 t3
t0

Voltage ripple reduction:
V
A
T
v(t) = V*sin(theta)
V
HALF-WAVE
RECTIFIER
R=0
V
240 kV, 50 Hz
AC Source
Vsource
LOAD
100
[ohm]
1.0
[uF]
Diode

For 100 𝝁𝑭:

V
A
v(t) = V*sin(theta)
V
FULL-WAVE
RECTIFIER
240 kV, 50 Hz
AC Source
Vsource
LOAD
100 [ohm]
T
T
T
T
R=0
V
Vlaod2

Simulated results (full-wave rectifire):

V
A
v(t) = V*sin(theta)
V
FULL-WAVE
RECTIFIER
240 kV, 50 Hz
AC Source
Vsource
THYRISTOR FIRING PULSE
CONTROL CIRCUIT
LOAD
100 [ohm]
T
T
T
T
R=0
V
Vlaod2
1000 [uF]
Ia

Power Thyristor (SCR):
Fig. Structure of thyristor
Fig. Symbol of thyristor Fig. Equivalent circuit of thyristor
Ig
Ia
A
G
K

Mode of Current Flow in a Thyristor:
 Reverse Blocking Mode
 Forward Blocking Mode
 Forward Conduction Mode

• The cathode is made positive with respect to anode by supplying
voltage and keeping G open.
• Here, Junctions J1 and J3 are reverse biased whereas the junction
J2 is forward biased.
• The behavior of the thyristor here is similar to that of two diodes
are connected in series with reverse voltage applied across them.
• As a result only a small leakage current of the order of a few
μAmps flows. This is the reverse blocking mode or the off-state of
the thyristor, (open circuit function).
• If the reverse voltage is now increased, then at a particular voltage,
known as the critical breakdown voltage VBR, an avalanche occurs
at J1 and J3 and the reverse current increases rapidly.
• A large current associated with VBR gives rise to more losses in
the SCR, which results in heating of the thyristor.
Reverse Blocking Mode :

Forward Blocking Mode:
• Considering the figure:-
• J1 and J3 are forward biased; but junction J2 goes into reverse
biased.
• The thyristor conducts currents from anode to cathode with a very
small voltage drop across it.
• A thyristor is brought from forward blocking mode to forward
conduction mode by turning it on by exceeding the forward break
over voltage or by applying a gate pulse between gate and
cathode.
• In this mode, thyristor is in on-state and behaves like a closed
switch
• Voltage drop across thyristor in the on state is of the order of 1 to
2 V depending beyond a certain point

• Considering the anode is positive with
respect to the cathode, with gate kept in
closed condition.
• J1, J2 and J3 are forward biased
• Once the thyristor is turned on, we can see
from the diagram for characteristics of
thyristor, it continues conduction.
• In this mode of operation, the thyristor
conducts maximum current with minimum
voltage drop, this is known as the forward
conduction or the turn-on mode of the
thyristor, (closed circuit function)
Forward Conduction Mode:

Forward Conduction…
•The SCR can be brought back to the forward
blocking state from the forward conduction state
by reducing the anode or forward current below
the holding current level.

Fig. static characteristics of Thyristor

Methods of Thyristor Turn-on
Forward high voltage triggering
Thermal triggering
Light triggering
Gate triggering... Very common
dv/dt triggering

•An SCR is said to be ‘turned OFF’ if there is no flow of
forward current and even if the SCR is once again forward
biased (positive voltage at anode), the SCR will not conduct
without any Gate Signal.
•The reverse voltage, which causes to commutate the SCR, is
called the Commutation Voltage.
•The commutation methods are classified into two major
types:
Natural Commutation
Forced Commutation
Thyristor Turn-Off Methods

Fig. switching
characteristics of
thyristor

The transition of an SCR from forward conduction state to forward
blocking state is called as turn OFF or commutation of SCR.
Once the SCR starts conducting, the gate has no control over it to
bring back to forward blocking or OFF state.
To turn OFF the SCR, the current must be reduced to a level below
the holding current of SCR.
SCR turn OFF is achieved by reducing the forward current to zero.
But if we apply the forward voltage immediately after the current
zero of SCR, it starts conducting again even without gate triggering.
 This is due to the presence of charge carriers in the four layers.
Therefore, it is necessary to apply the reverse voltage, over a finite
time across the SCR to remove the charge carriers.

Hence the turn OFF time is defined as the time between the instant
the anode current becomes zero and the instant at which the SCR
retains the forward blocking capability.
The excess charge carriers from the four layers must be removed to
bring back the SCR to forward conduction mode.
This process takes place in two stages.
In a first stage excess carriers from outer layers are removed and in second
stage excess carriers in the inner two layers are to be recombined.
Hence, the total turn OFF time tq is divided into two intervals; reverse
recovery time trr and gate recovery time tgr.
• The figure above shows the switching characteristics of SCR during
turn ON and OFF.
• The time t1 to t3 is called as reverse recovery time; at the instant t1the
anode current is zero and builds up in the reverse direction which is
called as reverse recovery current.

• This current removes the excess charge carriers from outer layers
during the time t1 to t3.
• At instant t3, junctions J1 and J3 are able to block the reverse voltage
but, the SCR is not yet able to block the forward voltage due to the
presence of excess charge carriers in junction J2.
• These carriers can be disappeared only by the way of
recombination and this could be achieved by maintaining
a reverse voltage across the SCR.
Hence , during the time t3 to t4, the recombination
of charges takes place and at the instant t4, junction J2
completely recovers. This time is called gate recovery time tgr.
 From the characteristics figure, the turn OFF time is the time interval between
the t1 and t4. Generally, this time varies from 10 to 100𝜇𝑠𝑒𝑐.

This turn OFF time tq is applicable to the individual SCR.
 The time required by the commutation circuit to apply the
reverse voltage to commutate the SCR is called the circuit turn
OFF time (tc). For a safety margin or reliable commutation, this
tc must be greater than the tq otherwise commutation failure
occurs.
 The SCRs which have slow turn OFF time as in between 50 to
100𝜇𝑠𝑒𝑐 are called as converter grade SCRs. These are used in
phase controlled rectifiers, cycloconverters, AC voltage
regulators, etc.
 The SCRs which have fast turn OFF time as in between 3 to
50𝜇𝑠𝑒𝑐 are inverter grade SCRs. These are costlier compared to
converter grade and are used in choppers, force commutated
converters and inverters.

• Single-phase half-wave SCR controlled rectifier with a firing
angle of α
Firing or triggering of SCR
V0

Firing angle(α)
Firing angle(α): is an angle at
which the SCR is triggered or
fired, and measured with respect
to the zero crossing point of the
mains that supply the rectifier
after which the SCR becomes
forward bias. It is also
called triggering angle.

Thyristor based halfwave rectifier _ PSCAD
V
A
T
2
v(t) = V*sin(theta)
V
HALF-WAVE
RECTIFIER
R=0
V
240 kV, 50 Hz
AC Source
Vsource
Diode
GP
10
[ohm]

300 firing angle 900 firing angle

𝛽
α
• Extinction angle(𝜸): It is
the angle at which the SCR
gets turned off. For AC
resistive load applications
normally, the commutation
takes place at every zero
crossings.
• In RL load application it
will vary irrespective of
zero crossing
Conduction angle (𝜷): an angle that measures
the SCR period of conduction (how long it
conducts in one cycle of time).
This angle depends on the firing angle α and
the load impedance angle φ . For purely
resistive load, the conduction angle β = (π- α ).

when the load contain inductor:
α
𝛽

𝑅𝑀𝑆 𝑙𝑜𝑎𝑑 𝑣𝑜𝑙𝑡𝑎𝑔𝑒, 𝑉0
𝑉0 =
𝛼
2𝜋
𝑉𝑚𝑠𝑖𝑛 𝜔𝑡 2 𝑑(𝜔𝑡)
=
𝑉𝑚
2
1
𝜋
{ 2𝜋 − 𝛼 +
𝑠𝑖𝑛𝛼
2
}

SCR based three-phase rectification: PSCAD
V
A
T
2
v(t) = V*sin(theta)
240 kV, 50 Hz
AC Source
Vsourcea
FP1
THYRISTORs with FIRING PULSES
T
2
T
2
FP2 FP3
10
[ohm]
A
B
C
Vsourceb
Vsourcec
Io
T
2
T
2
T
2 FP1
FP2 FP3
Load
Vo
1 3 5
2
6
4

𝐅𝐢𝐫𝐢𝐧𝐠 𝐚𝐧𝐠𝐥𝐞𝐬
𝐟𝐨𝐫 𝟎 ≤ 𝜶 ≤ 𝟑𝟎0

400 firing angle
4𝟎𝟎
𝜶

1100 firing angle
𝟏𝟏𝟎𝟎
𝜶

Beyond 1200 firing angle, proper triggering of the whole valves fails; i.e.,
commutation failure
Thus, it demands robust control system

Commutation failure
• Commutation failure is a common fault of converters.
• It is the failure of an incoming valve (due to insufficient extinction
angle) to take the control of load current when commutating voltage
reverses its polarity.
• During the operation of inverters, when the time (angle γ) of
applying negative voltage on the valve that has just been turned off is
shorter than the time required for it to restore the blocking capability
and the valve is conducted again when positive voltage is applied,
commutation failure of the converter valve would occur.
• The causes might be:
Low AC side voltage (for example due to fault)
Late of firing angle( experiences phase shift )
or the DC current increases,
Thus the angle extinction angle γ would be decreased.

𝛾 = 𝑒𝑥𝑡𝑖𝑛𝑖𝑐𝑡𝑖𝑜𝑛 𝑎𝑛𝑔𝑙𝑒
𝛽 = 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑎𝑛𝑔𝑙𝑒
𝜇 = 𝑜𝑣𝑒𝑟𝑙𝑎𝑝 𝑎𝑛𝑔𝑙𝑒

• Fig. 1 shows the basic equivalent circuit of line commutated six pulse bridge converter.
• Under normal circumstances, commutation process between valve 1 and valve 3 is
illustrated in Fig. 2, when the valve 3 is fired, valve1 is forced to a negative bias and get
switched off for a certain period of time after the extinction of its current (denoted by the
extinction angle 𝛾), and if the valve become positively biased before complete switch off
occurs, this thyristor will regain current and it will increase the risk of commutation failure.

•Additively, in the HVDC inverter station, the firing angle is large
(around 1300-1400), that's why the inverter is more prone to
commutation failure compared to rectifier which is rarely fails
to commutate because its firing angle is less than 900.
•For proper operation of valves during commutation, extinction
angle 𝛾 must be at a minimum value and it is expressed as:
𝛾 = 1800 − 𝛼 − 𝜇

Gate Turn-Off Thyristor (GTO):
 A gate turn-off thyristor (GTO) is a special type of thyristor. GTOs,
are fully controllable switches which can be turned on and off by
switching the polarity of the gate signal.
• Turn on is accomplished by a "positive current" pulse between
the gate and cathode terminals.
• Turn off is accomplished by a "negative current" pulse between
the gate and cathode terminals.

 Slow switching speeds.
 Used at very high power levels.
 Requires elaborate gate control circuitry (needs a turn-off snubber
circuit).

GTO… cont’d
IV-Characteristics of GTO
(a) Symbol of GTO
(b) Equivalent circuit of GTO

Dynamics Characteristics of GTO

GTO:
Characteristic Description
Thyristor (1600 V,
350 A)
GTO (1600 V,
350 A)
VT on
On-state
voltage drop
1.5 V 3.4 V
ton, Ig on
Turn-on time,
gate current
8 µs, 200 mA 2 µs, 2 A
toff Turn-off time 150 µs 15 µs
Comparison of an SCR and GTO of same rating
 GTO and SCR are pulse triggering devices

IGBT: MOSFET: BJT:
 These all are level triggering devices

Comparing IGBT with BJT and MOSFET:
Characteristic Power BJT Power MOSFET IGBT
Voltage Rating High < 1kV High < 1kV Very High > 1kV
Current Rating High < 500 A Low < 200 A Very High > 500 A
Input Parameter Base Current, Ib Voltage, VGS Voltage, VGE
Input Drive Current gain (hfe)
20-200
Voltage, VGS
3-10V
Voltage, VGE
4-8V
Input Drive Power High Low Low
Input Drive Circuitry Complex Simple Simple
Input Impedance Low High High
Output Impedance Low Medium Low
Switching Loss High Low Medium
Switching Speed Low Fast Medium

Bipolar Junction Transistor (BJT)
 A BJT is a current-controlled switch that can be considered as two diodes with a shared
anode.
 A sufficiently large base current results in the device fully on (iB > IC/hFE, where hFE is the dc
current gain).
 The base current must be supplied continuously to keep the switch turned on.
 Used commonly in the past (now used in specific applications). Being replaced by
MOSFETs and IGBTs.

Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET)
 A MOSFET (type of FET) is a voltage-controlled device - easy to control. It uses an electric
field to control the shape and hence the conductivity of a channel of majority-charge-
carriers in a semiconductor material.
 MOSFETS have gate, drain, and source terminals that correspond roughly to the base,
collector, and emitter of BJTs. MOSFETs also have a fourth terminal called the body, base,
bulk, or substrate.
 Optimal for low-voltage operation at high switching frequencies.
 On-state requires continuous application of gate-source voltage of appropriate magnitude

 Like the BJT, the IGBT has a small
on-state voltage.
 Similar to the GTO, IGBT can be
designed to block negative voltage.
Its name also implies the fusion
between them.
“Insulated Gate” refers to the input
part of MOSFET having very high
input impedance.
 It does not draw any input current
rather it operates on the voltage at
its gate terminal.
IGBT
 Advantages over MOSFET, BJT and GTO:
 It is a voltage controlled device
 Similar to the MOSFET, the IGBT has a high impedance gate, thus requires
only a small amount of energy to switch the device.

Typical switching characteristics
IGBT characteristics:

Operation principle:
The two terminals of IGBT collector (C) and emitter (E) are used for the
conduction of current while the gate (G) is used for controlling the IGBT.
Its operation is based on the biasing between Gate-Emitter terminals and
Collector-Emitter terminals.
The collector-emitter is connected to Vcc such that the collector is kept at a
positive voltage than the emitter. The junction j1 becomes forward biased and
j2 becomes reverse biased. At this point, there is no voltage at the gate.
Due to reverse bias of j2, the IGBT remains switched off and no current will
flow between collector and emitter.

Applying a gate voltage VG positive than the emitter, negative charges will
accumulate right beneath the SiO2 layer due to capacitance.
Increasing the VG elevates the number of charges which eventually form a layer
when the VG exceeds the threshold voltage, in the upper P-region. This layer form N-
channel that shorts N- drift region and N+ region. The electrons from the emitter flow
from N+ region into N- drift region. While the holes from the collector are injected
from the P+ region into the N- drift region.
Due to the excess of both electrons and holes in the drift region, its conductivity
increase and starts the conduction of current. Hence the IGBT switches ON.
Therefore, IGBT gets turned ON and begins to conduct forward current IC.

Example:
V
A
2
T
v(t) = V*sin(theta)
V
HALF-WAVE
RECTIFIER
R=0
V
240 kV, 50 Hz
AC Source
Vsource
Diode
GP
10
[ohm]
IGBT based halfwave rectifier _ PSCAD

If the pulse duration is increased by 0.002sec

In general, once again the applications of PE
devices:

PE4MSc_Ch1_SC_devices.pptx

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