power electronics and instrumentation class …
TRANSCRIPT
Madhusudhan M
Assistant Professor
Dept. Of ECE
Dr.AIT , Bengaluru-56
POWER ELECTRONICS
AND INSTRUMENTATION
Class-01(Introduction)
DEPARTMENT OF ECE, DR.AIT, BENGALURU
Subject Tit le : Power Electronics and Instrumentat ion
Subject Code : 18EC35
No. of Credits : 03=03:0:0 ( L - T –P)
No. of Lecture Hours/Week : 03
CIE+Assignment+SEE : 45+5+50=100
Total No.of Contact Hours : 39
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COURSE OBJECTIVES:The Thyristor circuits.
The applications of power devices in controlled rectifiers, converters and
Choppers.
The types of errors in measurement and the Operation of different Transducers.
Multirange Ammeters, Voltmeters and Multimeters.
Principle of operation of digital measuring instruments and Bridges.
DEPARTMENT OF ECE, DR.AIT, BENGALURU 3
UNIT-01Int roduct ion : History, Power Electronic Systems, Power ElectronicConverters and Applications.
Thyr i s tors : Static Anode-Cathode characteristics Two transistor model ofSCR and Gate characteristics of SCR, Dynamic Turn On switchingcharacteristics, Turn-ON methods, Turn-OFF mechanisms.
Turn -OFF Methods : Natural and Forced Commutation – Class A Class Band Class C.
Gate trigger circuits: Resistance firing circuit, Resistance capacitance firingcircuit.
Uni -Junct ion Trans i s tor : Basic operation and UJT Firing Circuit.
DEPARTMENT OF ECE, DR.AIT, BENGALURU 4
UNIT-02Phase Control led Converter : Control techniques, Singlephase half wave and full wave controlled rectifier with resistiveand inductive loads, effect of freewheeling diode, NumericalExamples.
Choppers: Chopper Classification, Basic Chopper operation:step-down, step-up and step-up/down choppers, NumericalExamples.
Inverters : Classification of Inverters
DEPARTMENT OF ECE, DR.AIT, BENGALURU 5
UNIT-03Pr inc ip les o f Measurement : Static Characteristics, Error inMeasurement, Types of Static Error.
Voltmeters : Introduction, Multi range voltmeter.
Ammeters : DC Ammeter, Multi-range Ammeter.
Dig i ta l Vo l tmeter : Ramp Technique, Dual slope integrating Type DVM,Direct Compensation type and Successive Approximations type DVM.
DEPARTMENT OF ECE, DR.AIT, BENGALURU 6
UNIT-04Digital Instruments: Introduction, Digital Multimeter,Digital frequency meters, Digital measurement of time.
Signal Generators: Function generator, Block diagram ofOscilloscope.
Bridges: Wheatstone’s Bridge, Capacitance and InductanceComparison bridge, Maxwell’s bridge, Wien’s bridge.
DEPARTMENT OF ECE, DR.AIT, BENGALURU 7
UNIT-05Transducers : Introduction, Selecting a transducer, ElectricalTransducer, Resistive Transducer, Resistive position Transducer,Resistance Wire Strain Gauges, Resistance Thermometer,Thermistor, LVDT, Instrumentation Amplifier using TransducerBridge, Temperature indicators using Thermometer, AnalogWeight Scale
DEPARTMENT OF ECE, DR.AIT, BENGALURU 8
DEPARTMENT OF ECE, DR.AIT, BENGALURU 9
Text Book:1. M.D Singh and K B Khanchandani ,“Power Electronics”, 2nd Edition, Tata
McGraw Hill Education Private Limited, 2009
2. H. S. Kalsi, “Electronic Instrumentation”, 3rd Edition, McGraw Hill, 2012
Reference Books:1. Mohammad H Rashid, “Power Electronics, Circuits”, 3rd/4th Edition, Pearson
Education Inc, 2014
2. David A. Bel “Electronic Instrumentation & Measurements”, 2nd Edition,
Oxford UniversityPress PHI, 2006
Course Outcomes:CO1: Define and classify the Power electronic devices and its characteristicsand instrument errors
CO2: Explain the power electronic circuits and principles of measurementtechniques
CO3:Analyse and design different types of rectifiers and converters
CO4: Apply measurement techniques to measure different quantities.
DEPARTMENT OF ECE, DR.AIT, BENGALURU 10
DEPARTMENT OF ECE, DR.AIT, BENGALURU
POWER ELECTRONICS
Electronics
Control
Power
In Electrical Engineering can be divided into 3
types:
1.Electronics: study of Semiconductor circuits
for the processing of information at lower power
levels.
2.Power: deals with both rotation and static
equipment for the generation, transmission,
distribution and utilization of vast quantities of
electric power
3.Control: stability and response characteristics
of closed loop systems using feedback on either a
continuous or sampled-data basis.
P o w e r E l e c t r o n i c s “The application of solid state electronics for the
control and conversion of electric power”
Power Electronics is a combination of power,
Electronics and Control
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I N T R O D U C T I O N
DEPARTMENT OF ECE, DR.AIT, BENGALURU
Electron i cs vs Power E lect roni csElectronics deals with transmission of information/signals in the form of electrical
energy. Whereas power electronics deals with transmission of electrical energy itself.
As the name suggests, easiest answer would be, power electronic devices are used in
high power/ high current scenarios.
Role of power Electron i csThe basic functions of importance for power electronics are:
(1) power conversion, ac to dc, dc to ac, ac to ac,
(2) power conditioning to remove distortion, harmonics, voltage dips and over voltages.
(3) high speed and/or frequent control of electrical parameters such as currents,
voltage impedance, and phase angle
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DEPARTMENT OF ECE, DR.AIT, BENGALURU 13
HISTORY
Metal tank Rectifier, Grid controlled vacuum-tube rectifier, ignitron, phanotron
and thyratron were used for power control Until 1950
Mercury Arc Rectifier in 1900.
In 1948(first electronics revolution began), invention of the silicon transitors at
Bell Telephone Laboratories by Bardeen, Brattain and Shockley.
In 1956, invention of PNPN triggering transistor which was defined as a thyristor
or silicon-controlled rectifier(SCR)
DEPARTMENT OF ECE, DR.AIT, BENGALURU 14
Power source
Power Modulator
Motor Load
Sensing Unit Control Unit
Command
Block Diagram of Power Electronics
Madhusudhan M
Assistant Professor
Dept. Of ECE
Dr.AIT , Bengaluru-56
POWER ELECTRONICS
AND INSTRUMENTATION Class-02 (Introduction)
Power Electronic Converters•Power electronics circuits are also called power convertors
•Convertor- uses a matrix of power semiconductor switches to convert electrical at high efficiency
•Components - Switches, reactive components L,C and transformers
•Switches of two types
• Two terminal devices- Diodes.
• Three terminal devices- Thyristors or Transistors
DEPARTMENT OF ECE, DR.AIT, BENGALURU 2
Classification of ConvertorsThe power converters can be classified into 5 broad categories. They are:
1. Phase controlled rectifiers (AC to DC convertors).
2. choppers (DC to DC convertors).
3. Inverters (DC to AC convertors).
4. Cyclo converters (AC to AC convertors).
5. AC voltage Controllers (AC Regulators)
DEPARTMENT OF ECE, DR.AIT, BENGALURU 3
DEPARTMENT OF ECE, DR.AIT, BENGALURU 4
Phase controlled Rectifiers•These controllers converts fixed AC voltage to a variable DC output voltage.
•Input power from one or more AC voltage/current sources of single or multiple phase and delivers to a load.
•The output variable is a low-ripple dc voltage or dc current and these convertors uses line voltage for their commutation. Hence they are also called as line commutated or naturally commutated ac to dc convertors.
•Application:1. Battery charger circuits.• 2. regulated DC power supply
• 3. DC motor drives
Phase controlled Rectifiers
DEPARTMENT OF ECE, DR.AIT, BENGALURU 5
Choppers (DC to DC converter)•Converts fixed dc input voltage to a variable dc output voltage. The dc output voltage may be different in amplitude than the input source voltage.
• choppers are designed using semiconductor devices such as power transistors, IGBTs, GTOs, power MOSFETs and Thyristors.
•Application:
• Battery driven Vehicles.
• Electric traction
DEPARTMENT OF ECE, DR.AIT, BENGALURU 6
Inverters(DC to AC converter)•Converts a fixed dc voltage to an ac voltage of variable frequency and of fixed or variable magnitude.
•A practical inverter has either a battery, a solar powered dc voltage source.
•Used in very low-power portable electronic systems such as the flashlight discharge system in a photography camera to very high power industrial systems.
•Inverters are designed using semiconductor devices such as power transistors, IGBTs, GTOs, power MOSFETs and Thyristors.
•Applications:
• Uninterruptible power supply(UPS).
• Aircraft and space power supplies.
DEPARTMENT OF ECE, DR.AIT, BENGALURU 7
Cyclo Converters (AC To AC Convertors).•Convert input power at one frequency to output power at a different frequency through one stage conversion.
•Designed using Thyristors and are controlled by triggering signals derived from a control unit.
•Output frequency is a simple fraction such as 1/3, 1/5 and so on of the source frequency.
•They are mainly used for low speed and very high power industrial drives.
DEPARTMENT OF ECE, DR.AIT, BENGALURU 8
AC voltage controllers(AC Regulators)•Converts fixed ac voltage directly to a variable ac voltage at the same frequency using line communication.
•These converters employs a thyristorised voltage controller.
•Application:
• Lighting control
• Speed control of large fans and pumps
DEPARTMENT OF ECE, DR.AIT, BENGALURU 9
DEPARTMENT OF ECE, DR.AIT, BENGALURU 10
POWER ELECTRONICS AND INSTRUMENTATION
Class-03
Madhusudhan M
Assistant Professor
Dept. of ECE, Dr.AIT,
Bengaluru-56
Thyristor
• Two transistor model of SCR.
• Static Anode-Cathode characteristics.
• Gate characteristics of SCR.
• Dynamic Turn On switching characteristics.
• Turn-ON methods.
• Turn-OFF mechanisms.
CONTENTS,
2Dept. of ECE, Dr.AIT, Bengaluru
INTRODUCTION
• Thyristor is a general name given to the family of power semiconductor
switching devices.
• Derived from a combination of capital letters THYRatron and transISTOR
• Characterised by a bistable switching action depending upon the PNPN
regenerative feedback.
• Thyristor is almost universally referred to as the SCR(Silicon Controlled
Rectifiers)
3Dept. of ECE, Dr.AIT, Bengaluru
Construction of Thyristor• The structure and symbolic representation of
the Thyristor is as shown in the figure
• Four layer PNPN switching device,
• Three PN junctions J1, J2 and J3.
• Three external terminals namely the anode(A),
cathode(K) and gate(G).
• The anode and cathode are connected to the
main power circuit.
• The gate terminal is provided at the P layer
near the cathode, hence it is known as
cathode gate
• The gate terminal carries a low level gate
current in the direction of gate to cathode
4Dept. of ECE, Dr.AIT, Bengaluru
Forward Blocking
state OR off-state of
the device
• When the end P layer is made positive
w.r.t end N layer, the two junctions J1
and J3 are forward biased but, J2
becomes reversed biased(due the
presence of depletion layer current does
not flow through the device).
• Only leakage current(negligibly small in
magnitude)flows through the device due
to the drift if the mobile charges
• The current is insufficient to make the
device conduct.
• In the depletion layer, most of the
immobile charge do not constitute any
flow of current5
Dept. of ECE, Dr.AIT, Bengaluru
Reverse blocking state
OR off-state of the
devices
• N-layer is made positive w.r.t P-layer.
• PN junction J2 becomes forwardbiased whereas J1 and J3 becomesreversed biased.
• The junction J1 and J3 does notallow any current to flow through thedevice.
• Only a small amount of leakagecurrent may flow because of the driftof the charges.
• The leakage current is againinsufficient to make the deviceconduct.
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Dept. of ECE, Dr.AIT, Bengaluru
Conduction state
OR on state
• The width of the depletion layer at thejunction J2 decreases with the increasein anode to cathode voltage (width isinversely proportional to the voltage)
• When the Voltage between anode andcathode is kept on increasing, a stagecomes, the depletion layer at J2vanishes.(voltage gradient across itsdepletion layer)
• This phenomenon is known as theAvalanche breakdown.
• since J1 and J3 are already forwardbiased, there will be a free carriermovement across all the three junctionsresulting in an large amount of currentflowing through the device from anode tocathode.
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Dept. of ECE, Dr.AIT, Bengaluru
Static Anode-Cathode
characteristics of SCR• The figure shows a elementary
circuit diagram for obtaining V-I
characteristics of the SCR.
• Anode and cathode are
connected to the main source
through a load.
• The gate and cathode are fed
from the another source Eg.
• Va is the anode-cathode voltage
and Ia is the anode current.
Dept. of ECE, Dr.AIT, Bengaluru 8
Dept. of ECE, Dr.AIT, Bengaluru
•Reverse Blocking region
•Forward blocking region
•Forward conduction region
Regions of
operation
9Dept. of ECE, Dr.AIT, Bengaluru
Reverse Blocking
Region
• Cathode is made positive w.r.t anode,
were Thyristor becomes reversed biased.
• In reverse blocking region, Junction J1
and J3 are reversed biased and J2 is
forward Biased.
• A small leakage current flows.
• If reverse voltage is increased, a critical
breakdown called Reverse Breakdown
Voltage(VBR), an avalanche will occur at
J1 and J3 increasing the current
sharply.
• Power dissipation will lead to the
dangerous level that may destroy the
device.
10Dept. of ECE, Dr.AIT, Bengaluru
Forward
Blocking Region• Anode is made positive w.r.t
cathode.
• Junction J1 and J3 are forward
biased while junction J2 is
reversed biased.
• Anode current is small forward
leakage current.
• The region OM represent the
forward blocking region , where
the device does not conduct.
11Dept. of ECE, Dr.AIT, Bengaluru
Forward Conduction
Region.• Anode to cathode forward voltage is increased
with the gate circuit kept open, avalanche
breakdown occurs at the junction J2 at a critical
forward break-over voltage(VBO) at point M,
• Device is at conduction State.
• The region MN shows, the voltage across the
device drops from several hundred volts to 1-
2V with a very large amount of the current
flowing through the device.
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Dept. of ECE, Dr.AIT, Bengaluru-56
Forward Conduction Region.
• The region of NK of the characteristic
is called as the forward conduction
state.
• Where the anode current is
determined by the external load
impedance.
• When the Thyristor conducts forward
current, it can be regarded as a
closed switch.
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Dept. of ECE, Dr.AIT, Bengaluru-56
Latching Current(IL): the minimum anode current
required to maintain Thyristor in the ON-state
immediately after the Thyristor has been turned ON and
the gate signal has been removed
Holding Current(IH): the minimum anode current to
maintain the Thyristor in the ON-state. The holding
current is less than the latching current i.e, IH<IL.. It is
in the order of Milliampere.
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SCR using Two Transistor Model
Dept. of ECE, Dr.AIT, Bengaluru-56
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Dept. of ECE, Dr.AIT, Bengaluru-56
POWER ELECTRONICS AND INSTRUMENTATION
CLASS-04
Madhusudhan M
Assistant Professor
Dept. of ECE,
Dr.AIT, Bengaluru-56
Thyristor
• Static Anode-Cathode characteristics.
• Two transistor model of SCR
• Gate characteristics of SCR.
• Dynamic Turn On switching characteristics.
• Turn-ON methods.
• Turn-OFF mechanisms.
CONTENTS
2Dept. of ECE, Dr.AIT, Bengaluru
Two Transistor Analogy • The operation of an SCR can also be
explained by using the two transistor. Thisis know as two transistor analogy of theSCR.
• SCR can be considered as an npn and pnptransistor.
• Collector of one transistor is attached tothe base of the other and vice versa, asshown in the figure(b).
• The model is obtained by splitting the twomiddle layers of the SCR into twoseparate parts, can be seen in figure(a).
3Dept. of ECE, Dr. AIT, Bengaluru
• Collector current of the transistor
𝑇1 𝑏𝑒𝑐𝑜𝑚𝑒𝑠 𝑡ℎ𝑒 𝑏𝑎𝑠𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑜𝑓 𝑡𝑟𝑎𝑛𝑠𝑖𝑠𝑡𝑜𝑟 𝑇2 𝑎𝑛𝑑 𝑣𝑖𝑐𝑒 𝑣𝑒𝑟𝑠𝑎
𝐼𝑐1 = 𝐼𝑏2 𝑎𝑛𝑑 𝐼𝑏1 = 𝐼𝑐2
𝐼𝑘 = 𝐼𝑎 + 𝐼𝑔… . . 2.1
• We have 𝐼𝑏1 = 𝐼𝑒1 − 𝐼𝑐1…… 2.2
𝐼𝑐1 =∝1 𝐼𝑒1 + 𝐼𝑐𝑜1… . . 2.3
• Where ICO1is the reverse leakage current of the reverse biased
junction J2. substituting Eqn.2.3 in Eqn. (2.2), we get
𝐼𝑏1 = 𝐼𝑒1−∝1 𝐼𝑒1 − 𝐼𝑐𝑜1, 𝐼𝑏1 = 1− 𝛼1 𝐼𝑒1 − 𝐼𝑐𝑜1
• The anode current of the device Becomes the emitter current of
the transistor T1 that is 𝐼𝑎 = 𝐼𝑒1
𝐼𝑏1 = 1−∝1 𝐼𝑎 − 𝐼𝑐𝑜1…… 2.4
Also 𝐼𝑐2 =∝2 𝐼𝑒2 + 𝐼𝑐𝑜2
The cathode current of the SCR becomes the emitter current of the
transist T2, 𝐼𝑘 = 𝐼𝑒2
𝐼𝑐2 =∝2 𝐼𝑘 + 𝐼𝑐𝑜2… . . 2.5
𝐼𝑏1 = 𝐼𝑐2……(2.6)
Dept. of ECE, Dr. AIT, Bengaluru 4
Substituting Eqn. (2.4) and (2.5) in Eqn.(2.6), we have
1−∝1 𝐼𝑎 − 𝐼𝑐𝑜1 =∝2 𝐼𝑘 + 𝐼𝑐𝑜2… 2.7
Substituting Eqn.(2.1) in Eqn.(2.7) we get
1−∝1 𝐼𝑎 − 𝐼𝑐𝑜1 = ∝2 𝐼𝑎 + 𝐼𝑔 + 𝐼𝑐𝑜2
𝐼𝑎 =∝2 𝐼𝑔 + 𝐼𝑐𝑜1 + 𝐼𝑐𝑜2
1 − ∝1 +∝2… 2.8
Assuming that the leakage current of thetransistor T1 and T2 to be negligible small, wehave
𝐼𝑎 =∝2𝐼𝑔
1−(α1+α2)…....(2.9)
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Regenerative Action • From Eqn.(2.9), if (α1+α2)=1, the value of anode current Ia becomes infinite.
• The device suddenly latches into the conduction (ON)state from the non-
conduction(OFF) state. This characteristics of the device is known as
Regenerative Action.
• It can also stated as, the gate current Ig is of such a value that (α1+α2)
approaches unity, the device will trigger.
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Turn- ON conditions of the SCR
• Thermal. If the temperature of the device is very high, there is an increase in no. of electron-
hole pairs, which increases the leakage current. Increase in the current causes (α1 and α2) to
increase. Due to regenerative action (α1 and α2) may tends to unity and Thyristor may turned on.
• High Voltage. If the forward anode to cathode voltage is greater than the forward breakdown
voltage(VBO), a sufficient leakage current flows to initiate regenerative turn-on.(this type of turn-
on may be destructive and should be avoided)
• Gate Current. If gate current is injected into the base P in the same direction as current Ia across
the J2, the current gain of the NPN transistor be increased independently of the anode to cathode
voltage Va and current Ia.
since α2 depends on (Ia+Ig)and α1 depends on Ia. The total current will now depend on the
Ig and independent means of break over is obtained. The presence of the gate current modifies the
static V-I characteristics.
Dept. of ECE, Dr.AIT, Bengaluru-56
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Dept. of ECE, Dr.AIT, Bengaluru-56
The figure shows the gate characteristics of a typical SCR.
• First Region. OA lies near the origin and is defined by
the maximum rated junction temperature(usually 125C).
The gate must be operated in this region whenever
forward bias is applied across the Thyristor and triggering
is not necessary.
• Second Region. The minimum value of gate-voltage
and current required to trigger all devices at the minimum
rated junction temperature. This region contains the
actual minimum firing points of all devices. It is a
forbidden region(a signal in this may not always fire all the
devices or never fire any at all).from the figure, OL andOV are the minimum gate-voltage and gate-current limits
respectively.
• Third Region. Largest and shows the limits on the gate
signal for reliable firing. (OP and OQ)
Gate Characteristics of SCR
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Dept. of ECE, Dr.AIT, Bengaluru-56
Gate Characteristics of SCR From the Gate Characteristics of SCR, we have
• ON and OM – characteristics for SCRs of the same
Ratings.
• S,S1 and S2- operating points, most be close as possible to
the permissible Pg curve and must be within the max. and
min. limits of gate voltage and current.
• Es- gate source voltage, Es=OH is drawn as HD.
• Line HD- load line.
• The gradient of the load line, HD=(OH/OD), which will
give the required gate source resistance Rg.
• Line HE- maximum value of the series resistance.
• E- point of intersection of the lines indicating the
minimum gate voltage and gate current.
• Line HC- minimum value of the gate source series
resistance, obtained by drawing a line tangential to the curve
Pg
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Dept. of ECE, Dr.AIT, Bengaluru-56
Gate Power Dissipation • Higher the magnitude of gate current pulse, lesser is the time needed to inject the required charge for turning
on the Thyristor. Therefore SCR turn-on time can be reduced by using the gate current of higher magnitude.
• The gate pulse width is take as equal to or greater than SCR turn-on time,
• if T is the pulse width as shown in the figure, then we have
𝑇 ≥ 𝑡𝑜𝑛With pulse firing, if the frequency of firing f is known, the peak instantaneous gate power dissipation Pgmax can be
obtained as 𝑃𝑔𝑚𝑎𝑥 = 𝑉𝑔𝐼𝑔 =𝑃𝑔𝑎𝑣
𝑓𝑇… . . 2.10
𝑓 =1
𝑇1= 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑓𝑖𝑟𝑖𝑛𝑔 𝑂𝑅 𝑝𝑢𝑙𝑠𝑒 𝑟𝑒𝑝𝑖𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝐻𝑧
T= pulse width in second
Duty cycle : ratio of pulse-on period to the period time of pulse.
Duty cycle is given by,
𝛿 =𝑇
𝑇1= 𝑓𝑇… . 2.11
From Eqn.(2.10), 𝑃𝑔𝑎𝑣
𝛿≤ 𝑃𝑔𝑚𝑎𝑥
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Dept. of ECE, Dr.AIT, Bengaluru-56
POWER ELECTRONICS AND INSTRUMENTATION
CLASS-05
Madhusudhan M
Assistant Professor
Dept. of ECE,
Dr.AIT, Bengaluru-56
Thyristor
• Static Anode-Cathode characteristics.
• Two transistor model of SCR
• Gate characteristics of SCR.
• Turn-on methods.
• Dynamic Turn On switching characteristics.
• Turn-off mechanisms.
CONTENTS
2Dept. of ECE, Dr.AIT, Bengaluru-56
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Turn-on methods1. Thermal Triggering(Temperature Triggering).If the temperature of the device is very high, there is an increase in no. of electron-hole pairs, which
increases the leakage current. Increase in the current causes (α1 and α2) to increase. Due to regenerative
action (α1 and α2) may tends to unity and Thyristor may turned on.
2. Forward voltage Triggering (High Voltage).If the forward anode to cathode voltage is greater than the forward breakdown voltage(VBO), a sufficient
leakage current flows to initiate regenerative turn-on.(this type of turn-on may be destructive and should be
avoided)
3. Radiation Triggering(Light Triggering ).If light is allowed to strike the junctions of the Thyristor, the electron-hole pair increases, resulting which
Thyristor may be turned on. The light activated Thyristor are turned on by allowing the light to strike the
silicon wafers.
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4. dv/dt triggering.With the forward voltage across the anode and cathode of the device, the junction J1 and J3 are forward
biased and junction J2 is reversed biased. the reversed biased junction has characteristics of a capacitor due to
charges existing across the junction.
If the voltage impressed across the device is denoted byV, the charge by Q and the capacitance by Cj, then we
have
𝒊𝒄 =𝒅𝑸
𝒅𝒕=𝒅 𝑪𝒋𝑽
𝒅𝒕= 𝑪𝒋 ∗
𝒅𝑽
𝒅𝒕+ 𝑽 ∗
𝒅𝑪𝒋
𝒅𝒕the rate of change of junction capacitance may be negligible as the junction capacitance is almost constant.
The contribution of the charge current by the later term is negligible. Hence
𝑖𝑐 = 𝐶𝑗 ∗𝑑𝑉
𝑑𝑡Therefore, if the ia the rate of change of voltage across the device is larger, the device may turn on even
though the voltage appearing across the device is small.
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5. Gate Triggering. By applying a positive signal at the gate terminal of the device, it can be triggered much
before the specified break over voltage.
For gate triggering, three types of signal can be used, they are D.C signals, pulse signals or A.C signals
DC gate triggering. Dc voltage between gate and cathode is applied. Where gate becomes positive w.r.t the
cathode. When the applied voltage is sufficient to produce the required gate current, the device starts conducting.
Disadvantage:1. no isolation between the power and the control circuit..
2. continuous dc signal causes more gate power losses.
AC gate triggering. A reverse voltage is applied between the gate and the cathode terminal between the negative
half of the cycle.
Advantage: provides proper isolation between the power and control circuit.
Disadvantage: A separate transformer is required to step down the A.C supply.
Pulse gate triggering. Most popular methods of triggering. The gate drive consists of a singal pulse appearing
periodically or a sequence of high frequency pulses. This is known as carrier frequency gatting.
Advantage:1. gate losses are very much reduced.
2. isolation is been provided between main gate supply and gatting signals
Dept. of ECE, Dr.AIT, Bengaluru-56
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Dynamic turn-on switching Characteristics.
The turn-on time ton of the SCR is subdivided into three
distinct periods, called delay time, rise time and spread time.
These time periods are defined in terms of the waveforms of
the anode voltage and current obtained in the circuit.
Delay Time(td): time between the instant at which the gate
current reaches 90% of its final value and the instant at which
the anode current reaches 10% of its final value.
The time during which anode voltage falls from Va to 0.9Va,
where Va is the initial value of the anode voltage.
Rise time(tr): the time required for the anode current to rise
from 10 to 90% of its final value.
time required for the forward blocking off state voltage to fall
from 0.9 to 0.1 of its initial value.
The time is inversely proportional to the magnitude of the gate
current. For series RL circuit, rate of rise of anode current is
slow, tr is more. For series RC circuit, di/dt is high thus tr is
less.
Tr is minimized if high and steep current pulses are applied to
the gate
7
Dynamic turn-on switching Characteristics.
Spread Time (Ts): time required for the forward blocking
voltage to fall from 0.1 to its value to the on-state voltage
drop(1-1.5V). After spread time anode current attains the
steady state value and voltage drop across the SCR is equal to
the on-state voltage drop.
Turn-on Time(ton):the sum of the delay time, rise-time and
spread time. It is typically 1 to 4usec depending upon the anode
circuit parameters and gate signal waveforms. width of the
firing pulse should be in the range of 20 to 100usec.
SCR carries a large forward current which results in a high
instantaneous power dissipation creating hot-spots which could
destroy the device. Therefore it necessary to limit the rat of rise
of current
Normally, a small inductor called di/dt inductor is inserted in
the anode circuit to limit the di/dt of the anode current..
8
Turn-off Mechanism (Turn-off Characteristics)
9
Dept. of ECE, Dr.AIT, Bengaluru-56
POWER ELECTRONICS AND INSTRUMENTATION
CLASS-06Madhusudhan M
Assistant Professor
Dept. of ECE,
Dr.AIT, Bengaluru-56
Thyristor
• Static Anode-Cathode characteristics.
• Two transistor model of SCR
• Gate characteristics of SCR.
• Turn-on methods.
• Dynamic Turn On switching characteristics.
• Turn-off mechanisms.
CONTENTS
2Dept. of ECE, Dr.AIT, Bengaluru-56
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Turn-on methods1. Thermal Triggering(Temperature Triggering).If the temperature of the device is very high, there is an increase in no. of electron-hole pairs, which
increases the leakage current. Increase in the current causes (α1 and α2) to increase. Due to regenerative
action (α1 and α2) may tends to unity and Thyristor may turned on.
2. Forward voltage Triggering (High Voltage).If the forward anode to cathode voltage is greater than the forward breakdown voltage(VBO), a sufficient
leakage current flows to initiate regenerative turn-on.(this type of turn-on may be destructive and should be
avoided)
3. Radiation Triggering(Light Triggering ).If light is allowed to strike the junctions of the Thyristor, the electron-hole pair increases, resulting which
Thyristor may be turned on. The light activated Thyristor are turned on by allowing the light to strike the
silicon wafers.
Dept. of ECE, Dr.AIT, Bengaluru-56
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4. dv/dt triggering.With the forward voltage across the anode and cathode of the device, the junction J1 and J3 are forward
biased and junction J2 is reversed biased. the reversed biased junction has characteristics of a capacitor due to
charges existing across the junction.
If the voltage impressed across the device is denoted byV, the charge by Q and the capacitance by Cj, then we
have
𝒊𝒄 =𝒅𝑸
𝒅𝒕=𝒅 𝑪𝒋𝑽
𝒅𝒕= 𝑪𝒋 ∗
𝒅𝑽
𝒅𝒕+ 𝑽 ∗
𝒅𝑪𝒋
𝒅𝒕the rate of change of junction capacitance may be negligible as the junction capacitance is almost constant.
The contribution of the charge current by the later term is negligible. Hence
𝑖𝑐 = 𝐶𝑗 ∗𝑑𝑉
𝑑𝑡Therefore, if the ia the rate of change of voltage across the device is larger, the device may turn on even
though the voltage appearing across the device is small.
Dept. of ECE, Dr.AIT, Bengaluru-56
5
5. Gate Triggering. By applying a positive signal at the gate terminal of the device, it can be triggered much
before the specified break over voltage.
For gate triggering, three types of signal can be used, they are D.C signals, pulse signals or A.C signals
DC gate triggering. Dc voltage between gate and cathode is applied. Where gate becomes positive w.r.t the
cathode. When the applied voltage is sufficient to produce the required gate current, the device starts conducting.
Disadvantage:1. no isolation between the power and the control circuit..
2. continuous dc signal causes more gate power losses.
AC gate triggering. A reverse voltage is applied between the gate and the cathode terminal between the negative
half of the cycle.
Advantage: provides proper isolation between the power and control circuit.
Disadvantage: A separate transformer is required to step down the A.C supply.
Pulse gate triggering. Most popular methods of triggering. The gate drive consists of a singal pulse appearing
periodically or a sequence of high frequency pulses. This is known as carrier frequency gatting.
Advantage:1. gate losses are very much reduced.
2. isolation is been provided between main gate supply and gatting signals
Dept. of ECE, Dr.AIT, Bengaluru-56
6
Dynamic turn-on switching Characteristics.
The turn-on time ton of the SCR is subdivided into three
distinct periods, called delay time, rise time and spread time.
These time periods are defined in terms of the waveforms of
the anode voltage and current obtained in the circuit.
Delay Time(td): time between the instant at which the gate
current reaches 90% of its final value and the instant at which
the anode current reaches 10% of its final value.
The time during which anode voltage falls from Va to 0.9Va,
where Va is the initial value of the anode voltage.
Rise time(tr): the time required for the anode current to rise
from 10 to 90% of its final value.
time required for the forward blocking off state voltage to fall
from 0.9 to 0.1 of its initial value.
The time is inversely proportional to the magnitude of the gate
current. For series RL circuit, rate of rise of anode current is
slow, tr is more. For series RC circuit, di/dt is high thus tr is
less.
Tr is minimized if high and steep current pulses are applied to
the gate
Dept. of ECE, Dr.AIT, Bengaluru-56
7
Dynamic turn-on switching Characteristics.
Spread Time (Ts): time required for the forward blocking
voltage to fall from 0.1 to its value to the on-state voltage
drop(1-1.5V). After spread time anode current attains the
steady state value and voltage drop across the SCR is equal to
the on-state voltage drop.
Turn-on Time(ton):the sum of the delay time, rise-time and
spread time. It is typically 1 to 4usec depending upon the anode
circuit parameters and gate signal waveforms. width of the
firing pulse should be in the range of 20 to 100usec.
SCR carries a large forward current which results in a high
instantaneous power dissipation creating hot-spots which could
destroy the device. Therefore it necessary to limit the rat of rise
of current
Normally, a small inductor called di/dt inductor is inserted in
the anode circuit to limit the di/dt of the anode current..
Dept. of ECE, Dr.AIT, Bengaluru-56
8
Turn-off Mechanism (Turn-off Characteristics)
Dept. of ECE, Dr.AIT, Bengaluru-56
9
Dept. of ECE, Dr.AIT, Bengaluru-56
Power Electronics and Instrumentation
MADHUSUDHAN M
ASSISTANT PROFESSOR
DEPT. OF ECE,
DR. AIT, BENGALURU-56
CLASS-07
•Natural Commutation.
• Forced Commutation.
•Class A.
•Class B.
•Class C.
Turn-OFFMethods:
2Dept. of ECE, Dr.AIT, Bengaluru-56
3Dept. of ECE, Dr.AIT, Bengaluru-56
Turn-Off Methods
• A Thyristor can only operate in two modes: it is either in the OFF state or in the ON state. By itself it cannot
control the level of current and voltage in the circuit.
• Control can only be achieved, when the Thyristor is switched ON and OFF (Commutation is center to this
switching process)
• Commutation. The transfer of the current from one path to another path.
• In Thyristor circuit, the term is used to describe the process of transferring the current from one Thyristor to
another. Were the Thyristor current is reduced to zero and enables turn-off condition.
• All Thyristor circuit involves cyclic or sequential switching of Thyristors.
4
Turn-Off Methods
COMMUTATION
Natural Commutation
Forced commutation
Class A-self commutated by resonating the
load.
Class B- self commutation by
an LC circuit.
Class C-complementary Commutation.
Class D-Auxiliary
commutation.
Class E-External Pulse
commutation.
Class F- A.C line Commutation
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5
Turn-Off Methods
Natural Commutation
• In AC circuits the current always passes through zero every half cycle.
• As current passes through zero naturally, a reverse voltage will appear across the device. This immediately
turns-off the device. This process is called as natural commutation (Since no external circuit is required)
Forced commutation
• In DC circuits, for switching off the Thyristors, the forward current should be forced to be zero by means of
some external circuits. This process is called as forced Commutation.
• The external circuits required for it is known as commutation circuit.
• The components which constitute the commutating circuit is called commutating components (Inductor
and Capacitor).
• A reverse voltage is developed across the device by means of a commutating circuits that immediately brings
the forward current in the device to zero, thus turning off the device.
• The classification of the methods of forced commutation is based on the arrangements of the following:
• commutating components
• Manner in which zero current is obtained in SCR
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6
Forced Commutation
Class A- self commutation by resonating the load(Resonant
Commutation)
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7
• In the circuit, The forward current passing through the device is reduced to the less than
the level of holding current of the device. Hence this method is also known as the current
commutation.
• A underdamped resonant circuit which is exited by DC source is as shown.
• From the current waveform, when the current reached zero at the point K, a reverse
current flows in the circuit. Where the device ia automatically turned-off.
• When the Thyristor is in turn-on condition, the capacitor will charge. When the capacitor
reaches the value equal to the supply voltage, the device will soon decay to the value less
than the holding current.
Class-A
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Forced Commutation
Class B- self commutation by LC circuit.
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9Dept. of ECE, Dr.AIT, Bengaluru-56
Forced Commutation
Class C- Complementary commutation.
10Dept. of ECE, Dr.AIT, Bengaluru-56
Power Electronics and Instrumentation
MADHUSUDHAN M
ASSISTANT PROFESSOR
DEPT. OF ECE,
DR. AIT, BENGALURU-56
CLASS-08
•Resistance firing circuit
•Resistance capacitance firing circuit.
Gate Trigger Circuits
2Dept. of ECE, Dr.AIT, Bengaluru-56
3Dept. of ECE, Dr.AIT, Bengaluru-56
Gate Trigger circuit
RESISTANCE FIRING CIRCUIT
Fig(a):R-firing Circuit Fig(b):Voltage waveforms
𝑅𝑀𝐼𝑁 ≥𝐸𝑀𝐴𝑋𝐼𝑔𝑚
𝑅𝑏 ≤𝑅𝑣 + 𝑅𝑀𝐴𝑋 𝑉𝑔 𝑀𝐴𝑋
𝐸𝑀𝐴𝑋 − 𝑉𝑔 𝑀𝐴𝑋
∝
4
Resistance Firing Circuit
• Gate current is applied by the AC supply voltage via es, Rmin, Rv and D1.
• During the positive half of the input cycle, Anode and Cathode will be forward biased but the
Thyristor wont be under the conduction stage unless Ig(min) is obtained.
• During the positive half of the input cycle, the diode (D1) will be under forward biased. As the
supply voltage is increased, the gate current will be increased to Ig(min). Where the Thyristor
come under conduction stage. Voltage drop across the load will be equal to the supply voltage
i.e, EL=es.
• As the supply voltage reaches to the value of zero, the voltage drop across the load and the
Thyristor will be zero. as the supply voltage becomes further negative the voltage drop across
the load will be zero.
• Rmin. To maintain the minimum current for triggering the Thyristor.
• Rb. Is also called stabilizing resistance. Provides a maximum possible gate voltage.
• Rv varies the resistance in the gate circuit.
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Gate Trigger circuit
RESISTANCE-CAPACITANCE FIRING CIRCUIT:
Fig(b):Voltage waveformsFig(a):RC-firing Circuit
Dept. of ECE, Dr.AIT, Bengaluru-56
𝑉𝑔𝑡 = 𝑉𝑔 𝑚𝑖𝑛 + 𝑉𝐷1
6
Gate Trigger circuit
RESISTANCE-CAPACITANCE FULL-WAVE TRIGGERING CIRCUIT:
Fig(a):RC-firing Circuit Fig(b):Voltage waveforms
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UNIJUNCTION TRANSISTOR
Fig(A):Basic UJT Structure
Fig(B): Symbol and Equivalent Circuit of UJT Fig(C): UJT biasing Circuit.
𝑉𝑥 = (𝑅𝐵1∗ 𝑉𝐵𝐵)/(𝑅𝐵1 + 𝑅𝐵2)
𝑉𝑥 = 𝜂𝑉𝐵𝐵
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Fig(B): V-I characteristicsFig(A): Equivalent circuit of UJT
UNIJUNCTION TRANSISTOR
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Fig(B): Waveforms
Fig(A): Circuit Diagram
UJT Relaxation Oscillator
UNIJUNCTION TRANSISTOR
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10
UNIJUNCTION TRANSISTOR
UJT as an SCR Trigger
𝑉𝐵1 𝑜𝑓𝑓 < (𝐼𝑔 ∗ 1𝐾Ω + 𝑉𝑔)
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11
UNIJUNCTION TRANSISTOR
Synchronized UJT Triggering
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12
UNIJUNCTION TRANSISTOR
Synchronized UJT Triggering
Dept. of ECE, Dr.AIT, Bengaluru-56
13Dept. of ECE, Dr.AIT, Bengaluru-56
Power Electronics and Instrumentation
MADHUSUDHAN M
ASSISTANT PROFESSOR
DEPT. OF ECE,
DR. AIT, BENGALURU-56
CLASS-08
•Resistance firing circuit
•Resistance capacitance firing circuit.
•UJT
Gate Trigger Circuits
2Dept. of ECE, Dr.AIT, Bengaluru-56
3Dept. of ECE, Dr.AIT, Bengaluru-56
Gate Trigger circuit
RESISTANCE FIRING CIRCUIT
Fig(a):R-firing Circuit Fig(b):Voltage waveforms
𝑅𝑀𝐼𝑁 ≥𝐸𝑀𝐴𝑋𝐼𝑔𝑚
𝑅𝑏 ≤𝑅𝑣 + 𝑅𝑀𝐴𝑋 𝑉𝑔 𝑀𝐴𝑋
𝐸𝑀𝐴𝑋 − 𝑉𝑔 𝑀𝐴𝑋
∝
4
Resistance Firing Circuit
• Gate current is applied by the AC supply voltage via es, Rmin, Rv and D1.
• During the positive half of the input cycle, Anode and Cathode will be forward biased but the
Thyristor wont be under the conduction stage unless Ig(min) is obtained.
• During the positive half of the input cycle, the diode (D1) will be under forward biased. As the
supply voltage is increased, the gate current will be increased to Ig(min). Where the Thyristor
come under conduction stage. Voltage drop across the load will be equal to the supply voltage
i.e, EL=es.
• As the supply voltage reaches to the value of zero, the voltage drop across the load and the
Thyristor will be zero. as the supply voltage becomes further negative the voltage drop across
the load will be zero.
• Rmin. To maintain the minimum current for triggering the Thyristor.
• Rb. Is also called stabilizing resistance. Provides a maximum possible gate voltage.
• Rv varies the resistance in the gate circuit.
Dept. of ECE, Dr.AIT, Bengaluru-56
5
Gate Trigger circuit
RESISTANCE-CAPACITANCE FIRING CIRCUIT:
Fig(b):Voltage waveformsFig(a):RC-firing Circuit
Dept. of ECE, Dr.AIT, Bengaluru-56
𝑉𝑔𝑡 = 𝑉𝑔 𝑚𝑖𝑛 + 𝑉𝐷1
6
Gate Trigger circuit
RESISTANCE-CAPACITANCE FULL-WAVE TRIGGERING CIRCUIT:
Fig(a):RC-firing Circuit Fig(b):Voltage waveforms
Dept. of ECE, Dr.AIT, Bengaluru-56
7
UNIJUNCTION TRANSISTOR
Fig(A):Basic UJT Structure
Fig(B): Symbol and Equivalent Circuit of UJT Fig(C): UJT biasing Circuit.
𝑉𝑥 = (𝑅𝐵1∗ 𝑉𝐵𝐵)/(𝑅𝐵1 + 𝑅𝐵2)
𝑉𝑥 = 𝜂𝑉𝐵𝐵
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Fig(B): V-I characteristicsFig(A): Equivalent circuit of UJT
UNIJUNCTION TRANSISTOR
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Fig(B): Waveforms
Fig(A): Circuit Diagram
UJT Relaxation Oscillator
UNIJUNCTION TRANSISTOR
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UNIJUNCTION TRANSISTOR
UJT as an SCR Trigger
𝑉𝐵1 𝑜𝑓𝑓 < (𝐼𝑔 ∗ 1𝐾Ω + 𝑉𝑔)
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11
UNIJUNCTION TRANSISTOR
Synchronized UJT Triggering
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UNIJUNCTION TRANSISTOR
Synchronized UJT Triggering
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13Dept. of ECE, Dr.AIT, Bengaluru-56
Power Electronics and Instrumentation
MADHUSUDHAN M
ASSISTANT PROFESSOR
DEPT. OF ECE,
DR. AIT, BENGALURU-56
CLASS-11
•Uni-Junction Transistor
•Problems
Gate Trigger Circuits
2Dept. of ECE, Dr.AIT, Bengaluru-56
3
UNIJUNCTION TRANSISTOR
Fig(A):Basic UJT Structure
Fig(B): Symbol and Equivalent Circuit of UJT Fig(C): UJT biasing Circuit.
𝑉𝑥 = (𝑅𝐵1∗ 𝑉𝐵𝐵)/(𝑅𝐵1 + 𝑅𝐵2)
𝑉𝑥 = 𝜂𝑉𝐵𝐵
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4
Fig(B): V-I characteristicsFig(A): Equivalent circuit of UJT
UNIJUNCTION TRANSISTOR
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5
Fig(B): Waveforms
Fig(A): Circuit Diagram
UJT Relaxation Oscillator
UNIJUNCTION TRANSISTOR
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6
UNIJUNCTION TRANSISTOR
UJT as an SCR Trigger
𝑉𝐵1 𝑜𝑓𝑓 < (𝐼𝑔 ∗ 1𝐾Ω + 𝑉𝑔)
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UNIJUNCTION TRANSISTOR
Synchronized UJT Triggering
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UNIJUNCTION TRANSISTOR
Synchronized UJT Triggering
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UNIJUNCTION TRANSISTOR
10
Problems
Example-01:
The latching current of a Thyristor circuit is 50mA. The duration of the firing pulse is 50us. Will
the Thyristor get fired?
Solution:
𝑖 𝑡 =𝑉
𝑅(1 − 𝑒−
𝑡𝜏)
𝜏 =𝐿
𝑅=0.5
20= 0.025𝑠
At t=50us, the value of the current is given by
𝑖 50𝑢𝑠 =100
20 1 − 𝑒−50𝑢0.025
𝑖 50𝑢𝑠 = 9.99𝑚𝐴
Since the current value is less than the latching current, the circuit does not get fired or the
circuit wont enter its conduction stage .
11Dept. of ECE, Dr.AIT, Bengaluru-56
Power Electronics and Instrumentation
MADHUSUDHAN M
ASSISTANT PROFESSOR
DEPT. OF ECE,
DR. AIT, BENGALURU-56
CLASS-13
2
Problems - 01
Example-01:
The latching current of a Thyristor circuit is 50mA. The duration of the firing pulse is 50µs. Will
the Thyristor get fired?
Solution:
𝑖 𝑡 =𝑉
𝑅(1 − 𝑒−
𝑡𝜏)
𝜏 =𝐿
𝑅=0.5
20= 0.025𝑠
At t=50us, the value of the current is given by
𝑖 50µ𝑠 =100
20 1 − 𝑒−50µ0.025
𝑖 50µ𝑠 = 9.99𝑚𝐴
Since the current value is less than the latching current, the circuit does not get fired or the
circuit wont enter its conduction stage .
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Problems - 02
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Problems - 01
4
Problems - 03
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Problems - 04
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Vgs
Rg
Ig*Rg
6
Problems - 05
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Problems - 06
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Problems - 07
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Problems - 08
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10Dept. of ECE, Dr.AIT, Bengaluru-56