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Power Electronics

Thyristors and

gate turn-off thyristors

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Power electronics

dr inż. Andrzej Smolarz

Instytut Elektroniki i Technik Informacyjnych

Politechnika Lubelska

a.smolarz@pollub.pl

smolarz.pollub.pl

E313, 081 538 4337

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Sources

M. D. Singh, Power Electronics‎, 2008 http://books.google.pl/books?id=0_D6gfUHjcEC

J.S.Chitode, Power Electronics‎, 2008 http://books.google.pl/books?id=VMC5AYf1YFwC

NPTEL Project (India) http://nptel.ac.in/downloads/108105066

Some students’‎presentations were also used

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CONTENTS

Introduction to thyristors

Thyristor operating principle

Thyristor turn-on methods

Thyristor commutation(turn-off)

Switching thyristor characteristics

The gate turn-off thyristor

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INTRODUCTION TO

THYRISTORS

THYRISTOR Thyristor, a three terminal, four layers solid state semiconductor

device, each layer consisting of alternately N-type or P-type material, i.e; P-N-P-N, that can handle high currents and high voltages, with better switching speed and improved breakdown voltage

Name‎‘thyristor’,‎is‎derived‎by‎a‎combination‎of‎the‎capital‎letters‎from THYRatron and transISTOR

Thyristor has characteristics similar to a thyratron tube which is a type of gas filled tube used as a high energy electrical switch and controlled rectifier

But from the construction view point, a thyristor (pnpn device) belongs to transistor (pnp or npn device) family

This means that thyristor is a solid state device like a transistor and has characteristics similar to that of a thyratron tube

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THYRISTOR Thyristor (famous as Silicon Control Rectifier-SCR) can handle

high currents and high voltages

Typical rating are 1.5kA & 10kV which responds to 15MW power handling capacity

This power can be controlled by a gate current of about 1A only

Thyristor a three terminal (Anode, Cathode and Gate), three junctions and four layers solid-state semiconductor device, with silicon doped alternate material with P-N-P-N structure

Thyristor act as bistable switch; It conducts when gate receives a current pulse, and continue to

conduct as long as forward biased (till device voltage is not reversed)

They stay ON once they are triggered, and will go OFF only if current is too low or when triggered off

Structure of a thyristor

Structure on the physical and electronic level,

and the thyristor symbol.

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Two-transistor model of thyristors

Q1

Q2

Basic Structure Equivalent Circuit

Two-Transistor Model of Thyristors Two-transistor model is obtained by bisecting the two middle layers in two

separate halves, junctions J1–J2 & J2-J3 constitute pnp & npn transistors separately

In‎transistor’s‎off-state, IC is related to IE as IC = αIE + ICBO

where‎α‎is‎the‎common-base current gain and ICB0 is collector-base leakage current of transistor

1. For transistor Q1; IC1 =‎α1 Ia + ICBO1

Similarly, for transistor Q2; the collector current IC2 is given by

2. IC2 =‎α2 Ik + ICBO2

Sum of two collector currents given by Eqs. (01) & (02) is equal to the external circuit current Iα entering at anode terminal A. Therefore;

3. Ia = IC1 + IC2, Ia =‎α1 Ia + ICBO1+‎α2 Ik + ICBO2

When gate current is applied, then Ik = Ia + Ig

Substituting this value of Ik in Eq. (03) gives

Ia =‎α1 Ia + ICBO1+‎α2 (Ia + Ig ) + ICBO2

or Ia =‎α2 Ig + ICBO1 + ICBO2 /[1-(‎α1+‎α2)]

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Thyristor internal

constructional view

Vertical cross section of a thyristor

THYRISTOR OPERATING

PRINCIPLE

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Thyristor- Operation Principle Thyristor has three p-n junctions (J1, J2, J3 from the anode) When anode is at a positive potential (VAK) w.r.t cathode with no

voltage applied at the gate, junctions J1 & J3 are forward biased, while junction J2 is reverse biased As J2 is reverse biased, no conduction takes place, so thyristor

is in forward blocking state (OFF state) Now if VAK (forward voltage) is increased w.r.t cathode, forward

leakage current will flow through the device When this forward voltage reaches a value of breakdown

voltage (VBO) of the thyristor, forward leakage current will reach saturation and reverse biased junction (J2) will have avalanche breakdown and thyristor starts conducting (ON state), known as forward conducting state

If Cathode is made more positive w.r.t anode, Junction J1 & J3 will be reverse biased and junction J2 will be forward biased

A small reverse leakage current flows, this state is known as reverse blocking state

As cathode is made more and more positive, stage is reached when both junctions A & C will be breakdown, this voltage is referd as reverse breakdown voltage (OFF state), and device is in reverse blocking state

Characteristics of thyristors

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THYRISTOR OPERATING MODES Thyristors have three modes 1. Forward blocking mode: Anode is positive w.r.t cathode, but the

anode voltage is less than the break over voltage (VBO) Only leakage current flows, so thyristor is not conducting

2. Forward conducting mode: When anode voltage becomes greater than VBO, thyristor switches from forward blocking to forward conduction state, a large forward current flows If the IG=IG1, thyristor can be turned ON even when anode voltage is less than

VBO

⁻ The current must be more than the latching current (IL)

⁻ If the current reduced less than the holding current (IH), thyristor switches back to forward blocking state

3. Reverse blocking mode: When cathode is more positive than anode, small reverse leakage current flows However if cathode voltage is increased to reverse breakdown voltage ,

Avalanche breakdown occurs and large current flows

THYRISTOR TURN-ON

METHODS

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THYRISTOR TURN-ON METHODS

Thyristor turning ON is also known as Triggering

With anode positive with respect to cathode, a thyristor can be turned ON by any one of the following techniques:

Forward voltage triggering

Gate triggering

dv/dt triggering

Temperature triggering

Light triggering

Forward voltage triggering When breakover voltage (VBO) across a thyristor is

exceeded than the rated maximum voltage of the device, thyristor turns ON

At the breakover voltage the value of the thyristor anode current is called the latching current (IL)

Breakover voltage triggering is not normally used as a triggering method, and most circuit designs attempt to avoid its occurrence

When a thyristor is triggered by exceeding VBO, the fall time of the forward voltage is quite low (about 1/20th of the time taken when the thyristor is gate-triggered)

However, a thyristor switches faster with VBO turn-ON than with gate turn-ON, so permitted di/dt for breakover voltage turn-on is lower

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Gate triggering Turning ON of thyristors by gate triggering is

simple and efficient method of firing the forward biased SCRs

In Gate Triggering, thyristor with forward breakover voltage (VBO), higher than the normal working voltage is chosen This means that thyristor will remain in forward

blocking state with normal working voltage across anode and cathode with gate open

Whenever thyristor’s turn-ON is required, a positive gate voltage b/w gate and cathode is applied

Gate triggering With gate current established, charges are injected into the inner p

layer and voltage at which forward breakover occurs is reduced

Forward voltage at which device switches to on-state depends upon the magnitude of gate current Higher the gate current, lower is the forward breakover voltage

When positive gate current is applied, gate P layer is flooded with electrons from cathode, as cathode N layer is heavily doped as compared to gate P- layer

As the thyristor is forward biased, some of these electrons reach junction J2.

As a result, width of depletion layer around junction J2 is reduced This causes junction J2 to breakdown at an applied voltage lower than

forward breakover voltage VB0

If magnitude of gate current is increased, more electrons will reach junction J2, thus thyristor will get turned ON at a much lower forward applied voltage

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dv/dt TRIGGERING With forward voltage across anode & cathode of a thyristor, two

outer junctions (A & C) are forward biased but the inner junction (J2) is reverse biased.

The reversed biased junction J2 behaves like a capacitor because of the space-charge present there

As p-n junction has capacitance, so larger the junction area the larger the capacitance.

If a voltage ramp is applied across the anode-to-cathode, a current will flow in the device to charge the device capacitance according to the relation:

If the charging current becomes large enough, density of moving current carriers in the device induces switch-on

This method of triggering is not desirable because high charging current (Ic) may damage the thyristor

Temperature triggering During forward blocking, most of the applied

voltage appears across reverse biased junction J2

This voltage across junction J2 associated with leakage current may raise the temperature of this junction

With increase in temperature, leakage current through junction J2 further increases

This cumulative process may turn on the SCR at some high temperature

High temperature triggering may cause Thermal runaway and is generally avoided

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Light triggering In this method light particles (photons) are made to strike the

reverse biased junction, which causes an increase in the number of electron hole pairs and triggering of the thyristor

For light-triggered SCRs, a slot (niche) is made in the inner P - layer

When it is irradiated, free charge carriers are generated just like when gate signal is applied b/w gate and cathode

Pulse light of appropriate wavelength is guided by optical fibers for irradiation

If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR)

Light-triggered thyristors is mostly used in high-voltage direct current (HVDC) transmission systems

THYRISTOR COMMUTATION

(TURN-OFF)

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Thyristor commutation Commutation: Process of turning off a conducting thyristor

Current Commutation

Voltage Commutation

A thyristor can be turned ON by applying a positive voltage of about a volt or a current of a few tens of milliamps at the gate-cathode terminals

But SCR cannot be turned OFF via the gate terminal

It will turn-off only after the anode current is negated either naturally or using forced commutation techniques

These methods of turn-off do not refer to those cases where the anode current is gradually reduced below Holding Current level manually or through a slow process

Once the SCR is turned ON, it remains ON even after removal of the gate signal, as long as a minimum current, the Holding Current (IH), is maintained in the main or rectifier circuit

Thyristor turn-off mechanism In all practical cases, a negative current flows through the device

This current returns to zero only after the reverse recovery time (trr), when the SCR is said to have regained its reverse blocking capability

The device can block a forward voltage only after a further tfr, the forward recovery time has elapsed

Consequently, the SCR must continue to be reverse-biased for a minimum of tfr + trr = tq, the rated turn-off time of the device.

The external circuit must therefore reverse bias the SCR for a time toff > tq

Subsequently, the reapplied forward biasing voltage must rise at a dv/dt < dv/dt (reapplied) rated. This dv/dt is less than the static counterpart

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Thyristor commutation types

Commutation can be classified as:

Natural commutation

Occurs only in AC circuits

Natural Commutation of thyristor takes place in

AC Voltage Regulators

Phase controlled rectifiers

Cycloconverters

Forced commutation

Thyristor Turn-Off:

Line-Commutated Thyristor Circuit

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Forced commutation Applied to DC circuits

If a thyristor is used in a DC circuit, when first turned on, it will stay on until the current goes to zero. To turn off the thyristor it is possible to use a Forced commutation circuit. The circuit creates a reverse voltage over the thyristor (and a small reverse current) for a short time, but long enough to turn off the thyristor

A simple circuit consist of a precharged capacitor and a switch (e.g. another thyristor) parallel to the thyristor. When the switch is closed, the current is supplied by the capacitor for a short while. This cause a reversed voltage over the thyristor, and the thyristor is turned off

Commutation is achieved by reverse biasing thyristor or reducing the thysristor current below the holding current value.

Commutating elements such as inductor, capacitors are used for commutation purpose

Forced commutation is applied to choppers and inverters

Forced commutation methods

Class A- Resonant Load

Class B- Self commutation

Class C- Auxiliary commutation

Class D- Complimentary commutation

Class E- External pulse commutation

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THYRISTOR TURN-OFF: FORCED- COMMUTATED

THYRISTOR CIRCUIT

THYRISTOR SWITCHING

CHARACTERISTICS

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Thyristor switching characteristics

Thyristor‎Turn−ON‎time‎for‎a‎resistive‎Load Thyristor‎Turn‎−OFF‎time‎for‎a‎resistive‎Load

THYRISTOR turn-ON & turn-OFF

CHARACTERISTICS

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THE GATE TURN-OFF

THYRISTOR

TYPES OF THYRISTOR SCR: Silicon Controlled Rectifier DIAC: Diode on Alternating Current TRIAC : Triode for Alternating Current SCS: Silicon Control Switch SUS: Silicon Unilateral Switch SBS: Silicon Bidirectional Switch SIS: Silicon Induction Switch LASCS: Light Activated Silicon Control Switch LASCR: Light Activated Silicon Control Rectifier SITh : Static Induction Thyristor RCT: Reverse Conducting Thyristor GTO : Gate Turn-Off thyristor MCT: MOSFET Controlled Thyristor ETOs: Emitter Turn ON thyristor

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THE GATE TURN-OFF THYRISTOR A gate-turn-off thyristor (GTO) like an SCR can be turned on by

applying a positive gate signal

GTO can be turned off by a negative gate signal

GTOs have certain advantages over SCRs: Elimination of the commutating components

Reduction of electromagnetic noise due to elimination of the commutation chokes

Faster turn-off times

Improved efficiency of the converters

A large initial gate trigger pulse is required to turn on a GTO

Once the GTO is turned on, forward gate current must be continued for the whole conduction period (1% of the turn on pulse)

A GTO requires a relatively high negative current pulse to turn off

It also has a higher on-state voltage of 3.4V for a 550A and 1200V device

STRUCTURE of GTOs

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GTO OPERATING PRINCIPLE A GTO thyristor consists of four layers, pnpn, as like

conventional thyristors

Functions except for turn-off are the same as those of conventional thyristors

When a GTO thyristor is in the on-stats, the central base regions are filled with holes supplied from the anode and electrons supplied from the cathode

If reverse bias is applied to make the gate negative in respect to the cathode, part of holes in the P-base layer are extracted through the gate, suppressing the injection of electrons from the cathode

In response to this suppression, more hole current is extracted through the gate, further suppressing the electron injection

In the course of this process, the cathode emitter junction (J3) is put into a reverse-bias state entirely, GTO thyristor is turned off

GTO OPERATING PRINCIPLE

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GTO turn-on and turn-off pulses

A GTO, like and SRC, turns on with a

positive gate signal

However, it can be turned off with a negative

gate signal

SUMMARY A thyristor is a latching device and it can be turned on with a

small‎gate‎pulse,‎typically‎100μs

Thyristors are generally off by line commutation due to the natural behavior of the input ac line supply

During the turn-off process, thyristors must be subjected to a reverse voltage for a certain minimum time known the turn-off

The Thyristor family: Double injection yields lowest forward voltage drop in high voltage

devices.

More difficult to parallel than MOSFETs and IGBTs

The SCR: Highest voltage and current ratings, low cost, passive turn-off

transition

The GTO: Intermediate ratings (less than SCR, somewhat more than IGBT).

Difficult to drive

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THE END … and they lived hapily ever after

Thank you for your attention

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