Power Switching Devices

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EE4030 Power Electronics and EE4030 Power Electronics and ApplicationsApplications

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Outline SyllabusOutline Syllabus1.0 Power Switching Devices

Overview of Thyristor, BJT, MOSFET, IGBT and other hybrid power devices,Basic structure, characteristic and application, comparison among powerdevices

2.0 Six StepVoltage Source Inverters

Structure and operation, Output voltage, current, harmonics and totalharmonic distortion (THD),

Input output modelling, voltage and frequency control

3.0 PWMVoltage Source Inverters

Square wave, sinusoidal and regular sampled PWM inverters,

Harmonic elimination and distortion minimization, Voltage vector and currentcontrolled PWM inverters , Voltage & frequency control, Implementationaspects

4.0 DC - DC Converters

Structure and operation of buck, boost, buck/boost and full bridge dc-dcconverters, Output characteristic and control,Application consideration

5.0ThyristorVoltage Converters

Single phase and three phase ac-dc converters, Current and voltage harmonics,

Line notching and their minimization, Selected application area2

ReferencesReferences

� Ned Mohan, Tore M. Undeland, and Willium P. Robbins, Power electronics converters, applications and design, John Wiley & Sons, Inc., 3rd edition, 2006.

� Muhammed H. Rashid, Power electronics circuits, devices and applications, Prentice-Hall of India Private Limited, 3rd edition, 2006.

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Power switching devicesPower switching devices

All power switching devices operate in ON and OFF states. This operation

�gives high energy efficiency for the converter

�permits flexibility in control

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Types of DevicesTypes of Devices

Devices currently in use are

� Power Diodes

� Standard Thyristors

� Gate Turn-Off Thyristors

� Power Transistors

� Power MOSFETs

� Insulated Gate Bipolar Transistor

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Power diodePower diode

i – v characteristics

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Power diode Power diode -- Basic functionsBasic functions

Basic functions served by power diodesand its variants are

�rectification

�feedback

�freewheeling

�protection

�transient suppression

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FreewheelingFreewheeling

Diode in this circuit serves freewheelingfunction. It allows inductive load currentcirculate after transistorT is switched off.

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FeedbackFeedback

Diode in this circuit serves feedback function. Ithelps to send the inductive energy of the loadback the source after switching off of transistors.

Diode D2 provides the path to send the storedenergy in the load back to the source afterswitching-off of transistor T1.

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Grades of diodesGrades of diodes

� Rectifier: used in line frequency (50 Hz or 60 Hz) converters

� Fast-recovery: used in high switching frequency circuits for feedback and freewheeling functions

� Power zenor diode: used in transient suppression of voltage spikes

� Schottky diode: used in drive circuits and in low voltage power supplies

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Diode during its switchDiode during its switch--offoff

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Diode during its switchDiode during its switch--offoff

� trr – Reverse recovery time

� Irrm – Maximum reverse recovery current

trr for a given diode is approximately aconstant and Irrm depends on trr and di/dtduring switch off.

Diode is called a fast recovery diode if trr isvery small, typically in ns period. Fastrecovery diode has a slightly higherconduction drop (typically 1V) compared torectifier diodes. 12

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Transient suppression diodeTransient suppression diode

This is the power zenor diode. In lowvoltage circuits, it is possible to use powerzenors to arrest transient voltage spikes.

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SchottkySchottky diodediode

These are diodes with low conduction drop, typically 0.3 V.

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Diode overDiode over--voltage protectionvoltage protection

Maximum reverse voltage acceptable to adiode is limited and provisions should bemade to limit any possible over-voltages.

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Diode overDiode over--voltage protectionvoltage protection

This type of over-voltage spikes can be arrested by using aparallel RC branch with the diode.

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Standard Standard ThyristorThyristor

This has a PNPN structure.

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ii –– v characteristicsv characteristics

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Modes of operationModes of operation

Thyristor has 3 modes of operation.

� Reverse blocking mode

� Forward blocking mode

� conduction mode

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ThyristorThyristor ratings ratings

Thyristor has three basic ratings for voltage.

�VDSM, VRSM – Direct/Reverse Single Maximum

�VDRM, VRRM – Direct/Reverse Repetitive Maximum

�VDWM, VRWM – Direct/Reverse Single Maximum

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ThyristorThyristor voltage ratings voltage ratings

� Single maximum ratings give the maximum single voltage spike the thyristor can accept. This type of spikes occur when the converter is switched on to power.

� Repetitive maximum ratings give the maximum repetitive (in each cycle) spike the thyristor can accept.

� Working maximum ratings give the maximum continuous voltage the thyristor can accept.

In practice we select thyristors for supply voltage crest ≈

5.22 to

VDWM

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ThyristorThyristor current ratingscurrent ratings

� There are two basic current ratings.

� IFSM� Continuous rating

� IFSM – Peak current of single half-sine pulse of 10 msduration, the thyristor can accept when it is alreadyoperating at its maximum temperature limit.

This rating is useful when designing over-currentprotection for the thyristor, e.g. Selection of fuses.

� Continuous rating gives maximum continuous currentthe thyristor can handle. Practical rating can be differentto the specified rating and it is decided in a thermaldesign. 22

dvdv//dtdt and and didi//dtdt ratingsratings

dv/dt rating:

This is the highest rate of change of voltage that the thyristor can accept between its anode and cathode without causing unintended switch-on.

di/dt rating:

This is the highest rate of change of anode current acceptable to the thyristor at the instant of switch-on without causing a permanent (thermal) failure of the thyristor.

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TurnTurn--off time (toff time (tQQ) rating:) rating:

� This is the maximum time for which a thyristor has to be held in reverse bias for complete turn off.

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Two transistor modelTwo transistor model

Behaviour of a thyristor can be explained using its two transistor model.

If Ig is zero, Q1 and Q2 are off (i.e. anode to cathode is open). If Ig isgiven, then Q1 is turned on and in turn Q2 is also turned on (becauseIC1 now acts as base current for Q2). IC2 of Q2 then acts as basecurrent for Q1 and Ig can now be withdrawn.

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Latching currentLatching current

This is the minimum anode current to which thethyristor must build up before the gate pulse canbe withdrawn. Typical latching current is about 50mA.

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Gate pulsesGate pulses

Long signal gate pulses such as that are required by tyristors driving highly inductive loads are replaced by a train of pulses. This is done to

�reduce losses at the gate

�allow the pulses to be passed through pulse transformers

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Holding currentHolding current

� This is the maximum anode current with which a conducting thyristor can continue to be in the ON state. Typically the holding current is about 40 mA.

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Switching ONSwitching ON

This is done with the aid of a gate drive circuit.

�D1 –To circulate inductive energy after transistor T is off.

�D2 –To stop reverse gate current

�D3 –To stop reverse gate voltage

�S –To suppress noise spikes 29

Switching OFFSwitching OFF

Gate current Ig can only bring a thyristor into conduction and it cannot stop the thyristor afterwards. To stop the thyristor, the following conditions must be satisfied.

�Bring the anode current below its holding current

�Then hold the thyristor in the reverse blocking mode for a time not less than tQ(thyristor trun-off time)

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Switching OFFSwitching OFF

In some circuits these conditions are satisfied automatically (naturally). But in some circuits we have to intervene and provide these conditions by force.

Method of commutation means method of providing these conditions. There are three methods of commutation.

�Line commutation

�Load commutation

�Forced commutation31

Line commutationLine commutationIn this method the conditions are satisfied due to the natural reversal of line voltage.

For successful turn-off,

T > tQ.

At t = t1, the conditions of

turn-off are satisfied.

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Load commutationLoad commutationIn this method, the conditions for turn-off are satisfied by the reversal of voltage induced by the load.

If Ig is given at t = 0,

then for t > 0

After ωt = π, thyristor is

turned off successfully. 33

Forced commutationForced commutation

This method is chosen if other methods of commutation are not available.

Assume the switch S to be closed at t = 0 to initiatecommutation. Thyristor current i is reduced as load current is

now diverted through the capacitor.34

ThyristorThyristor protectionprotection

Thyristor is a delicate device and must be protected against all possible disturbances if it is to perform reliably and successfully. There are several types of protection.

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dv/dt, di/dt and switching overdv/dt, di/dt and switching over--voltage protectionvoltage protection

If the dv/dt occurring at the anode is excessive, then the resulting displacement current across the reverse biased junction (described by the capacitor) can bring the thyristor into conduction automatically. This if happens, is a faulty condition.

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dv/dt, di/dt and switching overdv/dt, di/dt and switching over--voltage protectionvoltage protectionInitially only very small area in near the gate contact is available for conduction. This area propagates across the channel as time goes on. If anode current is applied at a rate in excess, there will be current crowding at the gate which can burn the gate. This is a permanent failure.

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SnubberSnubber circuits for circuits for thyristorthyristorThis circuit

� limits di/dt at turn on

� limits dv/dt in the off state

� limits over-voltage spikes occurring at instance of turn-off.

� limits dv/dt across the thyristor when recovering inverse parallel diode, in case such a diode is part of the circuit

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Behavior at turnBehavior at turn--off without off without snubbersnubber

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Behavior at turnBehavior at turn--off with off with snubbersnubber

The waveform indicates that transient switching voltage spikes are eliminated by the LCR snubber.

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dv/dt across the thyristor when dv/dt across the thyristor when recovering inverse parallel dioderecovering inverse parallel diode

In this circuit there is no voltage spike problem at thyristor turn-off, because the diode does not allow any reverse voltage. However there may be spikes due to the recovery of the diode after its turn-off.

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Recovering of inverse parallel diodeRecovering of inverse parallel diode

As the waveform indicates, the voltage felt by the thyristor during the diode recovery is damped down by the LCR snubber.

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Modified Modified snubbersnubber

The initial R.Irrm jump has high dv/dt and to ensure that the thyristor won’t mis-trigger on this, we can modify the snubber as shown below for still better reliability.

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Over current protectionOver current protection

Thyristors have excellent surge current capability and thus can be protected against over-current using fuses. Semiconductor fuses (which are extremely fast) can be used.

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Over current protectionOver current protectionA better choice of a fuse is one whose Ip for the given Im is less than (with adequate safety margin) the surge current IFSM for the thyristor.

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OverOver--voltage (power line surges) voltage (power line surges) protectionprotection

Typical protection for power line surges is to use surge arrestors i.e. Metal Oxide Varistors (MOV). Often we use one arrestor at the supply intake and one for each expensive thyristor.

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Crowbar protectionCrowbar protection

Crowbar thyristor is turned on automatically if over-current exceeds a preset level. This resulting current in thecoil in the crowbar path signals the input breaker to betripped off.

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Gate turn off Gate turn off thyristorthyristor (GTO)(GTO)

GTO is similar to the standard thyristor in most aspects and additionally it can be turned off by its gate control.

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Switch ON and OFFSwitch ON and OFF

� To switch ON: give in a narrow Ig pulse of small current (e.g. 200 mA, 10 µs)

� To switch OFF: withdraw a narrow Ig pulse of large current (e.g. 100 A, 20 µs)

For most GTOs turn-off gain is in the range of 3 to5.

offturncausetogatethefromoutdrawnbetocurrentGate

offturntopriorcurrentAnodegainoffTurn =−

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Mechanism of turnMechanism of turn--offoff

To turn off we should grab Ic2 entirely and divert it away from the base of Q1. This is done by negative Ig. This causes the turn off of Q1 first and Q2 next.

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Circuit support for turnCircuit support for turn--offoffTurn off snubber to provide

� an alternative path for the anode current during the turn-off transient

� a means of suppressing dv/dt peaks during turn-off and other operating conditions

The purpose of R is to provide apath for the discharge of capacitorvoltage during the ON state of theGTO.

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Main features of GTO Main features of GTO � its di/dt rating is much higher (in some cases di/dt inductor can be excluded)

� its turn-off can be initiated at the gate control. This permits to exclude bulky forced-commutation circuits and hence reduced converter size by about 60%)

� its turn-on is fast

� its conduction drop is slightly higher, about 1 V.

� its reverse voltage blocking feature is absent. GTOs must be used with inverse parallel diodes

� its dv/dt is some what lower. This requires the gate terminal to be held at a negative voltage throughout the OFF period to avoid dv/dt triggering

� its holding current is higher. This can cause partial turn-off at low anode currents, resulting in heavy heating of the GTO or sometimes unintended turn-off at too smalln anode current. To avoid this, we need to supply gate current throughout its ON period. 52

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Bipolar Junction Transistor (BJT)Bipolar Junction Transistor (BJT)

This is one of the widely used devices in low and medium power industrial applications. It has switching frequency capability up to about 30 kHz (15 kHz is a good practical limit).

They are available in 2 types, NPN and PNP.

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i i –– v v characteristicscharacteristics

VRBD (reverse breakdown voltage) is very small; i.e. the transistor is not meant to block reverse voltages. It should be used with inverse parallel diodes. VFBO is large but not a fixed value. It depends on the base-circuit arrangement. 54

ii –– v v characteristicscharacteristics

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Turn ON and OFFTurn ON and OFFIt is very easy to turn on and turn off a transistor.

�To turn on: giving IB of sufficient magnitude and hold it continuously.

�To turn off: bring IB to zero and optionally hold the base at a slightly negative voltage.

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Selection of Selection of IIB, ONB, ON

IB, ON is selected with the following considerations.

1.IB, ON should be large enough to saturate the transistor. i.e.

Ic,max – maximum likely collector current during the operation

hFE – transistor dc current gain

hFE is not a constant and it varies with Ic. We select hFE corresponding to Ic,max.

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2. IB, ON should be as large as possible to keep VCE,ON at a lower value.

Overdrive – giving base current in excess of the minimum needed

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3. IB, ON should not be too large to avoid long delays at turn-off due to excessive stored charges.

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Safe operating area (SOA)Safe operating area (SOA)

This is area in the Ic vs VCE plane designated to be safe for operation. There are two such areas.

�Forward bias safe operating area (FBSOA)

�Reverse bias safe operating area (RBSOA)

The circuit designer must ensure that the operating point of the BJT is always inside the relevant SOA. ON and OFF points must be placed inside SOA and also the transient variations (during switchover).

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FBSOAFBSOA

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RBSOARBSOA

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Power MOSFETPower MOSFET

The power MOSFET is a strong competitor to the power BJT in medium power applications. In comparison to BJT, the power MOSFET

�is easy to drive because it is a voltage driven device

�has surge current capability

�has still higher switching frequency capability

�has wider safe operating area

�provides provisions to make modules by series/parallel combinations

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DrawbacksDrawbacks

� They are expensive

� They have higher conduction voltage drops

High conduction voltage drop is the main drawback of MOSFET. This drop is increased with the voltage rating of the device. In practice, therefore we use MOSFETs only for low and medium voltage applications, e.g. below 600 V. Fortunately majority of industrial applications fall within this range.

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MOSFET typesMOSFET types

There are two types, N-channel and P-channel.

In practice, only the N-channel type is used. The P-channel type gives still higher conduction voltage drop and hence not used for general applications. 65


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Switch ONSwitch ON

To switch on we apply a voltage between gate and source; typically 1.2 V. Turn-on time depends on how quick the capacitor Cgs can be charged above the threshold which is 4 V typically. For short turn-on time, we should use a drive circuit which has small output resistance.

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Switch OFFSwitch OFF

To switch off we apply Vgs=0, i.e. discharge Cgs.

There are number of commercial MOSFET driver ICs including aditional features such as over-current protection etc.

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ProtectionProtectionMOSFET is a rugged device and needs minimal protection. Basically over-voltage and gate protection are sufficient.

Gate over voltage protection

Possible over voltage protection 69

Insulated Gate Bipolar Transistor Insulated Gate Bipolar Transistor (IGBT)(IGBT)

This is perhaps the most popular power switching device at the moment. This is a hybrid of MOS and bipolar technology which combines the good attributes of the BJT and MOSFET.

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The IGBTThe IGBT� has BJT like low conduction voltage drop

� handles high current density than BJT or MOSFET

� can operate at switching frequencies higher than that of BJT

� requires simpler gate drive similar to that of MOSFET

� is easier to protect and can operate sometimes without snubbers

� has good surge current capability (i.e. no secondary breakdown problems and has wider safe operating area)

� is more attractive than BJT or MOSFET in general power electronic applications. 72

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Latching of IGBTLatching of IGBT

� IGBTs can latch itself if it is subjected to severe over-current. If latched, the gate will have no control and the only way to stop the IGBT is by forcing the IGBT current to zero.

� If not detected early, a latched state can result in a destructive breakdown of the IGBT thermally. A good over-current limiter should be used to stop the IGBT on over-current. Modern IGBTs are however much improved and the latching problem is almost eliminated. They are called latch-free IGBTs.

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Power Switching Devices