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ET502POWER ELECTRONICS

CHAPTER 1: OVERVIEW OF POWER ELECTRONIC DEVICES

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1.0 OBJECTIVE • To familiar with type of devices used in power electronic.

• To recognize the symbol and structure of each devices.

• To know the I-V characteristic of each devices.

• To be able to explain the turn-on and turn-off characteristic of each devices.

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POWER ELECTRONIC DEVICE

Thyristor

SCR GTO TRIAC

Transistor

BJT FET IGBT

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1. SILICON CONTROLLED RECTIFIER (SCR)SYMBOL AND STRUCTURE• Build from four layers of alternating P- type and N-type material.

1.1 SCR Symbol 1.2 SCR Structure diagram

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Terminals of the SCR• Consist of 3 terminals : Anode, Gate and Cathode• The control terminal, called the gate, is attached to p-

type material near to the cathode.

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• Alternatively, SCR also can be generated using two- transistor model

1.3 Two-transistor model of a SCR

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THE REGENERATIVE ACTION USING TWO-TRANSISTOR MODEL

• When the gate current, IG, is zero, both transistors in the off state. • When a positive pulse of current (trigger) is applied to the gate, the value is

enough to “on” Q2 (VBE2=VG).

• Current collector Q2 will increase and will “on” Q1 (IB1=IC2).

• When Q1 “on”, IC1 will increase and also increase IB2.

• IB2 at Q2 increase and it also increase IC2. This process is called “Regenerative Action” where the current collector for each transistor will increase and this will make the process continuous for each transistor.

• In this condition, we can assume that the SCR will “on”.• The collector current of Q1 provides additional base current for Q2, so that

SCR stays in conduction after the trigger pulse is removed from the gate.

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1.4 THE V-I CHARACTERISTIC CURVE OF THE SCR

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• The SCR blocking voltage are: • (i) forward breakover voltage (VBO),

• (ii) reverse breakdown voltage (VBD).

• at gate current IG = 0, if forward voltage is applied on the device there will be a leakage current.

• If the voltage exceeds its critical limit or equal to forward breakover voltage, (VBO) then SCR is ON-STATE.

• By increasing IG, VBO is reduced, and if IG = IG3, SCR behaves like a diode with the entire forward blocking region removed.

• SCR will turn on successfully if a minimum anode current, called a latching current (IL), is maintained.

• During ON-STATE, if the IG =0 and the anode current (IA) falls below a critical limit, called the holding current (IH), SCR reach its forward blocking state.

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• As the reverse voltage (VR ) across the SCR increases from zero, only a small reverse current (IR) will flow through the device due to leakage.

• This current will remain small until VR becomes large enough to cause the SCR to breakdown.

• Then IR will increase rapidly if VR increases even slightly above the breakdown point (the curve is almost vertical and straight).

• If too much reverse current is allowed to flow through the SCR after breakdown occurs, the device could be permanently damaged.

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Forward-breakover voltage,• This is the voltage at which the thyristor enters the forward-conduction region. Reverse-breakover voltage• It is the maximum reverse voltage value to stay in blocking state (off). If the

reverse voltage through thyristor is more than VRBO. This will damage the P-N junction and causes reverse current unexpectedly flow.

Latching Current (IL) • The minimum anode current required to maintain the thyristor in the on-state

immediately after switching from the off-state to the on-state has occurred and the triggering signal has been removed.

Holding current • The minimum anode current necessary to keep the device in forward-

conduction after it has been operating at a high anode current value. Or • The minimum anode current required to maintain the thyristor in the on-state.

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Method to ‘on’ SCR1) Increase the forward biased voltage more or equal

to forward breakdown2) Trigger positive supply when the device in forward

biased condition.(Gate control method)

Gate control method have 3 types of trigger: 3) Trigger with DC signal4) Trigger with AC signal5) Trigger with beat signal

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Method to ‘off’ SCR1) Reducing anode current less then holding current 2) Switch off the positive voltage supply at anode.3) Short anode and cathode.4) Change the polarity of anode to negative.

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2. GATE-TURN-OFF (GTO)SYMBOL AND STRUCTURE• Similar to SCR with 3 terminal: Anode, Cathode and Gate.

2.1 GTO Structure diagram 2.2 GTO symbol

P

N

N

P

ANODE

CATHODE

GATE

ANODE

GATE

CATHODE

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GTO I-V CHARACTERISTIC CURVE

2.3 THE I-V CHARACTERISTIC CURVE OF GTO

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• TURN- ON GTO– The GTO thyristor can be turned ON by a short

pulse of positive gate current.

• TURN-OFF GTO– GTO can be turned off by applying negative pulse

of approximate amplitude at the gate terminal.

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3. TRIACSYMBOL AND STRUCTURE

– Similar with SCR with extra features where it can conduct current in two direction.

3.1 SCR Structure diagram 3.2 SCR Symbol

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• Operation of a triac can be related to two SCRs connected in parallel in opposite directions. Although the gates are shown separately for each SCR, triac has a single gate and can be triggered by either polarity.

3.3 Equivalent circuit of TRIAC using 4 transistors and 2 SCRs

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• If terminal MT2 is positive with respect to MT1 , the triac can be turned on by applying a positive gate signal between gate G and MT1.

• If terminal MT2 is negative with respect to MT1, the triac can be turned on by applying a negative gate signal between gate G and MT1.

• If operated in quadrant 1, it needs a positive gate voltage and current, while if operated in quadrant III it needs a negative gate voltage and current

3.4 THE I-V CHARACTERISTIC CURVE OF TRIAC

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• TURN- ON TRIAC– Applying forward bias voltage larger or equal to

the threshold voltage (Vfb >VT) and a positive gate voltage

• TURN-OFF TRIAC– Reducing the anode current below holding current

(IH)

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Power Transistors (PT) • PT have controlled turn-on and turn-off characteristics.• Used as switching elements (operated in the saturation

region)• Low on-state voltage drop.• Switching speed of transistors is higher than that of

thyristors.• Lower voltage and current ratings than thyristor.• Employed in DC-DC and DC-AC converters (provide

bidirectional current flow).• The PT can be classified into 5 categories: BJTs, MOSFETs,

SITs, IGBTs and COOLMOS.

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Classification of power transistor

Power Transistor

BJT

NPN PNP

MOSFET

D-MOSFET

E-MOSFET

IGBT

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BJT• Transistor is equivalent to having two pn-diode

junctions in opposite directions to each other.• The two types of a transistor are termed NPN-

transistor and PNP-transistor.• The three terminals are named as collector, emitter,

and base.• A bipolar transistor has two junctions, collector-base

junction (CBJ) and base-emitter junction (BEJ).

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Structure Diagram & Symbol of BJTs

NPN-Transistor

PNP-Transistor

Structure Symbol

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I-V Characteristics Curve The characteristic of BJT can

be represent by value of IB, IC

and VCE as illustrate in the fig

For a fixed IB, IC will dependent to VCE, and thus when VCE is increase IC also increase.

However the increment of IC after the saturation region is small compared to the increment of VCE

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Steady-state characteristics

Three are three operating regions of a transistor: cutoff, active and saturation. • In the cutoff region

– The transistor is off or the base current is not enough to turn it on and both junctions are reverse biased.

• In the active region– The transistor acts as an amplifier, where the base current is amplified by

a gain and the collector-emitter voltage decreases with the base current.

• In the saturation region– The base current is sufficiently high so that the collector-emitter voltage

is low, and the transistor acts as a switch.

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TURN- ON BJT– Applying forward bias voltage to the

base-emitter junction and reverse-biased to the base-collector junction

TURN-OFF BJTWhen IB=0, the transistor is in the cutoff

region. BJT can be Turn- off either by:• Applying reverse bias • Turning off power supply or switch• Reducing forward current less then

holding current (IH)

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MOSFET• A BJT is a current controlled device and requires base current for

current flow in the collector.• A MOSFET is a voltage controlled device and requires only a small

input current.• The switching speed is very high and the switching times are of

the order of nanoseconds. • MOSFETs do not have the problems of second breakdown

phenomena as do BJTs.• However, MOSFETs have the problems of electrostatic discharge

and require special care in handling.– Difficult to protect them under short-circuited fault conditions.

• The three terminals are called gate, drain and source.

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MOSFET• The two types of MOSFETs are: P-channel and the N-channel MOSFET • Both the P-channel and the N-channel MOSFET is available in two basic

forms, the Enhancement type and the Depletion type.

Depletion MOSFETs

Enhancement MOSFETs

Symbols

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MOSFET• The voltage VGS applied to the gate controls the current

flowing between the drain and the source terminals. • VGS refers to the voltage applied between the gate and

the source.• VDS refers to the voltage applied between the drain and

the source. • Because a MOSFET is a VOLTAGE controlled device, "NO

current flows into the gate!" then the source current (IS) flowing out of the device equals the drain current flowing into it and therefore (ID = IS).

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I-V Characteristics Curve The characteristic of

MOSFET can be represent by value of ID, VDS and VGS as illustrate in in the fig

For a fixed VGS, ID will dependent to VDS , and thus when VDS is increase ID also increase.

However the increment of ID in the saturation region is small compared to the increment of VDS

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Steady-state characteristicsThere are three regions of operation: Cutoff, Pinch-off or saturation and Linear region • In the Cutoff region

– where VGS ≤ VT

• In the Pinch-off or saturation regio – the drain current remains almost constant for any increase in the value of VDS

and the transistor are used in this region for voltage amplification. • In the Linear region,

– the drain current ID varies in proportion to the drain-source voltage VDS

– Due to high drain current and low drain voltage, the MOSFETs are operated in the linear region for switching actions.

• The pinch-off occurs at VDS = VGS – VT

It should be noted that saturation has the opposite meaning to that for bipolar transistors.

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IGBT• IGBT combines the advantages of MOSFET and BJT. – BJT has low power losses but have long switching

time (especially at turn off). – MOSFETs have very fast switching characteristics

(low turn on and turn off times) but have higher power losses.

• Thus an IGBT has fast switching times like MOSFET and low power losses like BJT.

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The symbol of the IGBT• The IGBT is a three terminal device. • The power terminals are called the emitter (E) and collector

(C), using the BJT terminology, while the control terminal is called the gate (G), using the MOSFET terminology.

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Output characteristics of IGBT

The i-v characteristics appears qualitatively similar

to that of a logic level BJT except that the controlling

parameter is an input voltage. The transfer curve ic-vGE is identical to that of the power MOSFET except VGE(th)

and the slope values.

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TURN- ON IGBT applying a positive gate voltage to open the

channel for n carriers

TURN-OFF IGBT removing the gate voltage to close the

channel.

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Study other types of semiconductor devices• Draw the symbols of the – Diac, – Photo Diode, – Photo Thyristor, – Photo Transistor, – Optocoupler and – Programmable Unijunction Transistor (PUT).

• Identify the terminals of devices.