device descriptions

7/28/2019 Device Descriptions

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Overview of electric drivesConverter

Types of power electronic switches and their symbols

Diodes

Thyristors or SCRs

BJTs

MOSFETsIGBTs

GTOs

IGCTs

Other devices

Summary of Capabilities

Presentation summary


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Overview of electrical drives


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Converter

As the figure above suggests, a converter is a combination of power electronic

switches, inductors, capacitors, resistors (as snubbers) and transformers.


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Types of power electronic switches

Note:Most of these switches carry current only in one direction.

However in some switches (eg. MOSFET) there may be a diode to carry currentin the opposite direction.Most of the controlled switches are normally off.

However, there may be some switches (eg. Static induction transistor SIT)

that may be normally on.


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Device symbols

General symbol of a switch

Diode A K

A K

G

T2T1

G

C

E

B

TRIAC

SCR

BJT(NPN)


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Device Symbols (Continued)

D

S

GMOSFET (N- channel)C

E

GIGBT

A K

G

GTO

Note: AAnode, B- Base, C-Collector, D-Drain , G-Gate ,E-Emitter, KCathode, S-

Source, T1-Terminal 1, T2- Terminal 2


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Diodes

iD

vD

infinity

infinity

Ideal Characteristics

iD

vD

Reverse

blocking

region

Vrated

VFForward

Voltage

drop

Actual Characteristics

A K

Symbol


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Different types of diodes

1. Schottky diodes

They have low forward voltage drop ( 0.3 V).

They are used in low voltage high current circuits.

They have low reverse voltage capabilities (50100 V).

They are also very fast switching type of diodes with very less reverse

recovery time (trr).


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2. Fast/ Ultra-fast/ Hyper fast recovery diodes

These diodes have very low recovery time, typically 30ns trr 1 s.

Thus these diodes are used in high frequency circuits.

Different types of diodes (2)

3. Line frequency diodes

These diodes have larger recovery time trr 100 s.

They are available with large current/ voltage ratings (several kAs and kVs).

They are typically used for line frequency (50, 60 Hz) rectifiers.

Large on-state voltage drops up to 3 volts.


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Reverse recovery time of a diode

iD

trr(reverse recovery time)

In the circuit shown above (left) ,when S is turned on the current through D falls in

the manner shown (above, right). The reverse recovery time trr depends upon the speedat which the free electrons at the diode p-n junction is swept away.

Vbus

ID

S


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Thyristor or SCR (Silicon Controlled

Rectifier)

P1

P2

N1

N2

DC

K

A

L

O

A

D

DC

G

VG

Equivalent SCR structure

A=Anode, G=Gate, K= Cathode

A K

G

Symbol

( )


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SCR (2)They have four layer PNPN junctions.

Can be turned on by

(i) Applying +VGK(continuous or pulsed)

(ii)Light activation (very good for isolation of driver circuit from

power circuit)

(iii) Applying +VAK VBO

(iii) Applying large dVAK/dt

(iv) With high junction operating temperature.

To start conduction, SCR anode cathode current has to go above IL

(latching current) before the gate pulse can be removed. Otherwise

SCR stops conduction.

Once latched, the SCR current has to be maintained above IH

(holding current). Otherwise SCR stops conducting.

IH < IL.


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SCR Characteristics

G


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Commutation of SCRs

Commutation means stoppage of current conduction.

SCRs cannot be commutated by just removing the gate drive.

It can only be stopped from conduction by applying a negative voltage

across anode and cathode for a specified period of time (tq). This is called voltage/ line

commutation.

Any positive voltage reapplied within tq (recovery time) may cause the SCR

to start conduction again.

By forcing the current in SCR to go to zero. As current falls below holding current (IH)SCR switches off. This is called current/ load / forced commutation. This normally

requires an additional SCR, inductor and capacitor.


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Examples of SCR Commutation

(i) Example : Line Commutation (Halfwave controlled rectified DC drive)

(ii) Example : Load Commutation (Series inverter)

Note: R,L,C form an under-damped

circuit


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di/dt & dv/dt limitations for SCRsAt turn on, localized hot spots near gate connection may destroy an SCR unless di/dt

(rate of rise of anode-cathode current) is restricted. This can be achieved by including

an inductor in series with a SCR.High values of dVAK/dt can trigger SCR even in the absence of gate current.

Example:

. SCR is represented by a resistor RAKwhen it is off. Just after switch sw is closeddVAK/dt @ t =0 is equal to V/ (see below), where is the time constant. = L/RAK.Remedy: Connect Rs, Cs across SCR. Typical values of Rs, Cs (15 , 0.1 F).

V

RAK

RS CS


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Bipolar Junction Transistors (BJTs) and

Power DarlingtonsThe BJT is operated either in cut off or in saturation region.

To operate the device in saturation region, IB> IC/hFE where hFE is the DC

current gain of the device (hFE = 5 to 10 in saturation region for BJT).

Low hFE implies high base current requirements. Hence BJTs are not usedextensively for high power level switches.

Instead Darlington transistor is used reduce drive power (base current)rs are

used.

hFE of a Darlington transistor is around 75 to 100.

Typical operating frequency is between 1- 10 kHz.


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BJT and Darlington symbols and

CharacteristicsC

E

B

C

E

B B

C

E

NPN PNP Darlington

NPN CharacteristicsIdeal BJT Charatersistics

iC

vCE

iB4

iB1

iB2

iB3

vCE(sat)

IC

iB1


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Reverse or Forward Bias Safe Operating

Area

Under both transient (switch on & off) and steady state operating conditions the power

dissipation of the device has to be within the safe operating area. The limits of the safe

operating areas are imposed by

ICM (maximum collector current)

TJ max (maximum junction temperature)

Second breakdown (only in BJTs)

BVCEO (maximum collector emitter breakdown voltage)


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Second Breakdown

Second breakdown : It is the thermal runaway caused by localized hot spots

due to current crowding. This is because a BJT is a minority carrier device

having a negative temperature coefficient of resistance. Increased current will

thus cause higher temperature leading to lower resistance and hence more

current.


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Power MOSFETs

Comes with intrinsic body diode (Figure above, left) which is not very fast.

Can be replaced by a faster diode (Figure above, right) in the following way.

Recently some MOSFETs are available with fast body diode.

N Channel MOSFET

with body diode

Body diode bypassedP Channel MOSFET

with body diode

D

S

G

D

S

G

D

S

G


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MOSFET Characteristics

iD

vDS

infinity

infinity

Ideal MOSFET

Characteristics N-Channel MOSFET Characteristics

Note: BVDSS is forward breakdown voltage

iD

vGSvGS(th)

iD

vDS

vGS4

vGS3

vGS2

vGS1

vGS1< vGS2


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Other MOSFET Characteristics

Operating frequency 1-10 MHz

Rds(on) : The on state channel resistance between Drain and Source. This

is a very important parameter.

Rds(on) K.(BVDSS)2.5-2.7. Thus higher the higher the breakdown voltage

higher is Rds(on) .

Rds(on) is usually specified at room temperature. It can increase 2-3 times

once the device is heated up.

Recent development (COOLMOS) has higher voltage but lower Rds(on) .


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Insulated Gate Bipolar Transistor (IGBT)

IGBT is a hybrid device.

Has a comparable on-state conduction loss as a BJT.

Can switch faster than a BJT but is slower than a MOSFET.

Initially IGBTs used to have latch up problems due to a parasitic thyristor

(top fig. right) existing due to the nature of the IGBT construction

(M1, T1 are responsible for normal operation. T1, T2 cause latch up).

Once latched, gate control is lost. Then the only way to stop IGBT from conduction

was to commutate the device like an SCR.

D

S

G

M1

T1

T2

IGBT symbol Latch-up caused by parasitic thyristor


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Actual IGBT Characteristics

iC

vGEvGE(th)

Gate-emitter CharacteristicsCollector-emitter Characteristics


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Other Devices

1) Gate Turn-off Thyristor (GTO):

A K

G

The device has no reverse blocking capability.

The device can be turned-off by applying large negative gate current.Maximum controllable anode current: Maximum value of device current with

which the device can be successfully turned-off. This value is less than the

maximum current a GTO can carry.

2) Integrated Gate Commutated Thyristor (IGCT):

Similar to GTO with much less gate drive requirement to turn off the device.


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Other devices (2)

3) Power junction field effect transistor or JFET or static induction transistor (SIT). They

are not very popular as they are normally ON devices.

4) Field Controlled Thyristor (FCT) is also a normally ON device.

5) Mos Controlled Thyristor (MCT) failed device.

Summary of power electronic device


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Summary of power electronic device

capabilities

Ref: Mohan, Undeland and Robbins.Power Electronics, 2nd ed., 1995, John Wiley)

Note: Even though MCT (MOS Controlled Thyristor) had been predicted as the

device of the future by the arrow and the dotted box, the technology never matured.

Instead, IGCT (Insulated Gate Commutated Thyristor) technology seems more promising.

Check the following website for more details on this device:

http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/

ae8a8e1d424af97ec12576c40053ca34/$file/igct.pdf
http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/

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