Lecture-4 : Semiconductor Power Switching Devices-1

Semiconductor Power Switching Devices-1(Lecture-4)

R S Ananda Murthy

Associate Professor and HeadDepartment of Electrical & Electronics Engineering,

Sri Jayachamarajendra College of Engineering,Mysore 570 006

R S Ananda Murthy Semiconductor Power Switching Devices-1

Static i-v Characteristics of Ideal Switch

+ -

(a)

+ -(b)

ON

OFF

FullyControlled

ON

OFF

FullyControlled (c)

We use power semiconductor devices as switches inconverters.


Properties of Ideal Switch

When switch is OFF, i = 0 and −∞≤ v ≤+∞ which impliesthat PON = 0When switch is ON, v = 0 and −∞≤ i ≤+∞ which impliesthat POFF = 0It should be possible to easily turn the switch ON and OFF

by applying an appropriate control signal.The power terminals of the switch should be electricallyisolated from the control terminals.


Properties of Ideal Switch

The power required to keep the switch in a particular state,or to switch it ON/OFF should be infinitesimally small.Should be able to change state instantaneously whichimplies that tON = 0, tOFF = 0 and PSW = 0.Should be able to withstand infinite temperature whichmeans that its power handling capability is infinite, i.e.,PHmax → ∞.Should be able to withstand infinite value of di/dt duringturn ON and infinite value of dv/dt during turn OFF.


Static i-v Characteristics of Practical Switch

−V1

+V2

Limit

Power

Limit

Power

III

II I

IV

v

i

+ Ito

toOFF State close

v − axisi − axis

ON State close

− I

The switch is assumed to be bilateral while drawing this.


Classification of Power Semiconductor Devices

Bipolar voltage blocking

Pulse triggered (latched)

Continuously triggered

Unilateral

Bilateral

Semi−controlled Fully−controlledUncontrolled

Unidirectional

Power Semicondutor Switching Devices

Unipolar voltage blocking


Classification of Power Semiconductor Devices

Device Control Input Controllability Conduction Blocking

Diode Nil Uncontrolled Forward Reverse

SCR Pulse Can be turned on Forward Bipolar

LASCR Pulse Can be turned on Forward Bipolar

TRIAC Pulse Can be turned on Bilateral Bipolar

BJT Continuous Fully-controlled Forward Forward

MOSFET Continuous Fully-controlled Forward Forward

IGBT Continuous Fully-controlled Forward Forward

SITH Continuous Fully-controlled Forward Forward

GTO Pulse Fully-controlled Forward Bipolar

MCT Pulse Fully-controlled Forward Forward

SITH – Static Induction Thyristor; MCT – MOS Controlled Thyristor.


Power Losses in a Switch

t

p = vi

t

p = vi

i

Turn−on time Turn−off time

v

i

v

The total average power loss over a time interval T is

PD = PON +POFF +PG +PSW ≈ PON +PSW =1T

ˆ T

0pdt

as POFF +PG is negligible as compared to PON +PSW .


Static Characteristics Power Diodes

Reverse

leakage

current

VBR

VF( )

Forward

conduction

drop

A K

Circuit symbol

+ −

i

v

A K

Structure

p i n

i

v

i

v

Linear Approximation

i

v

Ideal Characteristics

Avalanche

breakdown

VF = Vγ + IF RF where RF is the ON-state bulk resistance.


Reverse Recovery Characteristics of Power Diodes

IR

t1 t2 t3

VF

IF

QRR

IRM

t 3

t 2SF =

VR

IRMdtdiQ

RR

QRR2

di/dttrr =

dtdi

< 0

Diode with snubber

Turn OFFstarts here

turn OFFends here

v

t

t

Area =

RMV - E

0.25 IRM

= 2

0

Reverse Recovery Charge

Reverse Recovery Timeis the time taken by theminority carriers in the diodeto recombine when the diodeis reverse biased


Important Specifications of Power Diodes

Reverse Blocking Voltage (VRRM).Forward Average Current (IF (AVE)).Forward RMS Current (IF (RMS)).Surge Current Rating (IFSM).Maximum On-state Voltage Drop (VFM).I2t Rating.Reverse Recovery Time (trr ).The maximum allowable junction and case temperatures(θJM and θCM ).Junction-to-case and case-to-sink thermal resistances(RθJC and RθCS).


Types of Power Diodes

1 Standard Recovery Diodes2 Fast Recovery Diodes3 Schottky Diodes4 Silicon Carbide Diodes.


Standard Recovery Diodes

trr ≈ 25 µs.Leakage current in the OFF-state is of the order of few µA.Have lower ON-state voltage drop.Available with ratings of several kV and kA.Typically used in rectifiers at power frequencies i.e., at 50Hz or 60 Hz.


Fast Recovery Diodes

trr ≤ 5 µsHave relatively higher ON-state voltage drop.Typically used in D.C.-to-D.C. converters and invertersoperating at higher frequencies as freewheeling diodes.


Schottky Diodes

A K

Barrier Metal

n−type silicon layer

Heavily dopedsilicon substrate

A K

Barrier Metal

n−type silicon layer

Heavily dopedsilicon substrate

n+

trr is typically around few ns.Have very low ON-state voltage drop of the order of0.15-0.45 V and consequently very low PON .Typically available in voltage ratings in the range 50-200 V.Used in circuits having very low output voltages likeswitched mode power supplies.


Silicon Carbide Diodes

These are Schottky diodes constructed from siliconcarbide.Have very low power loss.Have extremely fast switching behavior with ultra-low trr .Can operate at junction temperatures > 225◦ C.Switching behavior is independent of ON-state forwardcurrent and temperature.Have much lower reverse leakage current than siliconSchottky diodes, and higher reverse voltage rating.Very expensive as compared to Si Schottky diodes.


Some Diode Packages

Axial Pack Plastic Pack Plastic Pack

Stud Type Stud Type Disc Type

Source: www.irf.com


www.irf.com

Some Diode Packages

Stud type diodes (Source: www.china-rectifier.com)


www.china-rectifier.com

Some Diode Packages

Disc type diodes (Source: www.china-rectifier.com)



Some Diode Rectifier Modules

Single-phase Diode Bridge Module(Source: www.china-rectifier.com)



Some Diode Rectifier Modules

Three-phase Diode Bridge Module(Source: www.china-rectifier.com)



Next Lecture...

In the next lecture we will discuss some more powersemiconductor switching devices used in power electronics.

Thank You.


Lecture-4 : Semiconductor Power Switching Devices-1

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Transcript of Lecture-4 : Semiconductor Power Switching Devices-1