Download - Lecture-5 : Semiconductor Power Switching Devices-2

Semiconductor Power Switching Devices-2(Lecture-5)

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-2

Silicon Controlled Rectifier (SCR)

p+

p

Gate (G)Cathode (K)

Anode (A)

n-

J1

J2n+

J3

Structure of SCR

p+ indicates heavily doped p-type regionn- indicates lightly doped n-type regionn+ indicates heavily doped n-type region

A

KG

Circuit Symbolof SCR

ON

OFF

= Latching Current= Holding Current

ReverseLeakageCurrent

ForwardLeakageCurrent

Thicker n- layer gives higher voltage blocking capability tothe device.The forward voltage at which the device turns ondecreases with increase in gate current.


Control Characteristics of SCR

vo

iG

vo

+

−

iG

V

t

R

−

+

V

t

SCR is semicontrolled and pulse triggered.


Two Transistor Model of SCR

p1

p2

n1

n2

p1

n1

2p

n2

p2

1n

T1

T2

+

−

Gate

Control

Circuit

IG

+

−

IK

IA

α1

IC 1

IC 2

α2 T2

IB 1

IB 2

T1

K

A

K

J1

J2

J3G

R

V

A

K

G

G V

RA

1.0

0.8

0.6

0.4

0.2

00 (mA)1

α

IE

(a) (b)

(c) (d)


Conditions for Turn ON and Turn OFF of SCR

In Figure-(c) of previous slide, by applying KCL to the device weget

IK = IA + IG and IA = IC1 + IC2 (1)

Considering each transistor we have

IC1 = α1IA + ICBO1 and IC2 = α2IK + ICBO2 (2)

Substituting Eq.(2) in (1) and by rearranging terms we get

IA =α2IG +(ICBO1 + ICBO2)

[1− (α1 +α2)](3)

When (α1 +α2)<< 1 the device is OFF as IA ≈ (ICBO1 + ICBO2).When (α1 +α2)→ 1 the device turns ON since IA→ ∞.


Factors Causing Turn ON of SCR

Forward voltage vAK > VBO. This should be avoided sinceit may permanently damage the device.Rise in device temperature can cause unwanted turn ON

and hence should be avoided by cooling the device.By injecting positive gate current IG until IA = IL where IL isthe latching value. This is the preferred method of turningON the device.Forward dvAK/dt > rated value causes undesirable turnON and should prevented by connecting a snubber circuitacross the SCR.Light radiation of specific wavelength incident on junctionsof SCR turns ON the device. LASCR’s are turned ON bythis method.


How to Turn Off SCR?

Gate current has no control over the SCR after it turns ON.IA should be reduced below the holding value IH in order tomake (α1 +α2)→ 0 to stop the internal regenerative actionto turn OFF the device.After IA drops to zero, the device should be reverse biasedfor a duration tq > tOFF where tOFF is known as the deviceturn OFF time and tq is known as the circuit turn-off time.


Gate Characteristics of SCR

Load Line

Equation for load line A

K

G

A

B Trigger Source

SCR

The VG-IG plot of SCR is similar to that of a diode.The VG-IG characteristics of individual SCRs belonging toa family will have a spread between A and B as shown.


Reverse Voltage Protection for Gate

A

K

G

Trigger Source

SCR

Connecting resitance between gate andcathode --- Increases dv/dt capability,lowers the turn off time of SCR,minimizes reverse voltage across the device,and increases holding and latching current.

(a) (b)

(c)

A

K

G

Trigger Source

SCR

A

K

G

Trigger Source

SCR


SCR Turn ON Characteristics

(a)

(b)

(c)

tON increases with increase in the inductance of the load.


SCR Turn OFF Characteristics

(a)

(b)

(c)

tq should be > tOFF and dvAK/dt should be less than ratedvalue for proper turn off of SCR.


Snubber Circuit for SCR

When SCR is turned ON di/dt throughSCR is limited to safe value byconnecting this inductor.

When SCR is turned ONthis resistor limits the capacitordischarging current throughthe SCR to safe value.

When SCR is turned OFF and forwardvoltage appears across the SCRD1 bypasses R2 and dv/dt acrossSCR is limited to safe value bythis R1C1 network.

A

K

SCR

G


Letter Symbols used to Specify Ratings of SCR

A – Anode, ambient J – Junction(AV) – Average K – Cathode(BO) – Breakover M, m – Maximum(BR) – Breakdown Q, q – Turn off

D – Off state or non-trigger R, r – Reverse, repetitived – Delay time (RMS) – RMS value

F, f – Forward, falltime rr – Reverse recoveryH – Holding S – surge, nonrepetitive

G, g – Gate terminal T, t – On-state, trigger

Examples:

VRRM – Reverse repetitive maximum voltage.

ITSM – On-state maximum surge current.

VGT – Gate voltage required for triggering.

IT (RMS)– On-state RMS current.


Specifications of BT145 Thyristor Series


Disc Type of SCRs

(Source: www.china-rectifer.com)


www.china-rectifer.com

Disc Type of SCR

(Source: www.abb.com)


www.abb.com

Stud Type of SCRs

(Source: www.china-rectifer.com)


www.china-rectifer.com

SCR Power Module

(Source: saishe.en.made-in-china.com)


saishe.en.made-in-china.com

SCR Power Module

(Source: http://theelectrostore.com)


http://theelectrostore.com

Specifications of SKKT 210/20E Module


Next Lecture...

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

Thank You.
