thyristors - samex ent

SCRs -1�W. P. Robbins

Thyristors (SCRs)

William P. Robbins

Professor, Dept. of Electrical and Computer Engineering

University of Minnesota

Lecture Notes

OUTLINE

• SCR construction and I-V characteristics.

• Physical operation of SCRs.

• Switching behavior of SCRs

• dv/dt and di/dt limitations and methods of improving them.

• SCR drive circuit considerations.


cathode gate

wafer

wafer

distributed gate

cathode area (metallization not shown)

Gate and cathode metallization for slow (phase control) thyristor. Gate and cathode metallization

for fast (inverter grade) SCR

Thyristor (SCR) Geometry Gate Cathode

Anode

N N

P

N

P

P+

++

10 cm19 - 3

10 cm19 - 310 cm

19 - 3

10 cm17 - 3

10 cm17 - 3

10µ

30-100µ

50-1000µ

J

J

J

1

2

3

10 - 5x10 cm14 - 313

30-50 µ

• Cross-sectional view showing vertical orientation of

SCR.

• SCRs with kiloamp ratings have diameters of 10 cm or greater.


• SCR triggerable from forward blockingstate to on-state by a gate current pulse.

• Thyristor latches on and gate cannot turn itoff. External circuit must force SCR off.

• Current to several kiloamps for V(on) of 2-4 volts.

• Blocking voltages to 5-8 kilovolts.

• VBO = breakover voltage ; IBO =breakover current

• VH = holding voltage IH = holding current

• Maximum junction temperature = 125 °C -limited by temperature dependence ofVBO.anode

cathode

gate

VAK

+ -iA

iG

Thyristor circuit symbol.

iA

vAK

I HIBO

VHVBO

forward on-state

forward blocking state

i > 0G i = 0G-V RW

M

Thyristor I-V Characteristics


N2

P2

P1

N1

G

K

A

J1

J2

J3

(N )-

(N )+

A

G

K

Q1

Q2

J1

J2

J3

• BJTs in equivalent circuit in active region.

• Use Ebers-Moll equations for BJTs

• IC1 = -α1IE1 + ICO1 ; IC2 = -α2IE2 + ICO

• IA = IE1 ; I K = -IE2 = IA + IG

• IC1 + IB1 + IE1 = 0

• IA = α IG + ICO1 + ICO2

1 - α1 - α2

• Blocking state α1 + α2 << 1

• At breakover α1 + α2 ≈ 1

Two transistor equivalent circuit

One dimensional SCR model.

SCR Model and Equivalent Circuit


• In forward blocking state, both BJTs active.

• If α1 + α2 < 1, connection is stable.

• If VAK = VBO or if positive gate current pulse is appliedα1 + α2 becomes equal to unity and circuit connection becomesunstable and SCR switches on.

Q1

Q 2

A

KG

Thyristor Turn-on Process

+-

+ +- -

-

J 1

J 2

J3

J depletion width - no gate current

2

J depletion width - with gate current

2

Electrons injected in response to gate current flow

+

- -

p1n1

p2

2n

Holes attracted by negative charge of injected electrons

-

--

+

+

+

• Negative charge of electrons swept into n1layer partially compensate positive chargeof ionized donors exposed by growth ofdepletion of junction J2.

• Growth of depletion reduces width ofbases of Qnpn and Qpnp and thusincreases α1 and α2.

• Holes attracted by first wave of injectedelctrons attract additional electrons and soon - regenerative action.


Thyristor On-state Latchup

• Negative gate current causes lateral voltagedrops as indicated which lead to currentcrowding in center of cathode.

• Conventional SCRs (phase control) have largearea cathodes - negative gate current cannotremove stored charge from center of largecathode area.

• SCR stays latched on in spite of negative gatecurrent.

• External circuit must force anode current tonegative values in order that enough storedcharge be removed from SCR so that it canturn off.

SCR with negative gate current


Thyristor On-state Operation

• On-state: all three junctions forward biased and BJTsin equivalent circuit saturated.

• On-state stable because saturated BJTs haveα1 + α2 << 1.

• On-state voltage VAK(on) = Vj1 - Vj2 + Vj3 + Vn

N P N P2 2 1 1

G

KA

total carrierdensity

x

x

ND2

NA11

ND1

NA2


t

I o

TA

TB

TC

control

vA

vB

vC

vCvBvA

iGtDelay or trigger angle

• Time intervals that T can be onA

t

t

t

i (t)A

v (t)AK

i (t)G

td(on) t rtp s

d id t

FI o

Thyristor Turn-on Behavior

• td(on) = turn-on delay time; time required for chargeinjection by gate current to make α1 + α2 = 1.

• tr = time required for anode current to reach on-statevalue. Anode current rate-of-rise diF/dt limited byexternal inductance.

• tps = time required for plasma to spread over wholecathode area from cathode periphery near gate.

• VAK does not attain on-state value until complete areaof cathode is conducting.


• SCR turn-off quite similar to power diode turn-off.

• Anode current rate-of-fall controlled by external inductance.

• Reverse voltage overshoot caused by external inductance.

• Junction J1 is blocking junction in reverse bias. J3 has lowbreakdown voltage (20-40 volts) because of the heavy doping onboth sides of the junction.

Thyristor Turn-off Behavior

Turn-off waveforms t1

3tdidt

R

VREV

IR4

t

t

v (t)AK

i (t)A

dvdt

F

I R

t2

recovery time t > t q 3


KGiG

A

P

N

PN N

1

1

22 2

Ai

• SCR first turns on at cathode periphery nearest gate.

• Current constricted to small areas during initial phases of turn-on, td(on) and tr.

• If anode current rate-of-rise, diF/dt, not kept less than somespecified maximum, current density in constricted area will betoo large.

• Localized power dissipation too high and thermal runawaylikely.

• Use shaped gate current pulse for rapid turn-on.

Thyristor di/dt Limit at Turn-on


Thyristor Re-applied dv/dt Limits

VREV

V F

d vd t

F

t

t

v (t)AK

i (t)A

forward recovery current

• Removal of all stored charge in SCR requires aminimum time tq.

• Application of positive dVF/dt larger than a specifiedvalue before tq results in a pulse of positive anodecurrent which may produce unintentioned turn-on ofthe SCR.

• Avoidance of unintentioned turn-on requiresdVF/dt < dVF,max/dt and remaining in reverse biasfor a minimum time tq.

A

K

G

Cj2

dvFdt max

<I BOCj2

dvFdt max

100 V/µs < < 2000 V/µs

Rate effect


main thyristor

pilot thyristor

K

G

A • Use of pilot thyristor to increase turn-on gate current to main thyristor.

• Larger gate current increases amount of initial conducting area of cathode and thus improves diF/dt capabiities.

• Diode allowes negative gate current to flow from main SCR.

waferdistributed gate

cathode area (metallization not shown)

• Interdigitated gate-cathode structure used to greatly increase gate-cathode periphery.

• Distance from periphery to center of any cathode region significantly shortened.

• Ability of negative gate current to break latching condition in on-state increased.

• Combination of pilot thyristor, diode, and iterdigitated gate-cathode geometry tgermed a gate-assisted turn-off thyristor or GATT

Methods of Improving Thyristor di/dt Rating


A

K

GCj2

dv

dtC j2 AK

J3

Cathode shorting structure

Improvement in dv/dt Rating Via Cathode Shorts

• Current thru Cj2 indistinguishable from positive gate current with respect to turn-on of SCR.

• If current thru Cj2 bypasses junction J3, then SCR will not be turned on by the large displacement currents.

• Cathode shorts provide this desirable bypass. Most effective withinterdigitaated gate-cathode geometry.

•dVF

dt max significantly increased.

N+N+N+N+

P

N -

P+

K Gcathodeshort

A


Thyristor Gate Trigger Requirements

IG

VGK

VGG

VGGRG

minimumtemperature

maximumtemperature

minimumtrigger current

IG1

IG2

maximum gate power dissipation

trigger circuit load line Equivalent circuit of

SCR drive circuit

thyristors - samex ent

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