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AN1008

Application Notes

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AN1008 - 4

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Thyristor Product Catalog

IGT: DC Gate Trigger Current

SCR

IGT is the minimum DC gate current required to cause the thyris-

tor to switch from the non-conducting to the conducting state for

a specified load voltage and current as well as case temperature.

The characteristic curve illustrated in Figure AN1008.6 shows

that trigger current is temperature dependent. The thyristor

becomes less sensitive (requires more gate current) with

decreasing junction temperatures. The gate current should be

increased by a factor of two to five times the minimum threshold

DC trigger current for best operation. Where fast turn-on is

demanded and high di/dt is present or low temperatures are

expected, the gate pulse may be 10 times the minimum IGT, plus

it must be fast-rising and of sufficient duration in order to properly

turn on the thyristor.

Figure AN1008.6

Normalized DC Gate Trigger Current for All

Quadrants versus Case Temperature

Triac

The description for the SCR applies as well to the triac with the

addition that the triac can be fired in four possible modes (Figure

AN1008.7):

Quadrant I (main terminal 2 positive, gate positive)

Quadrant II (main terminal 2 positive, gate negative)

Quadrant III (main terminal 2 negative, gate negative)

Quadrant IV (main terminal 2 negative, gate positive)

Figure AN1008.7

Definition of Operating Quadrants

VGT: DC Gate Trigger Voltage

SCR

VGT is the DC gate-cathode voltage that is present just prior to

triggering when the gate current equals the DC trigger current. As

shown in the characteristic curve in Figure AN1008.8, the gate

trigger voltage is higher at lower temperatures. The gate-cathode

voltage drop can be higher than the DC trigger level if the gate is

driven by a current higher than the trigger current.

Triac

The difference in VGT for the SCR and the triac is that the triac

can be fired in four possible modes. The threshold trigger voltage

can be slightly different, depending on which of the four operating

modes is actually used.

Figure AN1008.8

Normalized DC Gate Trigger Voltage for All

Quadrants versus Case Temperature

IL: Latching Current

SCR

Latching current is the DC anode current above which the gate

signal can be withdrawn and the device stays on. It is related to,

has the same temperature dependence as, and is somewhat

greater than the DC gate trigger current. (Figure AN1008.1 and

Figure AN1008.2) Latching current is at least equal to or much

greater than the holding current, depending on the thyristor type.

Latching current is greater for fast-rise-time anode currents since

not all of the chip/die is in conduction. It is this dynamic latching

current that determines whether a device will stay on when the

gate signal is replaced with very short gate pulses. The dynamic

latching current varies with the magnitude of the gate drive cur-

rent and pulse duration. In some circuits, the anode current may

oscillate and drop back below the holding level or may even go

negative; hence, the unit may turn off and not latch if the gate sig-

nal is removed too quickly.

Triac

The description of this characteristic for the triac is the same as

for the SCR, with the addition that the triac can be latched on in

four possible modes (quadrants). Also, the required latching is

significantly different depending on which gating quadrants are

used. Figure AN1008.9 illustrates typical latching current require-

ments for the four possible quadrants of operation.

1.0

2.0

3.0

4.0

-65

-15

+65

+25

+125

-40

Case Temperature (TC) – ˚C

MT2 POSITIVE

(Positive Half Cycle)

MT2 NEGATIVE

(Negative Half Cycle)

MT1

MT2

+ I

REF

QII

MT1

GATE

MT2

REF

MT1

MT2

REF

MT1

MT2

REF

QIV

QIII

ALL POLARITIES ARE REFERENCED TO MT1

(

GATE

(+)

GT -

GATE

(

GATE

(+)

NOTE: Alternistors will not operate in Q IV

1.0

1.5

2.0

-65

-15

+65

+25

+125

-40

Case Temperature (TC) – ˚C

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