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AN1002

Gating, Latching, and Holding of SCRs and Triacs

Introduction

Gating, latching, and holding currents of thyristors are some of

the most important parameters. These parameters and their

interrelationship determine whether the SCRs and triacs will

function properly in various circuit applications.

This application note describes how the SCR and triac parame-

ters are related. This knowledge helps users select best operat-

ing modes for various circuit applications.

Gating of SCRs and Triacs

Three general methods are available to switch thyristors to

on-state condition:

•

Applying proper gate signal

•

Exceeding thyristor static dv/dt characteristics

•

Exceeding voltage breakover point

This application note examines only the application of proper

gate signal. Gate signal must exceed the IGT and VGT require-

ments of the thyristor being used. IGT (gate trigger current) is the

minimum gate current required to switch a thyristor from the off

state to the on state. VGT (gate trigger voltage) is the voltage

required to produce the gate trigger current.

SCRs (unilateral devices) require a positive gate signal with

respect to the cathode polarity. Figure AN1002.1 shows the cur-

rent flow in a cross-sectional view of the SCR chip.

Figure AN1002.1

SCR Current Flow

In order for the SCR to latch on, the anode-to-cathode current (IT)

must exceed the latching current (IL) requirement. Once latched

on, the SCR remains on until it is turned off when anode-to-cath-

ode current drops below holding current (IH) requirement.

Triacs (bilateral devices) can be gated on with a gate signal of

either polarity with respect to the MT1 terminal; however, differ-

ent polarities have different requirements of IGT and VGT.

Figure AN1002.2 illustrates current flow through the triac chip in

various gating modes.

Figure AN1002.2

Triac Current Flow (Four Operating Modes)

Anode

Cathode

Gate

(+)

(-)

(+) I

IGT

Gate(+)

MT1(-)

IGT

MT2(+)

QUADRANT I

Gate(-)

MT1(-)

MT2(+)

QUADRANT II

IGT

Gate(+)

MT1(+)

IGT

QUADRANT III

Gate(-)

MT1(+)

MT2(-)

QUADRANT IV

IGT

MT2(-)

AN1002

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