Ingenieria Electrica

AN1001
AN1001

Fundamental Characteristics of Thyristors
The connections between the two transistors trigger the occurrence of regenerative action when a proper gate signal is applied to the base of the NPN transistor. Normal leakage current is so low that the combined hFE of the specially coupled two-transistor feedback amplifier is less than unity, thus keeping the circuit in an off-statecondition. A momentary positive pulse applied to the gate biases the NPN transistor into conduction which, in turn, biases the PNP transistor into conduction. The effective hFE momentarily becomes greater than unity so that the specially coupled transistors saturate. Once saturated, current through the transistors is enough to keep the combined hFE greater than unity. The circuit remains “on” untilit is “turned off” by reducing the anode-to-cathode current (IT) so that the combined hFE is less than unity and regeneration ceases. This threshold anode current is the holding current of the SCR.

Introduction
The thyristor family of semiconductors consists of several very useful devices. The most widely used of this family are silicon controlled rectifiers (SCRs), triacs, sidacs, and diacs.In many applications these devices perform key functions and are real assets in meeting environmental, speed, and reliability specifications which their electro-mechanical counterparts cannot fulfill. This application note presents the basic fundamentals of SCR, triac, sidac, and diac thyristors so the user understands how they differ in characteristics and parameters from their electromechanicalcounterparts. Also, thyristor terminology is defined.

SCR
Basic Operation
Figure AN1001.1 shows the simple block construction of an SCR.
Anode P N Gate P N Cathode Cathode
J1 J2 J3

Geometric Construction
Figure AN1001.3 shows cross-sectional views of an SCR chip and illustrations of current flow and junction biasing in both the blocking and triggering modes.

Anode

Gate (+) IGTCathode (-) N N P (+) Anode IT Forward Blocking Junction

Cathode (-)

Gate
P

Block Construction
Figure AN1001.1

Schematic Symbol

SCR Block Construction

(+) Anode

The operation of a PNPN device can best be visualized as a specially coupled pair of transistors as shown in Figure AN1001.2.
Anode P N N J2 N P Gate N Cathode Cathode Gate P J3 N P P J1 N J2 Load Anode

ForwardBias and Current Flow

Equivalent Diode Relationship

Gate

Cathode (+) N N P (-) Anode Reverse Biased Junction Reverse Biased Gate Junction

Cathode (+)

P

(-) Anode

Reverse Bias

Two-transistor Schematic
Figure AN1001.2

Two-transistor Block Construction Equivalent
Figure AN1001.3

Equivalent Diode Relationship

Coupled Pair of Transistors as a SCR

Cross-sectionalView of SCR Chip

©2004 Littelfuse, Inc. Thyristor Product Catalog

AN1001 - 1

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AN1001

Application Notes

Triac
Basic Operation
Figure AN1001.4 shows the simple block construction of a triac. Its primary function is to control power bilaterally in an AC circuit.

Geometric Construction
Figure AN1001.6 show simplified cross-sectional viewsof a triac chip in various gating quadrants and blocking modes.

GATE(+) IGT

MT1(-)

Main Terminal 2 (MT2)

N

P
N

N

P
N

Main Terminal 1 (MT1)
Gate

N

N

P N

MT1(-)

P

N

Block Construction

MT2

IT

MT2(+)

QUADRANT I
GATE(-) IGT MT1(-)

Blocking Junction

Gate

N

N

P

MT2(+)
N

MT1

P

N

Equivalent Diode RelationshipSchematic Symbol
Figure AN1001.4 Triac Block Construction

MT2(+)

QUADRANT II

Operation of a triac can be related to two SCRs connected in parallel in opposite directions as shown in Figure AN1001.5. Although the gates are shown separately for each SCR, a triac has a single gate and can be triggered by either polarity.
MT1

GATE(-) IGT
N N

MT1(+)

P N

P

N

MT1(+)...