AC08FSM datasheet(123/223 Pages) TECCOR

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Application Notes

AN1001

AN1001 - 3

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Sidac

Basic Operation

The sidac is a multi-layer silicon semiconductor switch. Figure

AN1001.7 illustrates its equivalent block construction using two

Shockley diodes connected inverse parallel. Figure AN1001.7

also shows the schematic symbol for the sidac.

Figure AN1001.7

Sidac Block Construction

The sidac operates as a bidirectional switch activated by voltage.

In the off state, the sidac exhibits leakage currents (IDRM) less

than 5 µA. As applied voltage exceeds the sidac VBO, the device

begins to enter a negative resistance switching mode with char-

acteristics similar to an avalanche diode. When supplied with

enough current (IS), the sidac switches to an on state, allowing

high current to flow. When it switches to on state, the voltage

across the device drops to less than 5 V, depending on magni-

tude of the current flow. When the sidac switches on and drops

into regeneration, it remains on as long as holding current is less

than maximum value (150 mA, typical value of 30 mA to 65 mA).

The switching current (IS) is very near the holding current (IH)

value. When the sidac switches, currents of 10 A to 100 A are

easily developed by discharging small capacitor into primary or

small, very high-voltage transformers for 10 µs to 20 µs.

The main application for sidacs is ignition circuits or inexpensive

high voltage power supplies.

Geometric Construction

Figure AN1001.8

Cross-sectional View of a Bidirectional Sidac Chip

with Multi-layer Construction

Diac

Basic Operation

The construction of a diac is similar to an open base NPN tran-

sistor. Figure AN1001.9 shows a simple block construction of a

diac and its schematic symbol.

Figure AN1001.9

Diac Block Construction

The bidirectional transistor-like structure exhibits a high-imped-

ance blocking state up to a voltage breakover point (VBO) above

which the device enters a negative-resistance region. These

basic diac characteristics produce a bidirectional pulsing oscilla-

tor in a resistor-capacitor AC circuit. Since the diac is a bidirec-

tional device, it makes a good economical trigger for firing triacs

in phase control circuits such as light dimmers and motor speed

controls. Figure AN1001.10 shows a simplified AC circuit using a

diac and a triac in a phase control application.

Figure AN1001.10

AC Phase Control Circuit

Geometric Construction

Figure AN1001.11

Cross-sectional View of Diac Chip

Equivalent Diode Relationship

Schematic Symbol

MT2

MT1

MT2

MT1

MT2

MT1

MT2

Block Construction

Schematic Symbol

Load

MT1

MT2

Cross-section of Chip

Equivalent Diode

Relationship

MT1

MT2

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