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Application Notes

AN1001

AN1001 - 5

http://www.teccor.com

Thyristor Product Catalog

+1 972-580-7777

When voltage is impressed suddenly across a PN junction, a

charging current flows, equal to:

When

becomes greater or equal to thyristor IGT,

the thyristor switches on. Normally, this type of turn-on does not

damage the device, providing the surge current is limited.

Generally, thyristor application circuits are designed with static

dv/dt snubber networks if fast-rising voltages are anticipated.

Voltage Breakover Turn-on

This method is used to switch on sidacs and diacs. However,

exceeding voltage breakover of SCRs and triacs is definitely not

recommended as a turn-on method.

In the case of SCRs and triacs, leakage current increases until it

exceeds the gate current required to turn on these gated thyris-

tors in a small localized point. When turn-on occurs by this

method, localized heating in a small area may melt the silicon or

damage the device if di/dt of the increasing current is not suffi-

ciently limited.

Diacs used in typical phase control circuits are basically pro-

tected against excessive current at breakover as long as the fir-

ing capacitor is not excessively large. When diacs are used in a

zener function, current limiting is necessary.

Sidacs are typically pulse-firing, high-voltage transformers and

are current limited by the transformer primary. The sidac should

be operated so peak current amplitude, current duration, and

di/dt limits are not exceeded.

Triac Gating Modes Of Operation

Triacs can be gated in four basic gating modes as shown in

Figure AN1001.17.

Figure AN1001.17

Gating Modes

The most common quadrants for triac gating-on are Quadrants I

and III, where the gate supply is synchronized with the main ter-

minal supply (gate positive — MT2 positive, gate negative —

MT2 negative). Gate sensitivity of triacs is most optimum in

Quadrants I and III due to the inherent thyristor chip construction.

If Quadrants I and III cannot be used, the next best operating

modes are Quadrants II and III where the gate has a negative

polarity supply with an AC main terminal supply. Typically, Quad-

rant II is approximately equal in gate sensitivity to Quadrant I;

however, latching current sensitivity in Quadrant II is lowest.

Therefore, it is difficult for triacs to latch on in Quadrant II when

the main terminal current supply is very low in value.

Special consideration should be given to gating circuit design

when Quadrants I and IV are used in actual application, because

Quadrant IV has the lowest gate sensitivity of all four operating

quadrants.

General Terminology

The following definitions of the most widely-used thyristor terms,

symbols, and definitions conform to existing EIA-JEDEC stan-

dards:

Breakover Point – Any point on the principal voltage-current

characteristic for which the differential resistance is zero and

where the principal voltage reaches a maximum value

Principal Current – Generic term for the current through the col-

lector junction (the current through main terminal 1 and main ter-

minal 2 of a triac or anode and cathode of an SCR)

Principal Voltage – Voltage between the main terminals:

(1) In the case of reverse blocking thyristors, the principal volt-

age is called positive when the anode potential is higher than

the cathode potential and negative when the anode potential

is lower than the cathode potential.

(2) For bidirectional thyristors, the principal voltage is called

positive when the potential of main terminal 2 is higher than

the potential of main terminal 1.

Off State – Condition of the thyristor corresponding to the high-

resistance, low-current portion of the principal voltage-current

characteristic between the origin and the breakover point(s) in

the switching quadrant(s)

On State – Condition of the thyristor corresponding to the low-

resistance, low-voltage portion of the principal voltage-current

characteristic in the switching quadrant(s).

Specific Terminology

Average Gate Power Dissipation [PG(AV)] – Value of gate power

which may be dissipated between the gate and main terminal 1

(or cathode) averaged over a full cycle

Breakover Current (IBO) – Principal current at the breakover

point

Breakover Voltage (VBO) – Principal voltage at the breakover

point

Circuit-commutated Turn-off Time (tq) – Time interval between

the instant when the principal current has decreased to zero after

external switching of the principal voltage circuit and the instant

when the thyristor is capable of supporting a specified principal

voltage without turning on

Critical Rate-of-rise of Commutation Voltage of a Triac

(Commutating dv/dt) – Minimum value of the rate-of-rise of prin-

cipal voltage which will cause switching from the off state to the

on state immediately following on-state current conduction in the

opposite quadrant

------

èø

æö

------

èø

æö

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

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