AN303 datasheet(7/13 Pages) STMICROELECTRONICS

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AN303

Latching current – the details

Doc ID 3654 Rev 3

7/13

Figure 8.

TRIAC triggering quadrants

2.3

Variation of the latching current

2.3.1

Typical variation of IL with device sensitivity and quadrant

The latching current IL is related to the triggering current, IGT. The IL/IGT ratio also depends

on the triggering quadrant as shown in Table 1.

Example

With a BTA/BTB12-600TW, IGT (QI) = 1 mA (measured),

then IL (QI) ≈ 3 mA, IL (QII) ≈ 6 mA and IL (QIII) ≈ 3.5 mA.

With a BTA/BTB12-600C, IGT (QI) = 15 mA (measured),

then IL(Q1) ≈ 22.5 mA, IL (QII) ≈ 67.5 mA, IL (QIII) ≈ 15 mA and IL (QIV) ≈ 7.5 mA.

In the case of TRIACs (as opposed to the SCRs), it is important to note that the current IL

(QII), is much higher than the IL current in the other quadrants. So, in the data sheets, two

maximum values are specified: one value for quadrants I, III and IV (if specified) and one

value for quadrant II.

2.3.2

Typical variation between IL and IH

The holding current value IH (refer to AN302 for holding current) is linked to the latching

current value IL. For the most TRIACs (rated current lower than 40 A), the IL value is higher

than the IH value. The IL /IH ratio is related to the TRIACs current rating, as shown Table 2.

+ +

QII

QIII

QIV

Table 1.

Approximate ratio between IL and IGT for sensitive and standard TRIACs

Sensitive TRIAC 12 A rms (TW type)

3.5

N.A.

Standard TRIAC 12 A rms (C type)

1.5

4.5

0.5

I (QI)

(QI)

I (QII)

(QII)

I (QIII)

(QIII)

I (QIV)

(QIV)

Table 2.

Approximate ratio between IL and IH according to devices current rating

Rating of TRIACs and SCRs

IRMS ≤ 6 A

6 A

≤ IRMS ≤ 40 A

I L / I H

(1)

First quadrant in the case of TRIACs

1.1 to 1.5

1.5 to 3

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