AN303 datasheet(9/13 Pages) STMICROELECTRONICS

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AN303

Latching current – the details

Doc ID 3654 Rev 3

9/13

Note:

The latching current of sensitive SCRs is always specified with a 1000 ohm gate-cathode

resistor.

Sensitive SCRs (IGT < 200 µA)

For sensitive SCRs, RGK has a large influence on the latching current as shown by

Figure 10. Thus, in certain applications, the designer may want to use a high-impedance

control circuit. The drawback will then be that the SCR sustainable dV/dt will be lower.

Standard SCRs, sensitive and standard TRIACs

RGK greater than 20 Ω will have no significant effect on the latching. But values lower than

Ω are not used in practice as they will excessively increase the current applied by the

control circuit to trigger the device.

2.3.5

Typical variation of the latching current IL with the control signal

The latching current IL varies with the amplitude and the pulse width tp of the gate current IG.

With a constant triggering pulse width (< 50 µs), the IG amplitude increase will lead to the IL

amplitude increase. And vice versa, if the IG amplitude is kept constant, the tp decrease will

lead to the IL amplitude increase (refer to Figure 11).

Figure 11.

Variation of IL with the width tp and the amplitude IG of the gate pulse

(quadrant I)

The latching current varies also according to the gate current pulse shape. The latching

current can:

●

Increase if a negative gate current is applied at the end of the pulse, as shown in

Figure 12.

●

Decrease if the decreasing slope of the gate current is low (compared to dashed line),

as shown in Figure 13. For a decreasing slope of the gate current lower than 0.5 A/µs,

the IL value is typically closed to the IH value.

IL (mA)

100

IG = 10 I GT

IG = 5 I GT

IG = 2 I GT

[pulse width]

IGT

Tj = 25 °C

tp (µs)

TRIAC BTB16-600B

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