AN436 APPLICATION NOTE

4/9

Figure 5. spurious firing of the triac

The influence of the transformer can be estimated by measuring the critical (di/dt)c of the triac with and

without the transformer.

Example: BTA06-400CW

The specified (di/dt)c of this triac is:

3.5A/ms min at Tj = 125°C

Measurement of a sample without transformer:

(di/dt)c = 6A/ms

Measurement with transformer:

(di/dt)c = 3A/ms

It is necessary to consider this phenomena and to take some safety margin (in some cases the critical (di/

dt)c of the triac + transformer can be lower than the specified (di/dt)c of the triac as shown in the previous

example). This is very important in the case of transient currents higher than the nominal value, as is the

case with the cold filament of incandescent lamp, load dispersion, etc...

One has to take into account the maximum (di/dt)c in the application in all cases, especially in the transient

state where (di/dt)c can be higher than it is in the steady state. The following example shows values for an

incandescent lamp and universal motor.

Table 1. Maximum (di/dt)c in the application

INCANDESCENT LAMP

UNIVERSAL MOTOR

1.35A

3.8A

STEADY STATE (di/dt)c

0.6A/ms

1.7A/ms

TRANSIENT STATE (di/dt)c

2.6A/ms

5A/ms

spurious firing

0

0

0

5A/div

50V/div

50mA/div