AN1966

Transil choice for efficient TRIAC voltage protection

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product portfolio is 50 mA ( we consider only Q1 and Q3 triggering as the TRIAC is triggered

in these quadrants here).

turn on the TRIAC will be higher at low temperature. According to ST TRIAC datasheets, the

The Transil current can be higher than 115 mA (as shown in Section 4: Experimental

validation example). This phenomenon is due to the delay of the TRIAC to switch on. This

delay is typically lower than 2 µs. But during this time, even if the Transil clamps the voltage,

the line voltage is still increasing. This results in a current increase through the Transil diode.

This current is higher if the load impedance is lower, i.e. when the load power is higher. For

a 2 kW load, the current can reach several amperes (refer to Section 4: Experimental

validation example).

This figure shows that for a current pulse width shorter than 20 µs (much higher than TRIAC

Note that this figure is given for a 25 °C initial junction temperature. For a higher initial

temperature, you can use the temperature derating factor (

αT) given in Transils datasheet,

as shown on the following formula. As the Transil is not operating before the surge, the initial

temperature is the ambient temperature. As usually in industrial and appliance applications,

the highest ambient temperature is 70 °C, the supplementary voltage is around 20 V for a

BZW04-376B and so can be neglected.

given for a 1.4 A peak current (328 V). This gives a maximum voltage of 656 V which is good

Figure 6.

Clamping voltage versus peak pulse current

0.1

1

10

100

1000

1

10

100

1000

10

100

BWZ04 5V8

BWZ04 8V5

BWZ04 33

BWZ04 213

BWZ04 376

V(V)

CL

Ipp (A)

t

%Ipp

100

50

0

t r

t< 10

s

r

t

-

-

-

-

-

Tj initial = 25°C

t = 20µs

p

t = 1ms

t = 10ms

p

p

Exponential waveform: