AN1966 datasheet(7/14 Pages) STMICROELECTRONICS

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AN1966

Transil choice for efficient TRIAC voltage protection

7/14

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

in these quadrants here).

Then, as Igt is increasing when the temperature is decreasing, the required Transil current to

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

Igt could be up to 2.25 times higher for a -40 °C junction temperature.

This gives a maximum Transil current of: Igtmax @ -40 °C = 115 mA

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).

To identify the VCL for such a current, Figure 6., given in Transil datasheets, can be used.

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

turn-on time) and a peak current lower than 1 A, the Transil VCL is around 550 V. So it is

lower than most TRIACs VDSM (700 V or 900 V).

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.

For an SMD application two SMAJ188CA-TR can be used in series. The maximum VCL has

then to be calculated as twice the value read in the data sheet graph or with the VCL value

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

even for 600 V TRIACs as ST guarantees a 700 V maximum peak voltage (refer to VDSM,

VRSM parameters).

Figure 6.

Clamping voltage versus peak pulse current

0.1

100

1000

100

1000

100

BWZ04 5V8

BWZ04 8V5

BWZ04 33

BWZ04 213

BWZ04 376

V(V)

Ipp (A)

%Ipp

100

t r

t< 10

Tj initial = 25°C

t = 20µs

t = 1ms

t = 10ms

Exponential waveform:

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