Electronic – Triac Turn-on Stress Calculation


When snubbing a triac, one of the concerns is minimizing turn-on stress as your snubber triac discharges across the newly-shorted terminals of the triac. This paper says that the main concern is keeping \$ \frac{dI}{dt}\$ below the datasheet maximums during turn-on.

How exactly do I work out what \$ \frac{dI}{dt}\$ will be theoretically given a particular RC snubber?

The datasheet for my triac indicates the "turn-on time" of the triac is \$ 2 \mbox{ } \mu s\$. Is the switching speed? Is the calculation \$ \left(\frac{dI}{dt}\right)_{max}=\frac{80\% \cdot 120V √2}{47\Omega \cdot 2µs} = 1.4 \frac{A}{µs} \$ (for a 47 Ohm snubber resistor) too simplistic somehow? It is based on the assumption that the voltage across the triac terminals drops approximately linearly from 90% to 10% of the peak AC voltage in 2µs. 1.4 A/µs is a good margin under my triac's rated \$ \frac{dI}{dt}\$ maximum of 10 A/µs.

I'm doubting my reasoning because the paper I linked to above says that \$47 \mbox{ } \Omega\$ is barely enough to limit \$ \frac{dI}{dt}\$ to \$ 50\frac {A} {\mu s} \$ at turn-on (see figure 6). Am I to understand from figure 6 that the switching time of STM triacs is much less than 2µs (the STM datasheets don't have a "turn-on time" field). If I'm eyeballing figure 6 properly, it looks like the snubber discharge current peaks only ~0.1µs after it begins to rise.

Best Answer

There sure is something wrong with your reasoning. With 120Vac, the peak voltage across the snubber network will be 120V x 1.414: 169.68V. this gives you a peak current of 3.6A. (The app note you linked to starts from 230Vac. Peak voltage: 325V.) Now you have to calculate the rise time but I think it's better to measure it after you build a proto-type. But you can make a guess on where to start based on the calculations you did.

It's difficult to see the current rise time in the app. note you linked to. It looks like 25A/µS to me. You should also take into consideration that the peak voltage appearing across the triac might become higher if you're switching an inductive load like a motor for example.

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