# 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.

• Good catch: I now know that "x VAC" means "AC with $V_{rms} = x$". However, this still leaves my $\frac{dI}{dt}$ unexpectedly well below the max $\frac{dV}{dt}$ of my triac. I'm still worried about the discrepancy between this result and the app note's assertion that $47 \Omega$ is barely enough to limit $\frac{dI}{dt}$ to $50 \frac{A}{\mu s}$. – Isaac Sutherland Jun 3 '11 at 0:39
• Furthermore, I'm trying to design this circuit to be robust to different loads (varying inductance and resistance) so I still need to understand the theoretical calculation for peak $\frac{dI}{dt}$. – Isaac Sutherland Jun 3 '11 at 0:43
• The $\frac{dI}{dt}$ contribution of my inductive load is trivial (on the order of 0.0012 A/µS). So, basically it boils down to the question: Can I assume a linear drop from 90% to 10% voltage (between the triac terminals) in 2µS? i.e. Is "switching speed" equivalent to the "turn-on time" in the datasheet? – Isaac Sutherland Jun 3 '11 at 17:23