On Thu, 10 Feb 2005 10:34:32 -0800, the renowned Jim Stewart
wrote:

I would have no problem designing, selling, and
standing behind a warranty on a product whose
parts are operating within the manufacturer's
specifications, even if it meant running a power
semiconductor at 80 or 100 deg C, as long as
I knew that worst-case, the value would never
be exceeded.

Triacs don't tend to have very good reliability in a fairly large
class of applications. The temperature is but a small factor (in a
decent design). Bigger factors (aside from the thermal cycling that I
already mentioned) are current and voltage surges. Ordinary fuses are
not fast enough to protect fragile semicondutors the way they are
often derated (eg. using a 10A or 8A triac at 5A). Ordinary MOVs may
be not sufficient to clamp voltage transients at non-damaging levels,
unles the semiconductors are rated at much higher than normal voltage.
They are really quite delicate compared to a hefty contact, and what's
often worse, they typically fail "on". Without massive overrating (eg.
using a 40A/1000V triac at 4A/240V, which costs a lot more money) it
may not be possible to protect them adequately at all.

It would be nice if a semiconductor manufacturer were to address the
issue by coming out with relatively inexpensive, yet rugged large-die
parts in low-cost packaging, but it's not happened yet that I know of.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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