Temperature measurements of ac operating triacs using a gate trigger current technique

A description is given for the use of the threshold gate trigger current (I
GT
) of an ac operating triac to indicate its junction temperature. Because of different triggering mechanisms, the I
GT
of the first-quadrant gating has been found to have a more consistent temperature dependence than that of third-quadrant gating. The temperature dependence of I
GT
can be described by an empirical formula of the form
I_{GT} = I_{GT_{0}} \exp [-a(T - T_{0})]
over the temperature range 25 to 125°C. The device constant a is generally of the order of 0.007 to 0.009 (°C)
^-1
. Junction temperature measurements can be made for a device during either the cooling or the heating period using the threshold gate trigger current as a temperature-sensitive parameter. Temperatures indicated by this technique are compared to surface temperatures measured directly with an infrared microradiometer. The gate trigger current has been found to be an easy parameter to measure for triacs in a self-gating circuit during transient or steady-state operation; therefore, it can provide triac users with a convenient and relatively accurate method of evaluating the thermal performance of their triacs.