The forward voltage technique to measure junction temperatures of ac operating triacs

The forward voltage drop Vfof an ac operating triac can be employed to indicate its junction temperature. The Vf, for a constant test current, is shown to decrease linearly with increasing temperature over the range of 20 to 125°C with measured slopes between -1.4 to -1.6 mV/°C. A sampling current must be used at the end of an applied power pulse and the cooling of the triac which takes place during the delay in measurement is taken into account by an extrapolation procedure. The indicated temperature using the Vfmethod is in excellent agreement With the maximum temperature measured with an infrared microradiometer at the surface of the active region of the triac. The Vftechnique requires a special power test circuit which is described. This technique provides an accurate indication of the maximum operating temperature and requires calibration of only a few triacs. Triac manufacturers can use the Vftechnique to accurately measure the thermal performance of their triacs and to screen production to eliminate marginal devices.