The forward voltage drop V f of an ac operating triac can be employed to indicate its junction temperature. The V f , 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 V f method is in excellent agreement With the maximum temperature measured with an infrared microradiometer at the surface of the active region of the triac. The V f technique 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 V f technique to accurately measure the thermal performance of their triacs and to screen production to eliminate marginal devices.

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

The forward V-I characteristics of p-n-p-n power switches usually consist of one or more voltage shifts having negative resistance characteristics at small forward current. This phenomenon is attributed to local turning on of the switch due to shorted emitter and/or shorting dots used to enhance the dv/dt capability of the device, since lateral current flow plays a significant role in determining the V-I characteristics at small current, It is shown that the threshold current responsible for the voltage shift phenomenon has an empirical temperature dependence of the form I_{T} \sim \exp [- a(T - T_{0})] over the temperature range 20-125°C. The hysteresis loop associated with each negative resistance region can be explained in terms of the difference in turn-on current and holding current of a localized area in the vicinity of shorted emitter or shorting dots.

Voltage shifts in the forward V-I characteristics of p-n-p-n power switches

In the course of determining the transient thermal impedance of power thyristors, step-like shifts are often observed in the temperature-sensitive voltage characteristic used to estimate the virtual junction temperature of the device. This correspondence describes investigations which have shown that these step-like shifts are in fact spontaneous voltage transients which are highly sensitive functions of current and temperature, and which cause small hysteresis effects in the voltage-current characteristics of the device. A simple method for adjusting the temperature sensitive voltage data to correct for such shifts is described; the method yields valid transient thermal impedance determinations in far less time than could be realized without the correction method.

P.A. Walters

Papers

Step-like shifts in the voltage—Current characteristics of power thyristors and their effect on transient thermal impedance determinations

P.A. Walters

Papers

Step-like shifts in the voltage&#8212;Current characteristics of power thyristors and their effect on transient thermal impedance determinations

Step-like shifts in the voltage—Current characteristics of power thyristors and their effect on transient thermal impedance determinations