FEC200 Thetaz Test
The THETAZ test is intended to do THERMAL RESPONSE tests especially but not exclusively on bipolar parts. It is a reverse test of course and uses the known temperature coefficient of VZ as a delta temperature indicator.
This test is not represented to give an absolute measurement nor does it comply with any known standard. The intention is to give a relative indication of THERMAL RESPONSE compared to known good parts of the same type.
It is not necessary for the user to enter a "K FACTOR." We use a built-in table of TC vs VZ, which of course is only approximately correct. This especially true with bipolar parts since the negative temperature coefficient of the forward biased junction partially cancels the positive coefficient of the reversed biased junction.
- TESTn THETAZ 100MA 100V <10/W T10
The 100V item is the estimated VZ at the test current. It is needed to force the tester to use an appropriate VZ scale.
The combination of 100V VZ and 100MA test current will apply 10 Watts to the part and if the 10 degrees Celsius per Watt is realistic, then the delta T will be about 100 degrees. This is about 50% of the maximum power of 20W that the tester can produce and so is appropriate from the view of the tester.
This estimated delta T however could be a bit high if the apparent TC is much lower than the value in our table. In such a case the actual temperature would be higher than expected and possibly damage the diode. The TC values in our table are typical values for UNIPOLAR ZENER DIODES.
It is a good idea to use as high a current as you feel comfortable with however, in order to get a large easy to read delta VZ and also to minimize the cancellation of TC effect in bipolar parts.
The T10 in the test makes the pulse width 10mS. You can use a wider pulse if you wish and you may need to do so if necessary to allow heat to flow out through the bond that you are trying to test.
HOW THE TEST WORKS
First the part is biased with 100ľA and the voltage ramps up to the VZ at that current. Next the actual programmed test current is applied and the VZ goes up a bit to VZ for that higher current. After 1mS at the higher current, the VZ is measured again and the delta VZ us calculated.
Finally using the IZ, VZ, delta VZ and the TC from the lookup table, the apparent THERMAL RESPONSE is calculated and reported in degrees per Watt.