FEC200 Theta Test
THETA is a THERMAL RESPONSE type of test. It is expressed in °C/W. In MAXX we abbreviate this to /W. Let's start right off with an example.
- TESTn THETA 10MA 1A <5/W T50 D100 K1900
The 10MA is IM which is the low current at which the reference cold VF is read.
The 1A is IH the heating current.
T50 is TH the heating time (50mS in this case)
D is TMD in µS. The delay from trailing edge of TH to the line when hot VF is read.
K is really 1/K in µV/°C. You need to measure this elsewhere. It is the change in VF at IM divided by the corresponding change in junction temperature.
In this example we measure and remember the VF at ambient temperature at 10mA. Then 1A is applied for 50mS. We measure and remember the VF at 1A. 100µS after the end of the 1A pulse we measure VF again at 10mA.
DVF is the difference between the two readings at 10mA. Watts is 1A times VF at 1A.
DT is the change in temperature, which is DV/K (really 1/K)
THETA is DT/Watts
The test in the example is a fairly easy one and should work well except possibly with very slow diodes, which might require a higher D.
Some combinations press against the limits of the tester, the test fixture or the diode itself. For example this common test:
- TESTn THETA 1MA 10A <2/W T10 D100 K1800
This combination will work with moderately fast diodes and with our typical 12-foot test leads tied together to minimize their inductance. The problem relates to the relatively high IH as well as the high ratio of IH/IM. Standard recovery rectifiers probably will not work with this test unless D is increased.
We encourage you to discuss any questionable spec with us and/or to evaluate it carefully as discussed below. If a test is working properly, you should be able to vary the D setting from a value 2 or 3 times higher than desired to the desired value or lower. The readings should change in a fairly gradual manner getting higher with decreasing D. If there is any abrupt change before getting down to the desired D, the reading is suspect.
The repeatability of this test is improved if you choose an IH value high enough to produce a DVF or 75 to 100mV. You can evaluate this by using the DVF test which is the same as THETA except that it reads out in DVF rather than /W.
We have a nice bench-top dedicated THETA tester, which would make a good standard against which to compare questionable readings.