I am tying to perform a thermal analysis in pspice. Resistors needs two temperature coefficients as TC1 and TC2 (if we assume linear change of R vs. T, it's zero) in pspice. But in resistor datasheets (like this) from Vishay, I've seen that TCR is for example ± 200 ppm/K. What does this ± mean? Why TCR is not a single positive or negative number? Thanks.
What does ±200 ppm/K mean?
These resistors ideally (are designed to) have a temperature coefficient of 0 (zero) meaning their value does not change at all over temperature. Of course this is impossible to produce reliably and in large quantities. So there will still be some temperature dependency.
That's what the ±200 ppm/K describes.
The manufacturer aims for a zero temperature coefficient but they cannot always make that so what they end up with is either a positive or a negative temperature coefficient.
They could make the resistors such that they always have a positive or always negative temperature coefficient but I'd rather have the "near zero" temperature coefficient instead.
If I absolutely must have a certain temperature coefficient then I'd use a PTC or NTC maybe combined with some resistors.
It means that an increase (+) of the resistor's temperature by one Kelvin (= one degree Celcius) will increase (+) it value at most 200 ppm or decrease (-) it at most by 200 ppm.
Ppm means parts per mission so 1 ppm is 1 per million so 200 ppm is 200 per million = 200 / 1000000.
Example: a 1 Mohm resistor will change +/- 200 ppm = +/- 200 ohms per Kelvin.
One degree isn't much, let's take 100 degrees then 100 * 200 ohms = 20 kohm. That means the 1 Mohm resistor will have a value change of less than +/- 20 kohm over that 100 Kelvin range.
If the resistor was exactly 1.000 Mohm at 30 degrees celcius then its value would be between 1.000 Mohm - 20 kohm = 980 kohm and 1.000 Mohm + 20 kohm = 1020 kohm when the temperature is between -20 and + 80 degrees Celcius (that's 30 C +/- 50 C).
Note the "If" at the paragraph above, that's because I ignore the initial tolerance (accuracy) of the resistor. To keep it simple I'm assuming I have an absolute accurate resistor.
Pspice is designed for simulating resistors which are on integrated circuits (remember that Spice means: Simulation Program with Integrated Circuit Emphasis). On an integrated circuit you generally cannot "mix" the materials (for making resistors) such that the temperature coefficient becomes zero. Resistors are made in a certain material which has a certain temperature coefficient and that's it.
If you're lucky there will be a different resistor type available as well made from a different material which results in a resistor with an opposite sign temperature coefficient, then you can combine the two (two resistors in series) and get a (near) zero temperature coefficient.