Can some please explain "What exactly is Derating?"
To derate is to take some operating rating and lower it to produce a safe/acceptable operating rating based upon other factors.
Take a typical SM0805 resistor. 0.125W power dissipation.
The datasheet states: 100% power rating upto 70C and then linearly de-rates to 0% at 155C
ie if you were to operate this resistor at 155C then it would only be able to dissipate 0W (ie it won't work).
Now some places then provide additional derating guidelines on top of manufacturer derating to improve product reliability.For a resistor is would include power and voltage rating
How it is related to positive and negative temperature coefficient.
In a similar way.
Where the temp coef comes into play is when you do any form of stress calculations.
1) you determine the maximum operating temperature (with some margin I would hope)
2) you work out the spread of resistance due to tolerance ( 10k 1% tol ==> 9k9 & 10k1)
3) you determine the additional variation due to temperature. THIS may increase or decrease the resistance due to it being a PTC or a NTC.
Determine the power dissipated.
Compare this to the maximum for the package including manufacture derating and any additional derating.
Derating is limited to only resistors and capacitors or it applies to ASICs also.
It covers pretty much everything (and not just electrical).
Some of the things I concider
Discrete Semi: Junction temp re-rating
Resistors: Power stress, Voltage stress (DC and transient)
Capacitor: Voltage stress, Power stress
Diodes: Power stress, blocking voltage stress, forward current
Zener: Power stress
MOSFET: Vds stress, Vgs stress, Power stress, drain current stress