25 degree C Junction Temperature

[quaternion00]

Datasheet specifies the electrical characteristics of some IC's under the condition of 25 degree C of junction temperature. The ambient temperature is commonly 25 degree C. I'm confused why the junction temp is equal to ambient temp. How come is the junction temp didn't changed temperature while it's operating?

Thanks in advance.

[newbrain]

Oh, but it will, according to the junction-to-ambient (including a possible heatsink) thermal resistance and the dissipated power.

The 25°C is just a handy reference point for ideal conditions.

For low power stuff, it may be close to the actual operating temperature.
For high power semiconductors, it's important to check the data sheet for derating information, and the absolute maximum rating are sometimes given for Tc rather than Tj.

e.g. IRF 520, Vishay:
The maximum dissipated power is 60 W, for Tc = 25 °C and R_thJC = 2.5 K/W this gives us a maximum Tj = 25 + 60 x 2.5 = 175 °C as stated in the same max rating table.

[Gyro]

The reason that they do a 25'C Junction spec is that this is the condition the dies/devices are in when they go through manufacturing parametric test. The test is so quick that the die doesn't have time to warm up.

Higher temperature limit specs are normally done on a sample basis.

[dmills]

There is also a 'trap for young players' here in that the parameters change with temperature, in particular for switching mosfets Rds(on) rises with increasing temperature and almost a factor of 2 between 25c and Tj(max) is not at all uncommon.

This matters when doing the thermal sums.

The good stuff has a proper datasheet with curves for all this stuff (But they tend to be 'typical' values, so stay well on the conservative side of the curve).

Regards, Dan.