difference between various temperature parameter.

[hiren_dave]

Hi,

i am finding a opamp with good temperature parameter. i want to know the difference between following parameters.

> Thermal resistance junction to ambient
> Thermal resistance junction to case
> Operating free-air temperature range

 

[chuckey]

Thermal resistance = deg C per watt. If the device is operated in the open (not bolted to a heat sink), It will have a thermal resistance of Ra . So if the power dissipated = P, then P X Ra + Tamb < maximum junc temp. So if the Tamb is high enough, P X Ra, would be zero, so you can not dissipate any power in the device.
Thermal resistance to case = Rc, if the heat sink has a thermal resistance of Rh to the ambient air. The total thermal resistance junction to ambient is Rc + Rh. So the new equation is P X ( Rc + Rh) + Tamb < maximum junc temp.
Typical figures Ra = 10 deg C/watt, Rc = .5 deg/W, Rh = .5 -> 5
Operating temperature take the power X thermal resistance figure at one end and the limits of the encapsulation and falling off of Hfe at the low temperature end.
Frank