See also: Resistance
The thermal resistance of an element expresses its resistance to the passage of a heat flow; in the system IF, it is given in (Kelvin by Watt). This concept is valid only in stationary mode, the transitory mode calling upon the more complex concept of thermal quadripole.
Thermal resistance Rth of a plate thickness L and surface S is worth where K is the thermal Conductivité material. If the 2 faces of the plate are maintained at temperatures T1 and T2, the heat flow crossing the plate is worth (T1 - T2)/Rth.
It is here about thermal resistance (its reverse being thermal conductance) which is expressed exclusively in the solids. In this case, one speaks about purely conductive transfer. So fluids (gas or liquids) contribute to the tranfert, total thermal resistance will utilize the concept of convectif transfer.
In electronics
The semiconductor elements of power are generally assembled on cooling (or squanderers) intended to support the evacuation of the energy produced on the level of the junction Anode - Cathode for the Diode S, the Thyristor S, the Triac S, and the collecting GTO or - transmitter for the bipolar transistors and the IGBT, or drain - source for MOSFET. In this case, thermal resistance between the junction and the ambient air are a sum of three thermal resistances:
; Thermal resistance junction-case
It is given in the sheets of characteristics of the manufacturer. Here some orders of magnitude of thermal resistances according to the types of current cases:
- small cylindrical, plastic or metal cases (TO-39/TO-5, TO-92, TO-18): between 20 and 175 °C/W;
- flat, plastic intermediate cases (TO-220, TO-126): between 2 and 6 C/W;
- cases average of components of power, plastics or metal (ISOTOP, TO-247, TOP-3, TO-3): from 0,3 to 2 °C/W
- cases of modular components of power: from 0,03 to 0,5 °C/W.
; Thermal resistance case-cooler
It depends on the surface of contact between the element and the cooler and on the presence or not of an electrical insulator. The transfer of heat enters the case and the cooler is done primarily by conduction. For example for a case TO-3: without insulator, dry: 0,25 °C/W; without insulator, with silicone grease: 0,15 °C/W; with insulator mica 50 µm and lubricates silicone: 0,35 °C/W.
; Thermal resistance cooler-environment
The transfer of heat enters the cooler and the ambient air is done primarily by Convection: the ambient air comes to lick the cooler; the air heated with its contact rises, it is replaced by colder air and so on. Thermal resistance depends on the surface of the cooler, of its type (flat, with wings, etc), of its orientation (the vertical parts dissipate the calories best that the horizontal parts), of its color (the black radiates more than the brilliance). It can be decreased by forcing a circulation of air (as in the personal computers) or while making circulate water in pipes envisaged for this purpose. Thermal resistance is given by the manufacturer.
For more information detailed concerning the orders of magnitude of thermal resistances and an example of use, to see the article thermal Law of Ohm.
In the building
In the field it building one can find the definition of thermal resistance of a material (for example an insulator) intrinsic, i.e. by not taking account of exposed surface:
where:
- E is the thickness in meters
- lambda is the thermal Conductivité in W.K -1 .m -1
- R is thermal resistance in K.m 2 . W -1
Recommendations ADEME/RT 2000
- Roof S and Roof S: 4,35 (or 4,5)
- Wall S outsides: 2,5
- full ground on ground: 1,4 (or 1,5)
- ground on room not heated: 3,3
- Opening S and Glazing S: 0,4
Bioclimatic recommendations
- Roof S and Roof S: 8
- Wall S outsides: 4
- full ground on ground: 3
- ground on room not heated: 6
- Opening S and Glazing S: 0,8
See too
- thermal Conductivity
- RT 2000 and Rt 2005
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