Thermal resistance
Thermal resistance may be a thermal property, a measure of the temperature difference at which an object or material resists the flow of warmth. Thermal resistance is that the reciprocal of thermal conductivity.
• (Absolute) thermal resistance R (Kelvin / Watt (K / W)) may be a characteristic of a specific component. for instance, the characteristics of a conductor.
• the precise thermal resistance or thermal resistance Rλ (Kelvin metric/watt (Km / W)) may be a material constant.
• The unit of warmth is Kelvin square meters per watt (m2⋅K / W) in SI units or square feet Fahrenheit (ft2⋅ ° F⋅h / Btu) in British thermal units. The thermal resistance of a unit area of the cloth. For insulation, it's measured by the R-value.
Absolute thermal resistance
Absolute thermal resistance is that the temperature difference of the whole structure when the unit thermal energy flows through the structure during a unit of time. The reciprocal of thermal conductivity. The SI unit of absolute thermal resistance is Kelvin / Watt (K / W) or equivalent Celsius / Watt (° C / W). the 2 are equivalent because they're evenly spaced: ΔT = 1 K = 1 ° C.
The thermal resistance of cloth is extremely interesting to electronic engineers because most electrical components generate heat and wish to be cooled. Since electronic components malfunction or break down when overheated, some components require daily countermeasures at the planning stage.
Analogy and nomenclature
Main article: Conflict between analogy model and Onsager
Electricians are conversant in Ohm's law and are often used as an analogy when making calculations involving thermal resistance. They often use Hooke's law as an analogy when making calculations involving thermal resistance.
Interpretation from an electronic perspective
Equivalent thermal circuits
The heat flow can be modeled according to the comparison of an electric circuit where heat flow is represented by a current, temperature is represented by a voltage, heat sources are represented by constant current sources, total thermal tolerance is a production by resistors, and thermal capabilities by producers.
Consider a component such as a silicon transistor that is attached to the metal frame of a piece of equipment. The manufacturer of the transistor specifies parameters in the datasheet called the total thermal resistance from knot to case (symbol :), and the maximum allowable temperature of the semicircle junction (symbol :). The design specification should include a maximum temperature at which the circuit should operate properly. Finally, the designer should consider how the heat from the transistor escapes to the environment: this could be by convection into the air, with or without heat sink support, or by carrying it through the printed circuit board. That is certain to be lower than above the ambient temperature. Note: THS appears to be undefined.
With the help of all this information, the designer may also build a model of the heat flow from the semicircular junction, where the heat is generated, to the outside world. In our example, the heat must flow from the junction to the case of the transistor, then from the case to the metalwork. As we are told that the metalwork will carry heat fast enough to keep the temperature below or above the temperature. environment: this is all we need to know.
Osbarr the engineer wants to know how much power can be fed into the transistor before heating. The calculations are as follows.
Total thermal resistance from junction to environment
where is the true thermal conductivity of the connection between the transistor case and the metalwork? This figure depends on the nature of the connection - for example, a thermal bonding pad or thermal transfer pad may be used to reduce the overall thermal resistance.
The maximum temperature drops from junction to environment.
We use the general principle that the temperature drops over a full thermal layer with a specific heat flow through it:
The designer now knows the maximum power that can allow the transistor to dissipate, so that they can design the circuit to limit the temperature of the transistor to a safe level.
Let’s replace some sample numbers:
(standard for silicon transistor)
(standard specification for commercial equipment)
(for standard TO-220 package [call required])
(standard value for elastomer heat transfer pad for TO-220 package [citation needed])
(standard value for heatsink for TO-220 package.
The result is then:
This means that the transistor can emit about 18 watts before overheating. A careful designer would operate the transistor at a lower power level to increase reliability.
This method can be universally applied to the incorporation of any number of layers of heat-conducting materials, simply by the addition of full thermal showers of the layers and the temperature drop over the layers.
From the Fourier Laws for heat conduction
From the Fourier Law for heat conduction, the following equation can be obtained and is valid as long as all the parameters (x and k) are constant throughout the sample.
where:
• whether the full thermal density (K / W) exceeds the thickness of the sample
• whether the sample thickness (m) (measured in parallel to the heat flow)
• whether the thermal conductivity (W / (K • m)) of the sample
• is the thermal stability (K • m / W) of the sample
• whether the cross-sectional area (m2) is perpendicular to the heat flow path.
Regarding the gradient of temperature throughout the sample and heat flux throughout the sample, the relationship is:
where:
• whether the full thermal density (K / W) exceeds the sample thickness,
• whether the sample is thick (m) (measured in a manner parallel to the heat flux),
• whether the heat flux is through the sample (W • m - 2),
• the temperature gradient (K • m - 1) throughout the sample,
• whether the temperature difference (K) is greater than the sample,
Problems with electricity parity
A 2008 review paper written by Philips Clemens researcher JM Lasance notes: “Although respiration is between the flow of heat by conduction (Fourier's law) and the flow of electric currents (Ohm's law), the corresponding physical properties of thermal and electrical conductivity the conductivity determines the conductivity of heat conduction is very different from electric current conduction. Unfortunately, although the equations of electrical and thermal difference are similar, it is erroneous to conclude that there is no tangible similarity between lightning and thermal stress " just about three orders of magnitude. The total range of thermal conduction is then equal to the difference in electrical conductivity of high-dop and low-density silicon.
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