thermal_resistance

This shows you the differences between two versions of the page.

Both sides previous revision Previous revision | Previous revision | ||

thermal_resistance [2019/06/03 18:30] |
thermal_resistance [2020/07/19 01:51] (current) stan_zurek |
||
---|---|---|---|

Line 1: | Line 1: | ||

+ | ====== Thermal resistance ====== | ||

+ | |< 100% >| | ||

+ | | // | ||

+ | |||

+ | ===== Definition ===== | ||

+ | |||

+ | **Thermal resistance** (// | ||

+ | |||

+ | Thermal resistance is often denoted as // | ||

+ | |||

+ | {{page> | ||

+ | |||

+ | <WRAP 40% right> | ||

+ | <box 100% right #f0f0f0> | ||

+ | [[Thermal resistance of ferrite cores]] depends on their size and shape[(Thermal_resistance)] | ||

+ | [[file/ | ||

+ | {{page> | ||

+ | </ | ||

+ | <box 100% right #f0f0f0> | ||

+ | Experimental measurement of thermal resistance of an [[ETD44]] transformer. The nominal power loss is applied until the transformer reaches [[thermal equilibrium]]. The [[temperature rise]] (difference between steady state and the initial temperature) divided by the applied power loss, in this case 34.4°C / 3.38 W gives the thermal resistance of 10.2°C/W for this particular prototype. | ||

+ | [[file/ | ||

+ | {{page> | ||

+ | </ | ||

+ | </ | ||

+ | |||

+ | Thermal resistance depends mostly on geometry and materials involved, and not on operating conditions (e.g. shape of current waveform, or [[switching frequency]]). For [[ferrite transformer|ferrite transformers]] it is found to be independent on operating frequency.[(Goldman> | ||

+ | |||

+ | For example, if thermal resistance of a given device is // | ||

+ | |||

+ | A similar concept is [[thermal impedance]] (// | ||

+ | |||

+ | ===== Ways of deriving ===== | ||

+ | |||

+ | The value of thermal resistance for a given device depends on the ratio of its total surface area to its volume, as well as [[thermal conductivity]] of the case, access to coolant (e.g. fresh air), emissivity of the surface, etc.[(Jung-Kyun Kima, Wataru Nakayamab, Yoshimi Itoc, Sang-mo Shina, Sun-Kyu Lee, Estimation of thermal parameters of the enclosed electronic package system by using dynamic thermal response, Mechatronics, | ||

+ | |||

+ | There are several empirical equations linking the size and shape of a given class of devices. For example for [[EE (ferrite)|EE]], | ||

+ | |||

+ | | $$ R_{th} = 53 · (V_{core})^{-0.54} $$ | (K/W) or (°C/ | ||

+ | |||

+ | where: // | ||

+ | |||

+ | So for instance for [[ETD44]] the core volume[[http:// | ||

+ | |||

+ | An experimental measurement for a particular winding configuration and type of [[bobbin]]) give a value of 10.2 K/W (see also the graph), which is also quite close, but shows that experimental verification on a given prototype can differ from the literature values. | ||

+ | |||

+ | Empirical data given by [[Epcos]] also suggest that the thermal resistance is roughly proportional to the reciprocal of square root (power of -0.5) of the ferrite core volume, which is in agreement with the previous equation: | ||

+ | |||

+ | | $$ R_{th} = \frac{x}{\sqrt{V_{core}}} $$ | (K/W) or (°C/ | ||

+ | |||

+ | where $x$ is the proportionality factor. | ||

+ | |||

+ | ===== Importance ===== | ||

+ | Thermal resistance is an important parameter used for correct design of most electronic power transformers. For instance, [[enamelled wire]] has a maximum rated operating temperature, | ||

+ | Depending on the cooling conditions and size, each transformer will have a specific value of thermal resistance.[(Thermal_resistance)] | ||

+ | |||

+ | Hence, knowing the highest expected ambient temperature (e.g. // | ||

+ | |||

+ | {{page> | ||

+ | |||

+ | ===== See also ===== | ||

+ | *[[Thermal resistance of ferrite cores]] | ||

+ | *[[Thermal impedance]] | ||

+ | |||

+ | ===== References ===== | ||

+ | ~~REFNOTES~~ | ||

+ | |||

+ | {{tag> |