Temperature effect of thermal resistance Rt
For the long-term reliability of the system, the effect of the generated thermal Ploss (electrical loss) on the hot spot temperature of the system must be optimized. The hot spot temperature may be called δ T (or? T). Proper thermal management design aims to minimize δ T values to maximize system life.
The relation between the heat or loss Ploss generated per unit area of the device surface area A and the resulting temperature rise (δ T) is expressed by the total thermal resistance Rt.
Ploss/A = Q heat flux
Rt is also the sum of all the insulation layers from “inside the system” to “outside the world”.
The heat flux Q is similar to the current I in the system; At the resistor Re, the current I will produce a voltage drop V = V1-V2.
Heat equivalent: The heat flux Q will produce a temperature difference T = T1-T2 over the thermal resistance Rt.
For a given heat flux Q, the higher the thermal resistance, the higher the δ T, and therefore the higher the hot spot temperature.
Thermal resistance from material to environment typically has three modes of heat transfer:
1. Inside the material: conduction (Rcond)
2. From surface to environment: Convection (Rconv) and radiation (Rrad)
Heat conduction
The thermal resistance Rth (conduction) within the system is proportional to the average distance D from the device surface and is the inverse function of the thermal conductivity K (W/mK) and the surface area A.
Rcond= d/K.A;
For bus bars, heat is generated on the entire conductor body and then transmitted to the surface. Distance D is the combined distance from the entire busbar to each generating hot spot (= each section of the current-carrying conductor).
The higher surface area A or thermal conductivity K is, the lower Rcond of thermal resistance is.
Post time: Mar-29-2022