When the power semiconductor component is working, it must produce heat loss itself. However, if the heat is too large, and it is too late to dissipate to the surrounding medium, the component will fail because it exceeds the guaranteed temperature for normal operation. Therefore, the selection of a suitable radiator is one of the important conditions for the reliable operation of components.

Concept

1, component working junction temperature Tj: that is, the maximum allowable working temperature limit of the component.
This parameter is provided by the manufacturer or mandated by the product standard.
2. Component loss power P: the average steady-state power consumption generated by the component itself during operation, defined as the product of the effective output current and the effective voltage drop.
3, dissipated power Q: the heat dissipation capacity of a specific heat dissipation structure.
4, thermal resistance R: when heat is transferred between the medium, the temperature rise generated by unit power consumption.
T/R = Δ Q

Radiator selection

Set the ambient temperature to Ta. The purpose of the configuration of the heat sink is to ensure that it can effectively conduct the heat loss of the component to the surrounding environment, and that the temperature of its heat source, that is, the node, does not exceed Tj. It is expressed by the formula
P<Q=(Tj-Ta)/R ①
(Of course, the dissipation of heat can also radiate in addition to convection. Discussed later)
The thermal resistance is mainly composed of three parts:
R=Rjc+Rcs+Rsa ②
Rjc: thermal resistance from junction to shell;
Rcs: thermal resistance from shell to radiator;
Rsa: Thermal resistance between heat sink and air.
Among them, the Rjc has a lot to do with the process level and structure of the component, which is given by the manufacturer.
Rcs is closely related to the interstitial medium (usually air) between the shell and the radiator, the roughness and flatness of the contact surface, and the installation pressure. The better the thermal conductivity of the medium, or the closer the contact, the smaller the Rcs.

(Reference value: The air-cooled installation of the convex components of our factory can generally be considered Rcs≈0.1Rjc)
Rsa is an important parameter for radiator selection. It is related to the material, the shape and surface area of the material, the volume, and the air flow rate.
Synthesize ① and ②, can be obtained
Rsa< [(Tj-Ta)/P] -Rjc-Rcs ③
The above type ③ is the basic principle of radiator selection.

General radiator manufacturers should provide the shape parameters and thermal resistance characteristic curves of specific radiator materials, according to which designers can calculate the surface area, length, weight of the required radiator, and further obtain the thermal resistance value Rsa of the radiator.

Precautions

The above theoretical analysis is a universal principle, and sufficient margin should be left in the actual design. Because the accuracy of the data provided, the installation condition from the element to the radiator, the state of air convection on the surface of the radiator, the unsteady distribution of heat, etc., are non-ideal factors that should be considered.
In addition, the heat radiation from the radiator surface to the air is also a way of heat dissipation. The anodic oxidation blackening and scouring process, which is widely used in self-cooling design, is an effective way to increase the thermal radiation. However, this method is obviously not applicable to the design requiring forced air cooling with convection conduction as the main way, because the brighter the radiator surface, the lower the thermal resistance, which is to remind the designer in particular.