Alumina ceramic plate application
1, the introduction
Ceramic spacer is a material with high thermal conductivity, mainly composed of alumina (alumina content up to 96%), the appearance is pure white, hard texture, mainly used for heat transfer and electrical isolation between power devices and radiators.It is closely combined with power devices (such as power MOS tube, power transistor, etc.), aluminum radiator, PCB board, excellent sealing performance, can achieve the ideal effect of dustproof, waterproof, heat conduction, insulation, and can adapt to high temperature, high pressure, dust poor working environment, improve the safety and stability of equipment operation.This paper analyzes the performance characteristics of ceramic gasket, and discusses its design of safety gauge, technology and structure in the application of power supply products.
2. Performance characteristics of ceramic gaskets
Ceramic gaskets have the following features:
1, the ceramic gasket thermal conductivity (20℃) up to 20 W/(m-k)~30W/(m-k), far more than the ordinary thermal conductivity of the thermal conductivity of the gasket, so in the power device heat dissipation requirements are very demanding conditions have been widely used.At present, the thermal conductivity of thermal pads is mostly below 2.0 W/(m-k), and the berg Sil Pad2000 series with high thermal conductivity is only 3.5W/(m-k).
High temperature and high pressure resistance.The breakdown strength of ceramic gasket is 10kV~12kV, and the maximum allowable temperature is 1600℃. It can adapt to the bad working environment of high temperature, high pressure, high wear and strong corrosion, and meet the application requirements of power supply products in various occasions.
The service life is long.It can reduce the maintenance times of equipment and improve the safety and stability of equipment operation.
In compliance with eu ROHS environmental standards.
3. Heat dissipation path
The ordinary thermal conductive gasket is composed of soft medium, which can fill the small gap between the power device and the surface of the radiator and reduce its contact thermal resistance.Ceramic gaskets are composed of hard alumina with a certain roughness on the surface. If directly assembled, there will be a lot of gaps between power devices and ceramic gaskets and between radiators and ceramic gaskets, which will seriously affect the heat dissipation efficiency and greatly reduce the performance of radiators, and even fail to play a role.Therefore, when using ceramic gaskets as thermal conductivity materials, it is necessary to apply thermal conductivity silicon grease on the two surfaces to fill the small gap between ceramic gaskets and radiators, ceramic gaskets and power devices, and reduce the contact thermal resistance between them.
After installing ceramic gasket, the thermal resistance of power device to ambient temperature is mainly composed of thermal conductivity silicon grease, ceramic gasket, thermal conductivity silicon grease and heat sink.Its heat dissipation path is divided into two parts:
Power device (heat source) → thermal conductivity silicone grease, ceramic gasket → thermal conductivity silicone grease, radiator (heat transfer to conduction based);
The radiator → ambient air (heat transfer to convection).
Figure 1 shows the thermal resistance model and heat dissipation path of power devices.Thermal resistance factors that affect power devices mainly include surface flatness of ceramic gaskets, thickness of ceramic gaskets and heat-conducting silicon grease, thickness and shape of radiators, pressure of fasteners, etc. These factors are related to practical application conditions, so the thermal resistance between power devices and radiators will also depend on actual assembly conditions.
4. Precautions for thermal design and structural design
The commonly used types of ceramic gaskets are single, double and so on.Double ceramic gasket is mainly used for parallel installation of two power MOS tubes.Figure 5 shows the parameters and sizes of the dual ceramic gasket in PCB and structural design (take the to-247 encapsulated power MOS tube as an example).
(1) double ceramic gasket two hole distance is fixed, so in PCB design to maintain the two MOS tube center distance of 19mm;In the structural design, the opening distance of two MOS tubes on the radiator is 19mm.
(2) if the two power MOS tube power consumption is large, and the radiator installed in the bottom of the PCB, the edge of the MOS tube is not too close to the edge of the radiator, according to the results of thermal simulation analysis to determine the size of the size, otherwise the cooling effect will be greatly compromised.
5. Application of ceramic gaskets in electronic loads
An electronic load (dedicated load for high-temperature aging of module power supply) consists of 16 circuits, and the working current of each circuit is 10A (one power MOS tube is used as the electronic load for each circuit). The power consumption of each power MOS tube is up to 32W, so effective cooling measures must be taken to ensure the safe and reliable operation of the power MOS tube.Because the power consumption of the whole system is very large, four 150mmx120mm large aluminum radiators are adopted (the manufacturer cannot process a long aluminum radiators, so they are replaced by four aluminum radiators). The radiators are located at the bottom of the PCB.Power MOS tube adopts IRFP150P (to-247 package), which is installed on the bottom radiator by lying down. Ceramic gaskets with high thermal conductivity are selected as its thermal conductivity materials (ordinary thermal conductivity gaskets cannot meet the design requirements).Since the product does not need to be insulated (the voltage difference between the internal circuit and the shell is 10V~65V), the screw method is adopted to fix the power MOS tube to meet the functional insulation.
Table 1 shows the operating temperature of the electronic load at room temperature of 25℃ and air cooling.According to the data in table 1, the operating temperature of the power MOS tube is between 45.3℃ and 66.3℃.The electronic load is required to work at a high temperature of 65℃, and then the temperature rise of 40℃ is added. It can be concluded that the operating temperature of the power MOS tube is between 85.3℃ and 100.3℃, and the temperature rise of all the power MOS tube (the highest operating temperature is 160℃) is within the allowable range.It indicates that the thermal conductivity of ceramic gasket is very excellent and can meet the heat dissipation requirements of high-power power supply products.
6, the concluding
Ceramic spacer is used as thermal conductivity material between power device and radiator, which has high thermal conductivity efficiency, high temperature/high pressure resistance, uniform heating, fast heat dissipation, simple and compact structure, and has a wide application prospect in high-power power supply products.
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