Application of ceramic gaskets in high-power power supply products

1. Introduction

Ceramic gasket is a material with high thermal conductivity, mainly composed of aluminum oxide (aluminum oxide content is as high as 96% or more), with pure white appearance and hard texture. It is mainly used for heat transfer and electrical isolation between power devices and heat sinks. After it is tightly combined with power devices (such as power MOS tubes, power triodes, etc.), aluminum heat sinks, and PCB boards, it has excellent sealing performance and can achieve the ideal effects of dustproof, waterproof, thermal conductivity, and insulation. It can also adapt to the harsh working environment of high temperature, high pressure, and dust, and improve the safety and stability of equipment operation. This article analyzes the performance characteristics of ceramic gaskets, and discusses its safety regulations, processes, structures, and other designs in power supply product applications. Finally, it gives its application in electronic loads (special loads for high temperature aging of module power supplies).

2. Performance characteristics of ceramic gaskets

Ceramic gaskets have the following characteristics:

⑴ The thermal conductivity of ceramic gaskets (20℃) is as high as 20 W/(mK)~30W/(mK), which is much higher than that of ordinary thermal conductive gaskets. Therefore, it has been widely used under the conditions where the heat dissipation requirements of power devices are very stringent. At present, the thermal conductivity of thermal conductive gaskets is mostly below 2.0 W/(mK), and the Bergquist Sil-Pad2000 series with a higher thermal conductivity is only 3.5W/(mK);

⑵Resistant to high temperature and high pressure. The breakdown strength of the ceramic gasket is 10kV~12kV, and the maximum allowable temperature is 1600℃. It can adapt to the harsh working environment of high temperature, high pressure, high wear and strong corrosion, and meet the application requirements of power products in various occasions;

⑶ Long service life. It can reduce the number of equipment maintenance and improve the safety and stability of equipment operation;

⑷ Comply with EU ROHS environmental protection standards.

3. Heat dissipation path

Ordinary thermally conductive gaskets are made of soft dielectric materials and can fill the small gaps between the power device and the heat sink surface to reduce their contact thermal resistance. Ceramic gaskets are made of hard aluminum oxide and have a certain degree of surface roughness. If they are directly assembled, there will be many gaps between the power device and the ceramic gasket, and between the heat sink and the ceramic gasket, which will seriously affect the heat dissipation efficiency, greatly reduce the performance of the heat sink, or even make it unable to function. Therefore, when using ceramic gaskets as thermal conductive materials, thermal conductive silicone grease also needs to be applied to their two surfaces to fill the small gaps between the ceramic gasket and the heat sink, and between the ceramic gasket and the power device, and reduce the contact thermal resistance between them.

After installing the ceramic gasket, the thermal resistance from the power device to the ambient temperature is mainly composed of the thermal resistance of the thermal grease, the thermal resistance of the ceramic gasket, the thermal resistance of the thermal grease, and the thermal resistance of the heat sink. The heat dissipation path is divided into two parts:

⑴ Power device (heat source) → thermal grease → ceramic gasket → thermal grease → radiator (heat transfer is mainly by conduction);
⑵ Radiator → ambient air (heat transfer is mainly by convection).
Figure 1 shows the thermal resistance model and heat dissipation path of the power device. The factors that affect the thermal resistance of the power device mainly include the surface flatness of the ceramic gasket, the thickness of the ceramic gasket and thermal grease, the thickness and shape of the radiator, the pressure of the fasteners, etc., and these factors are related to the actual application conditions, so the thermal resistance between the power device and the radiator will also depend on the actual assembly conditions.

Figure 1 Thermal resistance model of power device after adding ceramic gasket

[page] 4. Installation process, safety regulations and heat dissipation design

4.1. Notes on Radiator Selection

Compared with thermal conductive pads, ceramic pads have the following defects (points to note when selecting a radiator): Ceramic pads are hard, but brittle, and have poor resistance to bending and deformation. If the surface flatness of the radiator is very poor, they are easy to break during installation. Therefore, when using ceramic pads as thermal conductive elements, the manufacturer must be required to control the surface flatness of the radiator so that this indicator is within the allowable range.

4.2. Process assembly method (taking power MOS tube as an example for analysis)

The installation process of ceramic gaskets, power MOS tubes and heat sinks involves process and safety issues, which will be introduced one by one below.

4.2.1 Screw fixing method

After installing the power MOS tube, since the creepage distance between the screws and the metal part of the power MOS tube is limited, the screw fixing method can only be used in occasions of functional insulation (the heat sink is not connected to the casing ground), and cannot be used in occasions of reinforced insulation (the casing is used as a heat sink, and the safety distance requirements are larger), otherwise the safety regulations cannot meet the design requirements.

⑴ Use screws to fix the power MOS tube of TO-247 package

The power MOS tube of TO-247 package has metal only on the back heat dissipation part, and the other parts are plastic. Therefore, when fixing the power MOS tube, no special treatment is required. The ceramic gasket (both sides need to be coated with thermal conductive silicone grease) is sandwiched between the power MOS tube and the heat sink, and fixed directly with screws to meet the functional insulation requirements. As shown in Figure 2 (a) and (b), the creepage distance between the metal part of the power MOS tube and the screws is 1.3 mm ~ 1.5 mm (determined by the shape of the power MOS tube of the manufacturer).

(a) (b)
Figure 2 (a) Back view of the TO-247 packaged power MOS tube; (b) The TO-247 packaged power MOS tube is fixed with screws.

⑵ Use screws to fix the TO-220 packaged power MOS tube

When fixing the TO-220 packaged power MOS tube, a plastic pad needs to be added to the screw (as shown in Figure 3) to prevent the metal part of the power MOS tube from contacting the heat sink through the screw and causing a short circuit. After adding the plastic pad, the creepage distance between the metal part of the power MOS tube and the screw is ≥1mm. The ceramic gasket also needs to be coated with thermal grease on both sides to reduce its contact thermal resistance.

Figure 3 Screws fix the TO-220 package power MOS tube

4.2.2 Layering fixing method

When the pressure strip is used to fix the power MOS tube, it is mainly used in occasions with high safety requirements. There are two fixing methods: horizontal pressing and vertical pressing. Figure 4 (a) shows a schematic diagram of fixing the power MOS tube (TO-220 or TO-247 package) by vertical pressing, and Figure 4 (b) shows a schematic diagram of fixing the power MOS tube (TO-220 or TO-247 package) by horizontal pressing.

The following three points should be noted in process design: ① When using a pressure strip to fix the power MOS tube, choose a ceramic gasket without screw holes, otherwise the creepage distance from the power MOS tube to the heat sink will be greatly reduced; ② The size of the ceramic gasket must meet the creepage distance requirements from the power MOS tube to the heat sink; ③ Since the pressure strip is directly connected to the heat sink through screws, the pressure strip must be equipped with an insulating sleeve to increase the creepage distance from the power MOS tube to the heat sink.

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(a) (b)

Figure 4 (a) Schematic diagram of fixing the power MOS tube by vertical pressing; (b) Schematic diagram of fixing the power MOS tube by horizontal pressing.

4.3. Thermal and structural design considerations

Common types of ceramic gaskets include single-piece and double-piece. Double ceramic gaskets are mainly used for parallel installation of two power MOS tubes. Figure 5 shows the various parameters and sizes of double ceramic gaskets in PCB and structural design (taking TO-247 package power MOS tube as an example).

⑴ The distance between the two openings of the double ceramic gasket is fixed, so the center distance between the two MOS tubes should be kept at 19mm during PCB design; during structural design, the distance between the openings of the two MOS tubes on the radiator should be 19mm.

⑵ If the power consumption of the two power MOS tubes is large and the heat sink is installed at the bottom of the PCB, the edge of the MOS tube should not be too close to the edge of the heat sink. The specific size should be determined according to the results of thermal simulation analysis, otherwise the heat dissipation effect will be greatly reduced.

Figure 5 Various parameters and sizes of dual ceramic gaskets in PCB and structural design

5. Application of ceramic gaskets in electronic loads

A certain electronic load (module power supply high temperature aging special load) has 16 channels, each with a working current of 10A (each channel uses a power MOS tube as an electronic load), and the power consumption of each power MOS tube is as high as 32W. Effective heat dissipation measures must be taken to ensure that the power MOS tube works safely and reliably. Because the power consumption of the entire system is very large, four 150mmx120mm large aluminum heat sinks are used (the manufacturer cannot process a long aluminum heat sink, so four aluminum heat sinks are used instead), and the heat sink is located at the bottom of the PCB. The power MOS tube uses IRFP150P (TO-247 package), which is installed lying on the bottom shell heat sink. The thermal conductive material is a ceramic gasket with high thermal conductivity (ordinary thermal conductive gaskets cannot meet the design requirements). Because the product does not need to be reinforced with insulation (the voltage difference between the internal circuit and the shell is 10V~65V), the power MOS tube is fixed by screws to meet the functional insulation.

Table 1 shows the operating temperature of the electronic load at room temperature of 25°C and air cooling. From the data in Table 1, it can be concluded that the operating temperature of the power MOS tube is between 45.3°C and 66.3°C. The electronic load is required to work at a high temperature of 65°C, and a temperature rise of 40°C must be added. It can be concluded that the operating temperature of the power MOS tube is between 85.3°C and 100.3°C. The temperature rise of all power MOS tubes (whose maximum operating temperature is 160°C) is within the allowable range. This shows that the thermal conductivity of the ceramic gasket is very excellent and can meet the heat dissipation requirements of high-power power supply products.

Table 1 Operating temperature of electronic load at room temperature 25℃ and air cooling conditions

6. Conclusion

Ceramic gaskets are used as heat-conducting materials between power devices and heat sinks. They have high thermal conductivity, high temperature resistance/high pressure resistance, uniform heating, fast heat dissipation, and simple and compact structure. They have broad application prospects in high-power power supply products. This article analyzes the performance characteristics of ceramic gaskets, and discusses the safety regulations, process, structure and other designs in power supply product applications. Finally, the application results on electronic loads (special loads for high-temperature aging of module power supplies) are given. ■