Hybrid electric vehicles (HEVs) or rechargeable electric vehicles (PEVS) usually use motor-driven power modules, such as IGBT modules, which generate very large heat, usually about 1000W to 2000W. The only effective way to cool these automotive electronic power modules is water cooling, not air cooling, because the thermal conductivity of water is 20 times that of air. Water also has high heat (energy absorption capacity), which is four times that of air.
In most automotive applications, power modules have such thermal challenges. In fact, there is a special independent cooling cycle to cool it. The heat sink is used to transfer the heat of the power module to the coolant and becomes an important part of the cooling cycle. So, how to choose the best liquid cooling fins? Making this choice requires a trade-off between various considerations, such as thermal performance, weight, cost, reliability and manufacturability. Let’s see how the heat generated by IGBT chip is conducted through coolant.
Improve thermal conductivity
Firstly, the heat must be transferred out through the bottom layer of direct copper coating (DBC), then through the module base plate, then through the hot grease, and then into the bolted heat sink (see Figure 1). This part of heat transfer is completely realized through heat conduction.
In order to improve the heat transfer, you can either choose the material with the highest thermal conductivity (k), or reduce the layer thickness, or reduce the number of layers between the heat source and the coolant. For example, aluminum nitride ceramics (k = 160W / MK) is a good choice for DBC substrate material compared with alumina (k = 25 to 35 W / MK). Compared with aluminum (k = 220 w / MK) or aluminum silicon carbide (AlSiC) (k = 170 – 180 w / MK), copper (k = 390 w / MK) material is a better choice for module substrate. In short, if you don’t choose very expensive materials, the thermal conductivity of the materials you choose will not exceed that of copper.
It can simplify the selection of heat sink materials, which is actually between aluminum, AlSiC and copper. Aluminum is light and cheap, but its thermal conductivity is average, and it is important that there is no suitable manufacturing method to obtain high surface area. Aluminum mold can not obtain high density, and it is easy to produce pores, which will lead to coolant leakage. Aluminum also has a relatively high coefficient of thermal expansion (CTE ~ 23 ppm / C), so it is not suitable for use as power module substrate material.
AlSiC is very light, but very expensive. It is processed to reduce CTE to close to IC materials. It is usually made between 9 – 12 ppm / C. Moreover, the AlSiC casting process is difficult to achieve a larger cooling surface. Moreover, the mold wears out quickly, resulting in the cost of replacing the mold. Finally, the thermal conductivity of the material is relatively poor, about k = 170W / MK. For these reasons, AlSiC has never been used as the material of bolted cooling sheet. In practice, AlSiC is selected as the substrate material only when very high reliability is required.
Copper has high thermal conductivity, acceptable cost, and most importantly, advanced molding technology can be used to form heat sinks with very dense pin fin shapes (such as amulaire nanopins). The pin fin density of copper mold can achieve 3 to 5 times the heat dissipation surface area of aluminum or AlSiC heat sink. Although the CTE of copper is relatively high (17 ppm / C), it is still successfully applied in high reliability automotive applications. As the substrate / heat sink of power module, its manufacturing process is designed based on copper substrate.
One option to optimize power module cooling is to replace the power module substrate with a heat sink. This effectively removes the two layers in the assembly (the original substrate and thermal grease), and greatly improves the heat conduction from the chip to the heat sink wall.
Once the heat is transferred to the fin wall, the cooling of the fin by the coolant depends on convective heat conduction. The basic convective heat conduction equation is:
q = h A （Tw – Tf）
Where q is the conducted heat, in watts, h is the convective heat conduction coefficient, a is the surface area of the heat sink in contact with the coolant, TW is the temperature of the heat sink wall, and TF is the temperature of the flowing liquid.
As mentioned above, the size of TW depends on the heat transmitted from the IGBT chip to the heat sink wall through the power module device, and TF is determined by other parameters of the cooling system.
More effective heat conduction path can obtain higher TW and better convective cooling of coolant. For liquid cooling, no matter what heat sink is used, the H value will be relatively small. Then it is obvious that at any specified TW, the surface area a will mainly determine the effect of liquid cooling fins.
Summary of this paper
In order to provide the best cooling solution for HEV or PEV motor drive power module, it is first necessary to ensure that the module structure, material and heat sink wall have the highest thermal conductivity. When the heat sink is used as the base plate of the power module, its performance will be much higher than that of the bolted heat sink.
Secondly, the contact area between the heat sink and the coolant is maximized, and the pressure of the liquid flow is reduced in a reasonable range. Therefore, select a heat sink with large surface area and “flat”. Rough or angular fins will increase the pressure drop, forcing the use of high-power and high-cost pumps. Generally, the best structure is a circular or oval needle wing array.
The best choice for cooling HEV or PEV power modules is those made of copper, with large surface area and small pressure reduction to replace the module substrate. This structure can achieve the best combination of performance, weight, size, reliability and cost.
When making a comparison, remember to consider all the parameters of the heat sink. As the power module substrate, high-performance copper heat sink can be directly included in the automobile engine cooling loop in most cases because of its excellent performance. For the engine drive circuit, this structure saves the cost of special cooling loops (pumps, pipes, water tanks, etc.) and reduces the size and weight.
Responsible editor: GT