Optimization Tool for Direct Water Cooling System of High Power IGBT Modules Bahman, Amir Sajjad; Blaabjerg, Frede

Thermal management of power electronic devices is essential for reliable system performance especially at high power levels. Since even the most efficient electronic circuit becomes hot because of ohmic losses, it is clear that cooling is needed in electronics and even more as the power increases. One of the most important activities in the thermal management and reliability improvement is the cooling system design. As industries are developing smaller power devices with higher power densities, optimized design of cooling systems with minimum thermal resistance and pressure drop become important issue for thermal design engineers. This paper aims to present a user friendly optimization tool for direct water cooling system of a high power module which enables the cooling system designer to identify the optimized solution depending on customer load profiles and available pump power. CFD simulations are implemented to find best solution for each scenario. Introduction Power electronic devices like Insulated Gate Bipolar Transistors (IGBTs) generate heat losses up to several hundred watts during normal operation. The heat is generated in semiconductor chips due to conduction and switching losses. Although the generated losses are small portion of the total power converted, but it can result in large heat loads. Moreover, as the devices and packages become smaller, the generated losses lead to extremely large heat fluxes at chips – typically 300 W/cm2 and more [1]. Therefore, thermal management of power devices – e.g. cooling system design – becomes crucial for a reliable performance. This is more critical when designing a multi-chip power module for packaging of the power devices, because the thermal coupling effects of chips intensify the heat loads on the chips. The dominant failure mechanisms in the power devices do not only occur because of high absolute temperatures. The amplitude of temperature swings due to power cycling in normal operation may also lead to failures in a power device [2]-[4]. Cooling the high power modules with heat dissipations higher than 300 W/cm2 is beyond the capabilities of conventional air cooling systems [1]. On the other hand, liquid cooling systems surpass the air cooling systems by supplying heat transfer coefficient several orders of magnitudes higher. So, using liquid cooling system enables much higher power densities of power modules and more compact converter solutions. Today, various water cooling solutions have been introduced which generally are classified in two groups of indirect and direct liquid cooling [5]-[12]. In indirect cooling, the power module is mounted on a closed cooler, e.g. a cold plate, which is fabricated by pressed-in copper tubes in aluminum extrusions or gun drilling holes in aluminum plates. An example of which, is shown in Fig. 1 [13]. A Thermal Interface Material (TIM) is used between the power module and cold plate for higher heat spreading over the cold plate. Fig. 1: Cold plate for an IGBT module [13]. On the other hand, in direct liquid cooling systems, the coolant directly contacts the power module without any disturbing layer. A direct liquid cooling system is commonly done by various pin fin designs such as the one, which is shown in Fig. 2 that a high power IGBT module is equipped with a baseplate with pin fins [13]. In direct liquid cooling systems, the efficiency of cooling system increases by increasing the cooling contact surface and eliminating the TIM layer, thus reducing the junction-to-ambient thermal resistance [12]. Direct liquid cooling systems eliminates TIM that is traditionally needed between the power module and the cold plate. Because, the TIM layer accounts for 30%-50% of the junction to coolant thermal resistance, elimination of TIM results in an improved thermal management for the power module. Since the dominant failure mechanisms are due to thermal stresses, this will lead to a higher reliability. However, the design of power module and cooling system parameters should be calculated considering the load cycles and converter topologies. Fig. 2: A baseplate with pin fins for an IGBT module [13]. Today, commercial Computational Fluid Dynamic (CFD) simulators have facilitated the calculations for cooling system design, thus reduced the time-consuming and costly experimental tests. CFD can predict the water flow in fluid channels of cooling system in order to identify correct heat transfers and pressure drop conditions. Heat transfer rates, in turn, can be used for junction temperature calculations in dynamic operation of the power module. In this paper, a design tool for optimization of direct water cooling system is developed using Computer Aided Design (CAD) software. Various physical parameters which influence the performance of cooling system in different applications are studied and applied in the design tool. The design tool is tested by CFD for given operating conditions and performance indexes. Direct Liquid Cooling System Concept In the direct liquid system under study, which is named ShowerPower® [13], a plastic turbulator is placed in the layer between the main cooling bath tub and baseplate of the power module, as shown in Fig 3. The turbulator forces the coolant to flow along the power module baseplate in several isolated cells, and thereby supplying a homogenous cooling of a large baseplate of high power module. Actually the turbulator does not mean a turbulent liquid flow, but a laminar flow under normal flow rates of the coolant. The plastic turbulator consists of several cooling cells in the X and Y directions, which is supplied by a manifold structure on the backside for a uniform water temperature in all channels. Fig. 4 shows the plastic turbulator flow regime. The cold liquid enters the channels and flows through channel fins where heights are less than side walls. This feature provides the power module a two-step cooling: first the cold water enters the channel through the channel inlet and cools down the contact surface of the power module baseplate in the top region of the fins and channels. On the other hand, a portion of cold water flows through the lower region of the channel. The second cooling step of water occurs when the hot and cold water meet in the backside of the fins. Fig. 3: Power module and cooler [13]. Fig. 4: Plastic turbulator flow vectors and velocity magnitude contours done in CFD simulation. Considerations in the Design of Cooling System A cooling system enables more efficient heat transfer from a heat source to the adjacent fluid by using an extended surface area. The performance index that is generally used in the design of cooling systems is thermal resistance. Thermal resistance is a measurement of a temperature difference by which the material resists a heat flow and defines in the conductive heat transfer (Fig 5(a)) as: 1 2 k T T L R Q kA − = = [K/W] (1) where Rk is the thermal resistance in the conductive heat transfer, T1 and T2 are temperatures in two faces of the material, Q is the heat flux, k is the material thermal conductivity, A is the cross-sectional area, and L is the material’s thickness. The thermal resistance in the convective heat transfer (Fig. 5(b)) is defined as: 1 S k h T T R Q hA ∞ − = = [K/W] (2) where Rh is the thermal resistance in the convective heat transfer, TS is the material’s surface temperature, T∞ is the fluid temperature, Qh is the heat flux transferred by convection, h is the heat transfer coefficient and A is the surface area.