Atsuo Nishihara

Packaging technologies of direct-cooled power module

A power module, which includes an insulating plate soldered to a heatsink, is called a direct-cooled power module. There are several packaging technologies to improve a direct-cooled power module in terms of not only reduction of thermal resistance but also elongation of lifetime. Among those approaches, this paper describes optimization of heatsink thickness and prevention of ratchet deformation. The calculation results show that a thicker heatsink is better to avoid liquid leakage although thicker heatsink provides shorter solder-lifetime. A heatsink with Vickers hardness of greater than 60 MPa is expected to prevent ratchet deformation under the condition of thermal cycling between -40oC and 125oC. and hardness is verified as an effective approach to produce reliable direct-cooled power modules.

A novel direct water and double-sided cooled power module and a compact inverter for electrified vehicles

Direct Water and Double-Sided Cooled Power Modules (DWDSCPM) are especially attractive due to high thermal performances. The DWDSCPM was able to reduce the thermal resistance by 50% and increase power density of the traction inverter by 70% in comparison with a conventional power module.

Advanced direct-water-cool power module having pinfin heatsink with low pressure drop and high heat transfer

In the direct-water-cooled power module, there is a small gap between the heatsink and the channel wall. This gap results in bypass flow that reduces the pressure drop while maintaining high heat transfer. In this paper, we discuss the effect of this gap on both pressure drop and heat transfer over pinfin heatsinks using our semi-analytical model based on mass, momentum, and energy conservation within two control volumes. The first control volume in the model is located within the finned area, and the second one is located in the gap between the tip of the pins and the flow channel. Dimensionless pressure drops could be predicted within an error of 30%, and the predicted Nusselt numbers agreed within an error of 50%.

Multi-Objective Optimization of Pin-Fin Heatsinks

The multi-objective optimization of pin-fin heatsinks using a Kriging approximation model is presented based on systematic experimental results. Thermal resistance and pressure drop are the objective functions in this study. Pareto solutions to the objective functions are illustrated. We derived the design rules for the diameter, height, and pitches for the uniform staggered arrays of pin-fin heatsinks by correlating the objective functions with design variables. We also analyzed the contribution of all design variables to the thermal resistance as well as the pressure drop. We found that both the thermal resistance and the pressure drop are the most sensitive to the ratio of transverse pitch to pin-fin diameter.Copyright

Distribution of heat transfer coefficients from small surfaces cooled with submerged jets of fluorocarbon liquid determined by inverse analysis of heat conduction

Convective boiling from small surfaces with submerged impingement of fluorocarbon liquid is investigated for high-performance cooling systems of electronic devices. The local heat transfer coefficients from silicon heaters in submerged impingement are measured by two independent techniques. The silicon heaters used for this measurement are constructed of many cells which can independently generate heat on one side of the heater and are cooled on the other side. Each cell also contains a sensor to measure the temperature, One of the independent techniques we use is to control the distribution of the heater power to produce a uniform temperature. With this technique, the variation of the heat flux distribution between each side of the heater caused by the heat conduction can be eliminated and the heat transfer coefficients measured more accurately. The second technique we use is inverse analysis. Inverse analysis is used to correct the effect of heat conduction in the heater and to determine the distribution of heat transfer coefficients on the cooling surface. The distribution of heat transfer coefficients determined with these techniques agreed well with the equations used to describe the convective boiling.

Non-Intrusive Case Temperature Measurement Method of Direct-Water-Cooled Power Module

In this paper, we present a non-intrusive case temperature measurement method of the direct-water-cooled power module. It uses the structure function, which in this case comprises the cumulative thermal capacitance and the cumulative thermal resistance. Since the effective heat transfer rate of the pinfin heatsink varies with the water flow rate, in this study we assumed the inflection point of the structure function corresponding to the change in the flow rate was junction-case thermal resistance. We compared numerical simulation results with experimental results and present our findings. Finally, we show that the design area in which the heat spreading angle of 45 degrees, the well-known rule of thumb, is suitable.Copyright

Design of Cost-Effective Water-Cooled Pinfin Heatsinks

This paper presents the multi-objective optimization of cost-effective pinfin heatsinks. Heat transfer rate and pressure drop were the objective functions, and four parameters (height, diameter, longitudinal pitch, and transverse pitch) were the design variables. The relationship between the objective functions and the design variables for pinfin heatsink was calculated by using our own empirical equations using the aspect ratio and the minimum gap considering cost-effectiveness and manufacturability. Non-dominated solutions to the objective functions were illustrated, and we derived the design rules for design variables. We found the similarity of the optimum trade-off curves between the heat transfer rate and the pressure drop if the multiplication of the height and the minimum gap were the same for the different constraints. We validated our calculated solutions by comparing them with experimental results and attained reasonable agreement between the calculation and experiment.Copyright

A Novel Electronics Cooling System Using Water Heat Pipes Under Freezing Conditions

A novel electronics cooling system that uses water heat pipes under an ambient temperature range from −30°C to 40°C has been developed. The system consists of several water heat pipes, air-cooled fins, and a metal block. The heat pipes are separated into two groups according to the thermal resistance of their fins. One set of heat pipes, which have fins with higher thermal resistance, operates under an ambient temperature range from −30°C to 40°C. The other set, which have lower resistance, operates from 0°C to 40°C. A prediction model based on the frozen-startup limitation of a single heat pipe was first devised and experimentally verified. Then, a prediction model for the whole-system was formulated according to the former model. The whole-system model was used to design a prototype cooling system, and it was confirmed that the prototype has a suitable cooling performance for an environmentally friendly electronics cooling system.Copyright