Bo Tian

Date of Award

Spring 2023

Document Type

Open Access Dissertation


Electrical Engineering

First Advisor

Enrico Santi

Second Advisor

Chen Li


There is a widespread need for high performance wide bandgap power modules in both commercial and military applications. However, given the rapid advancements of wide bandgap power module technology, conventional cooling solutions have not kept up and do not provide the thermal management performance needed for high power density. Based on the two-phase cooling approach, two-phase microchannels operating at low fluid flow rates with low pressure drops have huge potential in enabling higher power density applications. Several studies have illustrated the potential great advantages of two-phase cooling compared to single-phase cooling in terms of maximum device temperature, spatial thermal distribution and required pumping power, but no experimental validation for a power module application has been presented.

In this study, the objective is to achieve a power module with high thermal performance with two-phase mini-channel cooling. Well known challenges are local dryouts, non-uniform temperature distribution, larger channel length causing two-phase flow instability, controlling flow rate to acquire the optimized flow boiling performance. To overcome the challenges above, the study concentrates on the design and optimization of two-phase mini-channel cooling on power module:

a) Simulation-based evaluation of performance of two-phase cooling applied to power module. This approach provides an approximate method to quantify the achievable thermal performance of two-phase cooling, which serves as a guide to subsequent experiments. Based on the simulation results and analysis, the advantages of two-phase cooling are summarized as: thermal power is increased by a factor of 3 for the same junction temperature rise; reduce difference between junction and case temperature ΔTjc increasing the number of allowable thermal cycles by more than 400X; reduction of spacing among SiC chips by a factor of 3 can lead to a 9X reduction of parasitic switching loop inductance; the 10,000 X reduction in cooling flow rate brings a huge reduction in sizes of the pump, heat exchangers and coolant piping.

b)Initial design of mini-channel coldplate. The initial design of mini-channel coldplate for power module is completed and the coldplate is fabricated to prove the feasibility of the approach. Hydrofluoroether (HFE), which has a low boiling point, is used as a dielectric coolant in test to demonstrate the advantages of two-phase cooling vs single-phase cooling. Experimental results show that up to 81% improvement in junction temperature rise and 2.41 times reduction of thermal resistance of coldplate are achieved in two-phase cooling.

c) Design improvements: slot structure, porous structure, and micro-gap. To solve these problems, the two-phase mini-channel coldplate is optimized by three solutions. The slot structure introduces a lateral flow path to avoid the formation of large vapor slugs, the maximum power dissipation increases ~115W achieved by experiment. The porous structure utilizes capillary force to hold more liquid on the surface to act as a reservoir. The porous structure has been experimentally demonstrated with a 300W increase of maximum power dissipation and a 37.5% reduction of the thermal resistance. Micro-gap structure allows rewetting flow and vapor expansion across the micro-gap, preventing slug formation. An optimized micro-gap 60 µm is achieved in the study, which makes a similar maximum power dissipation with porous structure.

d) Novel micro-gap structure. A novel micro-gap structure with reservoir channel is proposed to improve the overall thermal performance of two-phase mini-channel coldplate. The proposed micro-gap structure introduces reservoir channel allowing rewetting flow to avoid dryout. The novel design combined micro-gap and porous structure is demonstrated on a copper plate and reduced case temperature to 13 °C compared with conventional micro-gap mini-channel. An invention disclosure has been filed.

e) Integration of two-phase mini-channel structure on the module baseplate. A power module with integrated two-phase mini-channel on the baseplate is proposed, which has a significant reduction of thermal resistance by removal of baseplate and thermal interface material (TIM). Furthermore, to remove the baseplate with a function as a heat spreader, a higher HTC cooling is needed. Therefore, two-phase mini-channel is one of the best solutions in this case, which enhances the overall thermal performance of power module. The experimental results show that total thermal resistance from junction to coolant at inlet 𝑅𝑡ℎ(𝑗−𝑖𝑛) of proposed module is reduced 36.7%, which approximately matches the value from theoretical calculation. Approximately 2X increase of maximum power dissipation is achieved at 25ml/min flow rate.

f) Study on the effect of channel height on two-phase cooling performance by experiment. The channel height impact on the flow boiling performance was experimentally investigated based on heat transfer measurement and visualization via a high-speed camera. The case/wall temperature of the mini-channels was chosen as the main restraint for the testing. The experiment results are compared with a modified flow boiling model. The results show that the maximum heat flux ~70W/cm2 (heating power 1050W) is achieved in a designed channel height.