Film Capacitor Multi-Layer Resin Thermal Stress
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Solution Overview
Problem
Conventional film capacitors used in hybrid electric vehicle motor driving inverter circuits face issues with thermal stress-induced cracks due to mismatched coefficients of thermal expansion between potting resin and metal terminals, leading to poor humidity resistance, increased inductance, and reduced reliability under varying temperature and humidity conditions.
Innovation Solution
A film capacitor design utilizing multiple layers of epoxy resin composition, where the uppermost layer has the smallest coefficient of linear expansion, highest inorganic filler content, or highest thermal conductivity, to minimize thermal stress and enhance humidity resistance, combined with a manufacturing method involving two curing steps to optimize resin distribution and reduce cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single layer of potting resin is used to armor the film capacitor element, then the manufacturing process is simple, but thermal stress-induced cracks occur due to mismatched coefficients of thermal expansion between the resin and metal terminals
Solution Approach 1:
The potting resin is divided into multiple layers with different properties. The lower layer (first potting resin) has a coefficient of linear expansion closer to the film capacitor element, while the upper layer (second potting resin) has a coefficient closer to the metal terminal. This segmentation allows each layer to accommodate thermal expansion differences with adjacent components, preventing crack formation at interfaces during heat cycles.
Solution Approach 2:
Different regions of the resin structure are assigned different material properties tailored to their specific functional requirements. The lower layer is optimized for bonding and thermal expansion matching with the capacitor element, while the upper layer is optimized for bonding and thermal expansion matching with the metal terminal. This local differentiation of material properties resolves the thermal stress contradiction.
2Strength
If urethane resin is used to cover the film capacitor element, then the element size increases providing better protection, but repetitive stresses occur in the electrode portion increasing tanδ
Solution Approach 1:
The first potting resin layer is specifically designed to have mechanical properties that protect the film capacitor element while minimizing stress transmission to the electrode portion. This localized optimization of resin properties in the lower layer provides protection without causing repetitive stresses that would increase tanδ.
3Reliability
If inorganic filler is increased to decrease the coefficient of linear expansion of hardened matter, then heat cycle resistance is improved, but liquid viscosity becomes very high making pouring difficult and causing voids
Solution Approach 1:
The potting resin is divided into two layers with different inorganic filler contents. The lower layer has higher inorganic filler content to provide heat cycle resistance, while the upper layer has lower inorganic filler content to maintain low viscosity for easy pouring and void-free filling. This segmentation allows each layer to be optimized for its specific function.
Solution Approach 2:
Different regions of the resin structure have different inorganic filler concentrations tailored to their functional requirements. The lower layer is optimized for thermal expansion matching with high filler content, while the upper layer is optimized for pouring characteristics with lower filler content. This local differentiation resolves the contradiction between heat cycle resistance and manufacturability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively prevents resin cracks, enhances humidity resistance, and maintains low exothermic characteristics, resulting in a film capacitor with improved reliability and performance under extreme conditions.
Implementation Method 1
difference in coefficient of thermal expansion between the potting resin 24 and metal terminal 23, and the potting resin 24 may be cracked it not withstanding the stress
Implementation Method 2
blending inorganic filler by 40 to 95 vol. %, so that the difference in coefficient of linear expansion between the conductor or electric device and hardened matter of thermosetting resin for covering them directly may be 15 ppm/K or less
Implementation Method 3
a first step in which a first epoxy resin composition is poured, a second step in which the poured first epoxy resin composition is heated and cured for a specified time, a third step in which a second epoxy resin composition is poured on the first epoxy resin composition, and a fourth step in which the second epoxy resin composition is heated and cured
Data Source
AI summary
A film capacitor suited to car-mount application, excellent in heat cycle tolerance and humidity resistance, and high in productivity, while maintaining low heat generation and low inductance characteristic is provided. The film capacitor comprises a film capacitor element, a bus bar as metal terminal connected to electrode of this film capacitor element, and a case for containing them, in which the film capacitor element and bus bar are packed within the case by plural layers of epoxy resin compositions, and the plural layers of epoxy resin compositions are composed so that the coefficient of linear expansion is smallest in the composition disposed in the uppermost layer.