PTC Over-Current Protection Device Conductive Layer Thickness
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Solution Overview
Problem
Surface-mountable over-current protection devices using PTC materials face issues with resistance repeatability due to extreme expansion of the PTC polymer during temperature changes, leading to increased resistance after repeated tripping events.
Innovation Solution
The use of thicker conductive layers with low coefficient of thermal expansion (CTE) to restrict the expansion of the PTC material layer, improving resistance recovery and repeatability by creating a structural mismatch that mitigates excessive expansion and maintains initial resistance levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the PTC polymer is allowed to expand freely during tripping, then the resistance increases to suppress over-current, but the resistance cannot return to initial low resistance after repeated tripping events
Solution Approach 1:
The patent changes the physical state parameters of the polymer by introducing cross-linking to create a gel structure. This cross-linked gel structure allows the polymer to expand and contract reversibly without permanent deformation, enabling the resistance to return to its initial low state after tripping events, thus improving resistance repeatability
Solution Approach 2:
The patent creates a composite material system combining cross-linked polymer gel with conductive filler particles. The cross-linked gel matrix provides structural stability and reversible expansion, while the conductive filler maintains electrical conductivity pathways. This composite structure enables both over-current protection function and resistance recovery
2Reliability
If the conductive layer thickness is increased to restrict PTC material expansion, then resistance recovery improves, but device complexity increases
Solution Approach 1:
The patent optimizes the thickness parameter of the conductive layer to a specific range that provides sufficient mechanical constraint on the PTC material expansion while maintaining electrical conductivity. By carefully selecting this parameter, the patent achieves resistance recovery without excessive device complexity
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 reduces the resistance jump ratio (R3/Ri) to less than 1.4, ensuring superior resistance recovery and repeatability by utilizing conductive layers with sufficient thickness to manage the thermal expansion of the PTC polymer, thereby maintaining device performance across multiple tripping events.
Implementation Method 1
When the current make the device temperature increase to a temperature exceeding the phase transition temperature such as the melting point of the polymer, the polymer expands to change the crystalline state to amorphous state
Implementation Method 2
the polymer expands to change the crystalline state to amorphous state. As such, the carbon chains are broken and thus current is not allowed to pass therethrough
Implementation Method 3
The CTE of the copper foil or nickel-plated copper foil are about 17 ppm/K, and the CTE of the nickel foil is 13 ppm/K, both are much smaller than that of the PTC polymer material. At least one of the first and second conductive layers has a thickness sufficient to obtain a resistance jump R3/Ri of the over-current protection device less than 1.4
Data Source
AI summary
A surface mountable over-current protection device comprises one PTC material layer, first and second conductive layers, first and second electrodes, and an insulating layer. The PTC material layer comprises crystalline polymer and conductive filler dispersed therein. The first and second conductive layers are disposed on first and second planar surfaces of the PTC material layer, respectively. The first and second electrodes are electrically connected to the first and second conductive layers. The insulating layer is disposed between the first and the second electrodes for insulation. At the melting point of the crystalline polymer, the CTE of the crystalline polymer is greater than 100 times the CTE of the first or second conductive layer, and the first and/or second conductive layers has a thickness which is large enough to obtain a resistance jump value R3/Ri less than 1.4.