Solid State Switch Parallel Transistors Fuses Fail-Safe
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Solution Overview
Problem
Conventional transistor switches can remain in an activated state if they fail, leading to potential damage to connected loads, such as starting motors, due to the lack of a fail-safe mechanism.
Innovation Solution
A solid-state switch configuration using multiple transistors connected in parallel with fuses, where each fuse is connected in series with a separate transistor, allowing for a fail-safe mechanism that prevents the switch from remaining activated when a transistor fails, utilizing a combination of current ratings to ensure continued operation even after transistor failure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single transistor is used as a switch, then the device complexity is low, but the reliability deteriorates because the transistor may fail in an asserted state and remain stuck on
Solution Approach 1:
The patent divides a single transistor switch into multiple parallel transistor branches (e.g., three transistors Q1, Q2, Q3 with respective fuses F1, F2, F3). Each transistor-fuse combination forms an independent parallel path. This segmentation ensures that if one transistor fails in the asserted state, the other parallel paths can still be controlled to turn off, preventing the load from remaining continuously activated.
Solution Approach 2:
The patent incorporates fuses in series with each transistor as a preventive measure against transistor failure. The fuses are sized such that their combined current rating exceeds the load current, allowing normal operation with multiple transistors sharing the load. However, if a transistor fails in the asserted state, the corresponding fuse will blow under overload conditions, automatically isolating the failed transistor and preventing it from causing continuous activation of the load.
2Reliability
If multiple transistors are used in parallel to improve reliability, then the fail-safe mechanism improves, but the device complexity increases
Solution Approach 1:
The switch is segmented into multiple identical parallel modules, each consisting of a transistor and a fuse in series. This modular segmentation provides redundancy and fail-safe capability while maintaining a relatively simple and systematic structure that is easier to analyze and implement compared to more complex redundant architectures.
Solution Approach 2:
Each parallel transistor-fuse branch serves multiple functions: during normal operation, multiple transistors share the load current to reduce individual stress and heat generation; during failure conditions, the parallel structure ensures that the failure of one transistor does not cause complete switch failure, and the fuse provides automatic protection. This multi-functionality achieves high reliability without proportionally increasing complexity.
3Reliability
If fuses are added in series with each transistor, then the fail-safe mechanism improves, but the manufacturing complexity increases
Solution Approach 1:
The switch is constructed as multiple identical parallel segments, each containing a transistor and a fuse in series. This segmentation into standardized modules simplifies manufacturing and assembly, as each module can be designed and tested independently, and the same design pattern is repeated for each parallel branch.
Solution Approach 2:
The fuses are selected with specific current ratings that are proportional to the expected current distribution among the parallel transistors. By carefully selecting fuse parameters (current ratings) and transistor parameters (on-resistances), the system achieves automatic current sharing and failure protection. The fuse parameters are chosen such that during normal operation, no single fuse carries excessive current, but if a transistor fails, the corresponding fuse will blow to isolate the failure.
Data Source
AI summary
A solid-state switch includes a plurality of transistors connected in parallel and a plurality of fuses. Each fuse is connected in series with a separate one of the plurality of transistors. A combination of current ratings of the plurality of fuses is greater than a load current of a load connected to the plurality of transistors and the plurality of fuses, and a current rating of each separate fuse of the plurality of fuses is less than the load current.