Airbag Drive Circuit Active Region Transistor Energy Dissipation
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Solution Overview
Problem
Current air bag activation systems are costly, complex, and inefficient due to the need for external safing FETs and large high side FETs, which result in significant energy dissipation and size constraints, limiting integration and increasing the overall cost and complexity of the system.
Innovation Solution
A drive arrangement that integrates a high side switch FET, a low side switch FET, and a supply transistor, where the supply transistor operates in an active region to reduce energy dissipation and uses a blocking FET to minimize voltage drop, allowing for reduced size and increased integration while maintaining reliability and flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an external safing FET is used in series with the drive circuit, then reliability is improved through additional redundancy, but device complexity increases and manufacturing cost increases
Solution Approach 1:
The patent integrates the safing function directly into the high side FET by implementing current limiting capability within the FET itself, eliminating the need for a separate external safing FET. The high side FET is designed to limit current to approximately 2A, combining the switching and safety functions in a single component, thereby reducing device complexity while maintaining reliability.
Solution Approach 2:
The high side FET is designed to perform multiple functions: it acts as both the main switching element for activating the air bag and simultaneously serves as the safing mechanism through its current limiting capability. This multi-functional design eliminates the need for separate dedicated safing components, reducing overall circuit complexity.
2Reliability
If a large high side FET is used to handle the activation current, then reliability is improved, but the device area increases and manufacturing cost increases
Solution Approach 1:
The patent changes the operating parameters of the high side FET by implementing current limiting to approximately 2A. This parameter change allows the use of a smaller FET device area while maintaining reliable operation, as the FET is designed to operate within specific current constraints that prevent the need for oversized devices.
Solution Approach 2:
The high side FET is designed with dynamic current limiting capability that adjusts the current flow during activation. This dynamic control allows the FET to handle the activation pulse reliably while maintaining a smaller physical size, as the current is controlled and limited rather than requiring the FET to be oversized for peak current handling.
3Loss of energy
If the high side FET is designed to limit current to around 2A, then energy dissipation is reduced, but the activation pulse energy may be insufficient
Solution Approach 1:
The patent employs a periodic pulsed activation scheme where the high side FET switches on and off in controlled pulses. The current is limited to approximately 2A during these pulses, and the repeated pulsing action accumulates the necessary energy delivery to activate the air bag while keeping instantaneous power dissipation manageable and reduced compared to continuous high current operation.
Solution Approach 2:
The system performs preliminary charging of a capacitor through the high side FET before the actual activation pulse. This preliminary action stores energy in the capacitor, which is then discharged during the activation pulse. The current limiting during charging prepares the system in advance, allowing sufficient energy to be stored and delivered during the brief activation moment while keeping ongoing energy dissipation low.
Data Source
AI summary
A drive arrangement for activating a car safety device activation element, such as an air bag, comprises a drive circuit, which is coupled to the car safety device activation element. The drive circuit generates an activation signal which activates the car safety device. The arrangement includes a power supply transistor which is coupled in series with a power supply input of the drive circuit and an energy reservoir such as a capacitor. The arrangement further comprises control means which controls the supply voltage to the drive circuit by controlling the power supply transistor to operate in an active region to provide a voltage drop during activation of the car safety device activation element. Hence, a significant voltage drop and thus energy dissipation may be moved from the drive circuit to the power supply transistor. The drive circuit may therefore be reduced in size and the power supply transistor may be implemented in a cheap technology suitable for energy dissipation.