Electronic Circuit Protection Device with I2t Function
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Solution Overview
Problem
Conventional circuit protection devices for automotive electrical systems are inadequate in providing both instantaneous short circuit protection and time-delayed tripping to accommodate transient inrush currents, and they are often inconvenient and costly to implement.
Innovation Solution
A circuit protective device that includes a current sensing circuit, an overcurrent sensor, a function generator circuit to generate a signal based on the I2t relationship, and a control circuit to provide timed shutdown for overcurrent conditions, while enabling instantaneous shutdown for short circuits, using a piece-wise linear approximation circuit and latching control for power switch disablement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional fuses are used for wiring protection, then instantaneous tripping is provided for high fault currents, but time-delayed tripping capability is insufficient for applications like headlights and motors with high inrush currents
Solution Approach 1:
The protection device segments the protection response into two distinct pathways: an instantaneous trip pathway for high fault currents (above 1000% of rating) and a timed trip pathway for lower overcurrents (500-1000% of rating). This is achieved through separate sensing circuits and control logic that evaluate current magnitude and duration independently, allowing the device to adapt its response based on the specific fault condition.
Solution Approach 2:
The device dynamically adjusts the trip threshold based on the duration of the overcurrent condition. For brief inrush currents, the device tolerates higher current levels for short periods. For sustained overcurrents, the trip threshold decreases over time. This dynamic behavior is implemented through timing circuits and control logic that modify the effective trip point based on how long the overcurrent has persisted.
2Reliability
If dual element fuses or thermal-magnetic circuit breakers are used to provide both instantaneous and time-delayed protection, then both protective functions are achieved, but device size and installation complexity increase
Solution Approach 1:
The invention replaces mechanical trip mechanisms with electronic sensing and control circuits. Instead of using thermal elements and magnetic releases that require physical space and mechanical linkage, the device uses electronic current sensors, timing circuits, and semiconductor switches to achieve the same protection functions. This electronic approach dramatically reduces device size and simplifies installation while maintaining dual protective capabilities.
Solution Approach 2:
The protection device is designed as a universal module that can protect multiple circuits or loads through a single unit. The electronic architecture allows the same device to provide both instantaneous and timed protection for various types of loads (lights, motors, heaters) without requiring different fuse types or additional circuit breakers. This multi-functionality reduces the overall number of protection devices needed in the system.
3Reliability
If conventional fuses are used in automotive applications, then wiring protection is provided, but replacement is inconvenient due to difficult-to-reach locations
Solution Approach 1:
The protection device incorporates reset functionality that can be performed by the operator without requiring technical expertise or special tools. The electronic reset mechanism is designed to be user-friendly, allowing drivers or technicians to simply press a button or turn a knob to restore protection after a trip condition is resolved. This eliminates the need for fuse replacement procedures and makes the device self-serviceable.
4Adaptability or versatility
If the protection current is set at a high level for a given time after power switch is turned on, then transient inrush currents are accommodated, but protection current becomes too low for sustained operation
Solution Approach 1:
The device implements periodic evaluation of the current condition with different thresholds for different time periods. During the initial period after power-on, the device uses a higher current threshold to accommodate inrush. After this initial period expires, the device transitions to a lower, more sensitive threshold for sustained operation. This periodic threshold adjustment is controlled by timing circuits that track the operational state of the protected load.
Data Source
AI summary
A circuit protective device includes a current sensing circuit adapted to be coupled to an output power switch to provide a feedback signal representative of the output current, an overcurrent sensor responsive to the feedback signal exceeding a presettable reference level to provide an overcurrent alert signal, a function generator responsive to the feedback signal to generate a first signal related to the output current according to a preset functional relationship, an integration circuit coupled to the output of the function generator to provide a second signal representative of the product of the first signal and t, where t is the elapsed time following generation of an overcurrent alert signal, and a control circuit responsive to a presettable value of the second signal to disable a gating circuit for the power switch, the shut-down signal being provided at a time which depends on the magnitude of the overcurrent signal. The control circuit also shuts down the gating circuit if the output current exceeds a second preset limit representing a short circuit condition with substantially no delay. A related method, and a function generator in the form of a piece-wise linear approximation circuit are also disclosed.