Ignition Coil Current Soft Shutdown With Fast-Ramp IGBT Control
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Solution Overview
Problem
In ignition systems, existing coil current control circuits face challenges in performing a soft shut down without producing a spark, particularly when the coil current is below the current limit of the transistor, leading to uncontrolled current rises that can cause damage during thermal shutdown conditions.
Innovation Solution
A coil current control circuit that includes a transistor in series with the coil, a current sensing circuit, and a current limit control circuit, which compares voltage from the sensing circuit to a SSD signal from a ramp generator to reduce the coil current using a fast ramp profile when below the current limit and a slow ramp profile when at the current limit, minimizing uncontrolled current rises.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a transistor is controlled to abruptly switch OFF the coil current, then the coil is quickly discharged, but a large coil voltage is created that can transform to an even higher voltage across a secondary coil that in series with a spark gap, creating a spark
Solution Approach 1:
The patent applies dynamics by transitioning from a static abrupt switch-off control mode to a dynamic controlled ramp-down mode. The transistor gate voltage is gradually reduced according to a predetermined ramp profile rather than being suddenly switched off, allowing the coil current to decrease continuously. This dynamic control approach maintains the transistor in a linear operating region during the discharge process, preventing the large voltage spikes that occur during abrupt switching while still achieving effective coil discharge.
Solution Approach 2:
The patent changes the control parameter from a binary on/off state to a continuously varying gate voltage parameter. By controlling the transistor gate voltage to follow a predetermined ramp profile that gradually decreases from its initial value to zero, the coil current is controlled to ramp down smoothly. This parameter change approach transforms the harmful abrupt voltage transition into a controlled gradual transition, eliminating spark generation while maintaining effective discharge performance.
2Reliability
If a CCC circuit is configured to sense a coil current and control a transistor to reduce the difference between coil current and reference level, then the coil current is controlled during soft shut down, but performing a soft shut down before a coil current is charged to a current limit of the transistor can result in a period during a soft shut down in which the coil current is uncontrolled
Solution Approach 1:
The patent applies preliminary action by establishing a predetermined ramp profile for the transistor gate voltage before the soft shut down is actually initiated. This ramp profile is pre-calculated and stored, defining the exact trajectory the gate voltage should follow during discharge. When soft shut down is triggered, the controller simply executes this pre-planned voltage ramp-down sequence, ensuring the coil current follows a controlled path from the moment shutdown begins, eliminating any uncontrolled current period.
Solution Approach 2:
The patent implements feedback by continuously monitoring the coil current through a current sensing circuit and comparing it against the expected current trajectory defined by the ramp profile. The controller adjusts the transistor gate voltage in real-time based on this feedback to ensure the actual coil current follows the predetermined discharge curve. This closed-loop feedback mechanism eliminates uncontrolled current periods by continuously correcting any deviations from the expected controlled discharge path.
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
This approach effectively reduces unwanted current rises during soft shut down, preventing damage by ensuring controlled discharge of the coil current, even when triggered by thermal shutdown conditions, thereby enhancing system reliability.
Implementation Method 1
a current sensing circuit that is configured to sense a coil current flowing through the transistor
Implementation Method 2
The transistor is controllable to conduct current at or below a coil current limit
Implementation Method 3
a large coil voltage is created as the coil attempts to maintain the otherwise decreasing magnetic flux
Data Source
AI summary
A current control circuit for an ignition system (i.e., igniter current limiter) is disclosed. The current control circuit can reduce a coil current over a soft shut down (SSD) period using an insulated gate bipolar transistor (IGBT) that is controlled by a negative feedback loop, which controls the current limit of the IGBT according to a SSD profile. In order to prevent an unwanted current rise during the soft shut down period, the current control circuit compares a gate voltage of the IGBT to a reference signal and based on the comparison can enable the SSD profile to include a fast ramp. The fast ramp quickly lowers the current limit of the IGBT so that the coil current equals the current limit and can be controlled by the negative feedback loop.