Composite Transistor Overcurrent Clamping Without Sense Resistors
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Solution Overview
Problem
Existing over-current protection circuits for composite transistor devices face limitations such as reduced output voltage headroom due to sense resistors, high circuit temperatures, and slow response speeds, which can lead to damage from over-current events.
Innovation Solution
An over-current protection circuit comprising a control-terminal voltage-generation module, a composite transistor device, and an over-current protection module that applies a clamping voltage when the output current exceeds a preset limit, using a diode string and adjustable resistor to limit current and prevent overheating and overshooting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a sense resistor is used to sense output current, then over-current protection is achieved, but output voltage headroom is reduced due to voltage drop across the resistor
Solution Approach 1:
The patent extracts the current sensing function from the main current path by using a separate sensing transistor (third transistor) that mirrors the output current. This allows current protection without inserting a sense resistor that would cause voltage drop, thereby maintaining output voltage headroom while achieving over-current protection.
Solution Approach 2:
The patent introduces a fourth transistor as an intermediary element that acts as a current mirror to sense the output current indirectly. This intermediary transistor enables current detection without direct insertion of sensing components into the main current path, avoiding voltage headroom loss while providing protection.
2Reliability
If a sense resistor is used to sense output current, then over-current protection is achieved, but circuit temperature increases due to power dissipation
Solution Approach 1:
The patent removes the power-dissipating sense resistor from the circuit by using transistors configured as current mirrors to sense current. The sensing transistors operate with minimal voltage drop, dramatically reducing power dissipation and preventing temperature increase while maintaining over-current protection capability.
3Reliability
If a current-limiting feedback loop is used to limit output current, then over-current protection is achieved, but response speed is slow causing current overshoot
Solution Approach 1:
The patent implements preliminary protection action by using a current mirror configuration where the sensing transistor and protection transistor are directly coupled. When over-current is detected, the protection transistor immediately responds through direct coupling without waiting for feedback loop stabilization, preventing current overshoot and achieving fast response.
Solution Approach 2:
The patent uses a direct feedback mechanism where the voltage at the sensing transistor's drain is directly fed to the gate of the protection transistor. This immediate feedback path ensures that when output current exceeds the limit, the protection transistor responds instantly to clamp the current, eliminating the delay inherent in traditional feedback loops.
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 power dissipation across resistors, ensuring fast response times and preventing output current overshooting, thus protecting composite transistor devices from damage.
Implementation Method 1
an over-current protection module, connected between the composite transistor device and the load, wherein when the output current of the composite transistor device exceeds a preset limit, a clamping voltage is applied to the composite transistor device by the over-current protection module
Implementation Method 2
protects against over-current by including a sense resistor in its output current path to sense the output current
Implementation Method 3
the high power dissipated across the sense resistor will increase the circuit temperature
Data Source
AI summary
An over-current protection circuit for composite transistor devices is provided, connected between an input terminal and a load, and including: a control-terminal voltage-generation module whose output voltage varies with its input voltage when driven by a first voltage, wherein the output voltage of the control-terminal voltage-generation module serves as a control-terminal voltage; a composite transistor device, connected between the control-terminal voltage-generation module and the load, configured to conduct in response to the control-terminal voltage and a second voltage to generate an output current flowing through the load; and an over-current protection module, connected between the composite transistor device and the load, wherein when the output current of the composite transistor device exceeds a preset limit, a clamping voltage is applied to the composite transistor device by the over-current protection module to limit a current flowing through the composite transistor device, thereby limiting the output current of the composite transistor device.