Power IC Over-Current Protection Circuit Foldback Mechanism
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Solution Overview
Problem
Existing power ICs with over-current protection circuits suffer from high power loss and heat loss due to large voltage differences between input and output voltages, and are sensitive to temperature variations, with no fold-back current limit mechanism to prevent damage during short-circuits, leading to increased area requirements in CMOS manufacturing.
Innovation Solution
A two-stage current limit mechanism is implemented in the over-current protection circuit, comprising a constant current limit and a fold-back current limit, which clamps output current to a specified value and reduces power and heat loss during short-circuits, using a power transistor, feedback circuit, and over-current protection circuit with sensing and switch transistors to control output voltage and current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a sensing resistor is used to detect output current, then over-current protection is achieved, but power loss increases due to large voltage difference across the resistor
Solution Approach 1:
The patent replaces the traditional sensing resistor method with a transistor-based sensing mechanism. The sensing transistor detects output current through its base-emitter voltage characteristics, eliminating the need for a power-dissipating sensing resistor. This substitution reduces power loss while maintaining over-current protection functionality.
Solution Approach 2:
The patent changes the detection parameter from voltage drop across a resistor to base-emitter voltage of a transistor. By utilizing the transistor's inherent voltage characteristics, the system achieves current sensing without the continuous power dissipation associated with resistive sensing, thereby reducing energy loss.
2Reliability
If a current limit switch transistor is used, then output current is limited, but the circuit becomes sensitive to temperature variations
Solution Approach 1:
The patent implements a feedback mechanism where the sensing transistor's base-emitter voltage is continuously monitored and fed back to the control circuit. This feedback allows the system to compensate for temperature-induced variations in transistor characteristics, maintaining stable current limiting across different temperature conditions.
Solution Approach 2:
The patent utilizes the temperature-dependent base-emitter voltage characteristic of the sensing transistor as a sensing parameter. By monitoring changes in this parameter and adjusting the control signal accordingly, the system compensates for temperature effects and maintains reliable current protection.
3Device complexity
If no fold-back current limit mechanism is implemented, then circuit simplicity is maintained, but damage occurs during short-circuit conditions
Solution Approach 1:
The patent segments the current protection into two distinct stages: a first current limit stage for normal over-current conditions and a fold-back current limit stage for severe short-circuit conditions. This segmentation allows the circuit to provide appropriate protection levels for different fault scenarios while maintaining overall system reliability.
Solution Approach 2:
The patent implements dynamic current limiting where the current limit threshold is not fixed but adapts based on operating conditions. During normal operation, a higher current limit is maintained, but during short-circuit conditions, the limit dynamically folds back to a lower value, providing enhanced protection without requiring overly complex circuitry.
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 two-stage current limit mechanism effectively prevents over-current and short-circuit damage, reduces power and heat loss, and minimizes the area required in CMOS manufacturing by optimizing component usage, ensuring rapid recovery of output voltage during abnormal conditions.
Implementation Method 1
a sensing transistor, used for sensing the current flowing through the power transistor to form a sensing current
Implementation Method 2
a switch transistor, used as a turn-on/turn-off switch of the over-current protection circuit
Implementation Method 3
the voltage level control unit shifts the voltage level according to the sensing current and a bias current to control the switch transistor
Implementation Method 4
When the output current is larger than the fold-back current limit threshold, the fold-back current limit circuit limits the output current to a low current value to lower the output voltage to zero
Data Source
AI summary
A power IC with an over-current protection receives an input voltage and converts the input voltage into an output voltage to a load. The present invention controls a power transistor to provide an output current to the load, and uses the output control unit to control the power transistor. Furthermore, the over-current protection circuit has a constant current limit threshold and a fold-back current limit threshold for controlling the power transistor. When the output current is larger than the constant current limit threshold, the output current is clamped to a constant current value to descend the output voltage to a rated value. When the output current is larger than the fold-back current limit threshold, the output current is limited to a low current value to descend the output voltage to zero. Thereby, the inner circuit of the power IC and the load are protected.