Inrush Current Limit Circuitry Without Sense Resistor
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Solution Overview
Problem
Existing power over Ethernet systems face inefficiencies in limiting inrush current during device startup, as prior art methods using sense resistors dissipate heat and are costly due to the need for accurate resistance values.
Innovation Solution
An inrush current limit circuitry comprising electronically controlled switches, a ramp voltage circuit, differential amplifiers, and comparators to control the inrush current without an external sense resistor, ensuring a linear voltage ramp across the port capacitor, thereby fixing the inrush current magnitude and providing short circuit protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a sense resistor is used to sense and control inrush current, then the inrush current can be limited to the maximum acceptable value, but heat is dissipated which wastes power and the sense resistor must be very accurate which adds cost
Solution Approach 1:
The patent extracts and eliminates the sense resistor from the circuit by using a current mirror configuration where the inrush current itself is used to set the reference current for the mirror. This removes the need for a separate sense resistor that dissipates power, while still achieving accurate current sensing and limitation through the transistor-based current mirror mechanism.
Solution Approach 2:
The patent introduces a current mirror as an intermediary mechanism between the inrush current source and the control element. The current mirror acts as a mediator that copies the inrush current magnitude and uses it to control the main power transistor, enabling indirect current control without the need for direct sense resistor measurement and reducing power loss.
2Reliability
If a sense resistor is used to sense and control inrush current, then the inrush current can be limited to the maximum acceptable value, but the sense resistor must be very accurate which adds cost
Solution Approach 1:
The patent extracts and eliminates the sense resistor from the circuit by using a current mirror configuration where the inrush current itself is used to set the reference current for the mirror. This removes the need for a separate sense resistor that dissipates power, while still achieving accurate current sensing and limitation through the transistor-based current mirror mechanism.
Solution Approach 2:
The patent implements a self-service mechanism where the inrush current automatically sets the reference current for the current mirror without requiring external sense resistors or additional accuracy-critical components. The system uses its own operating current to establish the control reference, eliminating the need for high-precision external components.
3Reliability
If conventional current limiters are used, then inrush current can be controlled, but they require sense resistors which increase device complexity and cost
Solution Approach 1:
The patent merges the current sensing function and the current control function into a single integrated circuit block. The current mirror simultaneously senses the inrush current and generates the control signal for the power transistor, eliminating the need for separate sense resistors and reducing overall circuit complexity while maintaining effective current control.
Solution Approach 2:
The patent implements a multi-functional current mirror circuit that performs both current sensing and current control functions within a single circuit structure. This universal approach eliminates the need for separate dedicated sense resistors and control elements, reducing device complexity while achieving comprehensive current management.
Data Source
AI summary
An inrush current limiting method not requiring the use of a sense resistor, the method constituted of: generating a ramped voltage; comparing a first function of a voltage across a port capacitor with the generated ramp voltage; responsive to the outcome of the comparison of the voltage first function with the ramp voltage, controlling an electronically controlled switch coupled to the port capacitor such that the voltage across the port capacitor is a linear function of the generated ramped voltage; comparing a second function of the voltage at a terminal of the electronically controlled switch with a predetermined reference voltage; and responsive to the outcome of the comparison indicating that the second function of the terminal voltage is greater than the predetermined reference voltage, pulling the generated ramp voltage towards a predetermined shutoff voltage.