Sectioned FET for Wide-Range Current Sensing
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Solution Overview
Problem
Power over Ethernet (PoE) systems face challenges in accurately sensing a wide range of currents, particularly in detecting device disconnection and implementing current limiting, due to the limitations of existing current sensing methods which often result in excessive voltage drops and power loss.
Innovation Solution
The use of a dual-stage current sensing circuit with sectioned main and sense FETs, where the ratio of main FET to sense FET size is dynamically adjusted to accommodate both low and high current situations, allowing for efficient sensing of currents from 5-10 mA to 1.5 A without excessive voltage drops, by selectively adding or removing FET sections based on current thresholds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single FET configuration is used for current sensing, then the device structure is simple, but it cannot accurately sense both low currents (5-10 mA for disconnect detection) and high currents (up to 1.5 A for current limiting)
Solution Approach 1:
The main FET is divided into multiple sections (first main FET section and second main FET section) that can be selectively connected to the common gate. Similarly, the sense FET is divided into multiple sections that can be selectively activated. This segmentation allows the circuit to use different portions of the FETs depending on the current level, enabling accurate sensing across a wide current range while maintaining a relatively compact integrated structure.
2Measurement precision
If external sense resistors are used for current sensing, then the sensing range can be extended, but the device size increases and power loss occurs
Solution Approach 1:
The current sensing function is merged with the main power FET by integrating the sense FET and main FET sections within the same integrated circuit. This eliminates the need for external sense resistors and allows the sensing operation to occur within the low-side FET switch itself, reducing power loss and integrating the sensing capability into the controller.
3Power
If the main FET size is increased to handle high currents, then current limiting capability is improved, but voltage drop increases for low current operations
Solution Approach 1:
The FET configuration is made dynamic through selective switching of different FET sections based on the current level. A controller monitors the current and dynamically connects or disconnects specific main FET sections and sense FET sections to optimize the sensing ratio for the current operating condition, thereby minimizing voltage drop across the FET while maintaining accurate sensing across the full current range.
Data Source
AI summary
A sectioned field effect transistor (“FET”) for implementing a rapidly changing sense range ratio dynamically in response to changing load and main supply conditions. The sectioned FET may have multiple main FET sections, and multiple sense FET sections. These sections can be dynamically connected and disconnected from the sectioned FET. The sections may also be connected by a common gate. There may also be common drain or source connections for the main FET sections, and also common drain or source connections for the sense FET sections. The sectioned FET allows for the sense range to be extended by a multiple of k+1, where k is the size ratio or factor of the additional sense FET sections. This allows the current sense range ratio to be extended to (m+n)/n*(k+1).