Capacitor-Based Current Measurement Eliminates Shunt Voltage Drop
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Solution Overview
Problem
Current measurement methods in automotive systems face challenges due to undesirable voltage drops across shunt resistors, leading to measurement errors and feedback issues, particularly when using sense FETs, and the use of bipolar transistors becomes uneconomical as integration density increases.
Innovation Solution
A method and apparatus that utilize a capacitor to store charge based on the measurement current, with a reference timer controlling the charging time, allowing for accurate measurement without significant offset voltage influence, using a switched capacitor technology and a state machine to manage switch operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a shunt resistor is used for current measurement, then the current can be measured, but it causes voltage drop and feedback issues in the current path
Solution Approach 1:
The patent extracts the measurement function from the current path by using a capacitor connected in parallel with the sense FET. The capacitor measures the voltage across the sense FET without being in the current path, thereby eliminating the voltage drop and feedback issues caused by shunt resistors while maintaining measurement capability
Solution Approach 2:
The patent introduces a capacitor as an intermediary element that couples the sense FET to the amplifier. This capacitor mediates the measurement by transferring voltage information without requiring a direct resistive connection in the current path, thus avoiding the harmful voltage drop and feedback effects
2Object-generated harmful factors
If the shunt resistor value is reduced to minimize voltage drop, then voltage drop decreases, but the amplifier offset voltage becomes the dominant error source
Solution Approach 1:
The patent removes the shunt resistor from the measurement configuration and replaces it with a capacitor-based measurement approach. This extraction eliminates the trade-off between voltage drop and offset voltage errors by measuring through capacitance coupling rather than resistive voltage division
Solution Approach 2:
The patent changes the measurement parameter from resistive voltage drop to capacitive charge/voltage relationship. By measuring the voltage across a capacitor that was charged by the sense FET, the system avoids the offset voltage dominance issue that plagues low-value shunt resistor measurements
3Measurement precision
If bipolar transistors are used in the amplifier input stage to reduce offset voltage, then measurement accuracy improves, but chip area increases significantly
Solution Approach 1:
The patent uses simple capacitive coupling instead of expensive bipolar input stages. The capacitor-based measurement approach achieves adequate performance without requiring precision bipolar transistors, effectively replacing a costly component with a simpler, smaller capacitor that does not demand high offset voltage performance
4Object-generated harmful factors
If a capacitor is used to store charge for current measurement, then voltage drop and offset voltage influence are minimized, but the measurement requires additional circuit components and control logic
Solution Approach 1:
The patent merges the measurement capacitor with the existing sense FET circuitry by coupling it in parallel. This integration allows the capacitor to perform measurement functions without requiring completely separate circuitry, thereby reducing the net increase in device complexity while still eliminating voltage drop and offset voltage issues
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 minimizes voltage drops in the current path and eliminates the impact of amplifier offset voltage on measurement accuracy, ensuring precise current measurement by relying on the capacitance ratio and amplifier gain, independent of the measurement current's operating point.
Implementation Method 1
charging the first capacitor with the measurement current, as a result of which a charge which depends on the measurement current is stored in the first capacitor
Data Source
AI summary
A method is provided that comprises determining an amount of a first current from an amount of a charge stored in a first capacitor. Also, an apparatus is provided that comprises a reference timer circuit configured to generate a first signal indicating an expiration of a time period, and a sense circuit comprising a first capacitor and configured to sense, responsive to the first signal, a charge stored in the first capacitor, and to generate a second signal representing the sensed charge.