Low Temperature Coefficient Current Sensor Thermal Cage
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Solution Overview
Problem
Existing current sensors face challenges in accurately measuring large currents on printed circuit boards due to high temperature coefficient of resistance in materials like copper, leading to inaccuracies and increased complexity in temperature compensation, and the high cost of exotic materials with low temperature coefficients.
Innovation Solution
A system current sensor module that thermally couples a current sense resistor with a gain-setting resistor using a thermal cage to maintain similar temperatures, allowing for accurate current sensing and measurement by scaling the resistors' resistances and using an amplifier to control the current through the gain-setting resistor, while also employing trim adjustment techniques for calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If copper PCB trace is used as current sense element, then cost is reduced and manufacturing is simplified, but temperature coefficient of resistance increases leading to measurement inaccuracy
Solution Approach 1:
The patent creates a copy of the temperature effect by placing a gain-setting resistor in thermal coupling with the sense resistor. This copy resistor experiences the same temperature variations, allowing the measurement circuit to compensate for temperature-induced resistance changes without requiring exotic low-TCR materials or complex external temperature compensation circuits.
Solution Approach 2:
The patent changes the measurement approach by using a ratio of resistances rather than absolute resistance values. By measuring the ratio between the sense resistor and gain-setting resistor, the system becomes insensitive to common-mode temperature effects, as both resistors experience similar temperature coefficients that cancel out in the ratio calculation.
2Measurement precision
If exotic materials like iron-chrome or manganese-copper alloys are used for sense resistor, then temperature coefficient of resistance is reduced improving measurement accuracy, but cost increases significantly
Solution Approach 1:
Instead of using expensive exotic materials, the patent uses a copy approach where a standard copper gain-setting resistor thermally coupled to the sense resistor replicates the temperature behavior. This allows accurate temperature compensation using inexpensive standard materials throughout the circuit.
Solution Approach 2:
The patent employs homogeneous materials (standard copper for both sense and gain-setting resistors) throughout the circuit, eliminating the need for expensive exotic materials. The thermal coupling ensures both resistors experience identical temperature conditions, making the material choice uniform and cost-effective while maintaining measurement accuracy.
3Measurement precision
If temperature compensation circuits are added to compensate for copper's high TCR, then measurement accuracy is improved, but device complexity increases
Solution Approach 1:
The patent merges the temperature compensation function into the existing measurement circuitry by using the gain-setting resistor as both a circuit component and a temperature reference. The thermal coupling physically merges the temperature sensing function with the signal processing function, eliminating the need for separate temperature sensors and compensation circuits.
Solution Approach 2:
The patent uses the gain-setting resistor as a thermal copy of the sense resistor's temperature environment. This copy resistor enables the measurement amplifier to automatically compensate for temperature effects through its feedback mechanism, providing temperature compensation without additional active components or control logic.
4Measurement precision
If discrete sense resistors with low TCR are used, then measurement accuracy is improved, but power loss increases due to higher resistance values needed
Solution Approach 1:
The patent changes the measurement parameter from absolute voltage to a voltage ratio. By measuring the ratio of voltages across the sense resistor and gain-setting resistor, the system can use very low resistance values for the sense resistor, minimizing power loss while maintaining accurate measurement through the differential measurement approach.
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 maintains measurement accuracy despite thermal effects and aging, reducing errors and complexity, and is cost-effective by using copper as the material for both resistors, ensuring the ratio of their resistances remains stable regardless of temperature changes.
Implementation Method 1
The thermal coupling can include conducting heat from a first resistor layer (e.g., carrying the current sense resistor) to a thermal cage layer that can be located beyond a second resistor layer (e.g., carrying the gain-setting resistor)
Data Source
AI summary
A system current sensor module can accurately sense or measure system current flowing through a sense current resistor by shunting current through a gain-setting resistor and using an amplifier to measure a resulting voltage, with an output transistor controlled by the amplifier controlling current through the gain setting resistor in a manner that tends to keep the amplifier inputs at the same voltage. The resistors can be thermally coupled to maintain similar temperatures when a system current is flowing. The thermal coupling can include conducting heat from a first resistor layer carrying the current sense resistor to a thermal cage layer located beyond a second resistor layer carrying the gain-setting resistor. This preserves accuracy, including during aging.