Sensor Node Voltage Clamping Circuit for Battery Charger Thermistor
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Solution Overview
Problem
Existing temperature sensing circuits using thermistors in battery chargers often produce erroneous readings due to the non-linear relationship between thermistor resistance and temperature, leading to false signals indicating a battery has been removed when the temperature is too low for charging.
Innovation Solution
A voltage clamping circuit with a current source and a variable resistor, an amplifier, and comparators that adjust the current output to prevent the sensor node voltage from exceeding reference voltages, thereby distinguishing between low temperatures and a disconnected battery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a thermistor is used for temperature sensing in battery chargers, then temperature monitoring capability is provided, but erroneous readings occur at low temperatures due to the non-linear resistance-temperature relationship
Solution Approach 1:
The patent introduces an operational amplifier as an intermediary component that actively compensates for the non-linear resistance-temperature relationship of the thermistor. The op-amp senses the voltage across the thermistor and adjusts the current through it to maintain a linear voltage-temperature relationship, thereby eliminating erroneous readings at low temperatures while preserving the thermistor's temperature monitoring capability.
Solution Approach 2:
The patent changes the operating parameters of the thermistor by dynamically adjusting the current through it using the operational amplifier. By modifying the current parameter based on the thermistor's resistance characteristics, the system maintains accurate temperature sensing across the full temperature range, preventing the non-linear effects that cause erroneous readings.
2Reliability
If the second reference voltage is set higher to distinguish disconnected battery from low temperature, then false disconnection signals are reduced, but the voltage at sensor node quickly exceeds the reference voltage due to non-linear thermistor response
Solution Approach 1:
The patent implements a feedback mechanism where the operational amplifier continuously monitors the voltage across the thermistor and adjusts the current accordingly. This feedback loop ensures that the voltage at the sensor node remains controlled and does not quickly exceed reference voltages, allowing for reliable distinction between disconnected batteries and low-temperature conditions while maintaining precise temperature threshold detection.
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 solution effectively extends the operational temperature range for battery charging, reducing false disconnection signals and allowing safe operation at lower temperatures by maintaining the sensor node voltage within defined reference levels.
Implementation Method 1
the resistance of the variable resistor 120 increases rapidly in a non-linear fashion
Data Source
AI summary
A voltage clamping circuit includes a current source having a fixed current source and a variable current source and a variable resistor receiving current from the current source. The variable resistor varies its resistance in response to an environmental operating condition. The voltage clamping circuit also includes an amplifier configured to compare a sensor node voltage with a reference voltage, the sensor node voltage being in communication with the voltage drop across the variable resistor. The amplifier is configured and connected to provide a control output to control the variable current source to modify current output from the variable current source to at least in part prevent the sensor node voltage from exceeding a reference voltage when certain operating conditions are present.