Integrated MOSFET Current Sensing for Battery Fuel-Gauging
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Solution Overview
Problem
Existing fuel-gauging technologies for batteries require an extra sense element, such as a resistor, to accurately measure battery current, leading to power loss, increased device temperature, and reduced battery capacity due to heating and voltage drop across these components.
Innovation Solution
The solution involves measuring battery charging and discharging current without an extra sense element by using a switch-capacitor or analog-to-digital conversion technique to convert the voltage across the battery charge/discharge power MOSFET, allowing for digital representation of current, which is then used in fuel-gauging operations, thereby eliminating power dissipation and reducing device temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an extra sense element (resistor) is used to accurately measure battery current, then measurement precision is improved, but power loss increases
Solution Approach 1:
The patent extracts the current sensing function from a separate sense resistor and integrates it into the body diode of the MOSFET. By utilizing the inherent body diode structure, the need for an external sense element is eliminated, thereby removing the associated power loss while maintaining measurement capability.
Solution Approach 2:
The MOSFET body diode serves multiple functions: it provides the necessary current sensing capability for accurate battery current measurement while simultaneously serving as part of the normal charge/discharge current path. This multi-functionality eliminates the need for separate sense resistors and reduces overall power loss.
2Measurement precision
If an extra sense element (resistor) is used to accurately measure battery current, then measurement precision is improved, but device temperature increases
Solution Approach 1:
The patent removes the external sense resistor that generates heat, and instead uses the MOSFET's body diode for current sensing. This extraction of the heating element eliminates the primary source of temperature increase while preserving the current measurement function.
Solution Approach 2:
The patent converts the normally parasitic body diode (which is often considered a source of inefficiency) into a useful sensing element. By measuring the voltage across the body diode, the patent achieves accurate current measurement without the thermal problems associated with external sense resistors.
3Measurement precision
If an extra sense element (resistor) is used to accurately measure battery current, then measurement precision is improved, but battery capacity decreases
Solution Approach 1:
The patent extracts the sensing function from an external resistor that would require additional voltage headroom, and relocates it to the MOSFET body diode. This eliminates the voltage drop across a separate sense element, thereby preserving more of the battery's voltage range for actual capacity utilization.
Solution Approach 2:
The patent merges the current sensing function with the MOSFET's inherent body diode structure. This integration eliminates the need for separate sense resistors and their associated voltage drops, thereby maximizing the usable battery voltage range and effective capacity.
4Reliability
If multiple switches and sense resistors are used for current protection and measurement, then reliability is improved, but device complexity increases
Solution Approach 1:
The MOSFET body diode serves dual purposes: it enables accurate current sensing for measurement and simultaneously provides over-current protection capability. This multi-functionality reduces the number of discrete components needed while maintaining or improving system reliability.
Solution Approach 2:
The patent combines the current sensing function and over-current protection function into a single integrated approach using the MOSFET body diode. This merging of functions reduces component count and simplifies the circuit while maintaining the reliability benefits of both measurement and protection capabilities.
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 extends battery discharge time, lowers power requirements, and improves battery performance by eliminating the need for additional sense resistors, resulting in more efficient charging and monitoring of battery state of charge.
Implementation Method 1
using a switch-capacitor or analog-to-digital conversion technique to convert the voltage across the battery charge/discharge power MOSFET, allowing for digital representation of current
Data Source
AI summary
Integrated MOSFET current sensing for fuel-gauging. A 1st MOSFET through which the current is to be sensed is coupled to a 2nd MOSFET of the same type, the 2nd MOSFET being biased to have the same resistance as the 1st MOSFET. The 2nd MOSFET has a much smaller area than the 1st MOSFET, and is coupled to a current source representing a maximum current through the 1st MOSFET. The voltage across the 1st MOSFET relative to the voltage across the 2nd MOSFET provides a measure of the current through the 1st MOSFET. Various embodiments are disclosed, including embodiments in battery packs to eliminate the need for additional and expensive external components.