MOSFET Load Driver Current Sensing via Rds(on) and Temperature
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Solution Overview
Problem
Existing systems for sensing current in external MOSFETs driving a load, such as motors, incur additional costs and power dissipation due to the use of external shunt resistors, which are costly and cause voltage drops.
Innovation Solution
A method and circuit arrangement that measures drain-to-source voltage and operating temperature to calculate current flowing through external MOSFETs, eliminating the need for external shunt resistors by using internal circuit components like operational amplifiers and diodes to sense current indirectly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external shunt resistors are used to sense current, then current measurement capability is provided, but system cost increases and power dissipation increases
Solution Approach 1:
The patent uses the MOSFET's intrinsic on-resistance as an intermediary element to sense current. Instead of adding external shunt resistors, the solution leverages the existing Rds(on) parameter of the MOSFET, which naturally develops a voltage drop proportional to the current flowing through it. This voltage is then measured and used for current sensing, eliminating the need for separate sensing components while reducing power loss.
Solution Approach 2:
The MOSFET serves dual functionality: it acts as both the power switching device and the current sensing element. By utilizing the MOSFET's own on-resistance for current measurement, the system makes the power device self-sufficient for sensing purposes, eliminating external dependencies and reducing overall system component count and cost.
2Measurement precision
If external shunt resistors are used to sense current, then current measurement capability is provided, but system cost increases
Solution Approach 1:
The patent extracts the current sensing function from external components and integrates it into the MOSFET's inherent characteristics. By removing the external shunt resistor from the system architecture and utilizing the MOSFET's own on-resistance, the solution simplifies the bill of materials and reduces system cost while maintaining current measurement capability.
Solution Approach 2:
The MOSFET is designed to perform multiple functions simultaneously: power switching and current sensing. This multi-functionality eliminates the need for dedicated external sensing components, reducing system complexity and cost. The same device structure serves both primary power delivery and measurement purposes.
3Measurement precision
If external shunt resistors are used to sense current, then current measurement capability is provided, but voltage drop across the load increases
Solution Approach 1:
The patent uses the MOSFET's on-resistance voltage drop as an intermediary measurement signal rather than using external shunt resistors. This approach measures current through the voltage developed across the MOSFET itself, avoiding additional series voltage drops that would affect the load. The measurement is taken from the existing voltage drop necessary for MOSFET operation, not from additional external components.
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
Reduces costs and power dissipation by eliminating the need for external shunt resistors, providing accurate current sensing without voltage drops.
Implementation Method 1
measuring a drain to source voltage of the one or more Field Effect Transistors
Implementation Method 2
measuring an operating temperature of the one or more Field Effect Transistors
Implementation Method 3
obtaining said current as a ratio of the respective measured drain to source voltage over said calculated drain to source on resistance
Data Source
AI summary
A pre-driving stage drives one or more Field Effect Transistors in a power stage driving a load. A method for measuring current flowing in the Field Effect Transistors includes: measuring drain to source voltages of the one or more Field Effect Transistor; and measuring an operating temperature of the one or more Field Effect Transistor. The current flowing in the Field Effect Transistors is measured by: calculating the respective on drain to source resistance at the operating temperature as a function of the measured operating temperature and obtaining the current value as a ratio of the respective measured drain to source voltage over the calculated drain to source resistance at the operating temperature.