External MOSFET Current Sensing Without Shunt Resistors
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Solution Overview
Problem
Existing systems for sensing current in external MOSFETs driving loads, such as motors, require external shunt resistors, which increase cost and power dissipation due to voltage drops, especially at high currents.
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 resistance calculations and temperature-compensated diodes for accurate current sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external shunt resistors are used to sense current, then current measurement is achieved, but system cost increases and power dissipation increases due to voltage drops
Solution Approach 1:
The patent extracts the current sensing function from external shunt resistors and relocates it to the integrated circuit by utilizing the intrinsic on-resistance of the MOSFETs. This eliminates the need for separate external sensing components while maintaining measurement capability through voltage measurement across the MOSFET's channel resistance.
Solution Approach 2:
The MOSFETs themselves serve dual purposes: power switching and current sensing. The on-resistance of the MOSFET, which is necessary for its switching function, is also utilized as the sensing element. This self-service approach eliminates external components and reduces overall system power loss.
2Measurement precision
If external shunt resistors are used to sense current, then current measurement is achieved, but system cost increases
Solution Approach 1:
The patent merges the current sensing function with the existing power MOSFET structure and integrated circuit. By combining these functions, the patent eliminates the need for separate external shunt resistors and associated circuitry, thereby reducing component count, PCB space, and overall system cost while maintaining measurement accuracy.
Solution Approach 2:
The MOSFET is designed to perform multiple functions simultaneously: power switching and current sensing. The on-resistance that is inherent to the MOSFET's operation is repurposed as the sensing element, making the device universal and eliminating the need for dedicated external sensing components.
3Measurement precision
If series shunt resistors are used to sense current, then current measurement is achieved, but voltage drop across the load increases
Solution Approach 1:
The patent extracts the voltage measurement point from a series shunt configuration (which would add voltage drop) and relocates it to measure the voltage across the MOSFET's on-resistance instead. This extraction eliminates the additional voltage drop while preserving the ability to calculate current through the known resistance relationship.
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 reduces system costs and power dissipation by eliminating external resistors and voltage drops, while maintaining accurate current measurement, suitable for high-current applications.
Implementation Method 1
measuring an operating temperature of the one or more Field Effect Transistor, in particular MOSFET... measuring the forward voltage of diodes... calculating the operating temperature as a function of a known value of the forward voltage at room temperature and of the diode forward voltage temperature coefficient
Implementation Method 2
calculating the respective on drain to source resistance at the operating temperature as a function of the measured operating temperature... the on drain to source resistance varying with the temperature
Data Source
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AI summary
Described herein is a method for sensing at a pre-driving stage (12) driving one or more Field Effect Transistor (MHx, MLx), in particular MOSFET, comprised in a power stage (13) driving a load (DC1...DC4), a current flowing in said Field Effect Transistor (MHx, MLx), in particular MOSFET, the Field Effect Transistor being arranged external with respect to a chip on which said pre-driving stage (12) is arranged, said method comprising measuring (120) a drain to source voltage (VDS_HSx, VDS_LSx), of said one or more Field Effect Transistor (MHX, MLx), measuring (130) an operating temperature (TW) of said one or more Field Effect Transistor (MHX, MLx), measuring (140) a current (IHX, ILx) flowing in said one or more Field Effect Transistor (MHX, MLx) by calculating (142) the respective on drain to source resistance at the operating temperature (RdsONx(TW)) as a function (RdsONx(TW)) of said measured operating temperature (TW) and by obtaining (144) said current (IHX, ILx) as the ratio of the respective measured drain to source voltage (VDS_HSx, VDS_LSx) over said calculated drain to source resistance at the operating temperature (RdsONx(TW)).