MOSFET Rdson Current Sensing for Motor Winding Temperature Compensation
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Solution Overview
Problem
Conventional shunt current sensing techniques face challenges in accurately sensing motor winding currents over a wide range, especially at low speeds or light loads, due to high resistance values and temperature compensation issues with MOSFET junctions, which affect the reliability and efficiency of motor drive systems.
Innovation Solution
A controller is used to monitor voltages across circuit paths with shunt resistors and MOSFET switches to determine ON-resistance, allowing for real-time junction temperature measurement and compensation, thereby improving motor drive system reliability and current sensing accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If shunt resistance value is reduced to less than 0.5 mΩ to reduce power dissipation at high current sensing, then power loss is reduced, but current sensing accuracy deteriorates especially at low speed or light load
Solution Approach 1:
The patent combines shunt current sensing with Rdson sensing into a unified system. By measuring voltages across both the shunt resistor and the MOSFET's ON-resistance, the system leverages the strengths of both methods: shunt sensing provides accuracy at low currents while Rdson sensing contributes to overall current measurement, especially at higher currents where power loss concerns dominate.
Solution Approach 2:
The system uses the shunt resistor for multiple purposes: primary current sensing and calculating MOSFET junction temperature through power dissipation measurements. This multi-functional approach allows the same component to address both current measurement accuracy and thermal management, reducing the need for additional components that would increase power loss.
2Ease of manufacture
If three shunt resistors are used, one on each winding phase, then current sensing is simplified, but printed circuit board space increases, hardware cost increases, and power losses increase
Solution Approach 1:
The patent merges the functionality of multiple shunt resistors into a single shunt resistor combined with Rdson sensing. Instead of requiring three separate shunt resistors for three-phase current sensing, the system uses one shunt resistor alongside the existing MOSFET ON-resistances, significantly reducing PCB space while maintaining current sensing capability across all phases.
Solution Approach 2:
The single shunt resistor serves multiple functions: it provides reference voltage for ADC measurements, enables calculation of MOSFET junction temperature through power dissipation, and contributes to overall current measurement. This multi-functional design eliminates the need for multiple dedicated shunt resistors, reducing both space and cost.
3Reliability
If on-board temperature sensor is used to compensate Rdson change, then temperature compensation is achieved, but response speed is slow and steady state temperature errors increase
Solution Approach 1:
The system uses the MOSFET's own ON-resistance as a temperature sensor. Since Rdson naturally increases with junction temperature, the system measures this inherent property to determine temperature without requiring external sensors. This self-service approach provides immediate temperature feedback as the MOSFET operates, eliminating the lag and positioning errors associated with separate on-board temperature sensors.
Solution Approach 2:
The system exploits the natural change in Rdson parameter with temperature. By measuring voltage across the MOSFET during its ON-state and knowing the current through it, the system calculates Rdson and maps this resistance value to junction temperature using pre-characterized Rdson-vs-temperature curves. This parameter-based temperature sensing provides real-time, accurate temperature data without external sensors.
4Area of stationary object
If single shunt solution is used, then PCB space and hardware cost are reduced, but control complexity increases and performance deteriorates at low PWM duty cycle
Solution Approach 1:
The patent combines shunt sensing and Rdson sensing measurements into a unified control algorithm. By simultaneously measuring voltages across both the shunt resistor and MOSFET, the system obtains multiple data points that can be processed together to calculate phase current, eliminating the need for complex separate handling of different sensing methods and improving low-duty-cycle performance.
Solution Approach 2:
The system uses feedback from both shunt voltage and Rdson voltage measurements to continuously monitor and adjust current sensing calculations. This dual-feedback approach provides redundant information that helps maintain accuracy across the full range of PWM duty cycles, including low duty cycle conditions where single-shunt methods struggle.
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 enhances the accuracy of current sensing and temperature monitoring, enabling higher short-time current ratings and improved motor control by accurately determining ON-resistance and temperature, thus addressing the limitations of conventional methods.
Implementation Method 1
Shunt current sensing is a simple, relatively inexpensive and widely used technique in digital motor control
Implementation Method 2
Rdson is changing by MOSFET junction temperature
Implementation Method 3
monitor a first voltage across a first circuit path including a series connection of a first switch and a shunt resistor
Data Source
AI summary
An apparatus includes a controller that measures a first voltage across a first circuit path including a series connection of a first switch and a shunt resistor. The first voltage generated based on first current supplied from a first winding of a motor including multiple windings. Based on the magnitude of the first voltage, the controller determines an ON-resistance of the first switch. The ON-resistance can be used final office action any suitable purpose. For example, the controller can be configured to use the determined ON-resistance of the first switch to controller operation of a motor including the first winding. For example, the determined ON-resistance can be used as a basis to determine an amount of current through first switch and corresponding first winding of the motor.