Hybrid Current Sense System for BLDC Motor Control
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Solution Overview
Problem
Current three-phase brushless DC motor control systems face challenges with large, costly, and complex current sensing systems required for advanced control methods, such as sensorless field-oriented control, which increase component count, cost, and complexity, while also limiting the maximum pulse-width modulated signal frequency due to precise current measurement constraints.
Innovation Solution
The hybrid current sense system uses a shunt resistor and current-sense amplifiers to measure voltage drops across low-side switches, allowing for accurate current calculations with a single reading per pulse-width modulated signal cycle, reducing the need for multiple samples and enabling the use of slower, cheaper amplifier and analog-to-digital converter designs, and calibrates current information using calibration weights to provide accurate inputs for advanced control methods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a multi-resistor current sensing system is implemented, then accurate current measurements are provided, but component count, cost, and complexity increase
Solution Approach 1:
The patent merges the current sensing function into the existing power stage by using the low-side switches' inherent current-carrying capability. Instead of separate sensing resistors for each phase, the system uses a single shared sensing mechanism that leverages the power stage's existing structure, thereby reducing component count while maintaining measurement accuracy.
Solution Approach 2:
The low-side switches serve multiple functions: they act as both power switching elements and current sensing elements. The same switches that control motor phases are used to measure current, eliminating the need for dedicated separate sensing components and reducing overall system complexity.
2Device complexity
If a single shunt resistor system is used, then cost and complexity are reduced, but precise current readings are not provided
Solution Approach 1:
The system employs feedback through calibration weights that are determined based on measured current information. The calibration process uses the single shunt resistor measurement to generate correction factors that compensate for measurement errors, enabling accurate current readings to be derived from the simplified single-resistor architecture.
Solution Approach 2:
The patent changes the approach from direct measurement to calibrated measurement. By determining calibration weights based on measured current information and using these weights to calculate corrected current values, the system transforms the limited single measurement into multiple accurate readings, effectively improving precision without adding hardware complexity.
3Measurement precision
If multiple samples are taken per pulse-width modulated signal cycle, then accurate current readings are obtained, but the maximum pulse-width modulated signal frequency is limited
Solution Approach 1:
The system performs calibration actions in advance by determining calibration weights based on measured current information before the actual control operation. This preliminary calibration allows the system to use fewer samples during real-time operation while maintaining accuracy, as the calibration compensates for measurement limitations.
Solution Approach 2:
The patent changes the sampling strategy by using calibration weights to derive multiple accurate current readings from a single measurement. This parameter transformation allows the system to operate at higher pulse-width modulated signal frequencies without sacrificing measurement precision, as the calibration process compensates for the reduced sampling rate.
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 the component count and cost, simplifies the layout, and increases efficiency by allowing the use of advanced control methods at higher pulse-width modulated signal frequencies, while maintaining accurate current readings, thus addressing the limitations of previous systems.
Implementation Method 1
measuring a voltage drop across the low-side switch with the current-sense amplifier, and determining a current through the low-side switch based on the measured voltage drop
Data Source
AI summary
A hybrid current sense system and methods can comprise: a switch coupled to a phase output and coupled to a voltage rail; an amplifier coupled to the switch and the amplifier configured to detect current information of the switch; a current detector coupled between the switch and the voltage rail, the current detector configured to determine a measured current for the switch; and a current calibrator configured to calibrate the current information for the switch based on the measured current.