BLDC Motor Power Stage Short-Circuit Detection at Startup
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Solution Overview
Problem
Power tools with brushless motors using a triple half-bridge power stage are at risk of short circuits, which can damage the motor controller and battery if undetected, due to the potential failure of switching elements like MOSFETs.
Innovation Solution
Implementing a two-step short circuit detection process in the motor controller, where the controller charges bootstrap capacitors and measures current through switching elements during the bootstrap capacitor charging process, identifying and discontinuing charging if thresholds are exceeded, and individually checking each switching element for faults before normal operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a triple half-bridge power stage with 6 switching elements is used in a BLDC motor controller, then the motor can be efficiently controlled, but the risk of short circuit damage to the controller and battery increases
Solution Approach 1:
The patent applies preliminary action by performing short circuit detection during the bootstrap capacitor charging process, which occurs before normal motor operation begins. The controller proactively checks for short circuits in the switching elements during this initialization phase, preventing potential damage before it can occur during high-power operation.
Solution Approach 2:
The patent implements feedback by continuously monitoring the current during bootstrap capacitor charging and comparing it against threshold values. When the current exceeds the threshold, the system provides feedback to the controller, which then takes corrective action by preventing further operation until the fault is addressed.
2Reliability
If short circuit detection is performed during bootstrap capacitor charging, then damage to controller and battery is prevented, but the startup time and operational delay increases
Solution Approach 1:
The short circuit detection is performed during the bootstrap capacitor charging process, which is already part of the normal startup sequence. By integrating the detection into this existing preliminary action rather than adding a separate detection phase, the patent minimizes additional time loss while ensuring safety checks are completed.
Solution Approach 2:
The patent uses a threshold-based current monitoring approach that allows the system to quickly identify and respond to short circuit conditions. When normal operating current levels are detected (below the threshold), the system can proceed with startup without significant delay. Only when abnormal current levels indicate a short circuit does the system halt, making the detection process efficient and minimally intrusive.
3Measurement precision
If individual switching elements are checked for short circuits, then the specific faulty element can be identified, but the detection process complexity and time consumption increases
Solution Approach 1:
The patent applies segmentation by checking each switching element individually during the bootstrap capacitor charging process. Rather than attempting to detect faults in the entire power stage as a single unit, the system divides the detection into discrete element-level checks, enabling precise identification of which specific switching element has a short circuit condition.
Solution Approach 2:
The controller uses its existing current sensing capabilities and control logic to perform the short circuit detection without requiring external diagnostic equipment or additional complex measurement systems. The system leverages its own operational parameters (current draw during bootstrap charging) to self-diagnose switching element conditions, simplifying the overall detection approach.
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 method effectively reduces the risk of short circuit damage to the controller, power source, and other components by identifying and preventing short circuits during startup and each tool activation, ensuring continuous operation.
Implementation Method 1
measuring the current flowing through the half-bridge or the full power stage
Implementation Method 2
The controller charges bootstrap capacitors by turning ON the low-side switching elements
Data Source
AI summary
The present invention relates to switching element protection of a BLDC motor, such as used with a power tool. The present invention checks each switching element of a power stage individually for a short circuit when a trigger of the tool is actuated. Each switching element is turned ON for a period of time (such as 1-5 microseconds, for example), current flowing through the half-bridge or the full power stage is measured, and that switching element is turned OFF. When the current is greater than or equal to a threshold (such as 5 A, for example), the controller stops and indicates a fault condition. By testing each switching element in order, the controller is able to determine whether the shorted switching element is the opposite one in the half-bridge being tested.