BLDC Motor Driver Circuit BEMF Detection and Voltage Protection
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Solution Overview
Problem
Conventional BLDC motor driver circuits face challenges in accurately detecting small back electromotive force (BEMF) during startup, which can result in incorrect signal reading and require significant rotor disturbance, and are vulnerable to high voltages that can damage the controller.
Innovation Solution
A novel BLDC motor driver circuit and start-up control method that includes a driving power stage, current unidirectional circuit, biasing circuit, and sensor circuit to generate a detection signal during a test rotation, limiting voltage to a clamp level and providing a sensing signal indicative of the rotor's position, allowing for accurate BEMF detection and protection against high voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If voltage division resistors are used to retrieve BEMF during startup, then the BEMF signal can be detected, but the retrieved signal level becomes too small to be correctly read by IC when BEMF is small
Solution Approach 1:
The patent introduces a current unidirectional circuit as an intermediary between the motor phase and the detection circuit. This circuit includes a unidirectional control device (such as a MOSFET configured as a diode or a dedicated current unidirectional transistor) that allows current to flow in only one direction, effectively isolating the detection circuit from voltage fluctuations while preserving the BEMF signal. The biasing circuit then provides a stable reference voltage to enhance the small BEMF signal for correct IC reading.
Solution Approach 2:
The patent changes the electrical parameters of the detection circuit by introducing a biasing voltage through the biasing circuit. This biasing voltage shifts the operating point of the unidirectional control device, allowing it to operate in an optimal region where small BEMF signals can be detected with high precision. The biasing effectively amplifies the small signal without requiring physical modification of the motor or BEMF source.
2Measurement precision
If the BEMF is made large to ease detection, then the signal is easier to read, but it requires the rotor to have a considerably obvious disturbance which is not acceptable
Solution Approach 1:
The current unidirectional circuit serves as a mediator that selectively passes the BEMF signal while blocking harmful voltage spikes and disturbances. The unidirectional control device conducts only when the BEMF forward voltage exceeds the device's threshold voltage, naturally filtering out excessive disturbances without requiring large rotor perturbations.
Solution Approach 2:
The unidirectional control device provides beforehand protection by its inherent voltage threshold characteristic. When a voltage spike or excessive disturbance occurs, the device automatically blocks it if it exceeds the threshold, cushioning the detection circuit from harmful effects before they can cause damage or reading errors.
3Adaptability or versatility
If the BLDC motor driver circuit operates without voltage limiting, then the circuit can handle high voltage conditions, but the controller becomes vulnerable to high voltages that can damage it
Solution Approach 1:
The current unidirectional circuit acts as a protective intermediary between the high-voltage motor environment and the low-voltage controller. The unidirectional control device and biasing circuit together form a voltage-limited interface that allows the circuit to adapt to high voltage conditions while protecting the controller from damage.
Solution Approach 2:
The patent converts the potentially harmful high voltage condition into a beneficial protective feature. The unidirectional control device's threshold voltage characteristic, which might seem like a limitation, actually becomes a protective barrier that automatically clamps excessive voltages, turning what could be a harmful factor into a built-in protection mechanism.
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
Enables correct reading of BEMF at a predetermined ratio, protects the controller from high voltages, and detects small BEMF with minimal rotor disturbance, improving signal resolution and operational safety.
Implementation Method 1
a current unidirectional circuit configured to operably detect a BEMF generated during a period wherein the BLDC motor rotates for the test in the start-up mode
Implementation Method 2
when a voltage at the reverse end is higher than a voltage at the forward end, the current unidirectional circuit is configured to operably limit the voltage at the forward end not to be higher than a clamp voltage; and a biasing circuit coupled to the forward end of the current unidirectional circuit, wherein the biasing circuit is configured to operably bias the current unidirectional circuit in a forward operation state and is configured to operably provide the clamp voltage
Data Source
AI summary
A BLDC motor driver circuit includes: a driving power stage circuit configured to provide a start-up test signal in a start-up mode to excite a BLDC motor, to drive a rotor of the BLDC motor to rotate for a test; a current unidirectional circuit coupled to the BLDC motor at a reverse end for detecting a BEMF, to generate a detection signal at a forward end of the current unidirectional circuit, wherein when a voltage at the reverse end exceeds a voltage at the forward end, the current unidirectional circuit limits the voltage at the forward end not to be higher than a clamp voltage; a biasing circuit for biasing the current unidirectional circuit in a forward operation state and for providing the clamp voltage; and a sensor circuit for generating a sensing signal according to the detection signal to indicate a test rotation state of the BLDC motor.