Back EMF Sensing Circuit for Sensorless BLDC Motor Drive
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing circuits for sensing motor back EMF signals in brushless DC motors face challenges in protecting control circuitry from high voltages while maintaining signal integrity, particularly at low motor speeds, due to high signal attenuation and power dissipation issues in previous resistive attenuator and diode clamping circuits.
Innovation Solution
A circuit using an NMOS transistor with a biasing circuit, capacitors, and diodes is employed to dynamically adjust attenuation based on PWM states, providing low source impedance and minimizing power dissipation, allowing reliable zero crossing detection at both low and high motor speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a resistive attenuator circuit is used to protect control circuitry from high voltage, then the control circuitry is protected from damage, but the signal strength is significantly reduced especially at low motor speeds
Solution Approach 1:
The circuit dynamically switches between two attenuation configurations based on motor operating conditions. A first attenuation configuration with lower attenuation factor is used at low speeds to preserve signal strength, while a second attenuation configuration with higher attenuation factor is used at high speeds to protect the control circuitry. This dynamic adaptation resolves the contradiction by adjusting the protection level according to actual signal characteristics.
Solution Approach 2:
The circuit changes the attenuation parameter (attenuation factor) based on operating conditions. By switching between different attenuation configurations with different attenuation factors, the circuit optimizes the balance between signal preservation and circuit protection, enabling reliable zero crossing detection across the full motor speed range.
2Object-affected harmful factors
If a diode clamping circuit with current limiting resistor is used to protect control circuitry, then voltage protection is provided, but power dissipation becomes excessively high
Solution Approach 1:
The circuit dynamically selects between different attenuation configurations based on operating conditions. By using a switching mechanism controlled by a microprocessor, the circuit activates the appropriate attenuation configuration only when needed, minimizing continuous power dissipation while maintaining protection capabilities.
Solution Approach 2:
The circuit uses periodic PWM signals to control the switching between attenuation configurations. The microprocessor generates PWM signals that periodically activate the switching element, enabling the circuit to alternate between low-power standby mode and active protection mode, thereby reducing overall power dissipation.
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
The solution effectively protects control circuitry from high voltages, preserves signal integrity, and enables reliable zero crossing detection across a wider range of motor speeds with reduced power dissipation, enhancing the operational efficiency of sensorless BLDC motor drive systems.
Implementation Method 1
a capacitor coupled between the control terminal and a reference voltage terminal
Implementation Method 2
a first diode coupled between the output signal node and the reference voltage terminal
Data Source
AI summary
A back EMF signal from PWM driven motor is passed through an attenuation circuit. The attenuation circuit has a first mode of operation and a second mode of operation. The first mode of operation, used to sample a higher voltage back EMF signal during PWM on-time, applies the back EMF signal to a resistive divider formed of a first resistor and second resistor connected in series. The second mode of operation, used to sample a lower voltage back EMF signal during PWM off-time, applies the back EMF signal to a circuit comprised of a transistor conduction path in series with the second resistor. A control signal, responsive PWM on-time and off-time state, controls switching between the first and second modes.