Current Mode BEMF Sensing Circuit for Brushless DC Motors

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Solution Overview

Problem

Existing motor control circuits for brushless DC motors rely on resistor dividers and clamp diodes, which lead to noisy signals, commutation errors, and excessive current flow from the motor windings, causing voltage drops and inaccuracies in back electro-motive force (BEMF) detection.

Innovation Solution

A motor control circuit using a current mode sensing mechanism that detects BEMF without resistor dividers or clamp diodes, employing H-bridge drivers and current comparators to form sense currents representative of the BEMF, allowing for accurate zero-crossing detection without conducting significant current from the motor windings.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If resistor dividers are used to reduce voltage and form virtual neutral point, then voltage level matching is achieved, but signal noise increases and measurement precision deteriorates

Engineering Contradiction:

ImproveBEMF detection accuracyVSAvoidsignal noise

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The patent replaces the voltage-mode sensing mechanism (resistor dividers) with a current-mode sensing mechanism. Current mirrors are used to create sense currents that are proportional to the BEMF voltages, eliminating the need for resistive voltage division. This substitution of sensing modality fundamentally removes the noise introduction mechanism while preserving the voltage level matching function through current proportionality relationships.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the sensing parameter from voltage division to current mirroring. By using current mirrors with matched transistor parameters, the system achieves accurate BEMF sensing through current proportionality rather than voltage division. This parameter change transforms the sensing approach to one that is inherently less noisy while maintaining the necessary signal conditioning.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If clamp diodes are used to limit signal excursions, then circuit protection is improved, but commutation errors increase and measurement precision deteriorates

Engineering Contradiction:

Improvecircuit protectionVSAvoidBEMF detection accuracy

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The patent replaces the clamp diode protection mechanism with a current-mode sensing approach that inherently limits the impact of voltage excursions. The current mirrors and operational amplifiers in the current-mode circuitry provide natural signal clamping through their operating characteristics, eliminating the need for external clamp diodes that introduce commutation errors and measurement inaccuracies.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Ease of operation

If resistor dividers and clamp diodes are used for signal conditioning, then voltage level matching is achieved, but current flow from motor windings increases and power dissipation worsens

Engineering Contradiction:

Improvevoltage level matchingVSAvoidpower dissipation

Core Design Contradiction:

Ease of operationVSLoss of energy

Solution Approach 1:

The patent substitutes the high-current voltage-mode signal conditioning circuitry (resistor dividers and clamp diodes) with a current-mode sensing system. The current mirrors draw minimal current from the motor windings while still achieving the necessary signal conditioning and voltage level matching through current proportionality and operational amplifier action.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the operating parameters of the sensing circuit to use current-mode operation instead of voltage-mode operation. This parameter change enables the system to achieve voltage level matching with significantly reduced current draw from the motor windings, thereby reducing power dissipation in the sensing circuitry.

Inventive Principle:

Principle #35Parameter changes

4Power

If resistor dividers are used to sense BEMF, then voltage reduction is achieved, but device complexity increases due to external components

Engineering Contradiction:

Improvevoltage reduction capabilityVSAvoidexternal components

Core Design Contradiction:

PowerVSDevice complexity

Solution Approach 1:

The patent merges the voltage reduction function and the signal conditioning function into the integrated current-mode sensing circuitry. The current mirrors and operational amplifiers perform both the voltage level matching and the signal conditioning that previously required separate external resistor dividers and clamp diodes, thereby reducing device complexity and enabling full integration onto a semiconductor die.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The current-mode sensing circuit performs multiple functions simultaneously: it provides voltage level matching, signal conditioning, noise filtering, and protection against voltage excursions. This multi-functionality eliminates the need for multiple external components, reducing overall device complexity while maintaining the necessary voltage reduction capability.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Data Source

PatentUS11329586B1Semiconductor device and method therefor

Publication Date: 2022.05.10 SEMICON COMPONENTS IND LLC

AI summary

In one embodiment, a motor control circuit may be configured as a current circuit that forms a sense current that is representative of at least a BEMF voltage from a motor, and to use the sense current to detect a substantially zero-crossing of the BEMF voltage.