EMC Rectification for Brushless DC Motors
I. Introduction
In recent years, with the development of China's economy and the improvement of people's living standards, the continuous upgrading of consumption levels has led to increasingly stringent electromagnetic compatibility (EMC) requirements for electronic products. This necessitates that technical personnel consider not only the basic functionality of products during the initial design phase but also their EMC performance, particularly electromagnetic interference (EMI) emissions. Otherwise, rectifying issues during the later stages of product development will consume significant manpower and resources with limited improvement effects. As a critical component in many products—such as robotic vacuum cleaners, vacuum cleaners, electronic door locks, automotive wipers, air conditioning compressors, and various other electric devices—the electromagnetic radiation of brushless DC motors directly impacts whether the product meets EMC standards. This article focuses on analyzing the noise sources of brushless DC motors and proposes corresponding solutions, followed by a practical case study.
II. PWM Modulation Principle of Brushless DC Motors
The speed regulation of brushless DC motors via PWM (Pulse Width Modulation) involves chopping the DC power supply into PWM waves using a chopper, thereby adjusting the average voltage across the armature to control the motor's speed. For example, in the HPWM-LON modulation mode—a two-phase conduction star-type three-phase six-state operation—the upper bridge power switches (Q1, Q3, Q5) are modulated by PWM signals, while the lower bridge power switches (Q4, Q5, Q2) remain constantly conductive.
III. Generation of EMI
EMI manifests in two forms: conducted and radiated. Radiated EMI can be shielded using metal enclosures, while conducted EMI is mitigated with filters. Since both types originate from the same energy source, the energy must be dissipated somewhere. Blocking conducted EMI often increases radiated EMI. Therefore, effective EMI suppression relies on a thorough understanding of EMI mechanisms to reduce it at the source rather than merely blocking it after generation. The path of radiated EMI is difficult to pinpoint, making a complete analysis of its mechanism challenging. In contrast, the path of conducted EMI is well-defined, and analyzing it helps identify the EMI source and understand its generation mechanism. Reducing this source effectively diminishes both conducted and radiated EMI. Thus, the focus is on conducted EMI.
The coupled current in conducted EMI is the sum of currents in parasitic capacitances, caused by high du/dt and large square-wave voltage amplitudes. The coupled current comprises differential-mode and common-mode components, each propagating through distinct paths.
(1) Differential-Mode EMI
The motor controller employs PWM modulation, which, while generating a near-sinusoidal waveform, also superimposes high-order harmonics at the output, creating differential-mode interference. The primary cause of differential-mode interference is the pulsating current signal at the inverter bridge input due to the switching of power transistors. From the perspective of propagation paths, differential-mode currents mainly travel through the DC bus (positive and negative terminals) and AC cables on the input side.
(2) Common-Mode EMI
During the switching transitions of IGBTs, high du/dt is generated, leading to continuous charging and discharging through parasitic capacitances between components (e.g., switches, metal heat sinks, enclosures) and ground, thereby producing common-mode currents.
The common-mode current path consists of a DC bus, parasitic capacitance (Cc) between phases and ground, earth, AC power supply, and rectifier. A portion of the current flows through the rectifier's parasitic capacitance and DC capacitor. Since common-mode currents from different devices share the earth path, common-mode EMI levels are higher and more challenging to suppress.
IV. Rectification Measures
EMI suppression involves blocking the conduction path or confining it internally. Corresponding measures should be taken based on the interference source. The general principle is to first provide a local path and then increase high-frequency impedance.
Common Filter Circuits:
Option 1: Common-Mode Choke + Inductor + Capacitor
The common-mode choke effectively filters common-mode noise, while the capacitor targets differential-mode noise. Inductors L1 and L2, placed closest to the DC motor, effectively suppress voltage spikes generated during motor commutation.
Option 2: BDL + Ferrite Bead
BDL: The BDL device is a multifunctional integrated component with excellent EMI filtering performance.
Features:
Average component size reduction of 73%, meeting ultra-thin design requirements.
Optimized PCB area: >70%.
Directly replaces traditional LC filter circuits (common-mode choke + 2–3 filter capacitors).
Simultaneously filters common-mode and differential-mode noise. Due to its characteristics, BDL outperforms capacitors and common-mode chokes, with a filtering range extending to GHz and high voltage tolerance.
PE (Protective Earth) is typically the motor's metal casing. In addition to motor leads, the filtering sub-board requires an additional ground connection to the metal casing, and the quality of conductivity between these significantly impacts filtering effectiveness.
V. Case Study
A customer rectified a motor used in a vacuum cleaner with the following solution: BDL + ferrite bead + capacitor. The capacitor was added primarily to increase overall margin.
Pre-Rectification Data:
Vertical Direction:
Horizontal Direction:
Post-Rectification Data:
Vertical Direction:
Horizontal Direction:
VI. Summary
Electromagnetic compatibility is a critical issue in motor drives, especially with the widespread use of power electronic devices in transmission systems. In-depth research into its mechanisms and the implementation of practical technical measures during product design and development are essential to gradually mitigate EMI effects and achieve high reliability.
