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Conducted terminal disturbance
core concept
Conducted terminal disturbance refers to the electromagnetic disturbance noise emitted by electronic and electrical equipment through its wired connection ports such as power lines, signal lines, and control lines to the power grid or other connected devices.
Conduction: Refers to the propagation of noise through actual wires (such as power lines).
Terminal: Refers to the external connection port of a device, most commonly the power terminal (AC or DC input port), and also includes telecommunications/signal ports.
Harassment: refers to electromagnetic noise that exceeds regulatory standards and may affect the normal operation of other devices.
It contrasts with 'radiation disturbance', which refers to noise propagating through space in the form of electromagnetic waves.
The source of harassment
These noises are mainly generated by the internal switching power supply, digital circuits, motor drives, etc. of the equipment:
Switching mode power supply (SMPS) is the main source. The high-speed switching action of power devices (such as MOSFETs) can generate significant voltage and current changes (dv/dt, di/dt), and these high-frequency harmonics are transmitted out through power lines.
Digital circuits: The high-speed transitions of clock signals and data buses generate rich high-frequency harmonics, which are coupled to power lines through power distribution networks.
Inverter, motor driver: Switches such as IGBT will generate strong conduction noise.
The on-off of relays and contactors can generate transient pulse disturbances.
Types of noise
Conducted disturbances are mainly divided into two categories, which differ in frequency and generation mechanism:
Differential mode noise
Path: Noise current flows in opposite directions between the live wire (L) and the neutral wire (N).
Source: Usually generated directly by the switching current of the power supply. The circuit area is small and the frequency is relatively low (mainly in the low frequency range of<1 MHz to 30 MHz).
Analogy: It's like two people pulling a saw, with force transmitted back and forth between them.
Common mode noise
Path: Noise current flows in the same direction between the live wire (L) and the ground wire (PE), as well as between the neutral wire (N) and the ground wire (PE).
Source: Mainly caused by high-frequency voltage fluctuations (dv/dt) of switching devices (such as MOSFET drain) to the reference ground, coupled to the ground through parasitic capacitance. The circuit area is large and the frequency is high (mainly ranging from a few hundred kHz to 30 MHz, or even higher).
Analogy: It's like two people pushing or pulling an object at the same time, with force in the same direction.
In practical devices, two types of noise usually coexist and may be converted into each other.
Testing standards and limits
In order to ensure the harmonious coexistence of all electronic devices in the public power grid, countries have established mandatory EMC standards.
Common standards:
International: CISPR series standards (such as CISPR 32 corresponding to multimedia devices).
China: GB 9254 (corresponding to information technology equipment), GB 4824 (corresponding to engineering and medical equipment).
Europe: EN 55032 (corresponding to multimedia devices), etc.
Testing method:
Conducted in an electromagnetic shielding room.
The use of artificial power networks (LISN/AMN): connected in series between equipment and the power grid, has two key functions:
Provide pure 50Hz power supply for the tested equipment.
Provide a standard and stable high-frequency impedance (50 Ω) and separate the conducted noise generated by the device for measurement by the receiver.
Measure noise voltage using an EMI receiver, typically in the frequency range of 150 kHz to 30 MHz (for some devices such as automotive electronics, it may start from a few kHz).
Limit: The standard specifies the limit lines for both quasi peak and average detection methods. The test result must be below this limit line to be considered qualified.
suppression measures
Design engineers will use various filtering techniques to suppress conducted disturbances:
EMI filter: This is the most core and effective means. A typical power input EMI filter includes:
X capacitor: connected between L-N, mainly filters out differential mode noise.
Y capacitor: connected between L-PE and N-PE, mainly filters out common mode noise. Its capacitance is strictly limited (usually ≤ a few nF) to ensure leakage current safety.
Common mode inductance: Two identical coils are wound around a magnetic core, presenting high impedance to common mode current and canceling out magnetic flux to differential mode power supply current (equal in magnitude and opposite in direction), resulting in low impedance. It is a key component for suppressing common mode noise.
Differential mode inductor/magnetic bead: used to assist in suppressing differential mode noise.
Optimize PCB layout:
Reduce the high-frequency loop area.
Provide a clean, low impedance grounding path for switch devices.
Reasonably set power decoupling capacitors (large capacitors filter low frequencies, small capacitors filter high frequencies).
Use shielding: Shield internal noise sources (such as switch power modules) to prevent noise from coupling to cables.
Use ferrite magnetic rings: Install magnetic rings online to form impedance against high-frequency noise, especially effective in suppressing high-frequency common mode noise.
summary
| feature | Conducted terminal disturbance | Radiation harassment |
|---|---|---|
| transmission route | Through wires (power lines, signal lines) | Through spatial electromagnetic waves |
| Main frequency | Low frequency range (usually 150 kHz -30 MHz) | High frequency band (usually 30 MHz -1 GHz or above) |
| test object | Power port, signal port | Overall equipment and cables |
| Core countermeasures | Filtering (EMI filter, magnetic ring) | Shielding, grounding, filtering |
Simply put, the interference of conductive terminals focuses on the problem of "noise running out along the wire", which is the first hurdle that must be overcome in product EMC design and a mandatory testing item for product certification. Solving it requires a systematic consideration of three aspects: noise sources, propagation paths, and filtering.
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