How to Measure EMI and How to Suppress it？

A report by the World Health Organization states that in 2022 alone, there were more than 1,500 cases worldwide where EMI was blamed for medical device failures. In automotive electronic systems, 30% of intermittent faults can be attributed to EMI problems.How to measure EMI precisely and construct effective protective systems have become central challenges in the modern engineering field.

The Principles of EMI

Different types of electromagnetic interference are the main elements influencing the compatibility of electrical devices. Consequently, grasping the origins of electromagnetic interference is an essential condition for reducing it and improving the electromagnetic compatibility of electronic devices. The creation of electromagnetic emission can be classified into:

Internal Interference

This refers to the mutual interference among internal electronic components.

1.Interference caused by leakage due to distributed power sources and insulation resistance through power supply lines.

2.Effects resulting from signal coupling through ground lines, power supplies, and transmission wires, or mutual inductance between wires.

3.Interference arising from certain components within equipment or systems overheating, which affects the stability of both themselves and other components.

4.Interference caused by magnetic fields and electric fields generated by high-power and high-voltage components affecting other parts through coupling.

External Interference

These are influences exerted by factors outside electronic devices or systems on the circuits,device under testing, or systems themselves.

1.External high voltage or power sources interfering with electronic circuits, device under testing, or systems through insulation leakage.

2.Powerful external devices generating strong magnetic fields in space that interfere with electronic circuits, devices, or systems through mutual inductance coupling.

3.Interference caused by spatial electromagnetic fields on electronic circuits or systems.

4.Fluctuations in working environment temperature leading to changes in parameters of internal components within electronic circuits, nearby devices, or systems, thus causing interference.

EMI measurement System

1.EMI Measurement Equipment

The accurate measurement of electromagnetic interference is akin to performing an "electromagnetic health check" for electronic devices, requiring specialized instruments to establish a comprehensive diagnostic system:

A spectrum analyzer acts as the "stethoscope" for capturing interference signals. Its superheterodyne reception technology can cover frequencies from 9 kHz to 18 GHz, and with peak hold functionality, it can capture transient interference pulses.

Near-field probe sets function like an "electromagnetic microscope." The H-field scope probe can detect radiated emission from current loops at acceptable levels of 1 μA/m, while the E-field monopole probe can pinpoint sources of electric field leakage at 10 mV/m.

Standard antenna systems serve as the "yardstick" for far-field measurements. Biconical antennas provide omnidirectional response in the 30-300 MHz frequency range, while log-periodic antennas offer stable gain between 200 MHz and 2 GHz.

2.EMI testing procedure

The process of measuring electromagnetic interference (EMI) generally includes multiple essential steps:

2.1 Readiness

Equipment Selection: Select appropriate emissions testing tools like spectrum analyzers, antennas, and near-field probes.

Environmental Assessment: Verify that the emissions testing setting fulfills the specifications, like a shielded space or open environment, to prevent outside disturbances.

2.2 Calibration of Equipment

Adjust Equipment: Adhere to the manufacturer's guidelines to set up the emissions testing equipment, guaranteeing measurement precision.

Function Test: Ensure all equipment is functioning properly.

2.3 Configuration Setup for Testing

Establish Testing Criteria: Recognize emissions testing techniques and thresholds according to applicable standards like CISPR or FCC Part 15.

Position the Equipment Under Test (EUT): Arrange the EUT in compliance with regulations, confirming it is in standard operational condition

2.4 Conducting the Test

Frequency Range Scanning: Use a spectrum analyzer to scan the specified frequency range.Data Recording: Capture and document the frequencies and amplitudes of any interference signals detected during the scan.

2.5 Data Analysis

Result Comparison: Compare the measurement results with relevant standard limits.Identify Problem Sources: If limits are exceeded, attempt to identify potential interference external sources.

2.6 Remediation Measures

Implement Remediation: Take necessary corrective actions for non-compliant items, such as adding shielding or filters.Retesting: Conduct retesting to verify the effectiveness of the remediation.

2.7 Report Generation

Write Report: Document the testing process, data, and analysis results in a report.Conclusion and Recommendations: Provide a summary and suggestions for further improvements.

These steps may vary depending on different equipment, industry standards, or specific applications. Ensuring compliance with relevant regulations and standards is crucial.

The Hazards of EMI

From the perspective of current societal development, every aspect of our daily lives is closely intertwined with electronic devices. Consequently, the likelihood of harm caused by electromagnetic interference has significantly increased.

Certain conductive electromagnetic interference can degrade the inherent performance of electronic devices or lead to significant catastrophic consequences. For example, in the case of various medical equipment such as high-frequency surgical tools or microwave therapy devices, if such equipment emits high-frequency spatial radiated emission, it is likely to impair the performance of nearby equipment. In severe situations, if electronic devices are within the same circuit, they may experience sudden catastrophic failures.

Intense electromagnetic interference can damage sensitive electronic equipment due to overload. Typically, the reverse breakdown voltage between the emitter and base of a silicon transistor ranges from 2 to 5 volts, making it susceptible to damage. Additionally, this reverse breakdown voltage decreases as the temperature rises. The spike voltages caused by electromagnetic interference can increase impurity concentration at certain points in the emitter and collector junctions, leading to transistor breakdown or internal short circuits.

Measures to Suppress EMI

According to the fundamental principles of electromagnetism, electromagnetic interference (EMI) requires three elements: a source of interference, a pathway for the interference, and a system sensitive to electromagnetic interference. To prevent interference, both hardware and software anti-interference measures can be employed. Hardware anti-interference is the most basic and crucial measure, typically focusing on suppression and prevention to mitigate interference. The general principle is to suppress and eliminate interference sources, cut off coupling paths to the system, and reduce the system's sensitivity to interference external signals. Specific engineering measures include isolation, filtering, shielding, and grounding.

Isolation

Isolation refers to electrically separating the interference source from the part vulnerable to interference. In variable frequency drive systems, an isolation transformer is often used on the power line between the power source and amplifier circuit to avoid conducted interference. A noise isolation transformer can be employed for this purpose. In practical engineering, equipment should be arranged so that inverters are isolated as much as possible to eliminate their electromagnetic interference. However, due to space constraints, this may not always be feasible. Strategies such as placing power distribution cabinets between inverters and control devices can help minimize interference.

Filtering

High-frequency harmonics produced by inverters during operation can severely distort the power grid waveform, causing significant voltage drops and low power factors. Large-capacity inverters require particular attention. Solutions typically involve using reactive power compensation devices to adjust the power factor, along with installing inductive filters at both the power supply input and load side to reduce impact on the power grid.

Shielding

Shielding the interference source is one of the most effective methods to suppress interference. Enclosing inverters within a fully sealed metal case that is reliably grounded can minimize electromagnetic radiation and reduce interference with other equipment, particularly electronic circuits and devices. Inverters use high-performance microprocessors and integrated circuits, which are sensitive to external electromagnetic radiation. Interference can infiltrate through control cables, so adequate anti-interference measures must be taken during cable installation. Common measures include using shielded wires for analog control lines, with the shield connected to the common end (COM) of the control circuit near the inverter, while leaving the other end floating, rather than connecting it to the inverter's ground (E) or earth.

Grounding

Practice shows that grounding is a crucial method for noise suppression and interference prevention. Proper grounding can significantly reduce internal noise coupling and prevent external interference, enhancing the system's anti-interference capability. Various grounding methods exist for inverters, such as multi-point, single-point, and bus grounding, and should be selected based on specific conditions. It’s essential to ensure good grounding to prevent interference with equipment. Each device, when grounded alongside others or multiple inverters, should have its own independent connection to ground. Connecting the grounding of one device to another before grounding can lead to increased radiation and mutual interference, hindering normal operation.