What is Electromagnetic Compatibility?
Electromagnetic Compatibility (EMC) is the ability of an electrical device to function properly in its environment without being affected by electromagnetic interference from other devices. Therefore, EMC includes two test standards: Electromagnetic Interference (EMI) and Electromagnetic Susceptibility (EMS). Read on to learn more about EMC and how you can prevent it from negatively affecting your electronics.
EMC Certification Standards
Electromagnetic compatibility standards are important for manufacturers who have to deal with EMC. There are many different standards for EMC and many different industries that need EMC testing.
- IEC: The International Electrotechnical Commission, it includes 3 branches:
CISPR: International Special Committee on Radio Interference
TC77: Technical Committee on Electromagnetic Compatibility in Electrical Equipment (including Power Grids)
TC65: Industrial Process Measurement and Control
- ISO: International Organization for Standardization;
- ETSI: the European Telecommunications Standards Committee;
- CCIR: International Radiocommunication Advisory Committee;
FCC: Federal Pass;
VDE: German Association of Electrical Engineers;
VCCI: Japanese civil interference;
BS: British Standard;
ABSI: American National Standard;
GOSTR: Russian government standard;
GB, GB/T: Chinese National Standard.
Common EMC Metrics
Many different metrics can be used to measure EMC on an individual device or piece of equipment. However, there are a few metrics that are most common for EMC measurements for electronics.
- Electromagnetic field strength
This is a measurement of how much electromagnetic radiation is being emitted by a device. It can be useful to determine if EMI is being emitted by a device.
- Electromagnetic shielding effectiveness
This is a measurement of how well a device can deflect or block outside EMI. It can be useful to determine if a device will be affected by outside EMI.
- Electromagnetic coupling
This is a measurement of how well a device will be able to couple with another device’s EMI. It can be useful to determine if a device will be affected by other devices’ EMI.
How to protect electronic equipment from EMI?
1. EMC shielding design
The effectiveness of your EMC shielding design relies on the type of material you choose as well as how it is implemented. You can further improve its performance by combining different types of materials together or by choosing a certain orientation for each specific layer of your shielding.
1.1 Ventilation hole and opening design
1.2 Structural lap joint shielding design
1.3 The cable passes through the shielding body
If the conductors pass out of the shield, the shielding effectiveness of the shield will be significantly degraded. This penetration is typically when the cable exits the shield.
1.4 Design principles for cables going out of the shielding body
1.4.1 When shielded cables are used, when the shielded cables exit the shielding body, the clip wire structure is adopted to ensure reliable grounding between the shielding layer of the cable and the shielding body and provide a sufficiently low contact impedance.
1.4.2 When using shielded cables, use shielded connectors to transfer the signals out of the shielding body, and ensure the reliable grounding of the shielding layers of the cables through the connectors.
1.4.3 When using an unshielded cable, use a filter connector to transfer. Due to the high frequency characteristic of the filter, it is ensured that there is a sufficiently low high frequency impedance between the cable and the shield.
1.4.4 When using unshielded cables, the cables should be short enough inside (or outside) of the shield to prevent interference signals from being effectively coupled out, thereby reducing the impact of cable penetration.
1.4.5 The power line goes out of the shield through the power filter. Due to the high-frequency characteristic of the filter, it is ensured that there is a sufficiently low high-frequency impedance between the power line and the shield.
1.4.6 Using optical fiber outlet. Since the optical fiber itself has no metal body, there is no problem of cable penetration.
1.5 Poor grounding
1.6 Shielding materials and applications
The material that we need to shield includes conductive cloth, reed, conductive rubber, and more.
1.7 Cut-off waveguide ventilation plate
2. EMC grounding design
2.1 The concept and purpose of grounding
2.1.1 One is for safety, called protective grounding. The metal casing of electronic equipment must be connected to the ground, so as to avoid the occurrence of excessive ground voltage on the metal casing due to accidents, which may endanger the safety of operators and equipment.
2.1.2 The second is to provide a low-impedance path for the current to return to its source, that is, the working ground.
2.1.3 Lightning protection grounding to provide current discharge for lightning strikes.
2.2 Grounding provides signal return
2.3 Single point grounding
Suitable for systems with operating frequency below 1MHz.
2.4 Multi-point grounding and mixed grounding
3. EMC Wave filter design
3.1 Wave Filter Definition
A wave filter is a device that alters the frequency content of a signal by selectively attenuating certain frequencies while allowing others to pass.
3.2 Type of wave filters
The common filter types include: low pass filter, high pass filter, band-pass filter, and band-stop filter. As figure shows below:
If a filter passes low frequencies and blocks high frequencies, it is called a low pass filter. If it blocks low frequencies and passes high frequencies, it’s a high pass filter. There are also bandpass filters, which pass only a relatively narrow frequency range. And a band-stop filter, which blocks only a relatively narrow range of frequencies.
3.3 Wave Filter components
3.3.1 Capacitor (general capacitor, three-terminal capacitor);
3.3.2 Inductance (general inductance, common mode inductance, magnetic beads);
3.4 Differential mode filter and common mode filter design
4. EMC PCB Design
4.1 PCB design
4.1.1 Layout: similar circuits are arranged in one piece, the principle of controlling the minimum path, high-speed circuits should not be close to the small panel, and the power module should be close to the position of the single disk.
4.1.2 Layering: The high-speed wiring layer must be close to a ground layer, the power supply is adjacent to the ground, a layer of ground should be placed under the component surface, two surface layers may be placed close to the ground layer, and the inner layer should be indented by 20H compared to the surface layer.
4.1.3 Wiring: The 3W principle, the differential pair lines are of equal length, and the close walking, high-speed or sensitive lines cannot cross the partition.
4.1.4 Grounding: similar circuits are distributed separately and connected at a single point on the board.
4.1.5 Filtering: power supply module, functional circuit design board-level wave filter circuit.
4.1.6 Interface circuit design: interface circuit design filter circuit to achieve effective isolation between inside and outside.
4.2 The basic principles of layout
4.2.1 Referring to the functional block diagram of the principle, based on the signal flow, it is divided into functional modules.
4.2.2 Separate layout of digital circuits and analog circuits, high-speed circuits and low-speed circuits, interference sources and sensitive circuits.
4.2.3 Avoid placing sensitive devices or strong radiation devices on the welding surface of the single board.
4.2.4 The loop area of sensitive signals and strong radiation signals is the smallest.
4.2.5 Strong radiation devices or sensitive devices such as crystals, crystal oscillators, relays, switching power supplies, etc. should be placed away from single-board handle bars, external interface connectors, and sensitive devices. The recommended distance is ≥1000mil.
4.2.6 Sensitive devices: keep away from strong radiation devices, the recommended distance is ≥1000mil.
4.2.7 Isolation devices, A/D devices: the input and output are separated from each other, and there is no coupling path (such as adjacent reference planes), preferably across the corresponding partition.
4.3 Special device layout
4.3.1 Power part (placed at the power inlet).
4.3.2 Clock part (away from the opening, close to the load, wiring inner layer).
4.3.3 Inductive coil (away from EMI source).
4.3.4 Bus driver part (inner layer of wiring, away from the opening, close to the sink).
4.3.5 Filter components (separate input and output, close to the source, short leads).
4.4 Layout of filter capacitors
4.4.1 All branch power supply interface circuits.
4.4.2 Near components with high power consumption.
4.4.3 Areas with large current changes, such as input and output terminals of power modules, fans, relays, etc.
4.4.4 PCB power interface circuit.
4.5 Layout of decoupling capacitors
4.5.1 close to the power pins.
4.5.2 Appropriate location and quantity.
4.6 The basic principles of the layout of the interface circuit
Devices such as filtering, protection, and isolation of interface signals are placed close to the interface connector, and they are protected first and then filtered.
Isolation devices such as interface transformers and optocouplers are completely isolated from the primary and secondary.
No crossover of signal network between transformer and connector.
The BOTTOM layer area corresponding to the transformer should be placed as far as possible without other devices.
The interface chip (network port, E1/T1 port, serial port, etc.) should be placed as close as possible to the transformer or connector.
Short traces, wide spacing between different types of traces (except for signals and their return lines, differential lines, and shielded ground lines), fewer vias, no loops, small loop area, wireless head.
For traces with delay requirements, their lengths meet the requirements.
There is no right angle, and arc chamfering is preferred for key signal lines.
The signal traces of adjacent layers are perpendicular to each other or the parallel wiring of key signals of adjacent layers is less than or equal to 1000MIL.