To meet the EMC standards requirements, the design of electrical equipment must address several construction aspects related to wiring, grounding and shielding, metallic enclosure and PBC layout and the use of RFI suppression parts. Because of that, this article provides a list of the best EEE constructional tips and practices for ensuring EMC compliance.
Top EEE constructional tips and practices:
- Where cable crossings are unavoidable, a 90° crossing should be used for maximum decoupling.
- Cables should be shortened to the necessary length and routed in defined ways to minimise inductance and loop area.
- Be aware that inductive coupling can occur where cable loops of different cables are located too close to each other.
- Never cross input and output connections of a filter.
- Harmonic currents can cause overheating in the local supply distribution transformer. This is avoidable if it’s adequately rated or, at least, on the assumption of low harmonic levels.
- Use conductive painting and gaskets in the shielded enclosure.
- For cable shielding, you should use conduits, braided wires, metal films or two separate shields.
- Reduce the coupling from external radiated fields and between susceptible paths and paths with high emission.
- Ensure that each electrical circuit has an independent ground connection to avoid different potentials.
- Meet the emissions standard requirements by at least 3– 4 dB.
- To minimise coupling, keep the clock away from input or output circuitry.
- Each power pin should use high-frequency, low-inductance ceramic decoupling capacitors.
- Use triaxial cables for circuits with low and high-frequency signals.
- Keep in mind that equipment with timers and thermostats that cause frequent load changes will also cause voltage changes and fluctuations.
- Make all shielded enclosures of metal or other conductive materials.
- Ensure all shielded enclosures are grounded plainly.
- Use low-impedance material to achieve a high reflection loss at a sealing interface in an enclosure.
- Decouple bulk capacitors with a smaller capacitor with a lower effective series inductance.
- You can obtain the best effect of a cable shield by using a 360°ground connection on both ends of the shield.
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- Install the filter on the product enclosure in a surface part without painting.
- Use decoupling capacitors of 0.1 μ F for lower frequencies (below 15 MHz).
- Use decoupling capacitors of 0.01 μ F for higher frequencies (above 15 MHz).
- Where possible, use filtering components to limit the bandwidth of the clock signal.
- Make all ground connections directly to the ground plane to prevent ground loops.
- Ensure that the optimal trace width is 0.495 mm, or 20 mils, to reduce parasitic inductance.
- Ensure that the PCB filter provides a low-impedance connection to the protective earthing terminal in order to minimise the noise radiation voltage from the mains inlet connection.
- To diminish fluctuations in the output voltage in an SMPS, add a capacitor in parallel with the load.
- Place all parts of the PCB into functional groups (e.g. circuitry group).
- Avoid slots and bottlenecks
- To prevent signal reflection, use angles of less than 45 degrees.
- Place device decoupling capacitors very close (less than 1 mm) to the VCC and GND pins.
- Use a resistor in series with a clock output to reduce peak switching currents and limit the tendency to overshoot.
- For high frequencies, use cables with well-known characteristic impedances and ground them on both ends. For low frequencies, it’s enough to ground the cable only on the transmitter side.
- Separate all conducting traces so that there is at least 1 mm of ground space between them.
- To minimise the ground plane’s inductance, ensure that the PCB’s length/width ratio is less than 5.
- Place all traces with a high switching current at least 3 mm away from other parallel signal traces.
- Reduce cross-coupling and minimise the ground noise’s effects by keeping the fast logic away from connectors.
- Arrange the PCB’s ground lines in the form of a star, with the common point of origin at the power-feed entry point.
- Lastly, install an EMI filter as close as possible to the area in need of protection against the entering of unwanted signals.
