Electromagnetic Compatibility (EMC) In PCB Designs
Özge Memiş
Electrical & Electronics Engineer
Aktif Mühendislik
PCB design has become significantly more challenging with the increasing demand for high speed circuits. In addition to the design logic on the PCB, some other issues affecting the circuit such as power consumption, PCB size, environmental noise and EMC should be considered. In this article, the points to be considered in order to design a system in which EMC and EMI can be controlled / prevented at the PCB design stage are discussed.
1. Introduction
An electronic system consists of printed circuit boards (PCBs), integrated chips, interconnects, and input / output cables. Interconnections tend to act as antennas at higher frequencies depending on the length and the current carried. This situation results in electromagnetic interference (EMI). The EMI radiations generated interact with other devices in the environment. Devices exposed to this interaction can create safety problems for human and environmental health [1]. Therefore, there are international standards that limit the emission level. Devices must be designed in accordance with these standards and must meet the minimum requirements of these standards. For this reason, measuring electromagnetic radiation and controlling these radiations are extremely important [2].
2. Electromagnetic Compatibility (EMC) and Electromagnetic Interference (EMI)
Electromagnetic compatibility (EMC) is the ability of an electronic system to function satisfactorily in an electromagnetic environment without generating electromagnetic interference in nearby devices / systems. Electromagnetic compatibility ensures that the system operates as intended within the scope of defined safety measures [3]. Electromagnetic interference (EMI) causes electromagnetic noise signals generated by various sources to penetrate devices with electronic components in different ways and negatively affect system operation [3]. Checking EMI at the first stage of PCB design is essential. Controlling EMI in later production stages can be costly. The most important parameters to be considered for an EMC friendly electronic card design; component selection, circuit design and PCB layout design. Commercial products must meet the prescribed EMC standards to be ready for the market. Since 1996, EMC standards have been compulsory in the European Union market and are also a requirement for CE certification [4].
3. EMI / EMC Preventive Approaches in PCB Design
3.1. Ground Planes
One of the most important PCB design elements applied to minimize EMI is the low inductance ground system. Maximizing the ground area on the PCB decreases the ground inductance in the system. This reduces electromagnetic emissions and interference [5]. Signals can be connected to ground using different methods. The structure formed by connecting components to random grounding points is a poor PCB design. Such a design produces high ground inductance and leads to inevitable EMC problems [6].
In the design, it should be preferred that one of the PCB layers is ground as it will provide low impedance [3]. If it is not possible for a whole layer to be ground, then ground grids should be used. In this case, the ground inductance will depend on the space between the grids [4].
Using a Faraday cage is another mechanism used to reduce the problems caused by EMI. The Faraday cage is created by throwing ground around the PCB and not sending any signals beyond this limit. This mechanism limits the PCB to PCB emission and interference inside and outside the boundary defined by the cage [4].
The way a signal track rotates is very important in system grounding. This is because when a signal takes a longer track, it creates a ground loop that creates an antenna and radiates energy. Therefore, any track that carries the current back to the source must follow the shortest distance and be connected directly to the ground plane. It is not recommended to connect all ground paths in the circuit and then connect them to the ground plane [3].
Figure 1 Various ground plane applications [3]
High speed circuits should be placed closer to the ground, low speed circuits closer to the power layer. Copper fill areas must always be grounded. Otherwise, it may act as an antenna and cause EMC problems [3]. In cases where more than one power supply is required in the circuit, separating the power planes with ground planes can prevent the power sources from getting noise from each other [4].
3.2. Component Separation
For an EMC friendly electronic card design, the components on the PCB should be grouped according to their functionality such as analog, digital, power supply, low speed circuits, high speed circuits. Signal tracks for each component group must remain within defined areas. The filter can be used when a signal needs to be connected from one subsystem to another. [4]
Figure 2 Grouping of PCB components [4]
3.3 Arrangement of PCB Layers
The EMC performance of a PCB also depends on the arrangement of its layers. If a PCB with two or more layers is to be designed, an entire layer should be used as ground. In case of a 4-layer PCB, the lower layer of the ground layer should be used as the power layer [1]. Care must be taken that the ground layer is always between the high frequency signal tracks and the power layer. If a separate power plane is not used, ground tracks and power tracks should be designed in parallel to keep the supply clean. [3]
Figure 3 4-layer PCB layer arrangement [3]
When there are more than 4 layers, it is recommended to arrange PCB layers as signal layer › ground / power layer › signal layer › ground / power layer › signal layer › ground / power layer › signal layer [4].
3.4 Decoupling Capacitor
While ICs are working, they switch with high frequency due to their internal structure. This situation causes switching noise in the IC connected tracks. If this noise is not controlled, it will cause emission and therefore EMI [1]. By placing a decoupling capacitor near the IC, it is possible to reduce the spread of switching noise on the PCB and direct the noise to the ground. [2]
Figure 4 Decoupling capacitor layout [4]
3.5. Crosstalk
Crosstalk is used to identify disturbances caused by electromagnetic noise from one track on the PCB to another nearby track. Crosstalk in PCBs usually occurs in tracks that are side by side in the same layer or one above the other in adjacent layers. This situation appears as noise and can cause malfunctions if the amplitude is too large [7].
Figure 5 Crosstalk example [8]
When it encounters a bending of 90 ° while drawing the tracks in the PCB, the capacitance increases and this causes the characteristic impedance value to change. For this reason, 45 ° turning angle should be preferred in turns. [4]
Figure 6 Example of track drawn with 45 ° turning angle [4]
3.6. Digital Circuits
As you are working on digital circuits, high-speed signals should be extra paid attention to exactly. The tracks connecting such signals should be kept as short as possible and should be adjacent to the ground plane to control interference. While drawing such signals, it should be avoided to pass the PCB’s edge or close to the connectors. These signals should be kept away from the power plane as they can cause noise in the power plane [9].
While drawing the track for the oscillator in the circuit, care should be taken that no other tracks other than ground works in parallel or below the tracks of the oscillator. In addition, the oscillator should be kept close to the relevant IC [1]. It should also be noted that the return current always follows the least reactance track. Therefore, in order to keep the current loop as short as possible, ground tracks carrying return current should be kept close to the tracks carrying the relevant signal [9].
3.7. Analog Circuits
Tracks carrying analog signals should be kept clear of high speed or switching signals and should always be protected by a ground signal. A low-pass filter can be used to avoid high frequency noise surrounding analog tracks. In addition, it is important that the ground planes of the analog and digital subsystems are not common [2].
3.8. Shielding
Shielding is not an electrical solution, it is a mechanical approach to reduce EMI. Metallic packages (conductive or magnetic materials) are used to prevent electromagnetic emissions. These packs can be used to cover the entire system or part of it depending on the requirements. The protection is like a closed conductive enclosure connected to ground. It effectively reduces the emission by absorbing or reflecting some of the electromagnetic emissions. [6]. In this way, a shielding also acts as a partition between circuits / devices, weakening electromagnetic emission from one area to another. The shielding reduces EMI by weakening both the electric field and the magnetic field component of the spreading wave. [10].
Figure 7 Shielding example [4]
4. Results
An electronic circuit is made up of several electronic components arranged in a pre-defined manner. If the arrangement is not proper, then it may cause various EMI/EMC issues. The design of a PCB for any component has a major effect on its EMC performance and the amount of electromagnetic emission generated. While designing a PCB, you need to be mindful of each component’s EMI/EMC effect. Good EMC performance can be achieved with good PCB design applications where interference sources are eliminated or protect the circuit from its adverse effects. Ultimately, the goal is to maintain the intended functionality of the circuit board for better EMC performance. Electromagnetic compatibility of any electronic circuit is associated with the generation, spread of, and reception of electromagnetic noise. PCB designs with correct EMC improvements do not add extra cost to the final product. For this reason, it is recommended to be considered in the first design phase.
References
- Montrose, M.I. 1998. EMC and the Printed Circuit Board, Design, Theory, and Layout Made Simple, IEEE Press
- Williams, T. 2001. EMC for Product Designers, 3rd edition, Newnes, ISBN: 0-7506-4930-5
- https://www.electronicproducts.com/top-10-emc-design-considerations/ / Erişim Tarihi: 22.11.2020
- https://www.protoexpress.com/blog/7-pcb-design-tips-solve-emi-emc-issues/ / Erişim Tarihi: 22.11.2020
- Üstüner, F. 2011. EMI/EMC Elektromanyetik Girişim ve Uyumluluk, TÜBİTAK UME Sunum
- EMI/EMC Gürültü Azaltma ve Korunma, Ekranlama, Topraklama ve PCB Tasarımı, Çeviri Çiğdem Özşar, Aydın Bodur, TMMOB Elektrik Mühendisleri Odası Yayınları, 1.Basım, 2008, Ankara
- https://www.altium.com/solution/crosstalk-or-coupling / Erişim Tarihi: 22.11.2020
- https://eeestudy.com/what-is-crosstalk-in-pcb-design/ / Erişim Tarihi: 22.11.2020
- H. W. Johnson, M. Graham, High Speed Digital Design, a Handbook of Black Magic
- https://aktif.net/tr/Aktif-Blog/Teknik-Makaleler/emc-lvd-testleri-ve-uygulamalari / Erişim Tarihi: 22.11.2020
