Design for EMC
Basic concepts
The preferred and most cost-effective approach to the achievement of electromagnetic compatibility is to incorporate the control measures into the design. At the design inception stage some thought should be given to the basic principles of the EMC control philosophy to be applied in the design and construction of the product. The overall EMC design parameters can be derived directly or determined from the EMC standards to be applied, either as part of the procurement specification or as part of the legal requirements for market entry. There are usually two fundamental options for product EMC control:
● shielding and filtering (see Fig. 14.10(a))
● board level control (see Fig. 14.10(b))
For the shielding and filter solution, all external cables are either screened leads, with the screens bonded to the enclosure shield, or unscreened leads connected via a filter. The basic principle is to provide a well-defined barrier between the inner surface of the shield facing the emissions from the PCBs and the outer surface of the shield
interacting with the external environment. This solution requires the use of a metal or metal-coated enclosure. Where this is not possible, or not preferred, then the board level control option is appropriate, where the PCB design and layout provides inherent barriers to the transfer of electromagnetic energy. Filters are required at all the cable interfaces, except where an effective screened interface can be utilized.
Shielding
For design purposes, effective shielding of about 30–40 dB can be provided at high frequencies by relatively thin metal sheets or metal coatings on plastic. The maxi- mum shielding achievable is associated with the apertures, slots and discontinuities in the surface of the shield enclosure. These may be excited by electromagnetic energy and can resonate where their physical length is comparable with a wavelength, significantly degrading the performance of the shield. The following basic rules apply:
● the maximum length of a slot should be no greater than 1/40 of the wavelength at the highest frequency of concern
● a large number of small holes in the shield gives better performance than a small number of large holes
● the number of points of contact between two mating halves of an enclosure should be maximized
● mains or signal line filters should be bonded to the enclosure at the point of entry of the cable
Cable screens termination
For maximum performance, cable shields should be terminated at both ends with a 360° peripheral (i.e. glanded) bond. This is not always possible and in some cases it is undesirable because of the associated ground loop problem (Fig. 14.11). Noise currents, I, in the ground generate a voltage V between the two circuits A and B which can drive a high current on the outer surface of the interconnecting screen, thus permitting energy to be coupled into the internal system conductors.
In many applications involving the use of long conductors in noisy environments, the simplest solution is to break the ground loop by bonding the screen at one end only, as shown in Fig. 14.12. Here the ground noise voltage is eliminated but the shield protects only against electric fields and capacitive coupling. Additional or alternative measures, such as opto-isolation are required if intense magnetic fields are present.
PCB design and layout
Control at board level can be achieved by a careful design of the board involving device selection and track layout. As discussed in section 14.2.2, emissions from the PCB tracks may be reduced at high frequencies if the devices are chosen to have slow switching rates and slow transition (i.e. long rise and fall) times.
Device selection can also improve immunity by bandwidth control. The smaller the bandwidth, the less likely it is that high-frequency disturbances will be encountered within the pass-band of the circuit. Although rectification of the disturbance may occur in the out-of-band region, the conversion process is more inefficient and higher immunity usually results.
The tracks on PCBs can act as antennas and the control methods involve reducing their efficiency. The following methods can be applied to good effect:
● reduce the area of all track loops
● minimize the length of all high-frequency signal paths
● terminate lines in resistors equal to the characteristic impedance
● ensure that the signal return track is adjacent to the signal track
● remove the minimum amount of copper on the board, i.e. maximize the surface area of the 0 V (zero-volt ground) and VCC (power) planes
The latter two points are generally achieved where a multilayer board configuration is employed. These measures are highly effective at reducing board emissions and improving circuit immunity to external disturbances. Multilayer boards are sometimes considered relatively expensive but the extra cost must be compared with the total costs of other measures that may be required with single or double-sided boards, such as shielding, filtering and additional development and production costs.
Grounding
Grounding is the method whereby signal returns are managed, and it should not be confused with earthing which deals with protection from electrical hazards. Grounding is important at both the PCB level and circuit interconnection level. The three main schemes are shown in Fig. 14.13.
In the series ground scheme, noise from circuit A can couple into circuit C by the common impedance Z. This problem is overcome in the single-point ground, but the scheme is wasteful of conductors and not particularly effective at high frequencies where the impedance of the grounding conductors may vary, and potential differences can be set up.
The ideal scheme is the multipoint ground. Generally, single-point grounding is used to separate digital, analogue and power circuits. Multipoint grounding is then used whenever possible within each category of analogue or digital. Some series ground techniques may be employed where the coupled noise levels can be tolerated. Usually the overall optimum solution is derived from good basic design and succes- sive experimentation.
Systems and installations
The general principles discussed above may be applied at a system or installation level. Guidance on this topic is available in references 15 J and 15 K.