EMC & conducted emissions
Electromagnetic compatibility is ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to other equipment, system or services in that environment. Electromagnetic compatibility is an interaction of electrical and electronic equipment with electromagnetic environment and with other equipment, or electromagnetic environment interaction with electrical and electronic equipment.
Equipment and system is electromagnetically compatible with its environment and other equipment and systems, if it satisfies the following three criteria:
1. It does not cause interference with other equipment and systems;
2. It is not susceptible to emissions from other equipment, systems and environment;
3. It does not cause interference with itself.
Designing electromagnetic compatibility is not only important for the desired functional performance. Equipment and systems must also meet legal requirements before they can be offered in the market. There are no harmonized electromagnetic requirements all over the world. Requirements usually vary from country to country. Although, there are countries which agreed to harmonize the requirements to enable the free movement of goods, without country to country specific requirements. As an example European Union can be mentioned; it has developed harmonized electromagnetic compatibility standards and issued Electromagnetic compatibility directive that should be adopted in all member states.
Basically, electromagnetic compatibility is concerned with the generation, transmission and reception of electromagnetic energy. These three aspects are illustrated in Fig.1.1. Source generates the emission and a transfer or coupling path transfers the emission energy to a receptor, where it is resulting in either desired or undesired behavior. Electromagnetic interference (EMI) occurs if the received energy causes the receptor to behave in an undesired manner.
Transfer of electromagnetic energy occurs via couple of paths. Usually these paths are unintentional and electronic engineers are unaware of them. Paths represented in Fig. 1.1. can be observed as paths between two devices or systems as well as paths between components on PCB.
These paths are:
1) Conductive coupling;
2) Inductive coupling;
3) Capacitive coupling;
4) Electromagnetic coupling.
Fig. 1.1. Three aspects of electromagnetic compatibility– source, path, receptor
Conductive coupling happens when the coupling path between the source and the receptor is formed by direct contact with a conducting body. Conducting body can be formed out of transmission lines, PCB trace, wires, cables, heat sinks or enclosures. Inductive coupling happens when a varying magnetic field exists between two conductors in a close distance, inducing a current in nearby conductor. Capacitive coupling happens when a varying electrical field exists between two adjacent conductors in a close distance, inducing a change across the gap between these two conductors. Electromagnetic coupling happens when source and receptor are separated by a large distance, source emits electromagnetic energy and receptor receives electromagnetic energy in terms of electromagnetic waves. It should be noted that “close distance” in EMC usually is defined as distance where magnetic field is dominant and field strength varies as 1/r3, 1/r2, where r is distance from radiation source. “Large distance” is a distance in which the field strength varies as 1/r2 and wave impedance depends on medium it is propagating through (e.g. 377 Ω in vacuum).
To prevent electromagnetic interference three options exist:
- Decrease emissions at the source;
- Decrease coupling path efficiency;
- Increase receptor susceptibility.
The most efficient and economical way to reduce electromagnetic interference is to decrease the emissions at the source. This requires highly qualified engineers to modify the circuits to decrease emissions, but still provide the necessary functions. Receptor susceptibility improvement also requires highly qualified engineers to modify circuits to handle high disturbance impact and still provide its functions. Coupling path efficiency reduction is not as advanced as two tasks mentioned above. There is no need for source and receptor internal circuit management. It is only crucial to know the disturbance propagation path. Various techniques to decrease path efficiency exist, but one technique is common for all paths reduction. It is a filter application. In Fig. 1.2. the application of filters is explained using coupling paths defined in Fig. 1.1.
Fig. 1.2. Filter application to reduce coupling paths between source and receptor
Filter installed at the input and output cables of devices and systems reduce the conducted disturbance path efficiency. If the filter is installed in the right position and correctly wired, conductive disturbance reduction will lead also to inductive and capacitive noise reduction. Therefore, elimination of one disturbance path in the right region will lead to electromagnetic interference issue elimination. If the conducted disturbance is reduced at the right region, electromagnetic disturbances are reduced and there should be no further problems regarding electromagnetic interference.
Since unwanted disturbances usually are at much higher frequencies than useful signals (50Hz mains power, communication signals, sensor signals etc.) filter works by selectively blocking or attenuating unwanted higher frequencies. Basically, the inductive part of the filter is designed to act as a low pass device to enable low frequency useful signal transmission thru the line and attenuating high frequency signal components. Other parts of the filter use capacitors to bypass or shunt unwanted high frequency disturbance. Therefore, passive electromagnetic interference filters are a combination of inductors and capacitors, where each component has its purpose. Unwanted disturbance signal can also appear on antenna terminals, thus feeding unwanted signals to antenna, in combination with useful signals. In his case band pass filters are applicable– filter is designed to pass the useful signal, but attenuate unwanted signal components.
In general EMI problems that must be solved by filter application (or other method usage) are illustrated in Fig. 1.3.:
- Conducted emissions;
- Radiated emissions;
- Conducted susceptibility;
- Radiated susceptibility.
Fig. 1.3. Electromagnetic compatibility emission and susceptibility classification
It defines the conducted emission measurement methodology and defines the conducted emission limits. In Fig. 1.4. EN 55011 conducted emission limit lines are presented for class A and class B equipment.
Fig. 1.4. EN 55011 conducted emission limit lines
Conducted emission requirements are defined in range 150kHz– 30MHz. The measurement methodology is defined in Fig. 1.5. Line impedance stabilization network (LISN) is used to measure disturbances created by electronic products or equipment under test (EUT). EUT is connected to the mains network through LISN due to following reasons:
- LISN defines mains impedance;
- LISN filters acts as high pass filter and interface to EMI analyzer;
- LISN acts as filter and attenuates noise coming from mains network.
Fig. 1.5. Conducted emission measurements according to EN 55011
LISN impedance and construction parameters are defined by standards. Simplified LISN schematic is presented in Fig. 1.6. LISN actually is a high pass filter. 50Hz power frequency is transmitted through LISN power ports without any attenuation from side of mains network, but high frequency components are forwarded to high frequency output (50Hz component is not present at high frequency output). Disturbances from EUT are terminated in defined impedance by the LISN (at 30MHz line-ground impedance is 50Ω) and measured by EMI receiver.
Fig. 1.6. LISN internal circuit
To measure conducted emissions from one phase equipment, it is necessary to have LISN containing combination of two circuits defined in Fig. 1.6., one for line wire and second for neutral wire. For three phase EUT emission measurements there should be LISN with combination of four circuits defined in Fig. 1.6. One phase EUT connected to one phase LISN is shown in Fig. 1.7.
Conducted disturbances are flowing in all wires connected to EUT– line, neutral and grounding. To simplify disturbance analysis disturbances are divided in two groups:
- Common mode (CM) emissions;
- Differential mode (DM) emissions.
Differential and common mode currents are shown on simple case in Fig. 1.7. Impedances Z1 and Z2 represent the line impedance of EUT and Z3 represents EUT impedance to ground. Differential mode current iDM flows into line-neutral loop. Common mode current iCM flows in ground wire and creates two loops though line and neutral wires. Therefore, current in each wire separately can be calculated:
Voltage measured by LISN is (according to EN 55011 Line-Ground and Neutral-Ground impedance is 50Ω):
According to equations (1.4) and (1.5), LISN is measuring voltage drop created either by addition or subtraction of common and differential mode currents. To be in compliance with EMC standards these voltage drop values should be lower than limit line defined values.
Fig. 1.7. One phase equipment conducted differential mode and common mode emission measurements
The purpose of the EMI filter is to reduce voltage drop V1 and V2 to comply with EMC standards. Differential mode and common mode current magnitude depends on disturbance source– EUT. There could be a situation that one of disturbance mode dominates over the other one. Therefore, there is no universal filter circuit that guarantees the compliance with EMC standards. In each situation filter internal circuit should be engineered to effectively reduce disturbance emissions.
The basic principle of components in EMI filter is to bypass disturbance source using shunt capacitors and introduce high impedance to disturbance using series inductors. Filter can be made out of single component– first order filters– such as inductor or capacitor or more advanced topologies creating second, third etc. order filters introducing couple of inductors and capacitors. Each disturbance mode has its dedicated components and filter topologies, that are used to reduce the current disturbance mode emissions.
Differential mode current is flowing thru two wires (line-neutral in Fig. 1.8.), therefore DM filter is constructed and connected only to these two wires. Basically, DM filter consists of DM inductor and x-capacitor creating LC filters that can be connected in cascades. In Fig. 1.8. DM filter is introduced, containing two LC cascades. Inductor L2 creates high impedance for high frequency disturbances and capacitor CX2 is bypassing it. L1 and CX1 are accomplishing the same task, resulting in decreased voltage measured by LISN– V1 and V2. DM filter has no impact on CM disturbance current.
Fig. 1.8. Differential mode filter– two LC cascades
Common mode disturbance current is flowing thru all wires connected to EUT, therefore CM filter should be constructed to reduce current components in all wires. Basically, CM filter is created using CM chokes and y-capacitors. In Fig. 1.9. CM filter two LC cascade topology is introduced. Inductor L2 creates high impedance for high frequency CM current, while capacitors CY3 and CY4 are bypassing the CM current. The same task is accomplished by CM choke L1 and capacitors CY1 and CY2. It should be noted that y-capacitors have impact on DM disturbance current, as CY1, CY2 and CY3, CY4 are connected in series and connected between line and neutral. A possibility to connect inductor in series with ground wire also exists, but this kind of inductor application has drawbacks regarding electrical safety, due to limited ground current capability and raised grounding impedance.
Fig. 1.9. Common mode filter– two LC cascades
To reduce DM and CM disturbances it is necessary to create filter containing both upper mention filter characteristics– DM and CM filtering components. Finally, filter, even for one phase equipment, containing only single stage LC filter for each disturbance mode, consists of at least four components. If disturbance level is higher, the complexity of filter increases.