Using main EMI filters is a measure for ensuring electromagnetic compatibility. In most cases, the use of filters remains indispensable despite EMC design principles have been considered from the PCB through to the control cubicle. Moreover, the emissions caused by the ever-increasing power density and high processor performance of equipment do not get any less. As circuits become complex, they become more vulnerable and need protection against electromagnetic disturbances. The requirements of today’s society for high availability of equipment and installations contribute further to the ever-increasing importance of EMCs. The following shows the criteria for filter selection, as well as various concepts in connections with power entry and filter configuration.
It is time consuming to detect faults in an on-site installation, especially those irregular and seemingly random faults that are difficult to capture or simulate. It is therefore important to invest in protection against electromagnetic disturbances during the development phase. In choosing a filter, there are two fundamental questions to weigh: what kind of disturbances can be expected during operation, and what functional disturbances are tolerable. A simple example would be the noise that occurs with a television set when a mobile phone immediately next to it is receiving an incoming call. This disturbance is tolerable. In applications, however, such as medical or security technology, functional disturbances are intolerable.
PROTECTION AGAINST RADIATED AND CONDUCTED DISTURBANCES
Disturbances are transmitted into equipment or an installation through either airwaves or power supply lines. These disturbances can be either radiated or conducted. Metal housings, metalized synthetic materials and wire nettings are used against radiated disturbances. Line filters are used against conducted disturbances. All radiated disturbances have their roots in a cable and can be transmitted into a cable at any time.
Taking these into consideration, the appropriate interference suppression measures can be selected and applied for the optimum effect. For instance, the suppression effect in a stainless steel enclosure would be missing if the power line within that enclosure were inadequately shielded. The suppression effect of the built-in line filter would also be missing since the lack of adequate shielding would permit the unimpeded ingress of disturbance to take effect onto the load side of the filter, and thus bridge the filter. There are relevant design directives for this.
SELECTION CRITERIA FOR LINE FILTERS
Aside from physical properties, such as rated voltage, rated current and operating temperature, the selection of suppression filters is also influenced by pliant factors such as the installation method, resources and logistics (Figure 1).
FILTER CONFIGURATION
Between the aspects of power entry and filter configuration (Figure 2) there are some 10 meaningful combinations of how a filter can be configured (e.g. fixed connection with built-in filter, power entry module with integral line filter, fixed connection with a discrete filter configuration on the PCB, etc.).
These filter variations permit an appraisal based on the selection criteria and preferences listed earlier. This comparison process enables reduction in the possible power entry and filter configuration methods. Each one of these filter configurations has both advantages and disadvantages. Provided is an appraisal of the following methods:
1. Connector with integral line filter.
The advantage of this approach is that line filtering takes place immediately at the point of power entry (Figure 3). The filter is placed in a completely enclosed housing having wide metal flanges. Shielding and filter properties are ideal provided the equipment housing has a good electrical contact with the filter housing. There are no unfiltered power line connections with the equipment that could function as an ‘antenna’ and in addition, the approach is quite compact. On the other hand, some of the flexibility regarding filter design is lost, since a standard solution is given by default and the amount of available space within the filter is limited. This approach is only used with single-phase power entry modules.
2. Fixed power connection with line filter
This approach is practical for all power classes and is extremely flexible with regard to the dimensioning of the filter (Figure 4). A great number of different power categories and filter properties are available (filters having diverse numbers of stages, differing filter layouts). The disadvantage, however, is that shielding difficulties at the point of power entry are not taken care of. It is left to the user which shielding methods he wishes to employ (shielded cable and cable feeds, filter location, addition shielding plates, etc.).
3. Connecting and filtering on a PCB (discrete or with print filter)
The wiring expenditure does not apply when power entry and filtering are on a PCB (Figure 5). This has a positive effect on the assembly process. Print filters are recommended when the available space is limited. Special techniques in the print filter configuration give rise to shortage leakage paths and voltage clearances. The lengths of connections between connector and filter can be kept very short. In cases such as these, the short and unfiltered paths are usually of minor consequence. Otherwise, connectors with integrated line filters for PCB assembly can be used (Figure 6).
FILTER ELEMENTS IN POWER ELECTRONICS
For power electronics, particularly in the field of drives, further aspects of protection come into play in addition to just line filtering. Figure 7 depicts a possible configuration of a drive system with differing suppression elements together with their effects (qualitative voltage characteristics).
The high frequency switching of semiconductor elements gives rise to harmonics, commutation interruptions and transient voltage peaks. These place a burden on the power supply network. In addition to the relevant EMC standards, power supply authorities impose further directives to reduce interference effects on the power supply network to a tolerable level. Power chokes and line filters are used for this reduction. Together with line side EMC protection, protection measures are usually employed with the frequency converter output. The concern here is for an improvement in efficiency, an extension of the working life and for efficient cabling.Click here for the illustrations:
Figure 1, Figure2, Figure3, Figure4, Figure5, Figure6, Figure7 |
