EMI Filters / RFI Filters
What is an EMI Filter?
An EMI filter also called an RFI filter, is a passive electronic device used to suppress electrical noise from electrical equipment.
The filter works in both directions and will keep electrical noise from entering equipment also.
The first EMI filters were used to suppress electrical noise in vehicles. This electrical noise interfered with radio reception.
Filters today are quietly in use everywhere. It would be difficult to watch television with the dishwasher running if filters were not used to suppress the electrical noise from. the dishwasher. Conversely the Dishwasher electronic controls might not work well if the electrical noise from the television was not suppressed by a filter.
What criteria are used to select the proper EMI filter?
The proper selection of an EMI filter encompasses the entire electromechanical configuration of the filter. The filter’s mechanical footprint including mounting and terminations may impact the effectiveness of the filter in your system. Primary considerations also include leakage current, insertion loss, rated voltage and current, and agency approvals required for proper usage in the end application.
Basics of EMI filter selection:
- Voltage
- Current
- Space available
- How to mount
Next the system requirements:
- Leakage current
- Emissions restrictions
- Conducted
- Ratiated
- Susceptability requirements
- Conducted
- Radiated
- Transientsbelow
Can EMI filters help meet RE102 of Mil-Std 461?
The frequency measurement range of RE102 is 10KHz to 18GHz.
The standard EMI filter is designed to filter conducted emissions over the frequency range of 10KHz to 10MHz. This would lead one to think that the filter will not help much for the RE102 requirements.
This is true unless the filter is electrically and mechanically designed to attenuate the higher frequencies.
The power cord is often the major source of RE102 emissions. The good news is that this is generally an easy problem to solve. There are a few things that are necessary to implement the solution.
- The enclosure should be metal and “well-sealed”. Essentially you want to have a Faraday cage.
- The power line needs to have a filter that will block or send the high-frequency signals to a low impedance ground plane.
- An absolute requirement is to use feed through capacitors in the filter. These low impedance devices shunt the high frequencies to the ground. They also block the propagation of HF signals along current-carrying wires bypassing the wires through interior bulkheads that present orifices small enough to block signals below 20 GHz.
The figure below shows a possible installation of a Mil-Std-461 filter, but there are many more configurations that will also work. The key thing is that the input to the filter must be completely isolated from the output of the filter. JMK has many of these filters. The filters can also be modified to meet almost any installation.
What is Electromagnetic Interference (EMI)?
Electromagnetic interference, EMI, is any undesirable electromagnetic emission or any electrical or electronic disturbance, man-made or natural, which causes an undesirable response, malfunction, or degradation in the performance of electrical equipment.
What is Radio Frequency Interference (RFI)?
Radio frequency interference, RFI, is any undesirable electrical energy with content within the frequency range dedicated to radio frequency transmission. Conducted RFI is most often found in the low-frequency range of several kHz to 30MHz. Radiated RFI is most often found in the frequency range from 30MHz to 10GHz.
How does interference propagate?
EMI or RFI propagates through conduction over signal and power lines and through radiation in free space.
What are some common sources of conducted interference?
Typical sources of conducted interference include switching power supplies, ac motors, and microprocessors. In short, just about any electrical and electronic device has the potential to generate conducted and radiated interference.
What are some common sources of radiated interference?
The most common offender in the radiation of EMI is the electrical power cord of the electronic device itself. Since the power cord can act as an antenna, conducted EMI can also become radiated interference.
What is common-mode (CM) noise?
Common mode interference, a.k.a. asymmetrical interference, is a noise signal which is found in phase on both the line and neutral conductors with respect to ground. Common mode noise also typically has equal amplitude on both line and neutral conductors.
What is differential mode noise?
Differential mode interference, a.k.a. symmetrical interference, is a noise signal which exists between the line and neutral conductors.
Why is EMI regulated?
EMI is regulated to allow today’s sensitive equipment to function properly without suffering degradation in performance due to interference generated by other electronic devices. The EMI spectrum is a limited natural resource that must be maintained to allow reliable radio frequency communications. The successful regulation of EMI interference will allow future electronic devices to operate as defined, in the intended environment, without suffering any degradation in performance due to interference, and without disrupting the performance of other equipment.
What are the applicable US requirements?
Conducted and radiated emissions for computing devices are regulated by Part15, Subpart J of the US Federal Communications Commission, (FCC). The limits for conducted and radiated emissions are based upon the end application of the device.
Class A: Commercial, Industrial, and Business Applications
Class B: Residential Applications
What standards apply to EMI filters in the EU?
The EMC Directive 89/336/EEC mandates that all electronic equipment must comply with the applicable EN specification for EMI. Some typical EN specifications follow:
Electronic Equipment Spec. Industrial, scientific, and medical equipmentEN55011Broadcast receivers and associated equipmentEN55013Electrical motor-operated and thermal appliances for household and similar purposes, electrical tools and similar apparatusEN55014Electrical lighting and similar apparatusEN55015Information technology equipmentEN55022
How does the 50 Ohm data compare with actual performance in my system?
The 50 Ohm insertion loss data which appears in our catalog represents an industry standard to allow comparison of different manufacturers’ products against a known standard. This is generally the impedance used to test the conformance of a system to various military and other EMI standards. In practice, filters are the source and load impedance sensitive. The actual load impedance varies greatly from system to system. It is therefore often easier to test the filter and system to determine the actual combined performance.
How are EMI tests performed?
EMI tests are performed using passive networks called LISNs, Line Impedance Stabilization Network, connected in series with the power lines to the equipment. The LISN establishes a consistent impedance to allow for the repeatability of test results. Conducted emissions are measured via an RF connection to a port on the LISN. Radiated emissions may are measured using an antenna or current detecting probe.
What aspects of filter design and performance are regulated by safety agencies?
Safety agencies define limits for the maximum leakage current, operating temperature rise, dielectric strength, and the mechanical design to protect end-users.
What is Hi-pot testing?
Hi-pot or High Potential testing is also known as dielectric-withstanding testing. The testing is performed by applying a known potential or voltage between the power lines or between a power line and the case to expose any electrical deficiencies in the construction of the EMI filter. As part of our testing, all JMK products are tested for dielectric withstanding (Hi-Pot) between the power lines (DM) and between the power lines and the case (CM).
What is leakage current?
When a filter has capacitors connected between the conductors and the ground, small amounts of current flow through the capacitors to the ground. This current can create an electric shock hazard if the system is improperly grounded.
This current is limited by the international safety agencies to prevent a danger to personal safety.
What is insertion loss?
Insertion loss is quite simply a measure of the effectiveness of an EMI filter. Insertion loss is defined as the ratio of voltages across the load without the filter inserted in the circuit and with the filter inserted in the circuit.
Filter performance is also dependent on the input and output impedance seen by the filter. Insertion loss is therefore measured with a 50-ohm source and load impedances as a standard. While this provides a standard for comparison, it does not necessarily reflect the actual performance of the filter within your system.
Since filter performance is specific to the attached system, JMK maintains a test facility to test the individual needs of our customer’s power supply to select or design an appropriate filter. Normally such testing is offered by JMK free of charge.
Is it possible to build an EMI filter to enable a system to meet CE101 conducted emissions of Mil-Std-461?
The answer to this question is a very “qualified” yes.
We have built such filters and they are not particularly economical or efficient. The problem with designing a filter for a system to meet CE101 is the low frequency of the noise signals involved. Typically, the 3rd, 5th, and 7th harmonics are the problem frequencies. This means that for 60 Hz power the filter must attenuate 180, 300., and 420 Hz at a minimum. In the case of 400 Hz power, attenuation of 1200, 2000, and 2800 Hz will be necessary.
These filters will involve large components and some damping may be necessary, resulting in power loss. The best approach is to select a power supply that incorporates active harmonic attenuation. The harmonic attenuation will, indeed, add noise back on the incoming power lines. This noise will need to be attenuated but the frequencies are much higher, and the filter components required are much smaller.
The cost of filtering the active harmonic attenuation is considerably lower and the efficiency does not drag down the overall power factor. Our recommendation is to avoid trying to meet CE101 without harmonic attenuation built into the power supply.
This approach allows standard design approaches for the power supply and filters with reasonable size and cost.