EMI Filter Application and Test Requirements

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Choosing the right EMI filter for any application requires understanding several aspects of the equipment it will be used in. Will the equipment be subjected to extremes in electromagnetic energy, radical temperature swings, high levels of shock and vibration, or will the equipment be in an environment where only low levels of these will be experienced.

The equipment being filtered must also be understood in the respect of knowing how it is creating energy. Is the energy mainly differential mode or common mode? And what is the difference?

Differential mode energy, or currents, is that which will travel in one wire in one direction, and the equal and opposite energy flows in an adjacent wire. This is different than common mode, which can flow in one or more wires to a remote location, and then finds a return path by a remote path. This may be through a chassis or ground structure, or by radiation. Details of this are discussed in other papers.

If the equipment only produces low frequency electrical noise, which is typically differential mode noise or currents, then filtering may look different than if the equipment generates high frequency noise, which tends to be common mode. Often, electrical circuits will generate both at the same time, and both need to be considered.

Review of Test Requirements of Various Industries

All electromagnetic compliance standards and regulations require testing for emissions generated by the equipment under test. Many also require various immunity or susceptibility tests to be performed. Here is a short summary.

In the USA, Canada, and many other countries, commercial electronics require emissions testing to be performed, typically because of the use of digital circuitry. These tests are in the form of conducted emissions on the power lines from 150 kHz to 30 MHz, and radiated emissions from 30 MHz to at least 1000 MHz, and often  higher. These limits are typically in two categories: residential and commercial, which are lower emission limits than the industrial limits. The reason is that in the commercial and residential area, more sensitive components are used, including receivers and wireless transmission (Wi-Fi and Bluetooth). In industrial applications, large motors, generators, machinery, and electrical noise generating equipment can overwhelm lower emission equipment. In these applications, there is little need for tighter emission limits.

In Europe, Australia, and New Zealand specialized equipment such as test and measurement or medical, there is additional immunity testing. Immunity testing includes producing radio frequency voltages on power lines, radio frequency fields which are transmitted to the equipment, other transients, surges on power and interconnecting cables, electrostatic discharge, magnetic fields, and power line interruptions. These are intended to take on the nature of signals generated by other equipment and disruptive environments.

Consider the environment of commercial buildings; there may be significant usage of florescent lighting, motors for ventilation and air conditioning, elevators, along with equipment needed for manufacturing or other operation. Each can be a significant generator of electrical interference to computers, control systems, communication such as cell phones, Wi-Fi, and the like. The environment should be controlled to a tolerable level, and the sensitive equipment should be filtered and shielded to avoid interference which can disrupt normal operation.

In industrial applications, these noise generators can become very significant due to the high amounts of energy used. Other sources, such as arc welding can create high levels of radio interference which are difficult to control. These issues must be considered for equipment installed in high background environments.

Beyond these requirements are those for automotive, aerospace, and military applications. These requirements have wider frequency emission standards, which may have very low limits, and still must survive extreme electrical environments including lightning, and radio frequency fields which may be hazardous. Filtering for these radio frequency fields and currents is critical and can be difficult. High performance filters are needed, and the environmental conditions they are used in can be extreme.

Understanding Filter Performance

First consider the type of emissions which may be generated by the equipment. For switching power supplies, the demand on current at the switching frequency will result in a strong differential mode current at that frequency even in an operating range of up to 500 kHz. Having a capacitor from the input side of the power supply to the return line will supply the power supply with a local source of current which can reduce the emissions leaving the equipment. This will have little benefit on higher frequency common mode emissions.

For common mode energy, the current path is not line to line, but line to chassis, or line to some remote cable, wire, or object. Capacitance used must be from the line to chassis or other object, in order to complete the current path which also results in leakage current. Line to line capacitance would be ineffective for reduction of this energy.

Radiated emissions and radiated susceptibility and immunity testing create both common mode type of currents, and thus common mode filtering should be employed. Conversely, conducted emissions and immunity/susceptibility testing tends to have more differential mode applications. In addition, conducted testing is performed at a lower frequency than radiated testing.

When using inductors in filters, size and weight matter. High current differential mode inductors are massive and heavy as compared to common mode inductors. This is to avoid the saturation that may occur due to the large currents which flow through a differential mode inductor. Common mode inductors have all current carrying lines wound together, the sum of all the currents being zero or near zero. In addition, wiring used to wind any inductor must be adequate to carry the current over extended times without overheating.

Equipment with high voltage power requires filters rated for high voltages. The internal capacitors must be able to withstand those voltage, and the winding insulation on the inductors must be adequate. If the equipment operates on DC instead of AC, the types and sizes of filters can be larger as well.

For equipment operating on three phase (3Ø) power, it is best to use a single filter designed for three phases, not three individual filters, one on each phase. This allows for proper common mode inductors to be wound, and the use of differential mode filters (capacitors) which are most effective directly line to line, instead of line to neutral to line.

Dual stage filters, which have both differential mode filtering and common mode filtering built in, may be best when it is uncertain the nature of the energy being controlled. These filters can have better performance in general over single stage filters.

Since common mode filters typically have capacitors to chassis, the need to bond the filter to chassis is very important. This bond allows for a low impedance path back to the source through the common mode capacitors. However, if these filters are not bonded to the same metal as the source of the common mode energy, then the effectiveness of the filter will suffer. For this reason, it is important to provide a path, even a high frequency (capacitively coupled) path from chassis to the circuit generating the common mode energy.

Considerations for Filters

Unless specially designed, most filters do not control transient voltage caused by surge or lightning. These require the use of a type of transient suppressor, such as MOVs, Transorbs, or Gas Discharge Tubes. The use of such components should be carefully chosen based on the amount of energy to dissipate, the voltage standoff rating of the circuit, and the speed of the transient being controlled.

Other issues, such as ESD events to the chassis, magnetic fields induced to or from the equipment, voltage interrupts and transients on the power lines and the like, are not issues handled by most line filters. These events require other means of control.

Some environments may require special construction for filter design. When placed in an automotive or trucking installation, or used in aerospace or military applications, the location may have extreme environmental issues. The temperature of some installations may require operation from -40°C to +60°C Temperature, or even wider range. These environments must handle operation in high humidity and wet locations. Mechanical shock and vibration may be excessive over the life of the unit.

To survive these environments, the filter is often in a hermetically sealed case to keep out moisture. The case may be coated to minimize corrosion caused by salt or other chemicals, cleaners, oils, or hydraulic fluids. The filter may also be in a potting compound to protect it from mechanical shock and vibration as well as moisture. This, however, will then add weight which the aerospace market wishes to avoid.

Types of Ohmite Filters and Usages

One of the best ways to provide maximum benefit of filtering is to provide isolation from the input to the output of the filter. Mounting the filter into a chassis wall is an excellent solution. The Ohmite AF Series of IEC Inlet filters can provide this type of benefit. With available with and without fuses and with or without line switches.

With current ratings from 1 ampere to 15 amperes, these filters will work on: 

  • Electrical & Electronic Equipment
  • Testing & Measurement Equipment
  • Household equipment
  • Rack mounting equipment
  • Power Systems
  • Frequency converters
  • Stepper motor drives
  • UPS/Inverters 

And many other items which use an IEC power inlet.

For general purpose filtering in a modular design, consider the AG Series. These filters single phase and single stage filters, and are rated up to 250 VAC. With ratings from 1 ampere to 125 amperes, these filters will work on: 

  • Power Systems
  • Electronic Equipment
  • Frequency convertors
  • Stepper motor drives
  • UPS/Inverters 

These chassis mounted filters come in both regular and medical grade versions.

If additional filtering is needed, the AH Series Dual-Stage, and the AHL Series Low Frequency Dual-Stage filters should be considered. Coming in ratings from 1 ampere to 100 amperes and 1 Amperes to 60 amperes respectively, they can supply additional attenuation to high level interference and emissions. These are chassis mounted filters, and available in a medical grade version. 

In addition, a three phase (3Ø) filter is available, the TG Series general type EMI filter. They are available from 1 ampere to a massive 150 amperes per phase. The series includes a filtered neutral and should be considered for: 

  • CNC Machines
  • Electronic Equipment
  • Convertors
  • Rectifier
  • UPS/Inverters 

Also, in a three phase (3Ø) version is the TK Series which is excellent for Delta power and high voltage applications. These filters are rated up to 520 VAC and add line to line capacitance useful for differential mode energy. Available from 7 amperes to 150 amperes, these filters can work with: 

  • AC/DC Drives
  • Generator
  • Wind Turbine
  • Fuel Cells
  • Elevators

Finally, a series of circuit board mountable filters is also available from Ohmite. Rated from 1 to 6 amperes, and capable of up to 250 VAC, these filters can be used on: 

  • Switch Mode Power Supplies
  • Monitors
  • Programmable Logic Controller
  • Weighing Scales
  • Electronic Instruments
  • Personal Computer Periphery
  • Microwave Equipment
  • Office Automation Equipment 

When properly designed and utilized, EMI filters can greatly improve the ability to meet compliance requirements, and the needs of a wide range of equipment, installations, and environments. 

Apart from the above said series, Ohmite has multi-stage EMI Filters both in Star and Delta Power and both Bookshelf and Chassis mounting. Ohmite also has very high current filters ranging from 6A to 1000A. Ohmite can also customise the Filters to meet the customer requirements.