EMI filter solutions from Astrodyne TDI
Do you design or manufacture electronic devices or components? Are you becoming increasingly concerned with managing electromagnetic interference (EMI)?
When designing electronic devices, adding filters and shields can help control EMI. Both methods provide noise suppression and can be implemented without adding significant size or cost to a device.
Astrodyne TDI has shared insight into these two noise suppression methods and when you should consider using them.
EMI (sometimes referred to as radio frequency interference or RFI when in the radio frequency spectrum on the electromagnetic frequency spectrum) can disrupt the operation of an electronic product due to an electromagnetic field. The electromagnetic waves that result in interference get referred to as electromagnetic noise.
To function and to allow other devices to work, every product must be able to operate even when exposed to a certain level of noise and must not produce EMI at levels that impede the function of the devices. The International Special Committee for Radio Interference regulates it internationally in some sectors, with many industries setting their own requirements related to EMC.
Strong RF fields can cause phones, computers and even some medical devices to stop working correctly. Natural events such as electrical storms, solar flares and static electricity can also cause EMI.
That's why EMI shielding and filtering are so important.
What is EMI-RFI shielding and how does it work?
EMI-RFI shielding is the surrounding of an object with a metal plate or other form of protection to block electromagnetic fields. EMI shields are designed to prevent radiated emissions from getting past a certain point. EMI shielding solutions can both protect a device from external radiation and prevent that device from emitting radiation that could cause interference with other devices.
- The conductive surface of the shield reflects most of the energy from the EM wave in various directions.
- Exactly how the wave reflects depends on the qualities of the shield's material and the phase of the wave when it hits the shield.
- The EMI shield will also absorb some of the energy from the EM wave, which will get converted into heat energy.
- Depending on the power levels involved, this heat energy may necessitate thermal management - some EMI shielding materials are made to double as heat sinks.
- For shielding materials to work well, they also need good ground connections.
Examples of materials commonly used for EMI shielding include copper, aluminum and stainless steel.
More recently, manufacturers have also begun using composite materials, such as meshes and fabrics. These solutions often combine a metal with a polyester material. Some advantages of these newer materials are their light weight and flexibility.
What is EMI filtering and how does it work?
EMI-RFI filtering can remove unwanted components and let the necessary ones through in the electric current flowing in conductors. Noise gets diverted to the ground, absorbed or returned to its origin.
EMI filters consist of two kinds of components — capacitors and inductors — that work together to reduce EMI.
- Capacitors inhibit direct current, which is how a substantial amount of EMI gets carried into a device but let the alternating current pass through. The capacitors found in EMI filters get referred to as shunting capacitors. They redirect high-frequency current that would cause interference away from a circuit and feed it into inductors arranged in a series.
- Inductors are small electromagnets that can hold energy in a magnetic field as electric current passes through it, reducing the total voltage. As the current moves through this series of inductors, its voltage gets reduced. Ideally, the inductors will diminish the interference to nothing, which is also called shorting to ground.
There are various types of EMI filters you can use - the right type depends on the frequencies you want to block, the voltage you're working with and other factors. Two crucial types of EMI filters include single-phase and three-phase filters:
- Single-phase filters are best for smaller equipment such as consumer electronics, home appliances, fitness equipment and some industrial applications, such as power supplies, telecommunications equipment and food service equipment.
- Three-phase filters can block higher levels of noise than single-phase filters and are useful for more stringent EMI suppression. These kinds of filters are necessary for high-power applications, such as medical equipment, test equipment and various types of industrial machinery, like tools and motors.
Other classifications of EMI filters include:
- IEC inlet filters, which get used for power entry applications.
- DC filters, which block high-frequency currents but let DC and low-frequency currents pass through.
- Inverter EMI filters, which get used in applications that involve frequency inverters or inverter-based control systems.
- Feed-through filters, which get used for applications such as enclosures, base stations, mobile shelters and switching equipment and provide high insertion loss from KHz to GHz frequencies.
When should you use an EMI filter vs. a shield?
Often, using both a shield and a filter in conjunction is the most effective solution.
Sometimes the holes and gaps in EMI shielding can reduce its effectiveness. These openings, however, are necessary for heat reduction. This is an example of when to use EMI filters in addition to a shield.
Filters, however, can eliminate EMI. Filters can address many of the issues related to penetrations through shields, as well as the inputs and the outputs of an electrical system, which are usually the most vulnerable points of a shielded system. Filters are most effective at these locations.
Using EMI filtering techniques, as well as transient suppression, at a shielded enclosure interface is a highly efficient way to protect against compatibility issues.
Placing filters and filtered connectors at a system's input and output interfaces can help to eliminate EM noise from both internal and external sources at the connector interface. This placement sends the unwanted energy into the grounded shielded enclosure, making this an optimal location for eliminating higher frequency noise and mitigating EMI-related problems.
Filters protect against noise conducted through conductors, while shields mitigate noise conducted through space. A conductor that noise conducts through can also act as an antenna, however. When the conductor works as an antenna, the two types of conductions get transformed into each other due to the antenna.
To eliminate noise completely, it's essential to use both shields and filters at one location.