AC DC Power Supply
Power supplies are electrical circuits and devices that are designed to convert mains power or electricity from any electric source to specific values of voltage and current for the target device...
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An electromagnetic interference or EMI Filter is an electrical device or circuit that filters specific unwanted frequencies in power lines or offending frequencies that are detrimental to a system. They receive AC or main power signal input, modify it, and release output that has noise removed to create a clean signal to ensure device efficiency. An EMI filter, also referred to as a choke, mitigates high frequency electromagnetic noise in power and signal lines. They are classified as low pass, high pass, bandpass, or band reject and have passive components such as capacitors and inductors in LC form or complex configurations.
EMI filters use the properties of capacitive and inductive materials to remove high frequency noise from a signal. The noise that is mixed with the true signal is absorbed or reflected such that the released signal is clean.
Without sufficient protection, an EMI can cause poor performance in electrical equipment by generating unwanted currents and voltages in circuitry. An EMI can be conducted or radiated. A conducted EMI comes from a source to a receiver in a direct path and has a physical conduction route.
A radiated EMI does not require physical contact and travels through the air. These occur when a device produces electromagnetic energy in the form of an electrical field and causes damage to equipment by EMI radiated emissions. The emissions spread outward, travel long distances, and interfere with the operation of a device by overwhelming its circuitry.
Although an external EMI can cause damage to equipment, an intra system EMI can cause interference with circuits in a system. This occurs when sensitive components are placed close to power management circuitry that has high voltage and currents, a layout that leads to circuitry interfering electromagnetically.
There are an endless number of EMI filters that come in various sizes, designs, shapes, and configurations, each of which is capable of protecting sensitive equipment from being damaged by electrical noise. Active and passive are the two major categories for EMI filters.
With active EMI filters, an internal power supply generates electricity to remove the current causing a disturbance. Active EMI filters detect the voltage input and generate a current flow in the opposite direction, which removes any disturbance. It uses active electronic elements and internal circuits to block unwanted frequencies.
Unlike active EMI filters, passive EMI filters absorb unwanted energy. They reduce emissions in an electronic circuit using passive components that include capacitors, resistors, transformers, and inductors, which are tuned to produce electrical resonance at a single frequency or a band of frequencies. Passive EMI filters are capable of suppressing harmonic currents and decreasing voltage distortion in the sensitive elements of electronic systems.
The wide use of EMI filters is due to their ability to protect sensitive electronics from damage from radiation produced by electronic equipment and devices. They remove the current that can interfere with signal and power lines but allow acceptable current to flow through.
Single phase EMI filters are used with common and differential mode applications. It is a general-purpose filter designed to suppress noise in power lines. A single-phase EMI filter limits the amplitude of interfering voltages on AC power lines and prevents their spread.
For stronger suppression, 3 phase EMI filters are able to block higher levels of noise using a three-phase filtering system. They have a range of 47 Hz up to 400 Hz and are used with frequency converters, motor drives, and machine tools. The types of 3 phase EMI filters are Wye and Delta.
A common form of passive EMI filter is a ferrite choke, which is an inductor with capacitance that provides low pass filtration. There are several names for a ferrite choke and include ferrite clamp, ferrite collar, EMI filter bead, or ferrite ring filter. Ferrite chokes suppress high frequency signals and are placed around a power line for a particular device, such as a laptop.
Ferrite is a magnetic material. When it is placed around a power supply, it provides inductive resistance to signals that pass through it. It is a nonlinear EMI filter that changes resistance as the load current and voltage drop changes. Additionally, ferrite chokes change as a function of current and temperature changes.
The installation of a ferrite choke involves placing a small device round a wire connected to a piece of equipment. The size and type of ferrite choke depends on the thickness of the wire, which can vary from 3 mm for headphone cords up to 13 mm for 12 gauge or 120-volt power cords. In a normal installation, the ferrite choke is placed
5 cm from the connection to the device since it works better when placed closer to the device.
Differential mode passive EMI filters reduce the amount of high frequency AC current in circuits with low frequency AC or DC current. They are used individually or in groups with multiple inductors placed in different locations. The term differential mode refers to signals or noise that flows in opposite directions in a pair of lines.
A differential mode passive EMI filter reduces noise in a device through the use of a component that has wires wrapped around a magnetic core. The construction of the magnetic component opposes sudden changes in current creating a choking effect on DM noise. The amplitude of the frequency is reduced, which limits its disturbance to the operation of the circuits.
The various DM EMI filters come in a wide range of types to fit the needs of different applications. They are differentiated by the winding of the coils, their materials, and the type of magnetic core. DM EMI filters are used in power supplies and converters, solar inverters and wind powered systems, automotive electronics, and telecommunications.
A common mode passive EMI filter blocks high frequency noise that is common to power lines. Noise currents filtered by CM EMI filters are radiated from radio signals, unshielded electronics, inverters, and motors. In common mode, current travels in both lines in the same direction. The magnetic flux of the EMI filter creates an opposing field that blocks the noise and filters it out.
When common noise enters a common mode EMI filter, magnetic flux is induced in both windings of the core, which are structured to create a rotating magnetic field such that it acts like an inductor with resistance that is proportional to the resistance of the noise frequency. Low frequencies are allowed to pass while high frequencies are impeded.
Common mode EMI filters are classified by the frequency range of an application, which are radio frequency (RF) filters and audio frequency (AF) filters. Common mode AF filters are designed to suppress noise in the audio frequency range up to 30 KHz that include switch mode power supplies, AC/DC rectifiers, electrical ballast, power inverters, and variable frequency drives. With common mode RF filters, noise frequencies are higher above 30 KHz, such as USB, HDMI, LVDS, CAN bus, and ethernet applications.
The difference between the AF and RF common mode EMI filters is in their core materials with AF filters having solid iron cores while RF filters have powdered ferromagnetic core materials.
Line chokes are connected to the variable frequency drive (VFD) input or output circuit. They form a magnetic field that limits current by producing voltage that opposes the current. Line chokes provide protection against spikes, transients, and harmonics and can be placed on the input side or output side of the VFD. When they are placed on the input side, they are referred to as line reactors. On the output side, they are called load reactors.
The use of line chokes is found in VFDs with a rating of 1 KVA and are used for sinusoidal and harmonic current flow where harmonics are large current distortions. With a line choke, induced voltage has opposite polarity to the applied voltage and reduces the rate of current change and limits applied current.
Three phase line chokes are connected to the mains supply to the inverter and reduce harmonics that occur when the main is rectified for a DC link. They help reduce extra current drawn by an inverter or other devices and target input because of problems in the main.
In cases where there is a great deal of distance between the motor and VFD, over 100 feet, a load reactor is used since voltage spikes can be amplified due to the long distance. The use of a line choke in this scenario reduces the current to a safe level. The inductance protects the thyristors of the VFD from overheating and damage and is used when several drives are connected or when there is an imbalance in the power supply.
Line chokes are placed in series on AC power lines and are designed to improve power quality as well as reducing EMI effects. They are used in motor control systems and power lines.
RF chokes are fixed inductors that suppress high frequency AC signals from radio frequency devices and allow low frequency DC signals to pass. They reject all frequencies except DC signals. RF chokes have high impedance for a wide range of frequencies in order to suppress all frequencies. As frequencies get higher, they are met with equal resistance and blocked from passing.
The structure of a RF choke includes a coil of insulated wire wrapped around a magnetic core or circular bead made of ferrite material hung on wire. They have complex winding patterns to reduce their self-capacitance.
RF chokes are used in radio frequency circuits, such as antennas and transmitters, to prevent RF signals from interfering with circuit operation. Their high inductance and low resistance make them ideal for blocking RF signals and allowing low frequency signals to pass through.
As with RF chokes, power chokes are designed to filter out noise to provide a stable power supply. They filter out AC ripple in DC output to provide stable DC voltage. Power chokes are placed between the mains supply and inverter to smooth current peaks. They have high inductance and low resistance to filter out noise while allowing DC signals to pass through.
Active EMI filters are designed with operational amplifiers (op-amp) and complex circuitry. They can be configured for common mode and differential mode noise. Active EMI filters provide a simple and compact method for removing EMI using components with a small footprint. The circuit for an active EMI filter is built with an op-amp combined with capacitors and resistors. Active EMI filters reduce the size and cost of EMI filters without losing optimal performance.
The process for an active EMI filter involves sensing high frequency noise from the supply input and generating noise canceling currents that are injected back into the power supply. With a low resistance path of CM noise, an active EMI filter provides 15 dB up to 30 dB of CM noise rejection for frequencies ranging from 100 kHz up to 3 MHz.
Passive EMI filters are bulky and make up 30% of converter volume and weight while an active EMI filter takes up less room but offers high quality performance. An active EMI filter consists of a noise sensing circuit, resonant controller, and noise injection circuit. AEFs have a simple structure with a wide operating frequency range, exceptional stability, and reduced CM choke size with lower volume, weight, and cost. The design of AEFs matches the shift in next generation power electronics to higher densities, improved performance, and reduced weight.
Electromagnetic interference happens when unwanted electric currents interrupt currents that an electronic device is supposed to receive. The unwanted disruptive currents come in the form of noise or electromagnetic noise from external sources or may be created by components within the electronic device. An EMI disrupts the functions of an electronic device and causes it to perform unintended operations. They impact the quality of the signals a device receives causing component failure, temporary malfunctions, or permanent damage to a device.
Both types of EMI, conducted and radiated, can cause significant damage to a device and its performance. The impact on a device’s capabilities can involve impairment, degradation, malfunction, or complete and total system failure. It is for these reasons that EMI filters are essential and a necessity.
Natural causes of EMI are difficult to predict and happen suddenly without warning and have a severe and lasting impact on devices that are not protected. In more critical situations, natural EMI can damage military equipment, electronic tracking devices, and transportation technology, which can be very vulnerable due to society's dependence on electronic equipment.
Types of natural EMI
The sun causes disruption of satellite transmissions when it appears directly behind a satellite causing electromagnetic noise that blocks satellite communications. Snow storms can interrupt cell phone service, cause radio static, or create interference in laptops and computers.
Residential EMI comes from wireless signals and electronic appliances. They do not inflict significant damage but can cause annoying and infuriating interruptions of service and lead to the poor performance of electronics.
Society has radically shifted to dependence on electronic devices as new and more innovative designs are being introduced, which adds to the growth of EMI sources. The increased growth of electronics adds to the higher density of electromagnetic currents. Cell phones, laptops, and pad computers are constantly in use in close proximity to each other leading to the likelihood of the impact of an EMI.
The improved performance of modern electronics increases the electromagnetic interference they produce. The demand for perfect performance further exacerbates with devices operating at higher frequencies to produce more electromagnetic noise across a broader frequency range. This has led EMI filter manufacturers to produce products that can meet modern demands and provide protection for essential devices.
Types of Human Residential EMI
Due to the nature of industrial equipment, they are capable of causing large scale EMI and severe interference in essential technologies. The list of industrial sources of EMI is endless and can cause widespread impact such as disruption of hospitals, military operations, and power grids.
Examples of high-frequency sources of EMI include transmitters, transformers, inverters, microprocessors, and controls.
Types of Human Industrial EMI
Aside from natural and human Emi, there are noise sources that come from within a system. Digital circuitry that emits noise in a smartphone causes difficulty receiving and sending signals. Highly sophisticated circuits that are laid out close to one another where sensitive components are placed in close proximity to power management circuitry with high voltage and currents leads to circuitry interfering electromagnetically with the sensitive circuitry. Designers have to be aware of the density of components and EMI interference with susceptible circuitry.
A common circuit in modern electronics is the switch mode power supply that makes it possible to achieve high efficiency. Unfortunately, switching a field effect transistor on and off in a switch power mode power supply becomes a source of EMI, which can be alleviated with a DM EMI filter.
Prior to selecting an EMI filter, it is important to examine and evaluate the requirements of an electrical system. Each type of EMI filter has limits, frequency range, and suppression method. Reading filter data sheets that have insertion loss data and tables, which provide the loss in signal power between filter input and output and the loss a filter provides to a certain signal at a particular frequency measured in decibels.
Conducted noise, common or differential, is based on where it is and how it spreads. In the data collection phase, charts are available for common noise and one for differential noise. An examination of the data to determine the failure margin for each failed frequency will decide if the filter will provide sufficient attenuation to push the noise amplitude below the limit line. The same tests that need to be run for conducted noise should also be run for radiated noise.
After the collection of data, a review of the requirements and specifications of a system that includes voltage, current, operating and ambient temperatures, dielectric withstand voltage, leakage current, and type of feed system. Ratings of these requirements and specifications helps determine what a filter can accommodate without degradation of performance or dependability.
Single-phase EMI filters are rated for 250 VAC and work with any AC voltage below that. Three phase filters are rated for 480 VAC. Some single-phase filters have ratings of 277 VAC up to 300 VAC for higher voltage applications.
Current requirements range from less than 1 amp to over 1000 amps. The current rating is the maximum steady state current an EMI filter is designed to carry within its safety temperature range. It should be equal to or higher than the maximum input current a device will draw. Most filters can handle higher in rush current but fail if the rated current is exceeded for an extended period of time.
Although measuring the input current would be helpful in determining a devices current rating, an easier way is to examine the devices safety certificate for maximum current on its UL report. The report has the full load nominal input and the rating for the EMI filter. With additional power supplies and AC loads, the ratings for more filters would be required.
The temperature for a system takes two forms, which are ambient and operating. The ambient temperature is the highest temperature at which a filter can carry its full current. In most cases, it is between 40°C or 50°C. If the operating temperature is higher than the ambient temperature for the filter, the current must be derated.
The operating temperature is the temperature where the filter can safely operate, which can be, for most filters, -25°C to 85°C or 100°C. The operating temperature has to fall within this range since using a filter outside this range can damage components.
Leakage current flows from the line and neutral to ground when line voltage is applied to the filter and is caused by line to ground capacitor in the filter. EMI filters with Y capacitors add leakage current on top of a device’s contribution. Regulations limit the amount of leakage current and have regulatory safety standards that must be strictly followed.
Power systems vary from one country to another and come in special configurations apart from single phase, 3 phase delta, and 3 phase wye configurations. EMI filters are designed for single phase, 3 phase delta, 3 phase wye, and DC circuits but can be used in other systems with minimal adjustments. As may be expected, the input power supply type must be identified and match the selected filter.
The number of stages is in reference to the number of circuit repetitions, in a series, in the filter. Single stage filters have one circuit. When circuits repeat, the filters become dual and more stage filters. The performance of a filter improves as the stages increase.
The hipot test tests the dielectric strength of the insulation and is the application of DC voltage between the line and ground, which determines the weak points in the insulation or manufacturing defects. The applied high voltage is the rated voltage as specified by safety agencies. Depending on the application, hipot requirements may be higher and require filter adjustments.
EMI filters are available in a wide range of sizes, performance levels, interconnections, and mounting types. In some cases, it is necessary to custom design a filter to meet the needs of an application.
The type of equipment and its components are a crucial aspect in the selection of an EMI filter. AC/DC converters, manufacturing equipment, medical devices, RF modules, and other types of equipment necessitate the use of an EMI filter that fits the nature of the application. Clock frequencies and switching frequencies affect the emission profile of an EMI filter.
The cables and wiring inside a power system act like antennas for high frequency noise especially if the power supply is the open frame type without the shielding an enclosed power supply has. The design of power supplies can create a serious challenge for eliminating and mitigating radiated and high frequency conducted emissions. High frequency noise, filtered by a filter circuit, can couple back via the radiated mode, which makes the installation of an EMI filter a critical necessity.
Using the following EMI filter installation recommendations can assist in proper EMI Filter performance:
EMI sources generally do not have one solution and need to be approached from multiple directions. During the mounting process, it is advisable to have a selection of different performance EMI filters. This makes the installation process efficient and lowers filter costs. All EMI filter manufacturers and suppliers offer support services to assist with the selection, design, and installation of EMI filters as well as provide advice on cable routing and noise suppression methods.
An important part of an EMI filter is the components used to construct it. The basic components of an EMI filter are the capacitor and inductor where capacitors shunt noise away from the load while the inductor blocks or reduces noise. Additionally, there are circuit protection components including a fuse, metal oxide varistor, input surge current limiting resistor, and output transient voltage suppressor.
The fuse protects the power source and conductors feeding into the power supply. They are placed in a series with the input. The fuse is placed with the non-ground input terminal such that when the fuse opens, no voltage is on the power supply. Fuses are selected by the voltage, current, response time, and operating temperature of an application.
The term varistor is a combination of variable and resistor that is used to describe how a varistor works. The varistor provides over voltage protection by voltage clamping. They change their resistance value automatically as the voltage changes and are designed to absorb transient energy. An input fuse is placed between the varistor and the input power source.
The input surge current limiting resistor limits the input surge current when AC voltage is applied to the power supply. It is necessary because of the initial rapid charging of the capacitor when the input voltage is first applied.
When the load changes rapidly, the output voltage produces a reaction where the output voltage rises or falls, which is called a transient response. The TVS shunts voltage transients created by an external source on the output terminal to protect the power supply. Although a varistor can be used as a TVS, the output on a power supply is lower than on the input side and does not require the use of the varistor.
The DM choke forms a low pass LC filter with the input capacitor to reduce the conducted noise voltage on the input conductors from reaching the power source. It is designed to tolerate the maximum input current.
The CM choke creates high resistance to reduce common mode current that flows along the input conductors. The dots on the CM choke indicate the direction of the pair windings and its orientation. CM chokes are chosen to handle maximum current flow and have acceptable power dissipation.
The input capacitor is placed across the input power lines to shunt differential conducted voltage noise such that it does not continue to the voltage source. Capacitors have X or Y safety class construction, are designed to connect directly to the AC input line, and can withstand surges.
The Y capacitor is placed between the input and output to reduce mode voltage noise on the output power supply. It is designed to fail as an open circuit when the integrity of the input to output fails. The use of the Y safety isolation capacitor is necessary due to the waveform caused by the primary side switching transistor and the parasitic capacitance between the primary and secondary side of the isolation magnetic.
The filtering components are placed at the output terminal of the power supply and are placed near the load. They are selected for their ability to reduce the output ripple voltage to a level acceptable for the load.
The varied types of EMI filters on the market include panel mounts with resin or glass seals, surface mounts with a PCB footprint, board mounts, d-subminiature connectors, and single and 3 phase power line filters. Essentially, there is an EMI filter for every electronic, power, signal, and AC/DC application. The protection they offer is critical to the quality performance of all types of electronic devices.
The protection of electronic devices is the main reason for the use of EMI filters. They ensure that sensitive equipment will not be impacted by conducted or radiated noise, which can be damaging and cause malfunctions. Every type of electronic device has or should have an EMI filter installed.
There is a long list of codes, regulations, and standards that have to be met to ensure the safety of electronic equipment. Many of the regulations are applicable to the industrial use of EMI filters, which produce high amounts of EMI and RFI. All EMI filter manufacturers are aware of the list of regulations and produce their products in compliance and adherence to the standards and regulations. In many countries, failure to comply with EMI regulations can lead to fines, penalties, and production shutdowns.
Although it may not be true for residential electronics, system shutdowns due to high frequency interference can shut down a production or manufacturing operation. The use of EMI filters ensures continuous operation of electronic equipment without interference, errors, or malfunctions. The efficiency of EMI filters reduces maintenance and repair costs that affect the bottom line.
EMI filters help with long term savings, increase system reliability, and ensure uptime to meet national and international standards. As the complexity of electrical circuits increases, the number of electromagnetic disturbances greatly increases adding to the likelihood of errors and mistakes. The use of EMI filters can prevent and eliminate the possibility of such occurrences.
An EMC plan outlines details regarding a new product and how it will be tested. It carries all the details about a product, outlines EMC tests, and their results. An EMC plan provides an organized process for testing and acts as a record to enable laboratories to present accurate and cogent data. With the introduction of sensitive electronics, EMI filters offer control of electrical input current that can be recorded and protect items being tested.
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