What is EMI Filter in Power Supply and How Does It Work
Switching power supply is widely used in computers and peripheral equipment, communications, automatic control, household appliances and other fields. It has significant advantages such as low power consumption, high efficiency, and small size. It is currently the most commonly used power supply device in electronic equipment.Both ac dc power supply and dc power supply types are essential, with the dc power supply providing direct current for electronic devices and testing, while ac power supplies deliver alternating current to loads.
The outstanding disadvantage of switching power supplies is the generation of extremely strong electromagnetic interference (EMI). EMI signals will pollute the electromagnetic environment through conduction and radiation, affect the efficiency of power grids and power generation systems, and interfere with communication equipment and electronic products. The local power grid can be a significant source of widespread EMI, impacting critical systems such as hospitals and military operations. EMI can also disrupt residential and industrial devices, causing anything from minor performance issues to serious malfunctions or failures. It is a recognized power hazard.
Basic Working Principles of Switching Power Supply
The switching power supply is mainly connected to the power grid through a rectifier, which directly rectifies and filters the mains power into high-voltage DC, then converts it into low-voltage high-frequency alternating current through an inverter, and then undergoes secondary rectification and filtering to become the required low-voltage DC. The ac dc power supply plays a key role in converting mains or electrical source power into specific voltage and current outputs suitable for various devices.
The principle of EMI generated by Switching Power Supply
There are many reasons why switching power supplies produce EMI, among which the current high-order harmonic interference generated by the fundamental wave rectifier and the peak voltage interference generated by the transformer-type power conversion circuit are the main reasons. Electrical circuits within the power supply are fundamental sources of EMI, generating both conducted and radiated emissions.
The rectification process of the fundamental rectifier is a common cause of current harmonics.
Harmonic interference in primary rectifier circuit
The power frequency sine wave current passes through the rectifier diode of the full-wave rectifier circuit and becomes a unidirectional pulse current. This current can be decomposed into the sum of the DC component and a series of AC components of different frequencies.
Harmonics (especially high-frequency harmonics) will produce conduction interference and radiation interference along the transmission line. On the one hand, it will distort the front-end power supply waveform, and on the other hand, it will produce radiation interference through the power line. Voltage distortion can result from inadequate filtering, impacting power quality and system reliability.
Harmonic interference of secondary rectifier circuit
When the rectifier diode is conducting in the forward direction, the charge in the PN junction is accumulated. Because the diode works in a high-frequency on-off state, when a reverse voltage is applied to the diode, the accumulated charge cannot disappear immediately, resulting in a reverse surge current. Due to the presence of distributed capacitance and inductance in lines, high-frequency surge currents generate high-frequency oscillations when flowing through them, so these disturbances have a rich spectrum. Parasitic capacitance between circuit elements, especially in high-frequency switching circuits, can further affect noise attenuation and waveform integrity.
What is Electromagnetic Interference（EMI）?
Electromagnetic interference (EMI) occurs when unwanted electrical currents interfere with the electrical currents that an electronic device is supposed to receive. These interfering currents, often called "noise" or "electromagnetic noise," can come from external sources or be generated by other components within the device. The effects of EMI can range from minor glitches and signal distortion to severe failures and even permanent damage to sensitive equipment.
When EMI occurs, it interferes with the normal functioning of a device and can cause unexpected operation. Depending on the severity of the interference, EMI can affect the quality of the signal received by the device, cause temporary malfunction or failure of components, or even permanently damage the electronic device. Therefore, it is critical to understand the causes and effects of EMI. By identifying potential EMI sources and their effects on electronic devices, engineers can design effective solutions, such as installing EMI filters, to protect sensitive equipment from the harmful effects of electromagnetic noise.
EMI Filter to suppress EMI of switching power supply
Interference signals are injected into the public power grid from the power input terminal, causing conduction disturbance. Conducted interference signals can be divided into two forms: differential mode and common mode. Advanced EMI filters are designed to suppress both common mode and differential mode noise, improving system performance and EMC compliance. Differential mode noises, which occur between conductors in the power line, are mitigated by EMI power line filters to protect equipment and prevent noise pollution back into the grid.
Differential mode interference is transmitted between two wires and is symmetrical interference; common mode interference is transmitted between the wire and the ground (chassis) and is asymmetrical interference.
Differential mode interference has small amplitude and low frequency, causing less interference; common mode interference has large amplitude and high frequency, and can also produce radiation through wires, causing greater interference. Output ripple voltage at the output terminal must be minimized through proper filtering to maintain signal integrity and EMI compliance. Output transient voltage suppression, using TVS diodes and similar components, is also critical to protect the output terminal from voltage spikes.
What is EMI Filter?
EMI filter is an effective measure to suppress interference, especially the conduction interference and radiation interference of switching power supply EMI signals. EMI filtering in switch-mode power supplies is achieved using X- and Y-class ceramic capacitors, which are placed to reduce both common-mode and differential-mode noise, ensuring equipment safety and electromagnetic compatibility. The input capacitor, typically an X or Y class, is placed across the input power lines to shunt conducted noise and protect the input power source. Ferrite chokes are often used on power supply lines and ac power lines to further reduce EMI. Passive filters, consisting of inductors and capacitors, are commonly used for noise suppression, with their internal configuration determining the frequency range they can effectively attenuate. Low pass LC filters, combining an inductor and a capacitor, are effective in minimizing conducted noise voltage and protecting the power source. Active filters, which require an internal power supply, can analyze noise signals and generate anti-noise to cancel out high-frequency electromagnetic interference, offering improved noise suppression compared to passive filters.
The most effective way to weaken conducted interference and control EMI signals below the limit levels stipulated in relevant electromagnetic compatibility (EMC) standards is to add an anti-interference (EMI) filter to the power input end. The EMI filter is installed between the power line and the switching power supply. It only allows power frequency current to pass through, which plays an important role in improving the reliability of switching power supplies. Circuit protection components such as fuses, varistors, and surge protectors are integrated within EMI filters to safeguard circuits from voltage spikes and transient noise. Varistors, especially MOVs, are used to absorb transient energy caused by voltage surges or lightning strikes, protecting the power supply components. The input fuse is typically installed in series with the non ground input terminal to ensure that, if it blows, no voltage reaches the power supply, enhancing safety. Proper placement of protective components at the input power source is essential to guard against transients and surges.
The limit values of conducted interference levels stipulated in many EMC standards start from 10kHz. The operating frequency of the switching power supply is about 10kHz~100kHz. For high-frequency EMI signals generated by switching power supplies, satisfactory results can be obtained by selecting an appropriate decoupling circuit or an EMI filter with a relatively simple network structure. The effectiveness of EMI filters is evaluated by their impact on signal power and overall system performance, particularly regarding insertion loss and attenuation at specific frequencies. Output isolation, often achieved with Y capacitors, is important for maintaining electrical isolation and attenuating common mode noise at the output terminal.
How to Select an EMI filter?
When choosing a filter, the 3 most important parameters are: rated current, rated voltage and cutoff frequency. It is crucial to consider the maximum input current and voltage ratings to ensure safe and reliable operation under normal and transient conditions. The availability of different EMI filters allows for selection based on specific requirements such as frequency range, noise suppression capabilities, and industry standards. Choosing the best EMI filter for high-frequency suppression is essential for protecting sensitive electronics and maintaining power quality.
Rated current refers to the signal current passing through the filter. The maximum input current specification is critical for selecting inductors and capacitors to prevent saturation and ensure reliable operation.
Rated voltage: If the wires that need to be filtered will not be impacted by high voltages such as electrostatic discharge, current pulses, surges, etc., the working voltage of the circuit is the rated working voltage of the filter. Generally the rated working voltage should be above 200V. The impact of ac voltage on surge current and filter selection must also be considered, especially during power-up and filtering stages.
Cutoff frequency: The cutoff frequency of the signal line filter is defined as the frequency when the insertion loss is 3db.
The selection of the cutoff frequency must ensure that the passband of the filter covers the bandwidth of the functional signal and ensures the normal operation of the equipment. At the same time, filter out unnecessary high-frequency interference to the greatest extent. EMI filters are designed to target high frequencies for noise attenuation, as most electromagnetic noise occurs at higher frequencies.
DOREXS Manufacturing on EMI Filter
DOREXS has many years of experience in EMI filter manufacturing and is proficient in applying EMI filters to switching power supplies. Their products are integrated into various electrical systems to safeguard equipment from EMI and ensure reliable operation. EMI filter applications span a wide range of industries, including communications, automation, medical, and industrial sectors, each requiring tailored solutions. Power electronics, such as EV chargers and renewable energy inverters, play a vital role in modern energy management, efficiency improvements, and EMI mitigation strategies.
Moreover, DOREXS has multiple product certifications, including CE, ISO9001, UL and other certifications, which meet the technical requirements of European and American countries.
Contact us to get more EMI filters suitable for use in switching power supplies.
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