EMI Filter Components: what they are and how they work



As your project progresses, you have reached a critical juncture where you must choose the appropriate internal AC-DC and DC-DC power supplies. In your previous project, you opted for supplies with internal EMI and EMC components. However, the current project requires external EMI and EMC components for the supplies you are evaluating. In this discussion, we will guide you through the reasons behind this requirement and how to select the appropriate components.

Before delving into the specifics of EMI and EMC components selection, let’s clarify their fundamental concepts. EMI and EMC are acronyms that stand for Electromagnetic Interference and Electromagnetic Compatibility, respectively. Essentially, these concepts boil down to two primary goals: avoiding disruption to other systems and functioning correctly in the presence of external electrical disturbances.

Now, let’s dive into the nitty-gritty of EMI and EMC component selection. Our focus will be on external components since your current project demands their use. First and foremost, it’s essential to understand that the selection of EMI and EMC components is not one-size-fits-all. The components required will vary depending on factors such as the project’s electromagnetic environment and the specific power supply being used. Therefore, it’s crucial to consult with a knowledgeable engineer or supplier to determine the appropriate components.

It’s also important to note that selecting the wrong EMI or EMC components can result in poor system performance, leading to increased cost and delayed project timelines. Therefore, it’s crucial to invest time and resources in proper component selection to ensure optimal system performance.


Power Supply Block Diagram

Simplified block diagram of ac-dc switching power supply

Fig. 1 – Simplified block diagram of ac-dc switching power supply


The block labeled EMI/EMC filter in the diagram above is located at the input of the power supply, as depicted in Figure 2 below. However, in practical scenarios, some of the EMI/EMC filter components are positioned on the input of the power supply, some are placed on the output of the power supply, while others are installed between the input and output of the power supply.

EMI/EMC filter components are utilized to perform various functions such as reducing radiated and conducted noise on the input or output of the power supply, minimizing the impact of voltage transients on the input or output, lowering input surge current during voltage application, and protecting the input power source and conductors in case of power supply failure.


External EMI and EMC components

Fig.2 – External EMI and EMC components


Circuit Protection Components


A fuse is a safety device that serves to safeguard the power source and conductors that are connected to the power supply. It is situated in series with the power supply input, and it is essential that no other components are present between the fuse and the conductors from the upstream power source. To ensure safety, the fuse should be placed in series with the non-ground input terminal. In the event that the fuse opens, there should be no voltage present on the power supply.

If an excessive input current flows into the downstream power supply, and the correctly sized fuse opens, the downstream power supply could be damaged. In this case, simply replacing the fuse will not be sufficient, and the downstream power supply should be repaired or replaced. To select the appropriate fuse, it is crucial to consider the voltage, current, response time, and operating temperature of the application.

Figure 3 depicts a variety of package options available for selection, which can assist in further optimizing the design. These options should be selected based on the specific requirements of the application.

Fuse symbol (left) and mounting styles (right)

Fig. 3 – Fuse symbol (left) and mounting styles (right)


Metal Oxide Varistor (MOV)

The Metal Oxide Varistor (MOV) is an essential component placed across the input terminals to safeguard against sudden voltage transients that could originate from the input voltage source. Typically, in an AC-DC power supply, the input voltage source comes from the AC power lines, and the MOV’s primary function is to absorb transient energy caused by events such as lightning strikes or damage to the AC power network. During normal operation, the MOV component exhibits a high impedance, but when the rated voltage is exceeded due to input transient voltage, it quickly transitions to a low impedance state.

It is essential to include an input fuse between the MOV and the input power source to prevent any mishaps. The fuse acts as a protective measure that may blow if the MOV transitions to a low impedance state due to a transient input voltage. Selecting an appropriate MOV component requires considering both the operating voltage and the transient energy that may be applied to the power supply.

While MOVs are common components used for transient voltage protection, alternative components such as Zener diodes, Transient Voltage Suppression (TVS) diodes, or Gas Discharge Tubes (GDT) can be used for similar purposes. These protection components provide varying degrees of protection, and selecting the most suitable component depends on the specific application and requirements.


Input Surge Current Limiting Resistor

In the schematic diagram of EMI/EMC components, the resistor denoted as R1 is specifically designated for ac-dc power supplies. Its purpose is to limit the surge current that occurs during the initial application of ac voltage to the power supply. The surge current arises due to the swift charging of the bulk capacitor when voltage is first applied to the input. The absence of a current limiting resistor may result in a surge current up to 100 times greater than the regular operating current. However, a higher value of current limiting resistor can lead to unacceptable power losses in normal operations. Therefore, caution must be exercised when choosing the construction of the input surge current resistor. It must be able to handle the high energy burst that occurs during the flow of the input surge current. Wire wound construction resistors are typically preferred for the input surge current limiting resistor due to their ability to tolerate such energy bursts. In contrast, film construction resistors are often unsuitable to handle the large input surge energy.

To conclude, when selecting the current limiting resistor for the input surge current, a balance must be maintained between the reduction of the input surge current and power losses during normal operations. Wire wound construction resistors are generally recommended as they are better equipped to handle high energy bursts.

Input Surge Current Limiting Resistor

Fig, 4 – Input Surge Current Limiting Resistor


In certain circuit designs, an effective means of controlling input surge current is through the use of a negative temperature coefficient (NTC) resistor. This type of resistor is characterized by a high resistance value when it is cool, effectively limiting the input surge current. As the input current begins to flow, the NTC resistor begins to heat up, causing its resistance value to decrease, and ultimately reducing the associated power dissipation by the resistor.

It’s important to note that after the power supply is turned off, the NTC resistor requires a sufficient amount of time (tens of seconds) to cool down and return to its high impedance value before the input power is reapplied.

It’s worth mentioning that not all dc-dc converter applications require an input surge current limiting resistor. In certain instances, the input surge current may already be inherently limited, either due to the relatively low input voltage (peak input voltage for 240 Vac is 340 V) or because the source impedance of the input supply will naturally limit the maximum available input current.


Output Transient Voltage Suppression

The utilization of a Transient Voltage Suppression (TVS) diode on the output of a power supply can serve a dual purpose. One primary reason for integrating the TVS diode on the output of the power supply is to redirect voltage transients that are produced by external sources on the output terminals of the power supply. This protection feature helps in safeguarding the power supply against such transients.

Typically, a Metal Oxide Varistor (MOV) is used for similar purposes on the input of the power supply. However, the voltage present across the output of the power supply, as well as the energy associated with the induced transients on the output of the power supply, is typically lower compared to the input. As such, using a TVS diode is a more suitable solution for safeguarding the output of the power supply.

If there is a desire to clamp the output voltage in the event of power supply failure, a Zener diode can be used as an alternative to the TVS diode. However, it is important to note that during such a failure, the power that needs to be dissipated by the Zener diode may exceed the power supply output rating.

The Zener diode will have a breakdown voltage greater than the power supply output voltage, and in case of power supply failure, it may deliver greater than the rated output current.

In conclusion, the TVS diode on the output of the power supply protects against external transients, while the Zener diode can help clamp the output voltage in case of a failure.


EMI Filter Components

Differential Mode Choke

The LDM inductor is positioned in a sequence with the input power trail, creating a low-pass LC filter with CX input capacitor. This filter serves to reduce the undesired conducted noise voltage and prevents it from interfering with the external power source. It is crucial to ensure that the saturation current of this inductor is adequate to withstand the maximum input current during standard operation, even if the inductor saturates during the inrush start-up current surge. The differential mode inductor must be chosen with a low DCR (parasitic dc resistance) to keep the power dissipation within acceptable limits.

To guarantee a proper functioning system, the inductor must be carefully selected and correctly placed within the circuit. Proper inductor selection will provide sufficient current-carrying capability while minimizing parasitic resistance, thus reducing power dissipation.

In conclusion, the LDM inductor serves a vital function in reducing undesired conducted noise voltage and ensuring proper system operation. The inductor’s saturation current and DCR must be carefully considered to maintain optimal performance.

Differential Mode Choke

Fig. 5 – Differential Mode Choke


Common Mode Choke

The LCM choke with dual windings functions as an input common mode choke, meaning it creates a high impedance to weaken common mode currents in the input conductors. While incorporating the common mode choke in the schematic, it’s crucial to ensure that the ‘dots’ are correctly oriented. These ‘dots’ indicate the relative winding direction for the pair of windings, and it doesn’t matter if they are on the input or output of the connections to the common mode choke. However, both dots must be on the same electrical side of the choke. Unlike the differential mode choke, the common mode choke does not require dot notation as it has only one winding, and the current flow direction does not matter.

It’s imperative to choose a common mode choke that can handle the maximum current flow during normal power supply operation while maintaining acceptable power dissipation. Usually, the common mode choke has minimal current flowing in common mode, making the saturation current rating insignificant.

Common Mode Choke

Fig. 6 – Common Mode Choke


X Safety Class Input Capacitor

The CX capacitor, placed across the input power lines, plays a crucial role in shunting the differential conducted voltage noise to prevent it from reaching the external voltage source. To ensure optimum performance, it is advisable to use a capacitor that belongs to the X or Y safety class construction. Such capacitors are designed to be directly connected to the AC input lines, making them capable of withstanding any voltage surges that may be present.

To better shunt the undesired noise, a larger value of capacitance is preferred as it results in a lower impedance. However, this also increases the input leakage current in an AC-DC power supply. It is essential to note that many AC-powered systems need to comply with regulatory standards that impose maximum input leakage current limits. Consequently, the amount of capacitance in the shunt capacitor is restricted.

EMI current path with (top) and without (bottom) X-Capacitor

Fig. 7 – EMI current path with (top) and without (bottom) X-Capacitor


Y Safety Class Isolation Capacitor

In the schematic diagram, there is a capacitor labeled CY1 that is placed between the input and output of the power supply to reduce common mode voltage noise on the output. This capacitor is selected to be of Safety Class Y since it is located across the isolation barrier. Safety Class Y capacitors are designed to fail in an open circuit, ensuring the integrity of the input to output isolation of the power supply in case of capacitor failure. Sometimes, two capacitors are required to be connected in series to provide further assurance of the isolation between the input and output of the power supply.

The need for capacitor CY1 arises due to the voltage waveform created by the primary side switching transistor, and the parasitic capacitance between the primary and secondary side of the isolation magnetics (as shown in Figure 8). To maximize its effectiveness, capacitor CY1 should be placed between the source of the primary side switching FET and the terminals of the secondary winding of the isolation magnetics. Additionally, since the output terminals of the isolation magnetics have large bypass capacitors between them, they are at the same AC potential. Therefore, the output terminal of capacitor CY1 can be connected to either output terminal.

The capacitance value of CY1 must be larger than the primary to secondary parasitic capacitance to ensure proper attenuation of the common mode voltage on the output of the power supply. However, it’s essential to note that a larger value of capacitor CY1 can cause greater AC leakage current between the input and output of the power supply.

Primary Side Switch, Isolation Magnetics, and Parasitic Capacitance

Fig. 8 – Primary Side Switch, Isolation Magnetics, and Parasitic Capacitance


Input Bulk Capacitor

The schematic diagram features a capacitor, labeled as C1, which is positioned directly across the input of a dc-dc converter. This capacitor comes after the bridge rectifier and, if present, the power factor correction circuit, in an ac-dc supply. Its primary function in a dc-dc converter is to serve as an input charge reservoir that helps to reduce input voltage disturbances due to input current transients. On the other hand, in an ac-dc supply, the capacitor filters the rectified ac input voltage and supplies energy to maintain the output voltage when the ac input voltage is disconnected.

To limit the maximum current drawn when the capacitor is initially charged, an input surge current limiting resistor is employed. This helps to prevent any excessive flow of current that may result in damage to the system. Therefore, the capacitor and the resistor work together to ensure the smooth functioning of the dc-dc converter and ac-dc supply.


Output Filtering

The power supply’s output terminals can have filter components installed to tackle any EMI and EMC problems that may arise. Typically, these components are placed near the load and are chosen based on their ability to reduce the output ripple voltage to a level that the load can tolerate. In some cases, for ease of use, L1 may be substituted with a short circuit. It is important to note that capacitor C2 offers a low impedance ac pathway between the output terminals of the power supply, allowing CY1 to be linked to either output terminal.

To address EMI and EMC issues that may arise, filter components can be placed on the output terminals of a power supply. These components are usually positioned near the load and selected based on their ability to reduce output ripple voltage to a level that meets the load’s requirements. In certain situations, L1 can be replaced with a short circuit to simplify the process. It should be noted that capacitor C2 provides a low impedance ac pathway between the power supply’s output terminals, enabling CY1 to be linked to either terminal.

If you want to maintain the quality of your power supply, you should consider the installation of filter components on its output terminals. These components can be positioned near the load to ensure optimal reduction of output ripple voltage. In some cases, L1 may be substituted with a short circuit for convenience. Capacitor C2 is crucial in providing a low impedance ac pathway between the output terminals, which allows for CY1 to be linked to either terminal.

Output Filter

Fig. 9 – Output Filter


EMC Component Selection

It is evident that choosing the appropriate components to avoid electromagnetic interference (EMI) and electromagnetic compatibility (EMC) problems related to power supplies is not a challenging task. However, it involves taking into account numerous factors. The ultimate decision regarding the components and their values is frequently a trade-off between performance, expense, size, and energy conversion efficiency.

Many power supply providers, including ALEXANDER ELECTRIC, have customer support engineers who are ready to offer further aid if needed during the implementation of your project. These professionals possess the expertise and knowledge to assist you in selecting the optimal components for your application, while also considering your project’s specific requirements and constraints.

In conclusion, while choosing the right components for your power supply might appear to be a simple task, it necessitates careful consideration of several factors. If you require additional support, do not hesitate to reach out to customer support engineers from reputable power supply manufacturers such as ALEXANDER ELECTRIC.