In electronic devices, a filter is typically present on the AC input line. This is because, for electronic equipment that incorporates a switching power supply, the primary source of electromagnetic interference (EMI) is the power supply unit itself. The sources of EMI are diverse, including natural phenomena like lightning and the Earth's magnetic field, as well as man-made sources such as motors, radio frequency (RF) technologies, and digital/analog signals, all of which can generate interference. Filters are indispensable components for preventing these interference signals from being transmitted outward from the device or from affecting other nearby electronic equipment. This article will explore the causes of electromagnetic interference and the countermeasures to address it.
1- Types of Interference Signals and Their Generation
Noise in electronic devices refers to unwanted electrical signals within the device. These signals are unavoidable voltage or current disturbances. If the interference is excessive, the following phenomena may occur:
① Hearing noise in radios or multimedia devices that is unrelated to the intended audio.
② Displaying distorted or cluttered images on television screens beyond the original content.
③ Digital devices may start incorrectly or fail to operate normally.
④ Communication equipment may be unable to transmit normal signals.
⑤ Other effects that interfere with the proper functioning of electronic devices.
For these reasons, countries and regions have established corresponding requirements and regulations for electronic equipment, mandating that the interference signals generated by these devices must not exceed a certain limit. Manufacturers are obligated to control the EMI generated by their products to within these specified limits.
In recent years, electronic devices have widely adopted digital and switching technologies. As long as a product uses these technologies, it will inevitably generate EMI signals. Using filters is an effective way to keep this interference within regulated limits. The interference limits can vary between countries or regions, meaning the characteristics of the required filters will also differ. Pictured below are examples of a power line filter used externally for industrial equipment and an internal filter (common mode choke, differential mode choke) installed within a power supply.
Figure 1 (Left): External Industrial Power Line Filter
Figure 2 (Right): Internal Switching Power Supply Filter (Common Mode Choke)
In a switching power supply, the switching transistor, high-frequency rectifier diode, and switching transformer generate higher levels of interference. The operating waveforms within a switching power supply are typically square waves or triangle waves (fundamental waveforms). These waveforms contain high-frequency components that are integer multiples of the fundamental frequency. When these high-frequency waveforms propagate outward, they become interference signals.
Furthermore, the switching speed of transistors is extremely fast. For example, a current of 2A at 12V might be switched ON/OFF at a frequency of around 300 kHz. As shown in the diagram below, during the switching transition state, the rate of change of current (di/dt) is very high. Since inductance exists not only in the inductor coil but also as parasitic inductance on the printed circuit board (PCB), this rapid change in current can generate interference voltage signals, which interfere with the surrounding environment or other electronic components. These interference signals are not just conducted along the PCB traces but are also radiated outward through electromagnetic waves and wires. The frequency of this EMI is not fixed; there are many di/dt components within a single switching cycle, resulting in a wide frequency spectrum of generated interference voltage.
Figure 3: Equivalent Circuit Model
Figure 4: Interference Voltage Signal Model
Figure 5: Interference Voltage Signal
Figure 6: Interference Current Signal
Figure 7: Diode Turn-off Short-Circuit Current Model
Not limited to just switching power supplies, we can broadly classify where interference is generated in an electronic device based on the voltage/current path. As shown in the diagram below, interference generated in a differential mode and a common mode are referred to as differential mode interference and common mode interference, respectively.
Figure 8: Interference Signal Model Diagram
Interference that appears between the lines of an AC power cord, or between the positive and negative terminals of a DC output, is differential mode interference. In contrast, common mode interference refers to the interference signal component that arises between any line in the circuit and the ground line (i.e., with respect to Earth). Interference generated by power circuits is almost always initially differential mode. However, as this differential mode signal propagates to other circuits, its impedance balance with respect to ground can be disrupted by electromagnetic or electrostatic influences, causing it to be converted into a common mode signal. Ultimately, a significant portion of the interference becomes common mode.
Additionally, external interference signals that enter equipment from the natural environment are typically common mode, as their generation is almost always related to the Earth (ground). Furthermore, when common mode interference enters a circuit, it can also be converted into differential mode interference under various conditions and device influences, which can have a direct and adverse effect on the circuit's operation.
In electronic devices or power circuits, it is necessary to consider and implement countermeasures for both common mode and differential mode interference, which are entirely different in nature.
2- Countermeasures for Electromagnetic Interference
From the perspective of interference signal propagation, interference can be broadly classified into conducted interference and radiated interference. From the perspective of interference signal types, it can be divided into common-mode interference and differential-mode interference.There are two main approaches to suppressing interference signals:
① Preventing the generation of interference signals.
② Blocking, absorbing, or eliminating the propagation of interference signals.
Modern electronic devices predominantly use switching power supply and digital technologies. Devices employing these technologies inevitably generate interference signals, which are difficult to suppress solely through technological upgrades. Currently, most solutions focus on blocking or mitigating the propagation of interference signals.
2.1 Using passive components to block (absorb or eliminate) the conduction of interference signals, such as combining common-mode inductors, differential-mode inductors, X-capacitors, and Y-capacitors to suppress conducted interference.
2.2 Using power inductors with ferrite beads or magnetic shielding structures to prevent radiated interference signals from propagating externally.
To address conducted EMI, Codaca offers a series of common-mode inductors for signal lines (SPRHS series, CSTP series, VSTCB series, etc.), common-mode inductors for power lines (TCB series, SQH series, TCMB series), and differential-mode inductors (SPRH series, PRD series, and other power inductors that can be used as differential-mode inductors). These common-mode and differential-mode inductors help electronic devices resist external electromagnetic interference and also prevent devices from emitting EMI generated internally.
The effectiveness of interference suppression is closely related to the inductor’s impedance. Please refer to the following specification tables and frequency characteristic graphs for details.
Table 1: Codaca Common Mode Choke Characteristics Table
Note: This table shows only a selection of inductor models, For more information, please visit the Codaca official website.
Figure 9: Impedance-Frequency Characteristic Graph for Signal Line Common Mode Chokes
Figure 10: Impedance-Frequency Characteristic Graph for Power Line Common Mode Chokes
For radiated EMI solutions, ferrite beads can be used. In some high-frequency circuits, such as RF and oscillator circuits, it is necessary to add a ferrite bead at the power input section. Codaca offers a series of ferrite beads, such as the RHD, RHV, SMB, and UUN series.
Table 2: Ferrite Bead Characteristics Table
Note: This table shows only a selection of models, For more information, please visit the Codaca official website.
As mentioned earlier, magnetically shielded power inductors can also block the propagation of radiated interference. For radiated EMI, Codaca offers a series of magnetically shielded components, including molded inductors, high-current inductors, digital amplifier inductors, and chip inductors. These power inductors can be used in the power lines of switching power supplies. The magnetic shielding structure effectively prevents interference generated by the inductor from radiating outward and also shields the inductor from external radiated interference. Such shielded inductors are also used in differential mode interference solutions for signal and power lines.
Table 3: Magnetically Shielded Inductor Characteristics Table
Note: This table shows only a selection of models, For more information, please visit the Codaca official website.
Figure11:Temperature Rise & Saturation Current Curves, Inductance-Frequency & Impedance-Frequency Characteristics for VSHB0421-4R7MC
3- Conclusion
With the increasing integration and complexity of electronic products, the EMI/EMC environment they operate in also faces significant challenges. To help electronic devices solve EMI/EMC issues, Codaca has developed various series of standardized signal line common mode chokes, power line common mode chokes, differential mode chokes, ferrite beads, and various magnetically shielded power inductors. Engineers can select suitable standardized common mode chokes, differential mode chokes, or power inductors from Codaca based on the specific requirements of their power circuit design.