Electromagnetic Compatibility Technology and Its Application in Switching Power Supplies

2023-02-07
Electromagnetic compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment without generating electromagnetic interference to any other devices in that environment. In short, all electromagnetic devices in the entire system can maintain normal working status without being affected by electromagnetic interference from other electronic equipment. Two key aspects of EMC need to be clarified: first, the electromagnetic interference generated by a device during normal operation must be strictly controlled within specified limits; second, the device should have a certain level of immunity to electromagnetic interference in its surrounding environment, meaning it has low electromagnetic susceptibility.
With the continuous advancement of radio and electronic technologies, EMC technology has achieved significant development and widespread application, with its scope of use expanding steadily and its role proving undeniable. Essentially, EMC technology is a type of anti-interference technology. Electromagnetic interference is mainly categorized into natural interference and man-made interference. Natural interference sources such as lightning and atmospheric phenomena can severely disrupt the normal operation of electronic devices, leading to serious consequences. To effectively resist electromagnetic interference and suppress its generation and transmission, the active application of EMC technology is essential.
Principles of Electromagnetic Interference Generation in Switching Power Supplies
A switching power supply is a strong source of electromagnetic interference, primarily due to its composition of components including filter inductors, high-frequency switching devices, output rectifier diodes, and pulse transformers.
From the perspective of switching tubes and rectifier tubes: under normal circumstances, these components generate large-amplitude pulses during high-frequency on-off operations. The electromagnetic interference produced by these pulses is highly harmful, making them major interference sources. From the perspective of high-frequency transformers: the switching tube is loaded on the primary coil of the high-frequency transformer. When the switching tube is turned on, the primary coil generates a large surge current, and even a high surge peak voltage may occur. When the switching tube is turned off, there is leakage flux in the primary coil, which prevents a portion of energy from being effectively transmitted to the secondary coil. This residual energy eventually forms damped oscillations through the capacitors and resistors in the collector circuit, thereby generating turn-off voltage spikes. These turn-off voltage spikes interact with the magnetization inrush current transients that occur when the primary coil is energized. In this case, noise is introduced at both the input and output terminals, and the resulting conducted interference can cause severe damage—even leading to switching tube breakdown in extreme cases. In addition, the high-frequency switching current loop (composed of filter capacitors, the primary coil of the high-frequency transformer, and the switching tube) may generate spatial radiation, resulting in significant radiated interference. If the electromagnetic wave capacity is not effectively controlled or the high-frequency characteristics are poor, high-frequency impedance issues may arise in the capacitors, leading to conducted interference in the AC power supply. It can be said that high-frequency transformers play a significant role in electromagnetic interference generation in switching power supplies and pose substantial challenges in practical interference mitigation.
From the perspective of how stray parameters affect the characteristics of coupling paths: circuit networks can describe most coupling paths of interference in switching power supplies, but components in switching power supplies (such as capacitors, resistors, diodes, and inductors) all have stray parameters, making these components relatively complex. In high-frequency conditions, the characteristics of coupling paths are largely influenced by stray parameters, and distributed capacitances can become channels for electromagnetic interference. Furthermore, when switching tubes have high power ratings, heat sinks must be attached to their collectors. These heat sinks can cause electromagnetic interference to both space and power line conduction, and this type of common-mode interference is particularly harmful.
Application of Electromagnetic Compatibility Technology in Switching Power Supplies
Design of Input End FiltersThe noise generated during the operation of switching power supplies mainly falls into two categories: differential-mode noise and common-mode noise. Common-mode noise is caused by the potential difference between the ground and current-carrying conductors, while differential-mode noise stems from potential differences between current-carrying conductors themselves. Typically, electromagnetic interference on power lines contains both differential-mode and common-mode noise components. A feasible solution is to add a filter at the power supply input terminal, ensuring that the filter is impedance-matched to the power supply. A high level of impedance matching can achieve ideal attenuation effects and optimal insertion loss characteristics. In simple terms: if the noise source has low internal impedance, the input impedance of the electromagnetic interference filter should be high; conversely, if the noise source has high internal impedance, the filter should have low input impedance. Currently widely used electromagnetic interference filters incorporate suppression circuits for both differential-mode and common-mode noise, delivering excellent application performance.
During the design of input end filters, it is important to note that the proportions of differential-mode and common-mode components generated by different devices vary. Therefore, the filter circuit can be adjusted by adding or removing filter components, but such adjustments must be made based on actual conditions and verified through electromagnetic interference tests to achieve the optimal circuit performance. Additionally, during the installation of the filter circuit, ensure reliable grounding, and isolate the input and output terminals of the filter circuit to maximize filtering effectiveness.
Effective Prevention of Radiated Electromagnetic InterferenceLong-term research has shown that to effectively reduce radiated electromagnetic interference levels, voltage snubber circuits can be employed—for example, connecting an RCD (Resistor-Capacitor-Diode) snubber circuit in parallel across the switching tube. Alternatively, current snubber circuits can be used, such as connecting a 20–80 μH inductor in series with the collector of the switching tube. In switching power supplies, the collector of the power switching tube is consistently a major source of interference. A heat sink should be connected to the collector to ensure that current between the heat sink and the collector can smoothly flow into the main circuit.
It should be noted that the heat sink should be placed as far away from the equipment casing as possible to minimize the distributed capacitance between the casing and the heat sink. If conditions permit, a heat sink with shielding capabilities should be installed. For rectifier diodes, priority should be given to those with low recovery charge and short reverse recovery time—such as Schottky diodes. When using Schottky diodes, ferrite beads should be placed over their leads, and an RC snubber network should be connected in parallel to further reduce electromagnetic interference. It is also worth noting that as load current increases, the reverse recovery time of diodes becomes longer, which significantly impacts peak currents. To mitigate this effect, multiple diodes can be used in parallel to reduce the influence on peak currents.
In addition, switching power supplies must be properly shielded, preferably adopting a modular fully enclosed structure with a well-grounded shielding layer. Long-term application practices have demonstrated that modular fully enclosed structures provide excellent shielding performance, confining electromagnetic interference to a minimal range. For example, for a switching power supply whose radiated electromagnetic interference exceeds the standard limit by 20 dB, the following five measures can be taken to bring the interference level within the allowable range:
  • Connect a 470 pF capacitor in parallel across each rectifier diode;
  • Connect a 50 pF capacitor in parallel at the gate (G) input terminal of the switching tube;
  • Connect a 0.01 μF capacitor in parallel across each output filter capacitor;
  • Install small ferrite beads on the leads of rectifier diodes;
  • Optimize the grounding of the shield based on actual conditions.
Proper Application of Grounding Technology
In the application of grounding technology, an appropriate grounding method should be selected based on specific requirements. For switching power supplies, grounding primarily involves two aspects: signal grounding of the equipment and grounding the equipment to the earth.
For equipment signal grounding, two methods are commonly used: floating ground and hybrid ground. Taking floating ground as an example, this method isolates the circuit or equipment from the common grounding system, enabling circuit coordination between different potential levels. Long-term applications have shown that floating ground offers good anti-interference performance, but it also has a drawback: it can lead to electrostatic charge accumulation between the circuit and the common ground. Once the charge accumulates to a certain level, it can trigger severe electrostatic discharge, which in turn becomes a new source of interference. To address this issue, a high-resistance discharge resistor can be connected between the floating ground and the common ground to release accumulated charges.
For grounding the equipment to the earth, three key requirements must be met:
  • Ensure the safety of equipment operators at all times;
  • Timely discharge electrostatic charges accumulated on the equipment chassis;
  • Prevent changes in the potential of the equipment relative to the earth in the electromagnetic environment.
  • In summary, grounding equipment to the earth not only ensures equipment and personnel safety but also effectively suppresses electromagnetic interference.
Application of Shielding Technology
Shielding technology is highly effective in suppressing electromagnetic interference from switching power supplies and is worthy of widespread promotion. To prevent magnetic field leakage from pulse transformers, a closed magnetic loop can be used to form magnetic shielding. In addition, electric field shielding should be applied to the entire switching power supply.
It should be noted that for electric field shielding to be effective, the shielding enclosure must be grounded; otherwise, the shielding effect will not be achieved. In contrast, magnetic shielding does not require grounding of the shielding enclosure. In general, for switching power supplies, shielding work should focus on four key areas:
  • Shielding of switching tubes;
  • Shielding of high-frequency transformers;
  • Shielding of equipment casings;
  • Shielding of rectifier diodes, with priority given to the use of optoelectronic isolation technology.
Conclusion
The application of EMC technology in switching power supplies involves high professionalism and complexity, requiring in-depth expertise in relevant fields. Therefore, extra caution must be exercised in practice. To achieve optimal EMC design in switching power supplies, the focus should be placed on four key aspects: designing input end filters, preventing radiated electromagnetic interference, applying proper grounding technology, and implementing shielding technology.
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