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How to reduce EMI in the design of step-down power converters.

  • September 16, 2020
  • 71

Power conversion


   All portable devices have one thing in common, that is, they are powered by some form of battery. However, the voltage from the battery is unlikely to be directly used by the semiconductor devices that form the core of the product design, so it needs to be converted to convert (and stabilize) the voltage to the correct level.


   The so-called "buck" converter has become very popular-it provides a simple and more effective way to convert the battery voltage into a stable semiconductor power source. The buck converter changes the level by "cutting off" the DC voltage from the battery, but this may introduce EMI-related issues.


   Although the topic of EMI may be very complicated, designers should pay attention to many issues when designing most buck converters:


   Current loop layout


  Component placement


   output ripple voltage


  The choice of input capacitor


  Radiated EMI suppression

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  In order to reduce the size of power converters and related magnetic devices, designers usually use higher switching frequencies. However, as the current rate of change (di/dt) increases, the possibility of EMI will increase. In order to minimize these effects, it is very important to understand how current flows between components. Once the current path is clearly understood, the designer should ensure that all return current paths, especially ground traces, are as short as possible and have low impedance.


       The current and the selected topology (buck topology in this case) also play an important role in determining the component layout.


   A key consideration is to ensure the shortest current loop when placing capacitors and inductors. When doing so, you need to wire the input power traces. Here, the designer should make sure that the inductance of these traces is greater than the ESR of the input capacitor-which explains why a low ESR capacitor is a good choice.


  At the same time, in terms of input, the choice of capacitor is also very important, and designers should not ignore it. Capacitors are an important part of power converters. It is a good practice to choose high-energy capacitors, such as KEMET's KO-CAP polymer capacitors or low-ESR MLCC devices. Combining these two types together can reduce ripple voltage and minimize the number of components.


   When choosing the input capacitor, remember that the output current will directly affect the input ripple voltage. It may be difficult to define the requirements of transient current, but it will directly affect the choice of input capacitance, so it is an important step in the design process.


   Generally, when solving EMI problems, although a large ripple voltage is likely to cause EMI problems, current is the primary consideration. According to Ohm's law, the AC current flowing through the power inductor flows into the output capacitor, and a ripple voltage is generated when it encounters ESR. Therefore, choosing an output capacitor with low ESR can reduce the output ripple voltage.


  Conducted EMI is usually the most common problem in buck converter design. However, especially in densely arranged designs, such as modern consumer devices, radiated EMI is also a problem. Although techniques such as good layout and wiring design can reduce this problem, it cannot be completely avoided.


   Ferrite devices, such as Kemet’s FlexSuppressor (flexible suppression sheet) products, are very useful for creating in-circuit barriers, which can effectively isolate sensitive circuit areas from other parts that may be coupled to undesired radiated noise.


       Efficient design


   After solving the potential noise problem, you need to consider the overall efficiency of the buck converter design. More efficient design can achieve longer running time; due to the reduced heat generation, components can also be placed closer together, so that designers can flexibly shorten the traces to solve the EMI problem.


   Power engineers can use step-down controller ICs and well-designed power inductors to minimize losses and improve efficiency. Choosing an inductor that can work at high frequencies and can provide high current saturation characteristics and low DC resistance (DCR) is critical to efficient buck converter design.


   METCOM's new series of inductors from Kemei Electronics can effectively support the development of more efficient step-down converters and are suitable for other power-related applications including EMI filtering. The metal composite magnetic core has a large current saturation characteristic, which can keep the inductor operating under a large ripple current. High permeability can achieve low DCR, which can significantly reduce self-heating during high current operation, thereby increasing system efficiency and reducing the need for heat dissipation design considerations.


  METCOM inductors have a shielding structure, so most of the magnetic flux can be controlled in the inductor body to achieve more efficient work. This can enhance radiated EMI performance and significantly reduce RF coupling with nearby circuit areas.


   The value range of Kemet's new inductor is from 0.10mH to 47.00mH, and the DCR value is as low as 1.5mW. They can handle currents up to 35.4A and operating temperatures range from -55°C to +155°C. The package area is as small as 5.3mm×5.00mm and the height is as low as 2.0mm, which is very suitable for dense circuits in modern power applications.


   Summary of this article


  In this interconnected era, we cannot do without a variety of high-tech equipment every day, and power conversion is essential to provide these products. If designers want to create more dense designs, improving efficiency and controlling EMI are key challenges. However, if you follow good design practices and carefully select components, these challenges can be easily solved.


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