Introduction

The fourth part of this series of articles elaborates on some viewpoints regarding the radiation emissions generated by power converters (especially those used in industrial and automotive fields) during switching.

Radiated electromagnetic interference（EMI）It is a dynamic problem that occurs in a specific environment, related to the parasitic effects inside the power converter, circuit layout, component arrangement, and the overall system in which it operates. Therefore, from the perspective of design engineers, the issue of radiated EMI is usually more challenging and complex, especially when multiple DC/DC power levels are used on the system motherboard. Understanding the basic mechanism, measurement requirements, frequency range, and corresponding limitations of radiated EMI is crucial. This article focuses on introducing these aspects and presents the results of the radiation EMI measurement device and two DC/DC buck converters.

Near-field coupling

Figure 1 provides an overview of the basic EMI coupling modes between noise sources and disturbed circuits, particularly inductive or H-field coupling, which requires a time-varying current source with high di/dt and two magnetic coupling circuits (or parallel wires with return paths). On the other hand, capacitive or E-field coupling requires a time-varying voltage source with high dv/dt and two adjacent metal plates. Both mechanisms belong to near-field coupling, where the noise source is very close to the disturbed circuit and can be measured using a near-field sniffer.

For example, modern power switches, especiallygallium nitrideGaN and SiC based transistors with output capacitance COSSLow, gate charge QGLess, able to switch with dv/dt and di/dt conversion rates. There is a high possibility of H-field and E-field coupling and crosstalk between adjacent circuits. However, as the mutual inductance or capacitance decreases, the spacing between the coupling structures increases, and the near-field coupling significantly weakens.

Far-field coupling

Typical electromagnetic (EM) waves propagate in the form of a combination of E-field and H-field. The field structure near the radiation antenna source is a complex three-dimensional pattern. Further analysis from the radiation source reveals that the EM waves in the far-field region consist of E-field and H-field components that are orthogonal to each other and to the propagation direction. Figure 2 shows this type of plane wave, which represents the main reference for radiated EMI and is constrained by various radiation standards.