一篇关于EMI的文章

Offline switching power supplies have high voltage and highcurrent switching waveforms that generate ElectromagneticInterference (EMI) in the form of both conducted and radiatedemissions. Consequently, all off-line power supplies must bedesigned to attenuate or suppress EMI emissions belowcommonly acceptable limits.

离线式开关电源拥有高压、大电流的开关波形，这些波形以传导和辐射的形式产生电磁干扰。因此，设计离线式电源时，必须衰减或者抑制EMI在正常接受的范围内。

Properly designed transformers, PCboards, and EMI filters not only reduce conducted EMI emissionsbut also suppress radiated EMI emissions and improve EMIsusceptibility. These techniques can also be used in applicationswith DC input voltages such as Telecom and Television CableCommunication (or Cablecom).

变压器、PCB、以及EMI滤波器设计适当，不仅仅可以限制传导EMI，而且可以抑制辐射EMI，以及改善EMI的敏感度。这些技术也可以用于通信类直流输入应用。

 The following topics will be presented：

以下主题将会呈现。

1. EMI Specifications for North America, EuropeanCommunity, and Germany

南美、欧洲。以及德国的EMI参数要求

2. Measuring Conducted Emissions with a LISN

用LISN测量传导辐射

3. Peak, Quasi-Peak, and Average Detection Methods

峰值、准峰值、以及平均值的测量方法

4. Safety Principles

安规原理

5. EMI Filter Components

EMI滤波元件

6. Flyback Power Supply EMI Signature Waveforms

Flyback电源EMI信号波形

7. Filter Analysis

滤波器分

8. Power Cord Resonances

电源线共振

9. Transformer Construction Techniques

变压器构造技术

10. Suppression Techniques

抑制技术

11. General Purpose EMI Filters

EMI滤波器的一般目的

12. EMI Filter PC Layout Issues

EMI滤波器Layout注意事项

13. Practical Considerations

实际考虑

Safety is a vital issue which determines EMI filter componentselection, the transformer reinforced insulation system, and PCboard primary to secondary spacing. In fact, safety is an integralpart of the power supply/EMI filter design and is difficult todiscuss as a separate issue. 

安规对于以下反面的确定，是一个至关紧要的因素：EMI滤波元件的选择，变压器加强绝缘系统，以及PCB初级对次级的空间距离。实际上，安规是电源EMI滤波设计不可分割的一部分，也很难作为一个独立的事件来讨论。

Flyback Power Supply EMI Signature

Flyback power supplies have a distinctive EMI signature caused by superposition of several waveforms shown in Figure 28. The transformer primary current IPRI, Drain voltage VDrain, diode voltage VDiode, and transformer secondary current ISEC waveforms each generate emission currents which may exceed the desired EMI specification limits without properEMI design technique.

反激变换器EMI干扰由图28所示波形引起。变压器初级电流，漏级电压，二极管电压，以及次级电流波形，都会产生辐射电流。在木有适当设计技术时，该电流可能超出所期望的EMI指标限制。

Primary Current Waveform

Primary current IPRI begins to flow when Mosfet turns on.Transformer primary current ramps to a peak value determinedby input voltage, primary inductance, switching frequency, andduty cycle. This trapezoidal (or triangular) current waveformis characterized in the frequency domain by a spectrum with a fundamental at the switching frequency and harmonics determined by the relative squareness of the waveform andcauses primarily differential mode emission currents to circulatebetween the AC mains and the power supply input. This currentwaveform can also create common mode emissions due toradiated magnetic fields if the current path defined by the PCboard layout encircles a large physical area.

Mosfet导通时，初级电流产生。变压器初级电流上升到一个峰值，该峰值由输入电压、初级电感、开关频率以及占空比确定。这个不规则梯形（或者三角形）电流波形频域特性为开关频率的基波与谐波，它们由相关的方波形成，引起差模辐射电流在AC输入线和电源之间循环流动。如果该电流流通面积过大，产生了辐射磁场，从而也会形成共模辐射。