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.

離線式開關電源擁有高壓、大電流的開關波形，這些波形以傳導和輻射的形式產生電磁干擾。因此，設計離線式電源時，必須衰減或者抑制EMI在正常接受的范圍內。

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).

變壓器、PCB、以及EMI濾波器設計適當，不僅僅可以限制傳導EMI，而且可以抑制輻射EMI，以及改善EMI的敏感度。這些技術也可以用于通信類直流輸入應用。

 The following topics will be presented：

以下主題將會呈現。

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

南美、歐洲。以及德國的EMI參數要求

2. Measuring Conducted Emissions with a LISN

用LISN測量傳導輻射

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

峰值、準峰值、以及平均值的測量方法

4. Safety Principles

安規原理

5. EMI Filter Components

EMI濾波元件

6. Flyback Power Supply EMI Signature Waveforms

Flyback電源EMI信號波形

7. Filter Analysis

濾波器分

8. Power Cord Resonances

電源線共振

9. Transformer Construction Techniques

變壓器構造技術

10. Suppression Techniques

抑制技術

11. General Purpose EMI Filters

EMI濾波器的一般目的

12. EMI Filter PC Layout Issues

EMI濾波器Layout注意事項

13. Practical Considerations

實際考慮

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. 

安規對于以下反面的確定，是一個至關緊要的因素：EMI濾波元件的選擇，變壓器加強絕緣系統，以及PCB初級對次級的空間距離。實際上，安規是電源EMI濾波設計不可分割的一部分，也很難作為一個獨立的事件來討論。

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.

反激變換器EMI干擾由圖28所示波形引起。變壓器初級電流，漏級電壓，二極管電壓，以及次級電流波形，都會產生輻射電流。在木有適當設計技術時，該電流可能超出所期望的EMI指標限制。

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導通時，初級電流產生。變壓器初級電流上升到一個峰值，該峰值由輸入電壓、初級電感、開關頻率以及占空比確定。這個不規則梯形（或者三角形）電流波形頻域特性為開關頻率的基波與諧波，它們由相關的方波形成，引起差模輻射電流在AC輸入線和電源之間循環流動。如果該電流流通面積過大，產生了輻射磁場，從而也會形成共模輻射。