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June 1996®�TOPSwitch®Power Supply DesignTechniques for EMI and SafetyApplication Note AN-15Offline 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.This application note presents design techniques that reduceconducted EMI emissions in TOPSwitch power supplies belownormally specified limits. 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). Refer to AN-14 and AN-20 foradditional information. The following topics will be presented:•EMI Specifications for North America, EuropeanCommunity, and Germany•Measuring Conducted Emissions with a LISN•Peak, Quasi-Peak, and Average Detection Methods•Safety Principles•EMI Filter Components•Flyback Power Supply EMI Signature Waveforms•Filter Analysis•Power Cord Resonances•Transformer Construction Techniques•Suppression Techniques•General Purpose TOPSwitch EMI Filters•EMI Filter PC Layout Issues•Practical ConsiderationsFigure 1. FCC Class A and B Limits (Quasi Peak).120Amplitude (dBµV)1008060402000.010.11Frequency (MHz)10100PI-1623-111695EN55022A QPEN55022A AVGEN55022B AVGEN55022B QPFigure 2. EN55022 Class A and B Limits (Average and Quasi Peak).120Amplitude (dBµV)1008060402000.010.11Frequency (MHz)10100PI-1622-111695FCCA QPFCCB QP120Amplitude (dBµV)1008060402000.010.11Frequency (MHz)10100PI-1834-042296Vfg243 QPVfg46 A...