(1) input filter: its effect is the power grid existing clutter filtering, also hinder the machine produces clutter feedback to public power grid。 

    (2) rectifier and filter: the power grid ac power directly for a smooth dc rectifier, for the next level transformation。 

    (3) inverter: will the dc after rectifying a high-frequency ac, this is the core of high frequency switching power supply, the higher the frequency, the volume, weight and the ratio of power output and smaller。 

   (4) output rectifier and filter: according to load needs, providing stable and reliable dc power supply。

 2) control circuit: on the one hand, from the output by sampling with set standards to compare, and then to control inverter, changing its frequency or pulse width, achieve output stability, on the other hand, according to data provided by the test circuit, the protection circuit differential, provide control circuit to the machine to various protection measures。 Including the output feedback circuit and sampling circuit, pulse width modulator。

 3) the detection and protection circuit: detection circuit had current detection, over-voltage detection, owe voltage detection, overheat detection, etc。; Protection circuit can be pided over current protection, over-voltage protection, owe voltage protection, the ground-clamp protection, overheating protection, automatic restart, soft start, slow startup, etc。 Various types。

 4) other circuit: if the sawtooth wave generator, offset circuit, optical coupler, etc。

 Essay three：

  Modeling, Simulation, and Reduction of Conducted Electromagnetic

Interference Due to a PWM Buck Type Switching Power Supply I

Abstract:Undesired generation of radiated or conducted energy in electrical systems is called Electromagnetic Interference (EMI)。 High speed switching frequency in power electronics converters especially in switching power supplies improves efficiency but leads to EMI。 Different kind of conducted interference, EMI regulations and conducted EMI measurement are introduced in this paper。 Compliancy with national or international regulation is called Electromagnetic Compatibility (EMC)。 Power electronic systems producers must regard EMC。 Modeling and simulation is the first step of EMC evaluation。 EMI simulation results due to a PWM Buck type switching power supply are presented in this paper。 To improve EMC, some techniques are introduced and their effectiveness proved by simulation。 

Index Terms:Conducted, EMC, EMI, LISN, Switching Supply 

I。 INTRODUCTION

  FAST semiconductors make it possible to have high speed and high frequency switching in power electronics [1]。 High speed switching causes weight and volume reduction of equipment, but some  unwanted effects such as radio frequency interference appeared [2]。 Compliance with electromagnetic  compatibility (EMC) regulations is necessary for producers to present their products to the markets。 It is important to take EMC aspects already in design phase   [3]。 Modeling and simulation is the most  effective tool to analyze EMC consideration before developing the products。 A lot of the previous studies concerned the low frequency analysis of power electronics components[4][5]。 Different types  of power electronics converters ae capable to be considered as source of EMI。 They could propagate the EMI in both radiated and conducted forms。 Line Impedance Stabilization Network (LISN) is required for measurement and calculation of conducted interference level [ 6]。 Interference spectrum at  the output of LISN is introduced as the EMC evaluation criterion [7][ 8]。 National or international  regulations are the references for the evaluation of equipment in point of view of EMC [7][ 8]。