Research On Suppression Of Input Harmonic Currents And EMI For The Switching Power Supply

The switching power supply usually uses the diodes rectifier as the front converter. The input currents of the rectifier contain many harmonics, which result in serious problems of power line pollution and EMI. In this paper, the research focus on the novel passive three phase high power factor rectifiers and the frequency jitter technology to improve the input currents of a charger/discharger for Ni-Cd battery.Three kinds of passive methods to increase the power factor of three phase rectifier are analysed in this paper, namely three-phase rectifier with near sinusoidal input currents (RNSIC), 24-pulses three-phase rectifier and three phase rectifier adopting third-harmonic injection. The RNSIC is equipped with diodes, inductors and capacitors, the operational principle, characteristics, design method, disadvantages and improved scheme of which are illustrated in detail. The novel 24-pulse rectifier connects the primary side of its phase-shifting transformer with the rectifier in series, so the input power of the transformer is only half of the input one of the rectifier. In the dc side, a voltage injection circuit is added to give the input currents into shape.The analysis, simulation and experiment on it are given in this paper. The high power factor three-phase rectifier adopting third-harmonic currents injection is researched then, in which the third harmonic is provided by LC resonant circuit and injected to the ac side. A novel circuit is given and analysed detailly in this paper. The simulation and experimental results indicate that the third is the suitable rectifier to replace the former of the charger/discharger.The switching frequency jitter techonology is researched and a simple circuit is proposed to help the PWM IC SG3525 to jitter its out-put pulses frequency. The effection to the EMI suppression of DC/DC converts has been verified by simulation and experimental in the half bridge converter.The experiment results of the improved and the former charger/discharger are given in the end. The results indicate that the harmonic currents of the improved one have been reduced obviously.