電信交換機中高功率因數(shù)的保持

　　電信應(yīng)用中的電力傳輸系統(tǒng)現(xiàn)在必須處理大電流的大CPU陣列，可能消耗數(shù)百瓦的電力。為了提高效率，電信電源采用開關(guān)拓?fù)滢D(zhuǎn)換電供應(yīng)48伏直流輸出，然后分發(fā)給其他系統(tǒng)的線路卡。問題在于開關(guān)電源由于輸入電路的結(jié)構(gòu)而對電網(wǎng)呈現(xiàn)非線性阻抗。

　　輸入電路通常包括半波或全波整流器，接著是存儲電容器，該電容器維持電壓接近輸入正弦波峰值電壓的值。它是由隨后的半波充電的。這種拓?fù)渲辉跂艠O供電峰值時從輸入中提取電流；這個脈沖包含足夠的能量來維持負(fù)載直到下一個峰值。電流脈沖的持續(xù)時間不超過周期的四分之一，脈沖期間所提供的電流必須至少是平均電流的四倍。

　　與正弦輸入相比，產(chǎn)生的電流波形被大量強諧波嚴(yán)重扭曲。只有基本分量產(chǎn)生實際功率，剩余的峰值負(fù)責(zé)表觀功率。這種差異用功率因數(shù)表示，除純正弦波外，所有的情況都小于功率因數(shù)。

　　The harmonic distortion has an adverse effect on other loads on the same supply. The distorted current causes additional heating in the wires and electrical distribution gear. As a result， legislation has come in worldwide to limit the harmonic distortion of switched-mode PSUs， in addition to controls imposed by the utilities themselves on high-power equipment. Electrical equipment with an input power requirement of 75 W or more， supplied in Europe and Japan， must comply with the IEC61000-3-2 standard. The standard specifies the maximum amplitude of line-frequency harmonics up to and including the 39th harmonic. As core telecom switches are responsible for high loads， they need to limit harmonic distortion and maintain a high power factor.