电信交换机中高功率因数的保持

　　电信交换机中高功率因数的保持

　　电信应用中的电力传输系统现在必须处理大电流的大CPU阵列，可能消耗数百瓦的电力。为了提高效率，电信电源采用开关拓扑转换电供应48伏直流输出，然后分发给其他系统的线路卡。问题在于开关电源由于输入电路的结构而对电网呈现非线性阻抗。

　　输入电路通常包括半波或全波整流器，接着是存储电容器，该电容器维持电压接近输入正弦波峰值电压的值。它是由随后的半波充电的。这种拓扑只在栅极供电峰值时从输入中提取电流；这个脉冲包含足够的能量来维持负载直到下一个峰值。电流脉冲的持续时间不超过周期的四分之一，脉冲期间所提供的电流必须至少是平均电流的四倍。

　　与正弦输入相比，产生的电流波形被大量强谐波严重扭曲。只有基本分量产生实际功率，剩余的峰值负责表观功率。这种差异用功率因数表示，除纯正弦波外，所有的情况都小于功率因数。

　　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.