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阻抗失配原则及根据此原则的电源滤波器选择|电磁兼容(EMC)滤波器

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发表于 2008-1-28 10:14:18 | 显示全部楼层 |阅读模式

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  当滤波器输出阻抗 Z<sub>0</sub> 和与之端接的负载阻抗 Z<sub>L</sub> 不相等时,在该端口 A 上产生反射,见图 1 二十二。反射系数 ρ 定义为: <p>ρ = ( Z<sub>0</sub>-Z<sub>L</sub> ) / ( Z<sub>0</sub>+Z<sub>L</sub> ) </p><table cellspacing="0" cellpadding="0" hspace="0" vspace="0"><tbody><tr><td valign="top"></td></tr></tbody></table><img src="http://www&#46;dianyuangu&#46;com/jiaocheng/tupian/154/1&#46;gif" border="0" /> <p>图 1 :反射系数的定义 </p><p>&nbsp;&nbsp;&nbsp; 从上式可以看出,则 Z<sub>0</sub> 与 Z<sub>L</sub> 相差越大, ρ 便越大,端口 A 产生的反射将越大。考虑到滤波器和产品实际情况,在端口元件的选取上可以参照此式,因此电源 EMI 滤波器的输出阻抗与噪声源阻抗失配越严重,滤波器性能越好。这就是滤波器结构选取的“阻抗失配”原则,这是与信号滤波器最大的区别之处。厂家给出的滤波器或元器件的插损曲线都是在源阻抗和负载阻抗为 50的情况下测得的,而实际应用时源阻抗和负载阻抗往往都不是 50 ,并且会随频率而变化,所以在标准 50 系统下具有良好插损特性的滤波器在实际电路中也不一定有效,甚至有时更差,所以必须按照阻抗失配的原则选择滤波器结构。 </p><p>&nbsp;&nbsp;&nbsp; 参考“ EMI 滤波器的基本种类和特点”一文对基本电源滤波器的分析和介绍 ,根据阻抗失配原则,我们给出图 2 的 EMI 滤波器网络的推荐结构。图中 Zs 是源阻抗, Zl 是负载阻抗。例如当负载阻抗 Zl 很低而源内阻 Zs 很高时,滤波器可选择 n 级 L 型( n=1 , 2 , 3 , &#46;&#46;&#46;&#46;&#46;&#46;&#46; )。 </p><p>&nbsp;&nbsp;&nbsp; 下面表示出了 EMI 滤波器与设备的正确与错误的端接示例。 </p><p align="center"><img src="http://www&#46;dianyuangu&#46;com/jiaocheng/tupian/154/2&#46;gif" border="0" /></p><p align="center">a 、 n 级 T 型级连 </p><p align="center"><img src="http://www&#46;dianyuangu&#46;com/jiaocheng/tupian/154/3&#46;gif" border="0" /></p><p align="center">&nbsp;</p><p align="center">b 、 n 级 LC 级连 </p><p align="center"><img src="http://www&#46;dianyuangu&#46;com/jiaocheng/tupian/154/4&#46;gif" border="0" /></p><p align="center">c 、 n 级 CL 级连 </p><p align="center"><img src="http://www&#46;dianyuangu&#46;com/jiaocheng/tupian/154/5&#46;gif" border="0" /> </p><p align="center">d 、 n 级&nbsp;⑥ 型级连 </p><p align="center">图 2 : EMI 滤波器的阻抗失配选择形式 </p><p>表 1 给出了端口的正确端接方法和错误方法的比较 </p><p><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">到</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: "Times New Roman"; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-fareast-font-family: 宋体"><a href="http://www&#46;emcgarden&#46;net/"><span lang="EN-US" style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><span lang="EN-US">电磁兼容</span></span>(EMC)<span lang="EN-US" style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><span lang="EN-US">和电磁干扰</span></span>(EMI)<span lang="EN-US" style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><span lang="EN-US">学习园地</span></span></a>( <a href="http://www&#46;emcgarden&#46;net/">http://www&#46;emcgarden&#46;net</a> )</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">学习各种电磁兼容</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: "Times New Roman"; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-fareast-font-family: 宋体">(EMC)</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">设计整改经验知识以及各种电磁干扰</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: "Times New Roman"; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-fareast-font-family: 宋体">(EMI)</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">解决措施,到</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: "Times New Roman"; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-fareast-font-family: 宋体"><a href="http://www&#46;studyemc&#46;net/"><span lang="EN-US" style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><span lang="EN-US">电磁兼容</span></span>(EMC)<span lang="EN-US" style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><span lang="EN-US">和电磁干扰</span></span>(EMI)<span lang="EN-US" style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><span lang="EN-US">下载园地</span></span></a>( <a href="http://www&#46;studyemc&#46;net/">http://www&#46;studyemc&#46;net</a> )</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">下载各种</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: "Times New Roman"; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-fareast-font-family: 宋体">EMC</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">标准和</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: "Times New Roman"; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-fareast-font-family: 宋体">EMC</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">资料以及安规防雷静电放电</span><span lang="EN-US" style="FONT-SIZE: 9pt; FONT-FAMILY: "Times New Roman"; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-fareast-font-family: 宋体">ESD</span><span style="FONT-SIZE: 9pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1&#46;0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">等资料。</span></p><p align="center"><img src="http://www&#46;dianyuangu&#46;com/jiaocheng/tupian/154/6&#46;gif" border="0" /></p><p align="center">表 1 :滤波器端口的正确端接方法和错误方法</p>
发表于 2008-2-3 08:34:53 | 显示全部楼层
恩学习ING,输入输出阻抗都随时间变化,是阶段性的容性感性怎么解决呢?是2个不同的配套滤波器串联么?
发表于 2008-9-13 21:03:33 | 显示全部楼层
是不是图片的开放会根据&quot;级别&quot;调整?没有了图片,此帖还不如删除
发表于 2008-9-17 15:37:38 | 显示全部楼层
[s:28] 小女子是为学习,不是为灌水,谢谢
发表于 2008-12-2 17:55:13 | 显示全部楼层
看不到图!
发表于 2009-7-20 22:38:27 | 显示全部楼层
没有图理解起来麻烦些

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