FDTD法研究散热片与器件间的耦合电容

Application of FDTD method to analysis of coupling capacitance between heatsink and devices

散热片与器件间的电容耦合是电路产生共模辐射的主要原因之一 ,其对研究电路的辐射发射特别重要。然而长期以来人们将该电容简化为平板电容 ,采用静电场推导的电容公式计算。但在高频端 ,散热片和器件的尺寸与波长相比拟 ,分布参数影响了该耦合电容的数值。所以本文提出了采用FDTD法计算散热片与器件间的高频耦合电容的构想。数值计算的结果表明 :该电容已经不能看作是一简单常数 ,而是随频率变化的量。频率较低时 ,耦合电容随频率升高快速减小。且耦合电容具有频率选择性。激励源位于散热片的中心耦合电容小 ;绝缘层厚度越薄 ,相对介电常数越大得到的高频耦合电容越大 ,但不是线性变化。在实际散热片的选择和安装过程前必须对其产生的耦合电容进行预测 。

Coupling capacitance between the heatsink and the devices produces common mode emission, which is considered as a constant calculated by a equation resulting from the static field for a long time. However, if the sizes of the heatsink and devices are approximate to wavelength in high frequency, Distributed parameters affect the value of the capacitance. In this paper the coupling capacitance between the heatsink and the devices is calculated by FDTD method. It is shown that the coupling capacitance is not a constant, but a function of frequency. Coupling capacitance lessens rapidly in low frequency. And it has frequency selectivity. Coupling capacitance of center excitation is little smaller than the off center excitation. The thinner the dielectric and the bigger relative permittivity, the bigger capacitance, but it is not linear. So coupling capacitance should be predicted before selecting and installing heatsink for getting the smallest capacitance.