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在MHz的DCDC和RF LNA电路中,被新手忽视的 自谐振频率点Self-Resonant Frequency
计算公式为
下图显示了 1μF,封装为 1206 的陶瓷电容器的阻抗 (
MLCC有经典的V型阻抗-频率曲线。随着频率升高,寄生电感的影响开始凸显,阻抗先变小再变大,这是MLCC的固有特性。曲线中的最低点就是MLCC的自谐振频率)
由于 ESL 的存在( MLCC内部高密度金属电极和焊接端子都能提供少量的寄生电感 ),在某个频率下阻抗实际上随频率开始上升,这个频率点又被称为自谐振频率点
电容等效电路
具体自谐振参数的影响表现在,工作频率超过自谐振点,电容失去电容作用,呈感性。
总之,在DCDCmhz和射频电路设计中需要考虑该参数,并且要知道尺寸越大的MLCC,自谐振频率就越低。然后为了电容发挥该有的作用,我们需要控制ESL,比如
引线和走线长度、避免串联使用情况和使用更小ESL电容器。还有就是当工作频率较高,而电路又需要较大电容时,可以将多个小电容(容值可以不一样的)并联
。并联后的总ESL会减小(电感并联相当于电阻并联),从而拓宽应用频率范围。