Decoupling capacitors and distributed dielectric media between power and ground layers in a PCB are the typical high frequency noise current draining paths to chassis. These paths do not directly drain the high frequency noise currents from power layers to the chassis, instead they daisy-chain them through the PCB ground layers. Draining noise currents from the PCB power layers directly to the chassis is more effective in minimizing radiated emissions. The effectiveness of decoupling capacitors at high frequency has been challenged by several researchers. This paper provides circuit models for explaining how a chassis plate sinks common mode noise current from a PCB as a parallel branching circuit and a differential current canceller. A method for directly draining noise current from the PCB power layer to the chassis plate using low inductance paths for reducing the radiated emissions is explained with experimental data.
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