CE Source Stray Capacitance

Active circuits located in the vicinity of a ground plane have capacitance between the circuit conductors and the ground plane.  Circuit conductors include all components inside the circuit package, including semiconductor dies and bond wires, as well as all printed circuit traces and wires connected outside the circuit package. 

One challenge in constructing an accurate conducted emissions model is determining the value of this stray capacitance.  It can generally not be directly measured.  Product data sheets can sometimes be helpful for larger semiconductor devices, particularly power switching components such as Field Effect Transistors (FETs) and Insulated Gate Bipolar Transistors (IGBTs).  When information is not available the capacitance may be estimated considering the semiconductor die size and its height above ground.  EMI Analyst provides a Parasitics Calculator to assist in calculating common printed circuit and wiring stray capacitances.

The parallel disks below the sphere in the model diagram on the Main form represent the stray capacitance that exists between the source conductors and the ground plane.  If there is no conductive ground plane this stray capacitance is ignored.

More:

CE Filter