Design007 Magazine

PCBD-Nov2014

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38 The PCB Design Magazine • November 2014 right axis corresponds to vertical location inside the capacitor with the front center correspond- ing to the bottom of the capacitor, closest to the PCB it is mounted on. The left horizontal axis corresponds to the left-to-right position inside the capacitor body between the two vertical ter- minals. With a macro you can sweep through the frequency values or you can plug in a spe- cific frequency selector number. The current distribution one decade below the series resonance frequency is uniform. This directly shows up in the dielectric current plot being a flat surface. The capacitor plate cur- rent shapes look triangular. Since the capacitor plates don't short the opposite terminals, they end in the open dielectric, hence the triangular shape. The equal-height triangular shape in fact corresponds to uniform current sharing among the plates. When we set the frequency selector to the 135 th point, which corresponds to 22 MHz, the current distribution plots change significantly. In general, the values in both 3D charts are much higher, already suggesting a resonating scenario. The resonance clearly shows up on the right-hand 3D plot (dielectric current), exhib- iting a notch halfway up inside the capacitor body. This comes from a vertical half-wave reso- nance, also shown as a local peak at 22 MHz on the top left impedance magnitude plot. If we re-simulate the model with different L c values, we will see that the structural reso- nance gets stronger as the ratio of inductances of the vertical terminal versus the external con- nection increase. With poor connection, when the connection inductance is much higher than the inductance of the capacitor terminals, this resonance is hardly noticeable. Strangely, as Figure 5: impedance magnitude on the top left chart. The current distribution is shown at 22 Mhz in the capacitor plates (lower left 3D chart) and in the dielectrics (lower right 3D chart). VERTICAL RESONANCES IN CERAMIC CAPACITORS continues quiet power

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