Now I’m sure as engineers we have all studied circuit modeling. SPICE is an amazing tool to emulate how your final circuit might perform… but what’s really interesting is that passive components such as resistors, capacitors, inductors and wires (conductors) are far more than that as frequency increases! It is also important to revisit the printed circuit board… it too is more than simply “wires”.
At DC, copper traces look like a resistor with the value dependent on temperature and the cross sectional area of the trace. As frequency increases, weird things start to happen. The trace begins to look more like an inductor limiting instantaneous current flow. At higher frequencies there is a phenomenon called the “skin effect” that pushes the electrons to the outside of the conductor. This appears as an attenuation that affects the signal at roughly the square root of the frequency. There is also electric field coupling to other traces… namely ground or power planes. The board material that sits between the two conductors forms a capacitor and this also attenuates the signal fairly linearly with frequency. As you can see, if you are building 1GHz clock distribution systems, the PCB is already providing you free resistors, inductors and capacitors!
But what about using intentional devices such as capacitors? The latest passive technologies have provided ceramic capacitor values of many microfarads in tiny little packages (e.g. 22uF 0805 ceramics using Y5V dielectric). Even though these devices are capacitors, they are really a capacitor with a low value series resistor and inductor along with a very high value parallel resistor. All of these “parasitic” sub-components are byproducts of the physical implementation and chemistry of the device. The series impedance is most important in switched mode power supplies where the impedance can affect the stability of the control loop. This is why you may want to parallel smaller values which have the effect of increasing capacitance and lowering series impedance.
What’s more interesting is that due to the chemistry of the capacitor, it can actually affect the high frequency response… which can be VERY bad if you are attempting to bypass high frequency transients. A good tutorial on these effects for ceramic chip capacitors can be found on Johanson Dielectrics web site. For instance, the dielectric constant for NP0 (NP – Zero) capacitors are extremely stable with temperature, but X7R dielectric can vary +/-15% over temperature… this obviously can affect your design… the NP0 dielectric is much more stable, so if you are building a filter, you will want to use NP0 dielectric capacitors.
Ultimately, the capacitor’s parasitic components will impede its ability to carry AC signals, so the PCB can actually become useful. This is called “planar capacitance” and is commonly created by sandwiching a ground and power plan very close using the board material as the capacitor’s dielectric. In this application, surface area provides the increase in capacitance and can help bypass high frequency nodes (vias that draw or sink current to the plane).
All in all, it is very important to remember that rapid high current transients and high frequencies can cause all kinds of chaos if the parasitic effects of the system are not considered… believe me… I have made those mistakes and have felt the consequences. Make sure you know the chemistry of the capacitors and the style inductors you are using… a customer once asked me how theycould tell if they were using the correct inductor in a switch mode power supply. I asked him how much it weighed… (no joke). Turned out he had the correct value of inductance, but was using an RF air core choke… needless to say the power supply had very poor load regulation! Just something to help you smile and get through the day… till next time!
