UltraCAD Design, Inc | |
Calculating the impedance of a power distribution system (i.e. all those parallel bypass capacitors) on a PCB has not been possible without some very high-end (and very expensive) analytical software. We created a tool for our own that greatly simplifies the process and makes it manageable for most engineers and technicians. A paper on PDS impedance has been prepared for publication and will be posted on this site very soon.
In the past, we have looked closely at the theory behind bypass capacitor self resonant behavior and have published some conclusions in our landmark paper in 2000 entitled "ESR and Bypass Cap Self-Resonant Behavior, How to Select Bypass Caps." We have recently written another article on Power Distribution Impedance that you can read here. The conclusions from these articles include:
1. As ESR goes down, the troughs get deeper and the peaks get higher.
2. The minimum impedance value is not necessarily ESR (or ESR/n, where n is the number of identical parallel capacitors); it can be lower than that!
3. The impedance minimums are not necessarily at the self resonant points of the bypass capacitors.
4. For a given number of capacitors, better results can be obtained from more capacitor values, with moderate ESRs, spread over a range than with with a smaller set of capacitor values, with very low ESRs, at even well-chosen specific self resonant frequencies.
The theoretical calculations for the impedance response of a power distribution system (i.e. a large number of parallel equivalent RLC circuits of different values) can be quite complex. We wrote our own program to help with this.
A description and mini-operator's manual for the program (in *.pdf form) is available here.
The calculator program itself is described here.
For information regarding a license for the full version of the program, look here.