The design of power distribution decoupling networks has become problematic due to the ever-increasing complexity of electronic circuitry. Power supply distribution on a typical Printed Circuit Board (PCB) is accomplished with an elaborate decoupling network consisting of a multitude of shunt capacitors and series dampening elements. A major concern in modern PCB design is power supply isolation between the various circuits on the PCB. To that end, complex decoupling networks have evolved to minimize direct coupling and Electro-Magnetic-Interference (EMI) between these circuits along the power supply distribution lines. These circuits consist of various bypass capacitors that collectively provide high shunt admittance across all frequencies on the PCB, and series dampening devices which are generally resistors and ferrite beads. Over time these decoupling circuits have become increasingly complex and are projected to become more so as PCB, Integrated Circuit (IC), and other substrate circuitry increases in diversity and bandwidth. The object of the present invention is to provide an elegant comprehensive solution to this problem.
The current popularity of multifunctional, small, thin, and light circuitry on a single board has spurred the need for miniaturization which in turn fuels the desire to eliminate the plethora of bypass capacitors used in both analog and digital circuits. Achieving such a goal would save component and assembly costs, as well as lower failure rates. In analog circuits both low and high frequency components require specialized large and small bypass capacitors, respectively. The mixing of high-power amplifiers, both RF and audio, with low-noise front-end circuitry also increases the number of bypass capacitors required. In the digital world the coexistence of high-speed components (signal processing) and high power (control) circuitry again increases the need for more capacitors.
The design of power distribution decoupling networks is fraught with problems and tradeoffs. First, capacitors are not capable of bypassing both low and high frequencies. The upper frequency is limited by internal resonances in the capacitors themselves. This upper frequency limit can be increased by reducing the capacitance. However as the capacitance decreases so does its admittance, and hence its ability to shunt low frequencies. To remedy this problem, that is, to overcome the bandwidth limitations of a single capacitor, a multitude of elaborate decoupling networks have evolved. The most common decoupling network solution consists of a number of strategically placed capacitors of different values connected in parallel combined with small appropriately placed series damping resistors. Given the need to decouple nearly every major circuit component on a board, and the desire to maximize power delivery efficiently, a complex set of resonances is often introduced as the capacitors interact with each other and the line lengths between them increases. The power distribution network consisting of these capacitors, resistors, and inductive line lengths, is therefore generally underdamped and prone to instabilities which compromise the performance and isolation of the various generally diverse circuits that they feed power to.
Providing adequate bypassing on the multifunctional circuit boards in use today requires many capacitors spread all over the board along with series decoupling resistors and/or lossy ferrite elements. This traditional method of decoupling is power inefficient as well as costly in terms of manufacturing, reliability and board space. Because DC current must travel through the series damping elements, this approach has the additional flaw of wasting power (I2R losses) in power-distribution circuits. Furthermore, a multitude of capacitors must be purchased, inventoried, and handled, requiring labor and machine time to install them. Once installed, the capacitors contribute to reducing the reliability of the whole assembly tremendously by introducing two solder connections per component (e.g., capacitors and dampening resistors). One of the highest failure mechanisms in a modern surface mount board are stress failures at the solder joints themselves as well as the growing of metallic oxide dendrites between the component solder pads when biased circuit boards are subjected to humid or wet environments. This problem may become worse with the advent of the new “Pb free” solders that manufacturers will be required to use by Jul. 1, 2006.
Even in the best case, where the circuit designers are aware of all the subtleties inherent in a well functioning power supply distribution system, the resulting, often underdamped, decoupling network is a compromise between economy, stability, reliability, and power delivery efficiencies.
In view of the above, it would be desirable to have a single-element capacitor that is integrated into the circuit board that could distribute power, as a power plane, while providing wide-band, resonant free, high shunt admittance everywhere on the circuit board surface. Such a buried capacitor could eliminate most, if not all, discrete bypass capacitors and their associated dampening resistors and ferrite beads. In theory, a large capacitor spread across a printed circuit board or other substrate could provide shunt admittance large enough that other bypass elements would not be necessary. However, internal reflections off the finite boundaries of such a capacitor induce many resonances in its admittance, destroying the decoupling characteristics of the capacitor at its parallel-resonant frequencies, where its admittance is relatively small. Resonances can be sorted into two types, parallel resonances where the admittance is small and the decoupling is poor, and series resonances where the admittance is large and the decoupling is excellent.
The principal requirement for wideband power-plane decoupling is low shunt impedance over the entire frequency range of interest. This is accomplished using charge storage devices using:                materials that exhibit high Q over a wide bandwidth to realize low Effective Series Resistance (ESR) over the entire band;        a large enough capacitance to provide charge storage for the low frequency band edge; and        a physical structure that has no in-band resonances.        
An additional requirement is to provide sufficient charge storage electrically close to the devices requiring it. In other words, the capacitor must be placed physically close to the devices needing to be decoupled, thereby providing low external inductance and series resistance between the physical charge storage and the circuitry requiring the charge. Whereas using several layers of a circuit board to create a whole board integral capacitor provides charge physically close to the components requiring charge, it does not solve the fundamental resonance problems that limit the upper frequency use of the capacitor.
The origin of the capacitor resonance problem is described below using the most common fundamental capacitor, which is called a parallel plate capacitor. This capacitor consists of parallel electrically conductive plates separated by free space or a dielectric medium. Resonance, in general, occurs when reinforcing in-phase feedback is provided. In this case the capacitor has a resonance at any wavelength λ where constructive feedback occurs. This constructive feedback causes the resonances that limit the upper frequency use of a parallel plate capacitor. The feedback is created by repeated, reinforcing, multiple, low loss edge reflections of laterally flowing Transverse Electric Magnetic (TEM) waves between the parallel plates. These parasitic laterally flowing waves are initiated whenever a pulse of charge is added to or taken from the capacitor. They are initiated at the point in the capacitor where the charge transfer occurs. Each wave travels laterally from its initiation point until it encounters one of the capacitor's highly reflective edges, whereupon the reflected wave continues traveling in the opposite direction until it encounters another reflective edge. The wave continues traveling, bouncing back and forth between edges, creating resonances whenever the round trip for the wave is in phase. These resonances are called “N*λ/2” resonances because they occur when the distance between reflective edges is a multiple number (N) of half (½) wavelengths (λ).
As the capacitor is made larger and larger to extend the low end of the frequency band, the upper frequency limit to the capacitor gets lower and lower as the boundary conditions, within the capacitor, which determine its internal resonances, get electrically further apart. So, once again, while converting several of the board's layers to one large integral capacitor solves the problem of the stored charge being physically far from circuits requiring the charge, it does not address the fundamental self resonance problems that result from attempting to use a single large capacitor physically big enough to be close to all board components and electrically large enough to bypass the lowest frequencies of the wide band circuitry.
As matter of fact, a large in-board capacitor promotes coupling between circuit elements at the frequencies where capacitor resonances occur such that a by passed component sees a parallel resonance where it is attached to the large buried capacitor. The circuit isolation fails at these points, frequently resulting in unacceptable levels of crosstalk and circuit instabilities due to unintended feedback.
Accordingly, a need exists for a charge delivery source implemented integral to an integrated circuit device (such as the integrated circuit die, the integrated circuit package, the printed circuit board, or the circuit substrate) that can deliver charge over a wide band of frequencies without inducing resonances due to the internal reflections of laterally flowing traveling waves. A need also exists for a wide-band, both low- and high-frequency charge delivery source that provides a reliable, low-inductance, low-impedance return (e.g., a ground return) to electronic components on the integrated circuit device. Furthermore, a need exists for a non-resonant, wide-band, high shunt admittance DC power plane that is available everywhere to the surface of a circuit substrate.
Additionally, a need exists for a wideband capacitor that can be mounted to a circuit board whose upper frequency limit has been increased because the capacitor's naturally occurring resonances have been eliminated without compromising the Q of the capacitor at any frequency.
Finally, a need exists for an integrated capacitive structure that replaces most, if not all, bypass capacitors on a chip and/or board, thereby greatly reducing the part count on all circuits in which it is implemented and decreasing the size of the chip and/or board.