High-Stability Capacitors for High-Frequency Circuits
Different ceramic classes, MLCCs, and their relationship to stability.
Quality factor/dissipative factor provides a metric to determine stability.
When to use alternatives to MLCCs in design.
High-stability capacitors are often associated with chip packages.
Capacitors are ubiquitous in electronics, and for good reason: they perform multiple critical functions for many different types of circuits. While high-stability capacitors are valuable in many instances, they shine in high-speed RF applications. As capacitors tend to leak more energy at high frequencies, preventing loss to the environment is energy efficient and prevents heat-related aging of components and the substrate.
Ceramic Capacitors: Classes and Packaging
Capacitors fill a wide variety of roles across a circuit. As one of the fundamental building blocks of electronics, capacitors can filter, decouple, smooth signals, act as power reserves, and provide many other essential features. To best match these different uses, capacitors are available in many different materials and constructions.
Ceramic capacitors are an excellent starting point when discussing stability, but they represent a multitude of styles. The International Electrotechnical Commission has defined three different classes of ceramic capacitors:
- Class 1 capacitors provide high stability and low losses over a wide range of operating and environmental conditions.
- Class 2 capacitors contain greater storage capabilities but operate in a nonlinear fashion at extreme temperatures.
- Class 3 capacitors historically offered even greater storage capabilities than Class 2, while possessing an even greater level of instability. As multilayer ceramic capacitors (MLCCs) have exceeded the technological capabilities associated with Class 3 capacitors, the classification has fallen out of use and is no longer standardized.
Regarding stability, Class 1 capacitors provide designers with the best option, suitable for an ample, triple-digit temperature range while maintaining tight tolerances. However, they are not universally applicable. Class 2 capacitors, rated at the same capacitance, require a far smaller footprint than an equivalent Class 1 capacitor. When a layout is cramped, Class 1 capacitors may be unable to meet the capacitance requirements of the circuit.
Form Follows Function: The Relationship Between Packages and Stability
In terms of packaging, the MLCC (ceramic disc for through hole) has become far and away the package most associated with high-stability capacitors. The former is formed by a homogenous, powdered mixture of the majority para or ferroelectric material as well as any metals necessary to form the dielectric. A series of electrodes alternatingly attached to the two terminals form a stack of miniature capacitors (when interspersed between layers of dielectric). The smaller the grain size of the powdered dielectric, the thinner the electrode plates, allowing for reduced package sizes or an increase in volumetric capacitance. Some rearrangements of the manufacturing process, such as plating along the long edges of the body or utilizing a chip array format, can reduce the equivalent series resistance (ESR) of an equivalently sized package. ESR forms an important diagnostic tool, providing a metric by which to evaluate the stability of a capacitor.
Using the Quality Factor to Measure High-Stability Capacitors
The stability of a capacitor can also be analyzed through its quality factor, referred to shorthand as its Q-factor. In simple terms, the Q-factor measures the efficiency of a capacitor; it’s thermodynamically impossible to have a perfect energy store, so capacitors (and many other devices) are evaluated by relating the losses to the total storage capabilities of the capacitor. It stands to reason that the greater the Q-factor of a capacitor, the better its performance at reducing leakage. Quantitatively, the equation relates lossy and lossless impedance components as so:
The Q-factor relates reactive (lossless) and resistive (lossy) opposition to current flow
Reactance, denoted XC, represents the imaginary-valued, lossless opposition to current flow, while resistance (ESR in the case of a capacitor) is the real-valued, lossy opposition. In other terms, the higher the Q-factor, the more closely a capacitor models its ideal form, minimizing losses from Ohmic heating. While capacitance remains constant (provided dielectric breakdown is avoided), both reactance and ESR are frequency-dependent values. Q-factor, therefore, represents a variable response, often reported in datasheets either at noteworthy frequencies or with a curve to cover a wider range of values. A related value, the dissipation factor (DF), may be reported instead of the Q-factor, with the two terms being reciprocals of one another.
When the Q-factor Is Only Skin-Deep
Is the Q-factor/dissipation factor a crucial consideration for all capacitors? Not necessarily, but for some applications, losses can become untenable if appropriate capacitors are not employed. This is most often seen in high-frequency circuits where an increase in the frequency results in a greater concentration of current density towards the surface of the conductor. This phenomenon, dubbed the skin effect, arises due to the back EMF in an alternating current that is strongest at the center of the conductor, which consequently pushes charges to the edge radially. Since resistance is an extrinsic property of the component, a reduction in the area means an effective increase in resistance. This increase in energy lost as heat can be great enough to cause desoldering events in appreciably high-frequency boards.
Generally, capacitance and the dissipation factor are inversely related, with highly stable capacitors lacking in bulk capacitance, and vice versa. The paraelectric titanium dioxide mixtures used to produce these high-stability capacitors lack the permittivity of ferroelectric materials commonly found in capacitors with greater volumetric efficiency. The material of a single ferroelectric capacitor may be rated at the relative permittivity of as many as 30-70 paraelectric capacitors. In dense designs that require appreciable capacitance, tantalum capacitors may prove to be the most effective option, but they are cost-prohibitive relative to ceramics.
Selecting the Right Capacitor
High-stability capacitors may or may not be essential to your design’s function, but it is likely that your board already contains these capacitors due to their excellent performance characteristics over a vast range of operating inputs. MLCCs are the vast majority of nonpolarized SMD capacitors, offering general coverage of most capacitor needs including resonance, smoothing, bypass, and (de)coupling. More pertinent to designers and engineers will be understanding the gaps wherein more specific, and often pricier, capacitor solutions are required.
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