Addressing the Limitations of Conventional Aluminum Electrolytic and Tantalum Capacitors
The basic structure of a tantalum electrolytic capacitor is broadly similar to that of an aluminum electrolytic capacitor. A sintered body made from tantalum powder serves as the anode. A layer of tantalum pentoxide is formed on its surface as the dielectric, while solid manganese dioxide is used as the electrolyte.
Tantalum electrolytic capacitors offer several advantages: they are smaller than aluminum electrolytic capacitors, have excellent frequency characteristics, and provide long operating life because their electrolyte is solid. However, their typical failure mode is short-circuit failure, which can create a fire hazard. Appropriate safety measures are therefore required in the circuit design.
Addressing the Limitations of Multilayer Ceramic Capacitors
Capacitance Reduction Under DC Bias and Temperature
One well-known characteristic of multilayer ceramic capacitors (MLCCs) is that their effective capacitance can decrease substantially under DC bias. Capacitance can also decline at high and low temperatures. These changes must be considered when selecting an MLCC for a power circuit or backup circuit.
In the example shown, applying a 15 V DC voltage reduces the effective capacitance by as much as 80%. High or low temperature can cause an additional reduction of approximately 10%.
Assume that a circuit requires approximately 47 μF of effective capacitance when a 15 V DC bias is applied. If the design uses the 20% effective-capacitance value as the usable capacitance basis, a nominal capacitance of approximately 47 μF must be selected. For a 22 μF MLCC, the effective capacitance under the stated conditions is calculated as follows:
22 μF × 20% = 4.4 μF
47 μF ÷ 4.4 μF ≈ 10.7 capacitors
Therefore, more than 10 nominal 22 μF MLCCs would be required to provide approximately 47 μF of effective capacitance in this example. The total nominal capacitance would need to be at least 220 μF.
Selecting an MLCC with a higher nominal capacitance can reduce the component count. However, for general-purpose 25 V chip MLCCs, approximately 47 μF is often near the upper end of the available capacitance range. When a larger effective capacitance is required, designers commonly use multiple inexpensive, lower-capacitance MLCCs in parallel.
Solution: Replace MLCCs with Conductive Polymer Capacitors
Conductive polymer capacitors offer a different behavior: their capacitance changes very little with DC bias and temperature compared with MLCCs. In the example above, ten 22 μF MLCCs could be replaced by one 47 μF conductive polymer capacitor, depending on the component's rated operating conditions and the actual circuit requirements.
Reducing the number of capacitors can lower the total cost, including assembly cost, and may also reduce the required PCB mounting area. The primary benefit is not simply the nominal capacitance value, but the ability to maintain a larger portion of that capacitance under the applied DC bias and temperature conditions.
A hard-disk-drive application provides a practical example. To maintain memory backup operation and enable a safe hard-drive shutdown during a power interruption, the circuit requires approximately 140 μF of capacitance at 12 V.
If 22 μF MLCCs are used and an approximately 80% capacitance reduction caused by DC bias is taken into account, 36 capacitors would be required. Their combined nominal capacitance would be 792 μF. By comparison, three 47 μF SP-Cap conductive polymer capacitors can provide the required capacitance with significantly fewer components. This reduces component count, mounting area, and total cost. Other conductive polymer capacitor types, including POSCAP devices, can provide a similar component-reduction approach when their electrical ratings are suitable.
MLCC Acoustic Noise and Mechanical Vibration
MLCCs exhibit an inverse piezoelectric effect. When a voltage is applied, the ceramic body deforms slightly. Under a steady DC voltage, this deformation is largely static. When the voltage contains a periodic component, however, the MLCC expands and contracts periodically, causing the PCB to vibrate.
If the vibration frequency falls within the audible range, the vibration can produce an audible ringing or buzzing sound. The output of an AC adapter or switching power supply may contain ripple voltage superimposed on the DC voltage. When the ripple frequency is within the audible range, the periodic mechanical deformation of the MLCC can produce acoustic noise.
Even when the vibration is not strong enough to produce audible noise, it can still affect equipment operation. In hard-disk-drive applications, there have been cases in which slight vibration from an MLCC installed near the magnetic head contributed to data errors.
Solution: Use Capacitors Without an Inverse Piezoelectric Effect
Several MLCC design approaches are available to reduce acoustic noise. These include materials with lower deformation, reverse-LW structures in which the electrode length is shorter than the component width, and designs that use extended metal terminals or metal frames. Such products can reduce acoustic noise and mechanical vibration, but they do not necessarily eliminate them completely.
Conductive polymer capacitors do not exhibit the inverse piezoelectric effect associated with ceramic capacitors. As a result, they do not produce the same type of MLCC-related acoustic noise or mechanical vibration.
Another example involves the buck converter in a laptop computer, where the voltage from an AC adapter is stepped down by a DC/DC converter. MLCCs used as input capacitors in the converter generated audible noise. MLCCs with metal terminals, developed as a noise-reduction option, reduced the sound pressure level. In the comparison, the red curve represents a conventional MLCC and the green curve represents an MLCC with metal terminals.
In the comparison, one 33 μF conductive polymer capacitor replaced eight 10 μF MLCCs with metal terminals. The SP-Cap configuration produced no detectable MLCC-type acoustic noise or mechanical vibration, thereby eliminating the ringing problem observed with the ceramic capacitor configuration.
Conductive polymer capacitors are therefore increasingly considered when a design must maintain effective capacitance under DC bias and temperature variation, reduce component count, or avoid MLCC-induced acoustic noise and vibration. The correct choice still depends on the required voltage rating, capacitance, ripple current, operating temperature, reliability target, and PCB implementation conditions.
