Part4 Product Trend (2)
Trends Beyond Size/Thickness Reduction
Continuing Development of Models Optimized for Specific Applications

Kiminori Yamauchi
Executive vice president and director of the Components Business Unit of Murata

Monolithic ceramic capacitors have various kinds of specifications - dimensions and capacitance are merely two of them. Other specifications can be more important depending on the application. The development of monolithic ceramic capacitors with electrical characteristics and reliability-related properties that are optimized for a specific application is now under way. For example, Murata Manufacturing Co., Ltd. commercialized products with a higher noise absorption effect by reducing the equivalent series inductance (ESL) value or by controlling the equivalent series resistance (ESR) value. The company also released larger models for use in the inverter circuits of hybrid cars, for example.

The development of monolithic ceramic capacitors continues from the 0.6 x 0.3 mm package (EIA0201) type to the 0.4 x 0.2mm package (EIA 01005) type and on to even smaller models. The development of products to be embedded in printed circuit boards by ultimately reducing the thickness is also in process.

However, the reduction of size and thickness is not the only direction for monolithic ceramic capacitor development. Targeting various kinds of applications, the development of products with optimized electrical characteristics and reliability-related properties is progressing steadily. For example, Murata developed and started selling monolithic ceramic capacitors specialized for the decoupling of semiconductor chips and for automotive applications.

Low ESL, ESR Control

The company launched two new products intended for the decoupling of semiconductor chips with an enhanced speed and a reduced driving voltage.

One such product is the LLL/LLA/LLM Series featuring a low ESL as well as a high capacitance. Thus far, products designed for decoupling applications were often manufactured by combining a monolithic ceramic capacitor with an aluminum or tantalum electrolytic capacitor because the monolithic ceramic capacitor alone could not provide a sufficient amount of capacitance. The new product can reportedly provide the required capacitance with just a monolithic ceramic capacitor.

Fig. 1: Noise absorption and power supply characteristics of low-ESL models
The LLL/LLA/LLM Series all have low ESL values, but the noise absorption and power supply characteristics vary because the ESL values differ depending on the model. The LLM Series has the lowest ESL value of 45 pH (green), thus resulting in the best noise absorption and power supply characteristics. The LLA Series has the second lowest ESL value of 90 pH (blue), which is followed by the LLL Series with 160 pH (red). The difference in ESL value results from the difference in electrode structure.(Click to enlarge.)

The LLL/LLA/LLM Series offers several models with different dimensions, ESL values and capacitances. The noise absorption and power supply characteristics vary depending on the model (Fig. 1). For example, the LLL31measures 1.6 x 3.2 mm(EIA0612) and has an ESL value of 180 pH and a maximum capacitance of 10 μF. The LLA31measures 3.2 x 1.6 mm(EIA1206), has an ESL value of 90 pH and a maximum capacitance of 2.2 μF, while the LLM21 measures 2.0 x 1.25 mm(EIA0805) with an ESL value of 45 pH and a maximum capacitance of 2.2 μF.

The difference in ESL value results from the difference in the electrode structure of each model. The LLL Series adopted a structure called "LW inversion," with the electrodes arranged not on the shorter sides but on the longer sides. The structure adopted for the LLA and LLM Series has a number of positive and negative electrodes that are arranged alternately.

The other new product is the LLR Series, which is based on the concept called "controlled ESR." The controlled ESR concept refers to a method to improve the noise absorption characteristics by deliberately increasing the ESR value. When the ESR value is low, a sharp resonance point is generated in the high-frequency region of impedance. As a result, a comparatively sharp anti-resonance point appears in a slightly higher frequency region. Thus, the noise cannot be absorbed effectively because the impedance rises around the anti-resonance point.

Fig. 2: Lower anti-resonance point enabled by deliberate increase in ESR
When the ESR value is low, the impedance around the anti-resonance point increases and the noise cannot be absorbed effectively. To cope with this problem, the anti-resonance point is lowered by deliberately increasing the ESR value, thereby reducing the impedance. As a result, the capacitor can absorb the noise effectively.(Click to enlarge.)

Controlled ESR is an approach to lower the anti-resonance point by deliberately increasing the ESR value so that the noise absorption characteristics can be improved through the reduction in impedance (Fig. 2). The electronics industry in general has believed that "the lower the ESR value, the better the characteristics." Controlled ESR is a new concept that will reverse this conventional notion. For this reason, the LLR Series is presently "not widely used, but it will spread as the concept becomes more accepted in the future," said Kiminori Yamauchi, director of the Components Business Unit of Murata.

The company currently offers the 0.8 x 1.6mm package(EIA0306) type. It has a rated voltage of 4 V, a capacitance of 1.0 μF and an ESL value of 120 pH. The ESR is selectable among four values: 100, 220, 470 and 1,000 mΩ.

High-reliability models for automotive applications

With the use of its proprietary materials and structures, Murata has developed two types of monolithic ceramic capacitors that can satisfy the reliability required for automotive applications.

Fig. 3: Large monolithic ceramic capacitor for hybrid and electric vehicles
Murata's EVC Series measures 32 x 40 mm and has a rated voltage of 200 V. The effective capacitance under 150 VDC is 38 μF.

One is the EVC Series, which is targeted at smoothing capacitors for use in inverters to drive the motors on hybrid and electric vehicles (Fig. 3). It has extremely large dimensions at 32 x 40 mm. It is about 100 times the volume of Murata's largest general-purpose model, which measures 5.5 x 5.0 mm. The EVC Series has a rated voltage of 200 V. It ensures an effective capacitance of 38 μF under a DC voltage of 150 V. The allowable ripple current exceeds 25 Arms.

There are two capabilities required in a capacitor when it is used in an inverter to drive a motor. One is to provide a sufficient amount of effective capacitance under a DC voltage (DC bias) and the other is to ensure a high allowable ripple current. However, satisfying these two requirements in a well-balanced manner is difficult for monolithic ceramic capacitors using barium titanate (BaTiO3). Specifically, the capacitance may be greatly reduced when a DC bias is applied or a large amount of heat may be generated due to dielectric loss. Although the details are proprietary, Murata made it possible to offer these two characteristics at the same time by using a ceramic material specially developed for the EVC Series.

The other example of a product developmed for automotive applications is a monolithic ceramic capacitor that uses a ceramic material with a lower risk of cracking, even when stress is applied, due to vibrations, for example. In standard models, stress concentrated on the connection between the metal electrode and the ceramic tends to cause cracks. Thus, the new high-reliability model adopted a structure where a conductive resin is sandwiched between the metal electrode and the ceramic. "The soft resin loosens the contact area and prevents the concentration of stress," said Akira Mouri, manager of the Sales Engineering Group / Planning & Market Promotion Department in the Components Business Unit of Murata. "Our new product was created based on a counterintuitive idea, that is, make it more vulnerable on purpose in order to make it more robust."

* Indicated company and product names are the trademarks or the registered trademarks of each company.