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Development Trends with Ceramic Capacitors - Hi, we are the New Guys!

Created by Jürgen Geier, Technical Support, and Pascal Kundelius, Product Sales Manager Ceramic Capacitors, both at Rutronik |   Knowledge

There are many demands placed on ceramic capacitors: smaller construction, large temperature ranges, higher efficiency, stricter tolerances, optimized functions, and higher frequencies, not to mention the desire for application-specific versions.

The manufacturers do a lot to satisfy all these requests.

In order to continue optimizing their ceramic capacitors, manufacturers develop new materials and mixtures as well as new approaches to the construction, engineering, and interior structure (i.e. the forms of the electrode surfaces). They also deliberately make use of certain properties such as the widely negatively viewed effect of DC bias. This effect occurs in ferromagnetic ceramic materials such as barium titanate, which is currently the material most commonly used for MLCCs (multilayer ceramic capacitors).

Alongside the more widely known designs such as high-frequency, HiQ, RF, microwave, low-inductance, and low-loss capacitors, a range of specific and novel ceramic capacitors has recently started appearing on the market that are still broadly unknown—given the current deluge of new developments, this is hardly a surprise. We introduce some of the most important features here.

 

Variable Capacity Thanks to DC Bias

“Variable capacitors” do just as the name suggests — they offer variable capacitance. Using their DC bias reduces the effective capacities when a DC current is applied to their control electrodes, so they could also be referred to as VACs—voltage adjustable capacitors.

These electrically trimmable capacitors are available with capacitance ratings of 33 pF to 200 pF for control voltages of up to 3 or 5 V DC, allowing them to be adjusted to up to 50% of their base capacitance. Compared to regular trimmer capacitors, this provides them with unimagined opportunities — especially given that they can not only be set to fixed values, but also used to form control loops.

They are available in construction sizes of 0.6 mm × 0.6 mm and 1. mm × 0. mm for working voltages of 10 Vpp and 30 Vpp.. Applications, for which these are especially ideal include NFC antenna circuits (13.56 MHz band), such as those used in smartphones and check cards, because the variable capacitors here enable frequency adjustments to be performed by simply applying the appropriate voltage in conjunction with the NFC ICs. They also compensate for variance in the antenna’s L value, making f0 adjustment easier, and also facilitate debugging during certification testing and simplify deviation adjustments during installation in the housing.

 

Silicon in the Third Dimension

For “high-density silicon” or “3D silicon” capacitors, manufacturers use the third dimension to significantly enlarge the capacitor surface—and with it the capacitance—without increasing the base surface area of the capacitor. This is how capacitance values are achieved that would require around 80 layers in MLCCs with a component thickness of 100 µm.

The Murata SiCap, for example, offers a whole 100 nF with a size of 0402 and a thickness of just 100 μm - equivalent to ten Class 1 dielectric C0Gs of size 0603 and 400 μm thickness. Thinner versions are also available from Murata on request. Capacitance values range from several pF to a few µF, and voltages range from 5 to 450 V.

Their material and construction properties make these high-density silicon capacitors especially well-suited to high-frequency applications from 10 up to 110 GHz. Their electrical characteristics are similar to the known “ceramic” type NP0 (= C0G). Unlike the C0G MLCCs, however, they can already be used as standard versions up to 150°C and as high-temperature versions up to as high as 250°C.

Thanks to their construction and thicknesses of just 50 to 400 µm, they are available not just in soldered versions, but also as bonded versions and for embedded installation. This means that there are ideal designs available for automotive, medical, RFID, high frequency, and broadband applications, categorized as standard, high-reliability, high-temperature, and high-frequency models.

 

Capacitors with Antiferromagnetic Properties

If ceramic capacitors are used as snubber or DC link capacitors in a range of around 500 V to 900 V for working frequencies of several hundred kHz up to 1 MHz, MLCCs made of X7R ceramics (i.e. those that use the ferromagnetic barium titanate as their base material) are frequently pushed to the limits of usability. Due to their pronounced negative DC bias behavior, the necessary effective capacitance values in particular are almost impossible to achieve.

Antiferromagnetic properties are provided by the “CeraLink” capacitors from TDK. This means that they exhibit an increase in capacitance with the applied voltage, which enables much higher currents within the operating range. This is enabled by their lead (lanthanum) zirconate titanate (P(L)ZT) construction, referred to as a “ceramic.”

Thanks to their extremely low ESL and ESR, CeraLink capacitors support higher switching frequencies and currents. This allows for the use of cheaper, more robust semiconductors, for example high-speed IGBTs instead of MOSFETs. This method often enables the value of the capacitor, the space on the board, the magnetic components and the heatsink to be reduced, thus also reducing the total cost.

As snubbers, CeraLink capacitors are a superb solution to reduce the risk of semiconductors being damaged by voltage spikes.

CeraLink capacitors are based on chips (7.85 mm × 6.84 mm × 2.65 mm) from which the manufacturer assembles a variety of connection options and combinations.

 

Rechargeable Solid-State SMD Battery with MLCC Construction

The world’s first rechargeable solid-state SMD battery might not be a capacitor, but the basic principles of its engineering design, constructed as an MLCC, are consistent with one. “CeraCharge” batteries offer capacitance around one thousand times that of MLCCs of comparable physical size. In other words, they have a comparably high energy density with a minimal volume. Then there are the benefits of ceramic multilayer components, namely robust safety and large-scale serial production. The use of a solid ceramic electrolyte eliminates the risks of fire and explosion, and of electrolyte leakage.

CeraCharge supports a very large temperature range of –20°C to +80°C and is therefore suitable for outdoor use.

The properties open up new opportunities for CeraCharge, in particular for IoT applications, real-time clock syncing, and energy harvesting. If higher currents and/or voltages are required, these can be provided using parallel and/or serial circuits.

The CeraCharge is currently available in the size defined by EIA 1812 (approx. 4.5 mm × 3.2 mm × 1.1 mm) and offers a rated capacitance of 100µAh and a nominal voltage of 1.5 V.

 

Conclusion

Even if there are many other optimized ceramic capacitors alongside the types listed here, among them MLCCs with internal copper electrodes, MLCCs with end termination for conductive adhesives, or “X2Y” versions, those illustrated here show that it is worthwhile to be a little more adventurous on occasion when there are particular requirements. This is useful for you to remain aware of your options when development is being performed at a hurried pace, and even to set new trends with product and device designs.

 


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Substitute circuit diagram for variable capacitors (Image source: Murata)
Substitute circuit diagram for variable capacitors (Image source: Murata)