Overvoltages and the associated high surge currents can damage, or even destroy electrical and electronic devices. Reliable overvoltage protection is therefore essential. On the basis of a new ceramic material, TDK has now developed a high-surge series of multilayer varistors that combine compact dimensions with excellent protection properties.
Overvoltages that affect electrical devices have a wide variety of causes and different energy contents, and are injected via different paths. ESD pulses, for example, that are measured according to IEC 61000-4-2 mainly affect the I/Os of communications devices. These are measured with a test voltage of 8 kV (contact discharge) or 15 kV (air discharge).
The associated pulse shapes are characterized by an extremely steep edge with rise times in the order of nanoseconds. The energy content of the pulses, however, is relatively low at just a few millijoules. For example, TDK offers a wide range of miniaturized CeraDiode varistors in SMD packages for different voltages to protect against ESD events. The smallest design has a footprint of just 0.4 mm x 0.2 mm with an extremely low insertion height as low as just 0.1 mm. Such varistors are eminently suitable for use in mobile and increasingly compact applications such as smart phones, tablets and wearables.
Another form of overvoltage is mainly injected into power supply lines and may be caused by lightning strikes nearby or by load shedding. These events can cause surge currents of several kiloamperes with a duration in the order of microseconds. In the worst case scenario, the energy of these pulses can reach several thousand joules, making it several times greater than the energy levels encountered in the case of ESD events. The ability of components to withstand these high-energy pulses is measured in accordance with IEC 61000-4-5 using the pulse shape 8/20 µs for the short-circuit current and 1.2/50 µs for the open-circuit voltage (see Figure 1).
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Figure 1: Pulse shapes according to IEC 61000-4-5
1A: Measurement of the short circuit current (8/20 µs)
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Figure 1: Pulse shapes according to IEC 61000-4-5
1B: Measurement of the open-circuit voltage 1.2/50 µs
In order that they can offer sufficient protection against such events, protection devices have to be designed for the possible leakage currents that occur and the corresponding energy levels. Conventional varistors are relatively voluminous for just this reason.
New ceramic material enables more compact designs
In order to improve the performance and compactness of multilayer varistors, TDK has developed a new ceramic material for the multilayer varistors of the new high-surge series. The improved properties of the new material are based on a higher doping of the ZnO varistors with a special metal oxide, resulting in a finer granular structure of the ceramic material.
This produces significantly more grain boundaries per unit of volume. Consequently, it has been possible to increase the current density by more than a factor of three within the same active volume of the component. At the same time, the relative permittivity was several times higher, enabling a considerably higher electrical field strength (EV) – also within the same volume (see Figure 2).
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Figure 2: Comparison of conventional and new ceramic material
Devices like these EPCOS multilayer varistors feature a higher field strength, thanks in this case to the finer granular structure achieved with a special metal oxide doping of the ZnO varistor ceramic. The benefit is twofold: On the one hand, a higher number of internal electrodes can be integrated into exactly the same component volume, thereby improving the high-surge performance. On the other hand, the same performance can be realized in a smaller component, permitting further miniaturization of the varistors.
Thanks to the improved electrical properties, the number of internal electrodes can now be increased in order to design a varistor with a higher voltage class, significantly raise the surge current capability of the component, or realize the required performance in considerably smaller components. Standard varistors with a surge current capability of 1200 A (8/20 µs) are manufactured in EIA case size 2220.
With the new ceramic TDK has succeeded in achieving the same performance with a high-surge series component in EIA case size 1210. This corresponds to a reduction in volume by a factor of more than three. The new varistors are therefore ideally suited for use in Internet of things (IoT) or Industry 4.0 applications, as miniaturization is playing an increasingly crucial role in these applications too.
Lower clamping voltage and higher performance
Due to the higher permissible field strength of the new TDK ceramic material and the greater number of internal electrodes it has also been possible to reduce the clamping voltage of the components. The clamping voltage appears if an overvoltage event occurs during a specific surge current at the component. The higher the clamping voltage occurring at the varistor subject to the same current, the higher also is the electrical power and ultimately the energy that the varistor must absorb. Conversely, this means that with smaller clamping voltages a higher current capability is achieved, to attain the same energy absorption.
For example, the EPCOS type CN2220K50E2GK2 multilayer varistor from the existing surge protection series is specified with a clamping voltage of 135 V at 10 A. By contrast, the new EPCOS high-surge low-clamp type CT2220S50E3G with improved ceramic material achieves a surge current capability of 400 A at the same clamping voltage (see Figure 3). As a result, the new type offers a significantly higher degree of protection.
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Figure 3: Higher surge current capability at the same clamping voltage
Fewer components, better protection
In order to achieve the highest possible surge current capability with SMD multilayer varistors in practice, several components are often connected in parallel. As varistors exhibit voltage tolerances of up to ±20 percent however, it is necessary for such applications to use components that are precisely matched to one another.
This, in turn, represents a considerable cost factor. A further drawback is the fact that despite a narrow tolerance, the individual components differ slightly from each other in their electrical characteristics. Consequently, in the event of an overvoltage, components are subjected to different currents, which occasionally can cause the failure of the varistor subjected to the highest load.
Thanks to advances like the new TDK ceramic material, it is now possible to produce varistors that offer the correspondingly high surge current capability, and therefore provide the necessary protection in just a single component. So as well as improving reliability, it is possible to dramatically reduce the component count, which not only saves valuable space on the printed circuit board, but also reduces the costs of materials and assembly.
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