Overvoltage and associated high surge currents can damage or even damage electrical and electronic equipment, so reliable overvoltage protection is essential. At present, TDK Group has developed a high surge series of multi-layer varistors based on a new type of ceramic materials. These series of resistors are not only compact in size, but also have excellent protection properties.
Overvoltages affecting electrical equipment are produced for a variety of reasons, energy levels are different, and can be introduced through different routes. For example, an ESD pulse measured according to IEC 61000-4-2 primarily affects the input/output of a communication device, where the test level is 8 kV (contact discharge) or 15 kV (air discharge). The correlation pulse waveform is characterized by a voltage rise in nanoseconds, however the energy content of the pulse is relatively low, only a few millijoules.
To prevent ESD events, TDK Group offers a wide range of small CeraDiode® varistors (SMD packages) for different voltages with a minimum package size of 0.4 mm x 0.2 mm and an extremely low insertion height of only 0.1 mm. In other words, these varistors are ideal for mobile and compact applications such as smartphones, tablets and wearables.
Another type of overvoltage is mainly introduced through the power line, possibly due to nearby lightning strikes or load shedding. These events can cause surges of several thousand amps of up to several nanoseconds. In the worst case, the energy of these pulses can reach several thousand joules, several times higher than the ESD event. The ability of the component to withstand these high-energy pulses was tested according to IEC 61000-4-5 with a short-circuit current waveform of 8/20 μs and an open-circuit voltage waveform of 1.2/50 μs (Figure 1).
Figure 1: Pulse waveforms in accordance with IEC 61000-4-5
Short circuit current test (8/20 μs)
Open circuit voltage test (1.2/50 μs)
In order to adequately prevent these events from occurring, the design of the protection device must take into account the possible ground leakage current and the corresponding energy level. In this respect, the size of conventional varistor is relatively larger.
New Ceramic Materials Make Design More Compact To improve the performance and compactness of multi-layer varistors, TDK Group has developed a new ceramic material for the new high surge series of multi-layer varistors. The improved properties of the new material are based on the addition of a special metal oxide to the ZnO varistor, resulting in a fine-grained ceramic material with a significant increase in grain boundaries per unit volume. Therefore, the current density in the effective volume of the same component may increase by more than three times. At the same time, the relative dielectric constant is increased by several times, so that the electric field strength (E) is also significantly increased (the same volume, see Figure 2).
Figure 2: Comparison of traditional and new ceramic materials
Since ZnO varistor ceramics are doped with special metal oxides to form fine-grained structures, the new EPCOS multi-layer varistors have higher electric field strength. The benefits are twofold: on the one hand, more internal electrodes can be integrated in the same component volume, thus improving high surge performance; on the other hand, the same performance can be achieved on smaller components, making the varistor further compact It becomes possible.
With improved electrical performance, varistor with higher voltage levels can now be designed by increasing the number of internal electrodes and significantly increasing the surge current capability of the component or achieving the desired performance on components with significantly reduced size. Standard varistors with a surge current capability of 1200 A (8/20 μs) are produced in accordance with the EIA housing size of 2220. Through the use of new ceramic materials, TDK Group has successfully achieved the same performance through the EIA's 1210 high-surge series of components, which has more than tripled its volume. As miniaturization plays an increasingly important role in these applications, new varistors are ideal for Internet of Things (IoT) or Industrial 4.0 applications.
The lower the clamping voltage, the better the performance. Due to the higher electric field strength allowed by TDK Group's new ceramic materials and the higher number of internal electrodes, it is also possible to reduce the clamping voltage of the components. When a special inrush current is generated on the component, a clamp voltage is generated if an overvoltage event occurs. For the same current, the higher the clamping voltage on the varistor, the higher the electrical power, and the higher the energy that the varistor must absorb. Conversely, the lower the clamping voltage, the stronger the current capability required to absorb the same energy.
For example, when the current is 10 A, the existing surge protection series CN2220K50E2GK2 multi-layer varistor of EPCOS has a clamping voltage of 135 V. In contrast, the EPCOS high-surge, low-clamp CT2220S50E3G with improved ceramic material has a surge current capability of 400 A when the clamp voltage is the same (Figure 3). Therefore, the degree of protection of the new varistor is significantly improved.
Figure 3: Surge current capability is stronger at the same clamp voltage
The new EPCOS high surge/low clamp varistor clamp voltage of 135V can reach 400 A.
The fewer components, the stronger the protection
In practical applications, several components are usually connected in parallel in order to obtain the highest possible surge current capability with SMD multi-layer varistors. However, since the varistor has a voltage tolerance of up to ±20%, it is necessary for these applications to use components that are precisely matched to each other. Conversely, this is also a considerable cost factor. Another drawback is that although the tolerance range is very narrow, the electrical components of the different components are slightly different. Therefore, when an overvoltage occurs, the currents of the components are different, and occasionally the varistor fails due to excessive load.
With the new TDK ceramic materials, it is now possible to produce varistor with high inrush current capability, which provides the necessary protection for individual components. Therefore, in addition to improving reliability, it is also possible to significantly reduce the number of components, which not only saves valuable space for printed circuit boards, but also reduces material and assembly costs.
Table: Key data for EPCOS high surge series multi-layer varistors
Dimensions EIA 1210 to 2220
Maximum operating voltage [V DC] up to 65
Voltage of varistor [V DC] up to 85 at 1 mA
Surge current capability [A, 8/20 μs] up to 1 x 5000 and 10 x 3500
Clamping voltage [V] at 200 A is up to 135 (new ceramic material)
Maximum energy absorption [mJ, 2 ms] up to 15,000
Maximum operating temperature [°C] 125
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