Selecting Multilayer Varistors for High-Temperature Circuit Protection

Transient protection has always been a problem in electric vehicle charging and industrial systems, especially because they produce overvoltages far above their nominal operating voltage, and the protecting component is already operating in an extreme environment. In such conditions, the electrical circuits are exposed to ESD, switching transients, and other voltage spikes.

Multilayer varistors (MLVs) provide compact surface-mount protection by being highly resistive during normal operation and conducting when voltage rises into their nonlinear region. However, selecting one requires more than matching a maximum voltage. Engineers have to consider working and clamping voltage, transient current and energy, capacitance, signal speed, operating temperature, and package size.

KYOCERA AVX offers its MLV portfolio that addresses these electrical and environmental requirements. This article explains how multilayer varistors provide transient protection and EMI filtering, and how they compare with transient voltage suppression diodes (TVS) and metal oxide varistors (MOV). We will also look at how to select the right device for a given circuit before going into the specifics of how the KYOCERA AVX VT Series extends its capabilities into high-temperature environments.

Multilayer varistors with transient protection and EMI filtering

A multilayer varistor is a surface-mount voltage-dependent resistor built from zinc-oxide ceramic layers and internal electrodes. During normal operation, the zinc-oxide boundary network delivers high resistance, allowing only a small leakage current to flow.

When the applied voltage enters the nonlinear region of the device, its resistance drops sharply, and the MLV begins to conduct transient current, clamping the voltage. KYOCERA AVX positions its MLVs for events such as ESD, load dump, jump-start, and other electrical transients.

The multilayer construction is important because alternating ceramic and electrode layers create a large active region within a compact SMT package. This structure distributes transient current through multiple internal layers, improving energy dissipation and thermal performance during surge events.

The same electrode structure also gives the device inherent capacitance. At higher frequencies, that capacitance gives a lower impedance, allowing high-frequency EMI/RFI to be attenuated even when the voltage is too low to trigger the varistor’s nonlinear clamping behavior.

This shows how a single MLV can provide both transient suppression and useful high-frequency filtering, potentially reducing the need for a separate filtering component in some circuits.

MLVs and TVS diodes can both be connected across a protected line to divert transient current. However, they have different electrical characteristics. KYOCERA AVX MLVs provide bidirectional, nonlinear protection and can combine that function with capacitive EMI/RFI attenuation. TVS diodes may be preferable where very low capacitance or different clamping behavior is required, particularly on high-speed interfaces.

MLVs differ from traditional MOVs, even though both use nonlinear metal oxides. An MLV has thin ceramic layers and internal electrodes in a compact SMT package. On the other hand, bulk or disc MOVs provide much greater physical volume and are commonly used for higher-energy power line surges.

How to choose a multilayer varistor

Selecting an MLV starts with the circuit’s highest operating voltage. The device’s continuous working voltage rating must accommodate that voltage without entering significant conduction.

The varistor voltage identifies a reference point in the nonlinear V-I characteristic, while clamping voltage indicates the voltage remaining across the device at a specified transient current. An MLV must operate normally without excessive leakage yet clamp low enough for the residual voltage to remain within the tolerance of the protected circuit.

Peak current and transient energy ratings determine if the device can withstand the expected event. These values must be determined using specified waveforms. The VT Series specifies peak current using an 8/20 µs pulse and transient energy using a 10/1000 µs pulse. Larger devices can provide far greater energy capability, and capacitance also varies across the family.

It is important to note that a current rating alone does not establish protection because a longer pulse can dissipate considerably more energy. Other factors like package size, leakage current, and operating temperature range also have an influence on a suitable choice.

Capacitance adds another selection criterion when a shielded line carries data. The capacitance that enables an MLV to attenuate EMI can also load a fast signal, increasing rise and fall times and affecting insertion loss and impedance.

Higher capacitance may be useful on power rails, sensor inputs and lower-speed control lines, while faster interfaces require closer attention to capacitance and the S21 behavior of the device. There is no single data rate limit because suitability depends on the bandwidth, impedance, and signal integrity requirements.

High temperature protection without derating

The KYOCERA AVX VT Series extends multilayer varistor protection across a range of electrical and thermal requirements.

The family includes VTA3 devices rated from -55°C to +150°C and VTA7 devices rated from -55°C to +175°C, with working voltage options extending to 56 VDC and SMT case sizes from 0603 through 2220. Both families are AEC-Q200 qualified for automotive component applications.

The selected 56 VDC VTA3 devices are designed and tested for 48 V vehicle power supplies in accordance with VDA 320 requirements, with available package and energy options extending to larger 2220 devices.

The temperature capability is more significant than having a high maximum operating temperature rating. The derating curves of the KYOCERA AVX VT Series maintain 100 percent of the specified current, transient energy, and power ratings through +150°C for VTA3 and +175°C for VTA7.

A protection device selected solely by its room temperature surge rating offers less usable capability once installed in a variable operating environment.

Conclusion

The KYOCERA AVX VT Series highlights how multilayer varistor technology can provide both bidirectional transient protection and EMI/RFI attenuation in a compact SMT package while maintaining reliable performance in automotive and industrial environments up to 150°C and 175°C. As part of the broader KYOCERA AVX varistor portfolio, the VT series gives designers the flexibility to select circuit protection solutions optimized for specific voltage ratings, transient energy requirements, capacitance characteristics, operating temperatures, and packaging constraints.

To learn more, visit the KYOCERA AVX Varistors and MOVs portfolio.