Multilayer Varistors Will Keep Evolving
Recent advances move metal oxide varistor tech into fast-transient suppression apps.
Due to recent advances in multilayer varistor (MLV) design, metal oxide varistor (MOV) technology is increasingly being employed in fast-transient suppression applications. MLVs utilize zinc oxide grains as the varistor material in combination with a monolithic multilayer electrode construction that is analogous to multilayer ceramic chip capacitors (MLCCs).
Figure 1 shows a cross section of the construction of a typical MLV element. The low K (low dielectric constant) dielectric, when sandwiched between a pair of electrodes, results in a parallel plate capacitor.
When a number of these are stacked on top of one another, the intrinsic capacitance will multiply with the number of layers. The capacitance is also a function of the total surface area, meaning that increases in part size also result in increased capacitance.
The intrinsic capacitance of MLVs can range from sub-pF in small 0201 to 0402 sizes to greater than 1,000 pF in 1206 sizes and larger, which suits them to applications such as integration or EMI filtering. However, the primary function of the zinc oxide grains is to act as bidirectional Schottky diode elements, which provide transient protection. Consequently, an MLV can provide both ESD protection and secondary EMI attenuation within a circuit.
As a transient suppression device, MLV technology provides some significant advantages over competing solutions, such as silicon transient voltage suppression (TVS) devices, due to the fact that the transient switching occurs throughout the bulk of the material rather than across a single junction. The first is with respect to the activation or turn-on time. Since junction devices have an intrinsic response delay in a typical human body model discharge, they can miss the peak voltage spike, which can occur within the first nanosecond (ns) of a 50 ns pulse.
Alternatively, MLV can turn on within 0.3 to 0.7 ns, depending on the configuration, effectively eliminating the entire spike before it has ramped up (Figure 2). Secondly, the MLV can absorb more energy associated with the transient, typically 0.3 J for a 0805 size device, which is approximately five times more than competing solutions. Thirdly, the MLV has no wear out mechanism when sized correctly, so it can easily accommodate tens of thousands of repetitive strikes.
As MLV technology has evolved, advances in material systems and electrode design have resulted in increased performance capabilities in terms of clamping voltage and current handling. For example, MLV technology has benefitted from incorporating parallel advances in MLCC design, ranging from geometric electrode designs that enable multi-element arrays to feed-thru designs in which different internal channels can be inductively coupled to increase filtering capabilities. Figure 3 shows four major directions of MLV technology expansion that have resulted from the convergence of these capabilities: high density, ultra-low capacitance, maximum energy, and harsh environment.
High-density circuitry benefits from the recent development of 0201 sub-miniature MLV devices, which offer bi-directional ESD protection in the smallest package available today, and high-density arrays, such as the four-element 0612 TransFeed – an array of four LC-T configured filters. Handheld devices, ranging from smartphones to PDAs and GPS devices, use touch sensitive interfaces that are obvious targets for ESD. Meanwhile, the increasing megapixel ratings of camera (and, in some cases, dual camera) and frame rates for video, coupled with the increasing resolution of device displays, is driving both CCD and CMOS content, which subsequently requires more effective filtering from both an emissions and susceptibility standpoint. MLV arrays are ideal for these applications, as several multi-element chips placed around the RGB lines from the IC can provide effective filtering at the PCB level. In fact, such filtering has been so effective that the Federal Communications Commission (FCC) is now working with the Federal Aviation Administration (FAA) to extend the permissible use of these devices for text, email, and social network communications while in-flight.
The ultra-low capacitance of MLVs is particularly significant for protection due to the continual increase in data rates (the I/O data ports) for nearly all devices ranging from servers, PCs, and notebooks to PDA docking stations, which thus require additional ESD protection. In applications like these, the intrinsic capacitance of MLV devices can sometimes result in signal distortion that can be detrimental to the data rate through the ports. Historically, this has been an advantage associated with silicon TVS technology, which has a low parasitic capacitive element (below 1 pF).
A new generation of sub-pF MLVs that match the typical parasitic capacitance of silicon TVS (~0.8 pF) have not only been developed and released to the market, but have also been extended down to ~0.4 pF. The developments have extended the range of ideal MLV applications from USB and HDMI port protection to antenna protection in a variety of wireless devices (Figure 4).
The sub-pF characteristic pushes the self-resonant frequency (SRF) of these next-generation MLVs into the GHz range; 0201 and 0402 size MLVs now include ratings that can provide protection in RF applications to 5.3 GHz, where common mode (CM) and differential mode (DM) noise can exist (e.g., in unshielded twisted pair [UTP] applications in 10 Gb/s Ethernet infrastructures).
For harsh environment, high temperature, and high current applications, larger size MLVs have been developed to 2220 and 3220 cases sizes. These devices have higher energy capabilities than their smaller counterparts, exhibiting maximum energy absorption capabilities up to 25 J. Larger MLVs can also handle industrial related transients associated with load dump and jump start applications at operating temperatures up to 150°C.
In telecom applications, the same high current technology has enabled the development of large size 1812 devices tailored to provide interference immunity for telecom equipment in compliance with the International Telegraph and Telephone Consultative Committee (CCITT) 10 x 700 μs test (10 repetitive pulses of 700 μs duration). These MLVs, rated at 60 Vrms, can reduce the interference of the equipment from 2 kV to less than 200 V.
One market that benefits from all of the aforementioned MLV developments is automotive, as these advances dovetail ideally in a number of emerging automotive applications. Next-generation MLV devices satisfy the high temperature and high-energy requirements of underhood engine control unit (ECU) applications in both hybrid/electric drives and internal combustion engines (ICE).
The extended-feature MLVs now available on the market are nearly all based on the AEC-Q 200 qualification (Figure 5), meaning that their design has been optimized for use in automotive applications. The noise specifications of vehicles rival those of military high reliability electronic systems, as radiated emissions from vehicles can not only interfere with anything in close proximity but can also destroy the functionality of other modules in the offending vehicle. Consequently, implementation of MLVs on critical I/O lines, sensors, and power lines ensures uninterrupted system performance.
Although primarily discussed and utilized as surface mount devices (SMDs), MLVs, like MLCCs, can also be used as inserts in through-hole components in which the enhanced encapsulation can benefit the device in harsh environment and specialty applications, including feed-thru for low noise amplifiers (LNAs). Leaded MLVs are also ideal for turbocharger transient suppression, as well as for all types of single and multiphase electric motors (e.g., electric water pumps, electric power assist steering motors, fuel pumps, lift gates, etc.).
Continued advancements in MLV technology and new product expansions show no sign of slowing at the present time and, considering the enhanced performance capabilities already available, it is estimated that MLV use will expand at double-digit growth in automobile applications due to their continuously increasing employment of electronic systems. MLVs are also expected to continue evolving in opposing trends, including high and low capacitance, high and low energy, large and small case sizes, and specially packaged configurations such as notch filter varistors, RF varistors, power DC varistors, and SMT AC signal varistors. Further, these advancements, combined with the available software simulations of these devices, are expected to both ensure expanded use in future automotive designs and fuel further utilization in industrial, consumer, telecom, harsh environment, and instrumentation electronics applications, among several others.
This article originally appeared in the November/December print issue. Click here to view the full issue.