In Part III we saw that PTC thermistors protect against overcurrent and excessive temperatures. In Part IV of this series from Epcos, one of the few varistor suppliers worldwide to offer the complete range of radial-lead disk, block, strap and energy varistors as well as SMDs, we note that the varistor is a versatile component: it is found in automotive, consumer and industrial electronics, in medical engineering and mobile communications.
Varistors
This variety of varistors offered by Epcos ensures customised, cost-effective and reliable solutions. Varistors protect against overvoltages, which can be caused both internally, eg, by switching inductive loads or by flashovers, and by external effects such as lightning strikes, strong electromagnetic fields or contact after electrostatic charging. Disk varistors protect SMPS and other sensitive loads against voltage surges on the input side.
"Disk varistors are mainly used in white goods, entertainment and industrial electronics. They are connected in parallel with the electronic circuitry to be protected and form a low resistance shunt if the voltage rises. This prevents the overvoltage from rising further and damaging the load. Disk varistors are distinguished by high energy absorption up to 410 J," explains Wolfgang Kefer, product marketing manager for ceramic semiconductors.
Their surge current capability at 8/20 µs extends, depending on disk diameter, from 0,4 to 12 kA, and they can handle voltages from 11 to 1100 V r.m.s. Disk varistors have multiple load capability and can thus absorb surge currents several times. Their leakage currents are low and remain stable over an extended period, thus ensuring long service life and high reliability. For applications demanding even higher surge current capability, Epcos offers the 'AdvanceD/SuperioR' series of disk varistors, which have the same dimensions as the standard disks, but can absorb up to 45% higher surge currents (eg, S07k150 Standard 1200 A, AdvanceD Series 1750 A, 8/20 µs). As both series have the same lead spacing, AdvanceD varistors can be substituted for StandardD components without changing board layout or circuit design.
Special types
Special varistors for telecom and for automotive applications round off the range. They can withstand jump starts, absorb high energy, especially from load dumps, and operate at temperatures up to 125°C.
A new development by Epcos is the EnergetiQ series, whose shape is square instead of round. This design ensures maximum surge current protection in a minimum of space. The voltage range of EnergetiQ varistors extends from 130 to 680 V. They are ideal for miniaturised applications or where headroom is at a premium (eg, TVSS power strips). Buildings and industrial installations call for protection concepts with even higher surge current limiting, eg, for protection against lightning. For this purpose, disk and strap varistors can be connected in tandem. The input current is limited by strap varistors of series L/LS 40. The reduced currents then reaching the line are limited by disk varistors. Strap varistors of series L/LS 40 therefore have a correspondingly higher surge current capability, eg, 40 kA 8/20 µs, and higher absorption capability up to 550 J.
Epcos also offers a wide range of energy varistors with diameters from 32 to 99 mm for porcelain or silicon arresters, and can supply the right types for classes 1 through 5 to IEC standard 60099-4.
Multilayer varistors in the form of SMDs have emerged as the most frequently used components for ESD protection. These multilayer varistors have become the worldwide standard for ESD protection in the mobile phone industry. They can perform all the functions of a transient voltage suppressor (TVS) diode and offer numerous additional advantages:
* Further miniaturisation: The standard version of a varistor in SMD size 0603 is only a third of the size of comparable TVS diodes. What is more, filter and protective functions can be implemented in a single component and total component count reduced.
* Lower cost: SMD multilayer varistors are less expensive because they are bidirectional components. To obtain bidirectional protection with TVS diodes, either two diodes or correspondingly larger and more extensive two-in-one components would have to be used.
* Shorter response times: Thanks to their reduced dimensions and multilayer design, SMD varistors have a very low parasitic inductance, leading to short response times of less than 0,5 ns. TVS diodes require 0,8 to 3 ns.
* Energy savings: SMD varistors have an intrinsically lower leakage current than diodes, and Epcos even offers versions with particularly low leakage currents - an essential property for portable entertainment electronics equipment.
* Simpler design-in: SMD varistors are unaffected by room temperatures up to 125°C. TVS diodes, on the other hand, are subject to derating from 25°C.
* Easier assembly: SMD varistors are bidirectional and symmetrical components that pick-and-place machines do not have to handle as cautiously as diodes. They are also less prone to failure. There is no need to observe the right polarity in varistors - they cannot be connected incorrectly.
SMD varistors are available as discrete components or arrays. Epcos currently offers them in sizes 0402 to 2220. For the mobile phone market, Epcos has developed special multilayer varistors that combine three functions in one 0402 package: ESD protection to IEC 61000-1-4-2, level 4 (8 kV contact, 15 kV discharge in air), EMC filter for dual-band mobile phones, and suppression of interference by carrier frequencies caused by two distinct resonant frequencies at 900 and 1800 MHz. Another innovation in size 0402 is a multilayer varistor with extremely low self-capacitance of less than 3 pF. This is an ideal antenna varistor with very low insertion loss in the RF range.
CeraDiodes
Beyond these product lines, Epcos has developed an entirely new series of components for surge voltage protection: CeraDiodes.
Explains Martin Schulte, product marketing manager for multilayer technology: "These are predestined for state-of-the-art consumer electronics equipment, where in some cases more than 80 interfaces have to be protected against ESD. This low-cost series consists of several types in size 0603. Apart from these discrete components, a quadruple array in size 0612 is being developed - prototypes are now available. The product range comprises types for voltages from 5,6 to 22 V with capacitances from 2 to 470 pF."
In response to customer demand for more reliable ESD protection up to 25 kV in high-speed bus systems, Epcos has developed a series of high-speed multilayer varistors with low capacitance and high ESD compatibility. Thanks to their low capacitance, they prevent signal distortion on high-speed data lines.
As Thomas Feichtinger, product development manager for multilayer ceramic technology, explains, "Special types have been designed for elevated temperatures up to 150°C, for example for hybrid circuits close to the engine. Further advantages are their compact dimensions, short response time of less 0,5 ns and excellent solderability thanks to Ag/Ni/Sn contacts."
This series of articles will conclude next issue with Part V, where we will look at EMC filters.
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