Fighting ESD And EMI
With the number of automotive electronic functions rising steadily, the need for transient suppression and EMI control becomes increasingly important, particularly when many of the functions reside on a network bus such as CAN. Rising data rates on the bus tend to exacerbate the problem, creating a more urgent need for EMI suppression. A variety of methods can be applied to attenuate ESD transients and EMI. These include RC networks, LC networks, back-to-back Zener diodes with an EMI capacitor, and even on-chip transient protection.4
Another approach employs multilayer varistors (MLVs), such as those developed by AVX Corp. The MLV is a ceramic device fabricated from a conductive zinc-oxide (ZnO) material that's doped with bismuth, cobalt, manganese, or some other metal oxide. The device can be modeled as two back-to-back Zeners in parallel with an EMC capacitor.
The techniques employed in fabricating MLVs are similar to those used in multilayer chip capacitors (MLCCs) with some added proprietary methods that are unique to the ZnO materials. The same process capability that allows MLCCs to be produced with very thin dielectrics (less than or equal to 1 mil) also permits MLVs to be made with working voltages of as low as 3.3 V.
While working voltage is a function of ZnO dielectric thickness, the MLV's ability to absorb transient energy is a function of the number of layers of di-electrics and electrodes. The device's capacitance, which results from the combination of electrodes with the ZnO ceramic dielectric, ranges from about 3 pF up to several thousand picofarads. As with chip capacitors, packaging for MLVs is in standard EIA surface-mount cases.
In comparison to silicon transient-voltage suppressors (TVSs), MLVs are claimed to have somewhat faster response times due to the inductance introduced by the silicon TVS's lead frame and wire bonds. Moreover, MLVs will tolerate a greater number of transient strikes and handle more inrush current than the diode TVS. Plus, in the off-state, the MLV's capacitor can provide as much as 40 dB of EMI attenuation across a frequency spectrum of several hundred megahertz.
Housed in packages as small as the 0612 standard, AVX's TransFeed is a four-terminal version of the technology that improves upon the MLV's turnon time, broadband EMI attenuation, and size, versus MLV and filter combinations (Fig. 3a). In the TransFeed version of the MLV, some of the device's parallel inductance has been converted to series inductance. This effectively lowers the injected transient peak current and the clamping voltage, while also reducing turnon time down below 250 ps.
For surface-mount components such as the MLV, there are extensive opportunities in developing automotive designs because as Ron Demcko of AVX says, "You need a varistor on just about everything on the CAN bus," as well as on every airbag. Newer cars tend to include more of these safety devices.
Another interesting aspect of the MLV technology is that it creates an attractive path to passive integration. For example, AVX has just announced a four-element varistor housed in 0508 and 0612 packages that effectively combines four Zener pairs, four EMC capacitors, four inductors, and four resistors. It's fabricated by depositing thick-film nickel-barrium terminations on the MLV's ZnO body.
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