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| | | TransGuarcf /'\V»'< AVX Multilayer Ceramic Transient Voltage Suppressors »^» #/a\ Application Notes: The Impact of ESD on Insulated Portable Equipment | | |
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| | | The purpose or this discussion is lo recap the impact ESC has on portable, battery powered equipment. It will be shown thai tSIJ can cause tailutes in -fbaling ground systems" In a variety of ways. Sued cally. ESD induced :'ailures can be caused by one or more of its complex components: Predischarge - Corona Generated RF | | |
| | | In Ihe p'edisennree scenario (h gure 7) ;i human oha-qed lo -20 kV may approach a battery powered "system" on a table. As the person reaches toward the system electrostatics d.otate that flic system will have an equal and epposite charge on the system's surface nearest te Ihe person. Slrce the system we are approaching is iscla:od horn ground, the charge Is only redistributed among the device. (If the system were grounded a current would be generated by the loss of electrons to ground. The system would then become positive reative to ground). The rate of approach of the numar body model affects the charging current to a small extent: However, most impu'tanay. t .: the elecacs;at e "'ice are the unequal voltages which develcpes across the equip ment that cause the destruction c' eernponents whhiu tie system. In general, unprotected IC"s [particularly CMOS) are ..:::;:,■! :i:uie tc damage due to induced E field voltages. This problem is further complicated by the device Type an:: complexity and tne :ae: thai tie brea-wlosyn ,-oltage of a generic IC will vary greai'y from manafacturer to manufacturer [Figure '..>':. This one; discassicn should be ■ i i ii i'. 1 •'i..:11■ ii 'i i !■ i......i..:i. illy nduced E field can impact system reliab"ity. C protector enn oe acheveo' by placing a transient sap pressor on the most i pins of the sensi1. ve IC's [e.g.. Vcc and 10 pins, c | | |
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| | | Predischarge Discharge Discharge Discharge | | E Field Collapsing E Field Collapsing H Field 1 :i.irr- in! 'ii ■ i i .V; ■....... .A..... ■ .1 .. | | |
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| | | With this in mind it will be shown that the only way to Insure equipment survivability to ESD is to use a Transient Voltage Suocresse- (in adrii'br io proper circni; layout, decoupling, and shielding). In order to get a better understanding of what happens in an ESD event the charge developed by a human body should be defined. I he FSD schematic equivalent of the human body model is Shown in Figure I. Typically, the charge developed on a uerscr can be 'eo-esented by a ' bOoF eapacito: if so'ios will1 a icssbuae of 330 chins. Tlx; ene'gy an ESC waveform generated l:'o."'i tliis moeel is Q - '2 CV whore Q = total energy in Joules, C - capacitance of the human body model In farads and V = charging voltage in volts, '■■,"■:> tapes can be as high as 2o KV. however ;y\n ■., ;■■:> tages | | |
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| | | Where: model capacitance typically 150pF | | |
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| | | typically 330 U Figure 1. Human Body Model PREDISCHARGE E FIELD FAILURES | | |
| | | Rgure 3. IC Type E Field Susceptibility CONTACT DISCHARGE FAILURES | | |
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| | | (see Figure 4). I
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| | the c, | | a siege contact discharge. I:
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| | | I :)«<ivty mull p <! ilisrlMi(j(!S v.i I I killy oc::ur ;possiply saasee by a person's aard 'eacting to ar ESD spark and then touching the system again, etc.). In contact oacharge. when a charged person approaches the syo:er-\ r_ fields are induced As the person nets close' to the system, "is ibid intensify becomes greater eventually reaching a point large enough to draw an arc between the | | |
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