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 Electrostatic discharge immunity

Electrostatic discharge immunity

To fight ESD, we first must know what it is and how it gets into our equipment. Basically, ESD can happen anytime a charged conductive object approaches another conductive object. First, a strong electric field forms between the objects, which can cause feld-induced breakdown. Then an arc can occur when the voltage between the objects exceeds the breakdown voltage of the air and the insulation between them. In 0.7 to 10 ns, the current in this arc can reach tens of amps, sometimes exceeding 100 amps. The arc continues until the objects touch, shorting out the arc, or until the current drops too low to sustain the arc.

 

Depending on the initial voltage, resistances, inductances, and parasitic capacitances of the objects:

l  One arc may occur (human body model, charged device model, machine model).

l  One arc with an initial spike may occur (hand/metal model).

l  Multiple arcs of the same polarity or alternating polarities may occur (furniture model).

 

ESD can find its way into electronic equipment through five coupling paths:

1.         An initial electric field can capacitively couple to nets with a large surface area, looking like a signal to high-impedance analog circuits (rarely), and generate up to 4000 V/m 100 mm from the ESD arc.

2.         Charge/current injected by the arc can:

l  Break through thin insulating layers inside components, damaging the gates of MOSFETs and CMOS devices (very common).

l  Trigger latch-up in CMOS devices (common).

l  Short-circuit reverse-biased PN junctions (common).

l  Short-circuit forward-biased PN junctions (rare).

l  Melt bonding wires or aluminum traces inside active devices (rare).

3.         Current causes a voltage pulse on conductors (V = L * dI/dt), whether they are power, ground, or signal wiring, and gets into every device attached to that net (common).

4.         An intense magnetic field from the arc has a frequency range of about 1 MHz to 500 MHz, which inductively couples into every wiring loop in the vicinity, as high as 15 A/m 100 mm from the ESD arc.

5.         A magnetic field radiates from the arc, becoming an electromagnetic field, which couples into long wires that act as receiving antennas (rare).


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