ESD Protection Fundamentals

• Charged person - a person can become charged due to walking or other motion. ESD damage can be especially severe if the discharge from the person is via a metallic object, such as a tool.
• A cable rubbing across carpet - if a charged cable is plugged into a conductive contact with any source of charge ESD will occur.
• An electronic device sliding into or out of a bag or tube generates an electrostatic charge as the device's case and/or metal leads make multiple contacts and separations with the surface of the container.
  

Electrostatic damage to electronic devices can occur at any time, from the factory floor to the end-user’s home. ESD damage is generally classified as either catastrophic or latent failure.

Catastrophic Failure
A catastrophic ESD event may cause a metal melt, junction breakdown, or oxide failure, permanently damaging the circuitry and causing the device to fail. These types of failures are usually identified before the device is shipped, and represent approximately 10% of ESD failures.

Latent Failure
Latent failure is more difficult to identify and ultimately more costly.

• Device may be partially degraded after an ESD event, remaining operable, but with compromised performance characteristics.
• Devices with latent defects may experience intermittent or premature failure.
• Detecting cause of failure may be difficult or hazardous.
• Latent failures increase warranty and replacement costs.
  

ESD suppression devices attempt to divert a potentially damaging charge away from sensitive circuitry and protect the system from catastrophic failure. A variety of technologies are used for ESD protection. The PESD device is particularly attractive for high-frequency applications, due to its exceptionally low capacitance.

• Conductive particles are dispersed in a non-conductive matrix.
• The gaps between each particle behave like spark gaps when a high voltage ESD pulse occurs.
• When the voltage of the pulse reaches the "trigger voltage" these gaps spark over, creating a very low resistance path.
• In normal operation, the leakage current and the capacitance is very low, due to the physical gaps between the conductive particles.

ESD Application Example: USB 2.0
The USB 2.0, IEEE 1394, digital visual interface (DVI) and high definition multimedia interface (HDMI) protocols allow for high-speed data transfer rates, and may support plug-and-play hot swappable installation and operation. These external ports are susceptible to damaging ESD pulses from the operating environment and peripherals.

At these high-speed data rates the parasitic impedance of traditional protection devices can distort and deteriorate signal integrity. Low capacitance ESD protection is critical to maintaining data integrity in such applications.

The figure below illustrates a typical USB 2.0 circuit protection design utilizing PESD suppressor devices and a PolySwitch overcurrent protection device.

• The PESD devices help shunt ESD away from sensitive circuitry and provide exceptionally low capacitance compared to traditional MLV (multi layer varistor) or TVS (transient voltage suppression) diode technology.
• PESD devices perform better than other comparable components in transmission line pulse (TLP) testing, as well as IEC61000-4-2 testing, especially after multiple hits (up to 1000).
• The PESD device's low trigger voltage and low clamping voltage also helps protect sensitive electronic components.
  

Capacitance vs. frequency in the Raychem Circuit Protection PESD device is flat, and the device maintains a very low capacitance.

• Operating Voltage (VDC): Defined as DC voltage, under which device is in OFF state and leakage current below certain threshold.
• Leakage Current (IL): Current through device under Operating Voltage VDC
• Trigger Voltage (Vt): Voltage at which the device switches from the OFF to the ON state, during the IEC waveform or the TLP test system.
• Clamping Voltage (Vc): Voltage across device under 8 kV per IEC or measured by TLP test system. Typically measured 30 ns after initiation of the IEC ESD pulse, but 30ns and 60ns are sometimes used for TLP.
• Capacitance (Cp): Capacitance of the device measured at 1 MHz with 0 bias and 1 Vrms signal.
  

• The PESD device's low capacitance, low-voltage clamping levels, and high ESD tolerance are critical performance parameters for high data-rate transmission applications
• The device's form factor meets the board-space and surface mount installation requirements of new portable electronics designs.
• The PESD device performs better than other comparable components in IEC61000-4-2 testing, especially after multiple hits (up to 1000).