An Overview of ESD Protection Devices

Benny Lee
Protecting circuits from harmful ESD requires understanding andselecting the right type of protection device.
As electrostatic discharge (ESD) problems become more common in electroniccircuits, devices based on various technologies have also become readilyavailable to protect circuits from ESD. However, it is not always easyto select the appropriate protection device for a circuit, because thereare almost too many choices available. It is important to understandthe nature of each protection device and to evaluate its characteristicsagainst the requirements of the circuit to be protected.
ESD in this context is a high-voltage transient with fast rise timeand fast decay time. Several thousand volts of ESD with a high risetime (dv/dt) could break through the junction layer of protectivedevices easily and cause damage. ESD surge energy, however, is verysmall, and it does not require much energy-handling capability froma protective device. Electrical overstress (EOS) is a much slower phenomenonthan ESD. Therefore, the following factors should be considered whendesigning for EOS and ESD protection:
Voltage-clamping devices should limit the surge voltage to a safe level for the circuit or component being protected.
Voltage-clamping devices should withstand several thousand volts of a fast dv/dt impulse.
Protective devices should be small enough to fit into a limited space on a printed circuit board (PCB). Most components that require ESD protection are small surface-mount devices (SMD).
Current-limiting devices are sometimes not effective for ESD protection because ESD current is very small and does not induce much voltage across the device.
Limiting devices with high impedance are not effective for ESD protection. The stray capacitance in these devices could provide low reactance.
Many options are available for addressing EOS and ESD problems. Thisarticle looks at ceramic capacitors, zener diodes, transient voltagesuppession (TVS) diodes, multilayer varistors, and Schottky diodes (seeTable I). None of these devices, however, protects against the ESD causedby handling of a PCB during assembly.
Ceramic Capacitor
Zener Diode
Transient Suppression Diode
Multilayer Varistor
Schottky Diode
Barely withstands high-voltage ESD surges Strong resistance to surge than ceramic capacitors Stronger resistance to surge than ceramic capacitors Rugged; does not fail evenb at highest voltaghe level Protects limited parts of a printed circuit board
Surge energy does not dissipate against heat High clamping voltage; heat dissipation is slow Low clamping heat Low dc breakdown Low clamping voltage
—
Does not protect against nanosecond ESD events Does not protect against nanosecond ESD events —
Does not protect against nanosecond ESD events
Ceramic —
Available in small packages Ceramic; surface-mount packaging Surface-mount packaging
Table I. Basic characteristics of ESD protection devices.
Ceramic Capacitors
Some design engineers still prefer to use SMD ceramic capacitors forESD protection because of their simplicity and low cost. Capacitors,however, can barely withstand high-voltage ESD surges. For example,10 pieces out of 100 were damaged at 5 kV of ESD, and all 100 pieceswere damaged at 15 kV (see Figure 1). In addition, the surge energythrough a capacitor does not dissipate as heat, but rather filters througha device to the ground plane. This means that the filtered surge currentcan wander around the circuit via the ground plane unless it is dissipatedat the ground dc resistance. Figure 1 compares device survival ratesat various ESD voltage levels.

Figure 1. Comparative survival rates at ESD for devices protected by capacitors, zener diodes, and multilayer varistors.
Zener Diodes
Zener diodes are designed for voltage regulation, not for protectionagainst surge impulses. However, these devices are widely used by designengineers worldwide because of their low cost. A zener diode is moreeffective than a ceramic capacitor because it provides a stronger defenseagainst surge. Zener diodes have a higher clamping ratio (the ratiobetween impulse clamping voltage and dc breakdown voltage), which makesit difficult to lower the impulse clamping to a level safe enough forthe device being protected. However, these devices are too slow to protectagainst nanosecond ESD events. Some devices, including microprocessorchips, are sensitive to ESD even at 200 V. Heat dissipation of ESD atthe p-n junction is slower, which increases the clamping voltage level.For example, 10 pieces using zener diodes were shorted at 10 kV, andall failed at 20 kV (see Figure 1).
Transient Voltage Suppression Diodes
Also known as avalanche breakdown diodes, TVS devices have severaladvantages in ESD suppression—such as lower clamping ratio andstronger resistance to surges—over ceramic capacitors and zenerdiodes. TVS diodes are also too slow to protect against nanosecond ESDevents. The structure and characteristic curves are shown in Figure2. For ESD protection, a 500-W TVS diode is typically adequate. Thewattage rating is based on the maximum clamping voltage and peak surgecurrent at that moment, such as a 500-W, 5-V device with a peak surgecurrent of 52.3 A (10 x 1000 microseconds) and maximum clamping voltageof 9.6 V. The device wattage would be calculated as 52.3 x 9.6 = 502W. This number does not represent the conventional meaning of wattage(i.e., the energy during a 1-second period). Therefore, the energy ofa 500-W TVS diode could be estimated by multiplying the device wattageby surge duration (1000 microseconds in this case). The result is approximately0.5 J.

Figure 2. TVS diode structure and characteristic curve.
The question that arises is whether the joule rating is necessary fora TVS diode in ESD protection. As described earlier, although ESD hastens of thousands of volts of amplitude, it lasts only several nanoseconds,and the joule rating is almost negligible. Because of its high voltage,ESD is still a great threat even though the energy level is small. Tensof thousands of volts can cause dielectric breakdown of insulation,puncture a wafer junction, or burn off a tiny trace of a microprocessorcircuit. Therefore, protection devices should be strong enough to meethigh-voltage surges.
When a TVS diode is placed in a high-speed signal line, the capacitanceof the diode could upset the line impedance or attenuate the line signalconsiderably. Connecting a low-capacitance diode forward in series withthe device to be protected could reduce the capacitance of the TVS diode(see Figure 3).

Figure 3. Low-capacitance configuration with a TVS diode.
TVS diodes are available in small packages for ESD protection, in bothaxial leaded and SMD (TO-92, DO-215AA, DO-214AA, etc.). Array packagesand hybrid packages with diodes are also available.
Multilayer Varistors
Multilayer varistors are relatively new devices for ESD protection.They come in a surface-mount package ranging in size from 0402 to 1206.Single-layer devices are available with the same package sizes. Themain body substance is constructed of a ceramic material, which is ruggedagainst ESD surges. These devices will not fail even at the highestESD voltage level. The multilayer structure consists of very thin layersthat provide reasonable mechanical strength (see Figure 4). That meansthe dc breakdown of this device can go as low as <5 V dc.

Figure 4. Multilayer varistor structure.
Multilayer varistors lower device capacitance by adjusting the electrodesizes while still functioning in a low-voltage circuit protection modefor ESD. The design of these structures enables them to reduce devicecapacitance and still function as an ESD protector. These devices aresurge absorbers with rated voltages from 100 V dc to several hundredvolts with capacitance of about 1 pF.
Termination. Although the termination doesn't affect ESD protection,the termination process for these devices is pertinent because it differsgreatly from that for the other devices. The termination of these multilayervaristors and surge absorbers requires very sophisticated process controlcompared with other SMD devices such as capacitors or inductors. Metaloxide varistors contain a bonding material that could react easily withthe electroplating solution, forming a conductive layer.
Only a few manufacturers have been able to resolve this problem, withrather sophisticated processes. Other manufacturers simply create thetermination by dipping the varistor into silver alloy paste and thendrying it. Unfortunately, silver amalgam reacts with the solder pasteduring the solder process, which makes silver alloy paste terminationdifficult. The silver content of the terminal is drawn down during theamalgam, so nothing is left to hold down the terminal. This phenomenonis called the tombstone effect.
To minimize this tombstone effect, the maximum flow solder temperatureis typically set at lower than 240°C for this type of termination.
Schottky Diodes
These devices are also used for ESD protection in limited parts ofthe PCB. Two diodes can be forwardly connected in series between thepositive and negative power supply lines, with the center point connectedto the data and input/output ports where the protection is desired.Schottky diodes provide low clamping voltage. They are typically forwardvoltage biased.
When using these diodes, it is important to consider that the diode-forwardcharacteristic rating must meet the expected ESD voltage and currentratings. Like zener and TVS diodes, these devices are too slow to protectagainst nanosecond ESD events. Also, when a fast surge current is conductedthrough these diodes to the ground plane, radiated magnetic flux couldinduce noise on nearby circuitry. Schottky diodes are also availablein surface-mount package sizes from 0603 to 0805.
Conclusion
All of the devices described in this article are suitable only forsmall energy surges, not for lightning strikes or heavy-duty inductivesurges. Therefore, the location for the protection device—no matterwhich one is used—should be chosen carefully. The best locationis as close as possible to the circuitry to be protected.
Benny Lee is an engineer with World Products Inc. (Sonoma, CA).He can be reached at 707/996-5201 orblee@worldproducts.com.
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