An Overview of ESD Protection Devices
Protecting circuits from harmful ESD requires understanding and
selecting the right type of protection device.
As electrostatic discharge (ESD) problems become more common in electronic
circuits, devices based on various technologies have also become readily
available to protect circuits from ESD. However, it is not always easy
to select the appropriate protection device for a circuit, because there
are almost too many choices available. It is important to understand
the nature of each protection device and to evaluate its characteristics
against the requirements of the circuit to be protected.
ESD in this context is a high-voltage transient with fast rise time
and fast decay time. Several thousand volts of ESD with a high rise
time (dv/dt) could break through the junction layer of protective
devices easily and cause damage. ESD surge energy, however, is very
small, and it does not require much energy-handling capability from
a protective device. Electrical overstress (EOS) is a much slower phenomenon
than ESD. Therefore, the following factors should be considered when
designing 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
Many options are available for addressing EOS and ESD problems. This
article looks at ceramic capacitors, zener diodes, transient voltage
suppession (TVS) diodes, multilayer varistors, and Schottky diodes (see
Table I). None of these devices, however, protects against the ESD caused
by handling of a PCB during assembly.
Transient Suppression 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
|Available in small packages
||Ceramic; surface-mount packaging
|Table I. Basic characteristics of ESD protection
Some design engineers still prefer to use SMD ceramic capacitors for
ESD 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 pieces
were damaged at 15 kV (see Figure 1). In addition, the surge energy
through a capacitor does not dissipate as heat, but rather filters through
a device to the ground plane. This means that the filtered surge current
can wander around the circuit via the ground plane unless it is dissipated
at the ground dc resistance. Figure 1 compares device survival rates
at various ESD voltage levels.
|Figure 1. Comparative survival rates at ESD for devices protected
by capacitors, zener diodes, and multilayer varistors.
Zener diodes are designed for voltage regulation, not for protection
against surge impulses. However, these devices are widely used by design
engineers worldwide because of their low cost. A zener diode is more
effective than a ceramic capacitor because it provides a stronger defense
against surge. Zener diodes have a higher clamping ratio (the ratio
between impulse clamping voltage and dc breakdown voltage), which makes
it difficult to lower the impulse clamping to a level safe enough for
the device being protected. However, these devices are too slow to protect
against nanosecond ESD events. Some devices, including microprocessor
chips, are sensitive to ESD even at 200 V. Heat dissipation of ESD at
the p-n junction is slower, which increases the clamping voltage level.
For example, 10 pieces using zener diodes were shorted at 10 kV, and
all failed at 20 kV (see Figure 1).
Transient Voltage Suppression Diodes
Also known as avalanche breakdown diodes, TVS devices have several
advantages in ESD suppressionsuch as lower clamping ratio and
stronger resistance to surgesover ceramic capacitors and zener
diodes. TVS diodes are also too slow to protect against nanosecond ESD
events. The structure and characteristic curves are shown in Figure
2. For ESD protection, a 500-W TVS diode is typically adequate. The
wattage rating is based on the maximum clamping voltage and peak surge
current at that moment, such as a 500-W, 5-V device with a peak surge
current of 52.3 A (10 x 1000 microseconds) and maximum clamping voltage
of 9.6 V. The device wattage would be calculated as 52.3 x 9.6 = 502
W. This number does not represent the conventional meaning of wattage
(i.e., the energy during a 1-second period). Therefore, the energy of
a 500-W TVS diode could be estimated by multiplying the device wattage
by surge duration (1000 microseconds in this case). The result is approximately
|Figure 2. TVS diode structure and characteristic curve.
The question that arises is whether the joule rating is necessary for
a TVS diode in ESD protection. As described earlier, although ESD has
tens 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. Tens
of thousands of volts can cause dielectric breakdown of insulation,
puncture a wafer junction, or burn off a tiny trace of a microprocessor
circuit. Therefore, protection devices should be strong enough to meet
When a TVS diode is placed in a high-speed signal line, the capacitance
of the diode could upset the line impedance or attenuate the line signal
considerably. Connecting a low-capacitance diode forward in series with
the 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 both
axial leaded and SMD (TO-92, DO-215AA, DO-214AA, etc.). Array packages
and hybrid packages with diodes are also available.
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. The
main body substance is constructed of a ceramic material, which is rugged
against ESD surges. These devices will not fail even at the highest
ESD voltage level. The multilayer structure consists of very thin layers
that provide reasonable mechanical strength (see Figure 4). That means
the 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 electrode
sizes while still functioning in a low-voltage circuit protection mode
for ESD. The design of these structures enables them to reduce device
capacitance and still function as an ESD protector. These devices are
surge absorbers with rated voltages from 100 V dc to several hundred
volts 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 differs
greatly from that for the other devices. The termination of these multilayer
varistors and surge absorbers requires very sophisticated process control
compared with other SMD devices such as capacitors or inductors. Metal
oxide varistors contain a bonding material that could react easily with
the electroplating solution, forming a conductive layer.
Only a few manufacturers have been able to resolve this problem, with
rather sophisticated processes. Other manufacturers simply create the
termination by dipping the varistor into silver alloy paste and then
drying it. Unfortunately, silver amalgam reacts with the solder paste
during the solder process, which makes silver alloy paste termination
difficult. The silver content of the terminal is drawn down during the
amalgam, so nothing is left to hold down the terminal. This phenomenon
is called the tombstone effect.
To minimize this tombstone effect, the maximum flow solder temperature
is typically set at lower than 240°C for this type of termination.
These devices are also used for ESD protection in limited parts of
the PCB. Two diodes can be forwardly connected in series between the
positive and negative power supply lines, with the center point connected
to the data and input/output ports where the protection is desired.
Schottky diodes provide low clamping voltage. They are typically forward
When using these diodes, it is important to consider that the diode-forward
characteristic rating must meet the expected ESD voltage and current
ratings. Like zener and TVS diodes, these devices are too slow to protect
against nanosecond ESD events. Also, when a fast surge current is conducted
through these diodes to the ground plane, radiated magnetic flux could
induce noise on nearby circuitry. Schottky diodes are also available
in surface-mount package sizes from 0603 to 0805.
All of the devices described in this article are suitable only for
small energy surges, not for lightning strikes or heavy-duty inductive
surges. Therefore, the location for the protection deviceno matter
which one is usedshould be chosen carefully. The best location
is as close as possible to the circuitry to be protected.