Don Tuite
The Use And Misuse Of Circuit
Protection Devices
C
ircuit protection devices aren’t
the most glamorous part of
an engineer’s design. They’re
usually an afterthought to the
processors, FPGAs, memory, and other components, which most often are considered the
real “meat” of the device. But without circuit
protection, the entire product may be vulnerable to damage and can pose safety issues
for consumers.
The fuse is the most basic circuit protection device. When the current drawn by the
load rises above a certain point, it opens the
line carrying that current. A circuit breaker
is like a resettable fuse. Typically, it takes a
defect in the circuit being protected to “blow”
(open) a fuse. But if the fuse opens fast
enough, it should help protect parts of the
circuit that are not defective by sacrificing
itself. Alternatively, a fuse may open because
an excessive voltage is applied either by
mistake or because of a transient voltage on
the power supply line.
Fuses that have opened must be replaced. (Breakers have to be reset.) Often,
equipment needs to be protected during
short-duration overvoltage transients without
human intervention afterward. That’s where
different types of transient voltage suppression (TVS) come in.
Types Of Fuses
Consider different kinds of fuses and TVS
devices. Most designers are familiar with the
cylindrical fuses used in external ac-supply
lines. Surface-mount devices, which are soldered directly to circuit boards, come in fastacting and slow-blow (time-lag) varieties.
Fast-acting fuses are employed for user
safety on critical ac equipment that requires
frequent opening for maintenance operations. They can help prevent conductor and
Analog/Power Editor
Normal condition
Trip condition
Temperature rises during
short circuit condition
Temperature drops
after circuit resets
Polymer has
a crystalline structure;
conductive path is made
of carbon-black materials
Polymer expands
due to I2R heating;
conductive path breaks down
due to polymer expansion
1. In a polymer fuse device, the conductive particles form low-resistance networks at
normal temperature. If the temperature rises above the device’s switching temperature
(Tsw), either from
high current through the part or from an increase in the ambient
temperature, the crystallites in the polymer melt and become amorphous. This creates
an increase in volume that separates the conductive particles, resulting in a large nonlinear increase in resistance. After the fault clears and the polymer material shrinks,
these devices reset themselves.
component overheating and reduce the
severity of arc-flash events.
“Fast-acting” means the fuse clears very
quickly—as fast as a millisecond at high currents. Note that “high-current” caveat. With
lower currents, the fuse may take longer to
open. Check the fuse’s data sheet to see
what trip times are guaranteed or “typical.”
Smaller, fast-acting chip fuses help provide
overcurrent protection for systems using dc
power sources up to 63 V dc. Conversely,
slow-blow fuses help minimize the nuisance of
repeated replacements when a circuit experiences brief but recurring overcurrent spikes.
Fast or slow, chip fuse design is far
from trivial. The most common fuse type,
employed for secondary-level overcurrent
protection applications, uses a corrugated (zig-zag) wire element supported
within the ceramic fuse body. An alternative
is the self-supported “wire in air” element,
which provides consistent fusing and cutting
characteristics and is better able to withstand
high inrush currents.
There are also multi-layer designs that
expose more fuse element surface area to
the fuse-body’s glass-ceramic absorption
material. With multiple layers, when the fuse
elements open, there is more material for the
vaporizing fuse metals to absorb into, resulting in a very efficient and effective quenching
of the fuse arc.
Pulse-tolerant fuses have strong arc
suppression characteristics and withstand
high inrush currents. They are used in highperformance consumer electronics such as
laptops, multimedia players, cell phones, and
other portable devices.
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Monolithic, multilayer-design, highcurrent-rated chip fuses target overcurrent
protection of power supplies, servers,
communications equipment, voltage regulator modules, and other small, high-current
applications.
Normal operation
Rc
Resettable PPTC “Fuses”
Beyond fuses, there are resettable circuit
protection devices that behave like fuses.
They are made from a composite of semicrystalline polymer and conductive particles
(Fig. 1). These devices can be employed
to protect electric motors and transformers from damage caused by mechanical
overloads, overheating, stalls, and other
potentially harmful conditions. They can also
be used to help protect USB ports, telecom
interfaces, and many other power inputs
and outputs.
Known as polymeric positive temperature coefficient (PPTC) devices, they
help protect against damage caused by
overcurrent surges and over-temperature
faults. Like traditional fuses, they limit
the flow of dangerously high current during fault conditions. Unlike fuses, they
reset after the fault is cleared and power
to the circuit is removed. The tripping
point depends on current and ambient
temperature.
Rptc
Fault operation
Leakage current
2. During normal operation of the MHP-TA device, current passes through the bimetal
contact due to its low contact resistance. During an abnormal event, the device reacts
to the rise in cell temperature, causing the bimetal contact to open at the specified
temperature and its contact resistance to increase. The current then shunts to the
lower-resistance PPTC, which acts as a heater and helps keep the bimetal protector
open and in a latched position until the fault is removed. A typical MHP-TA device has
temperature characteristics with a 77°C thermal cutoff (TCO) rating. These resettable
TCOs can handle the higher voltages and battery discharge rates found in high-capacity
LiP and prismatic cell applications.
temperatures, high hold-current ratings,
and compact size that PPTCs alone can’t
always achieve (Fig. 2).
Resettable Thermal Cutoff/MHPs
(a)
ESD
0
10
20
30
While fuses and similar devices help
protect against overcurrent faults, several
other types of devices can help protect
against electrostatic discharge (ESD).
These devices can be zener or TVS diodes,
varistors, or polymer devices. ESD events
are very low-energy, short-duration events
lasting a few tens of nanoseconds. The
40
50
Time (ns)
60
70
80
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(b)
Burst
0
10 20 30 40 50 70 60 80 90 100
Time (ns)
TVS Diodes
TVS diodes offer protection up to 30 kV.
They are essentially Zener devices with a
cross-section that accommodates large
V OUT /V OUT-peak
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
ESD Devices
V OUT /V OUT-peak
I OUT/I OUT-peak
Another type of circuit protection device,
specifically for lithium battery cell protection, is the resettable thermal cutoff (TCO)
device. With battery designers increasingly
using large-surface-area, high-capacity,
envelope-shaped lithium-polymer (LiP) cells
in tablets and ultra-thin notebooks, they
need thinner TCOs for battery protection.
Metal Hybrid PPTC devices with thermal
activation (MHP-TA) combine low TCO
peak voltages can be very high, up to 20 to
30 kV. Transient-voltage protection devices
include gas-discharge tubes, multi-layered
varistors (MLVs), TVS diodes, thyristors, and
metal-oxide varistors (MOVs). TVS devices
may either clamp (voltage-limit) or crowbar
(short to ground) voltage transients that
appear at their inputs.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(c)
Surge
0
10
20
30
40
50
60
70
80
Time (µs)
3. The IEC1000 ESD test simulates the ESD of a human onto electronic equipment (a). The burst test simulates switching transients
due to relay contact bounce or the interruption of inductive loads (b). The surge test simulates transients resulting from lighting
strikes or the switching of power systems including load changes and short circuit switching (c).
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VCC
VCC
10k
RxD
MCU
DIR
TxD
1
2
3
4
R
VCC 8
RE
A
DE
D
XCVR
B
GND
R1
0.1μF
TVS
7
6
5
10k
R2
(a)
VCC
VCC
10k
RxD
MCU
DIR
TxD
1
2
3
4
R
RE
DE
D
VCC
XCVR
A
B
GND
8
R1
0.1 μF
TBU1
TVS
7
MOV1
6
MOV2
5
10k
R2
(b)
TBU2
4. Both of these circuits are designed for 10-kV ESD and 4-kV EFT transient protection.
The circuit in (a) provides surge protection for transients of up to 1-kV. The circuit in (b)
can handle surge transients of 5 kV and more.
reverse breakdown current flows. There are
single-channel and multi-channel silicon
ESD (SESD) TVS devices that may be unidirectional or bidirectional.
Bidirectional devices exhibit lower capacitance and insertion loss. They can be
placed on a printed-circuit board (PCB) in
any orientation, and they don’t clip signals
that swing below ground. Unidirectional
devices exhibit lower negative breakover
voltages. Multi-channel arrays make it
easier to protect multi-line data buses such
as Ethernet.
There are also polymer-enhanced,
precision-Zener-diode micro-assemblies,
consisting of a precision Zener diode and a
PPTC that are thermally bonded, that can
offer resettable protection against multi-watt
fault events without the need for multi-watt
heatsinks. The Zener diodes used in these
micro-assemblies are selected for a flat
voltage versus current response. This helps
improve output voltage clamping, even
when input voltage is high and diode currents are large.
As assembled, the Zener diode is in
series with and thermally coupled to the
PPTC layer, which responds to either extended diode heating or overcurrent events
by tripping, as described above. Polymerenhanced Zener diodes help protect circuits
in portable devices from inductive voltage
spikes, voltage transients, incorrect power
supplies, and reverse bias.
ally exceeds a voltage where avalanche
multiplication occurs, the impedance of the
device begins to decrease, and current flow
begins to flow through it. When that current
falls below a certain value, the thyristor
becomes a high-impedance device again.
Gas discharge tubes (GDTs) are placed
on power lines, communication lines, signal
lines, and data transmission lines to help
protect sensitive electronics from transient
surge voltages caused by lightning strikes
and equipment switching operations. These
devices exhibit high impedance in normal
operation. But in the event of an overvoltage surge, such as a lightning strike, the
gas ionizes and diverts the transient energy
to ground. After the fault condition has
cleared, the GDT then returns to its highimpedance state.
Thermal Circuit Protection
Devices/RTPs
Not all devices help protect against
overcurrents or overvoltages. Some protect
TVS devices do not have to be built using against heat, regardless of what causes it.
silicon technologies. Probably the most faReflowable Thermal Protection (RTP) devices
miliar is the MLV. MLVs are stacks of MOVs, help protect systems from thermal runaway
which, in turn, are arrays of zinc-oxide balls damage due to extreme environments or
in a ceramic matrix.
failed power components. They open if
In an MOV, each boundary is a diode
their internal junction temperature exceeds
junction. At each side of the array, there is a certain specified limits.
metal connection. One of these is connected
RTPs are surface-mount devices because
to the line being protected, the other to
they need to optimize thermal coupling
ground. Thus, an MOV is an array of backwith the PCB. After installation, a one-time,
to-back diodes that normally provide no
electronic arming process results in the
complete path through the array. Numerous device becoming thermally sensitive. Arming
breakdowns occur when a surge occurs,
can occur during manufacturing tests or
and the line is connected to ground for the
in the field. (Before the arming procedure,
duration of the surge.
the device can go through installation
Stacked into an MLV, the MOVs are
temperatures up to 260°C without opening.)
essentially in series. MLVs have a higher
After arming, the device will open when the
in-circuit resistance than a single MOV and
junction exceeds the open temperature.
faster response time. In high-data-rate
circuits, they may degrade or distort the
Beyond Fuses: Transient
signal due to high capacitance. Adding
Protection Basics
PPTC technology helps reduce current prior
Many ICs that interface with the world
to contact switching events.
beyond their own circuit boards are
fabricated with internal circuit protection
Thyristor Devices And Gas
devices. This raises the question of whether
Discharge Tubes
external protection (and proper use of
Thyristor surge protection devices have
personal shoe straps) is really necessary.
four layers of alternating conductivity: PNPN.
The short answer is that if the application
The four layers are designated the emitter
exposes the device to the outside world in
layer, the upper base layer (cathode), the mid- any way, then precautions against transient
region layer, and the lower base layer (anode). events are necessary.
When a transient voltage is applied to a
To get an idea of the energy in even the
thyristor, as the voltage increases, it eventu- most modest transient events, consider
MLVS And MOVs
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Volts
Power
Ground
Time
5. Injecting a large surge current into the circuit ground through the TVS limiter will
cause more or less ground bounce, depending on the connection inductance. Good
board design—that is, a low-impedance ground plane with good high-frequency
decoupling on the power bus—will maintain an essentially constant potential difference
during the ringing. In that case, the transient should not affect the behavior of the
system.
the test standards used to evaluate circuit
protection from various types of events,
from a simple static discharge to powerline transients. The International Electrotechnical Commission (IEC) recognizes
three major types of transients: ESD,
electrical fast transients (EFTs), and surge
transients (Fig. 3). The IEC61000-4 family
of electromagnetic compatibility (EMC)
standards specifies IEC61000-4-2 for ESD
immunity, IEC61000-4-4 for EFT immunity,
and IEC61000-4-5 for surge immunity.
The first of these, the ESD test, simulates the ESD of a human onto electronic
equipment. The standard describes an ESD
generator that generates pulses of less
than 100-ns duration and 1-ns rise time.
A sequence of 20 discharges of positive
and negative polarity is to be applied with a
one-second pause between pulses.
The second, the EFT or burst test,
simulates switching transients that often are
encountered in industrial environments, such
as factory automation and process control.
They may be caused by relay contact bounce
or the interruption of inductive loads.
Testing requires a generator that
produces a burst of test pulses. Each burst
provides roughly 15,000 transients with
rise times of around 5 ns. A test sequence
comprises six bursts of 10 seconds each
with a 10-second pause between bursts.
This adds up to several million pulses applied to the device under test (DUT) during
one minute.
Both ESD and EFT events involve relatively low energy levels. In contrast, the surge
test simulates much larger transients that
might be caused by lightning strikes (either
direct strikes or voltages and currents
induced by indirect strikes), or simply by the
switching of power systems with resulting
load changes and short-circuit switching.
At lower levels (500 V to 1 kV), these
transients occur in industrial automation. At
higher levels (5 to 6 kV), they occur in power
grid systems. In either case, due to the high
pulse energy, surge testing is usually limited
to five positive pulses and five negative pulses
with a one-minute pause between pulses.
Those test requirements illustrate the
different levels of protection needed.
Generally, the on-chip IEC-ESD protection
provided by manufacturers of various kinds
of bus receivers can absorb the energy of
single ESD and EFT pulses, but it breaks
down in the face of EFT pulse trains and
surge transients, even if the transient voltage is not very high.
The energy that’s in pulse trains does
not give chipmakers’ internal protection circuits time to recover. Instead, the
successively applied packages of energy
are converted into thermal energy—heat
that breaks down the protection cells,
destroying the internal transceiver input or
output circuit. Practical protection requires
external circuitry (Fig. 4).
Circuit Implementation
Both ESD and EFT transients have a wide
frequency bandwidth. Generally, frequency
components start at around 3 MHz and
reach as high a 3 GHz. Therefore, highfrequency layout techniques are essential:
• Place protection circuitry as close to the
bus connector as possible to keep noise
transients off the PCB to the greatest
extent possible.
• Design for multiple layers with full VCC
and ground planes to provide low-inductance paths for the transient currents to
return to their source.
• Because high-frequency currents follow
the path of least inductance, not necessarily the path of least resistance, be sure
to position the protection components in
line with the signal path. This prevents
the transient currents on the signal path
to the protection device.
• Place 100- to 220-nF bypass capacitors
as close as possible to the VCC pins of
the transceiver, UART, and controller
ICs. Use at least two vias for VCC and
ground connections of bypass capacitors and protection devices to minimize
effective via inductance. Use 1k to 10k
pull-up/down resistors to enable lines to
limit noise currents in these lines during
transient events.
• If the TVS clamping voltage is higher
than the specified maximum voltage
of the transceiver bus terminals, insert
pulse-proof resistors into the A and B bus
lines. They will limit the residual clamping
current into the transceiver and prevent it
from latching up.
• TVS protection alone is sufficient for
surge transients up to 1 kV, but higher
transients require MOVs, which reduce
the transients to a few hundred volts of
clamping voltage, along with transient
blocking units (TBUs) that limit transient
currents to less than 1 mA.
Non-destructive ESDs And
Signal Integrity
What does the TVS protection device do,
beyond shunting spikes to ground to actually maintain circuit functionality? That is, in
a digital circuit, how is it possible to avoid
false signal transitions caused by ESDs?
Due to circuit inductance, the real effect
of the current transient being bypassed to
ground is that the discharge pulse will ring
(Fig 5). That is less severe than the transient step, but it could still cause a missed
or false transition on the signal line. However, the ringing will be reflected on both
the ground and power busses, and external
signals input to the protected circuit will
reflect that ringing. So, signal transitions
referenced to the bouncing ground should
be masked. n
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