Download Circuit Protection Solutions for Security and Fire Alarm Systems

APPLICATION NOTE
Circuit Protection Solutions
for Security and Fire Alarm Systems
Security and fire alarm systems are designed for operation within
specified current and voltage ratings. If these ratings are exceeded,
due to short-circuit or voltage transients, components may sustain
permanent damage and the equipment may fail. Power supplies and
circuit traces must also be protected against faults that may occur
during installation or in the case of a shorted back-up battery.
Modems are frequently utilized in security systems to automatically
alert the fire or police department in an emergency. These telecom
lines must be protected from damage caused by lightning strikes,
power-line crosses or AC power induction. Alarm systems must
comply with the UL864 standard, which mandates that soldered-in
fuses are not allowed to qualify a power supply as inherently limited.
If the system has provisions for connection to a telephone line it
must comply with UL60950 and TIA 968-A in North America, as
well as ITU K.21 in Europe and other parts of the world.
Overcurrent Protection Options
Fuses have typically been used for overcurrent protection in
Many equipment manufacturers prefer resettable polymeric
security and alarm systems. However, UL864 and UL60950
positive temperature coefficient (PPTC) device protection.
pose difficult challenges for these devices, which can fatigue
Unlike fuses, these devices generally do not require
under certain test conditions. More importantly, fuses are
replacement after a fault event, and allow the circuit to return
single-use components that must be replaced after a fault
to normal operating condition after the power has been
occurs.
cycled and the overcurrent condition is eliminated.
PPTC Device Principle of Operation
PPTC devices are made from a composite of semi-crystalline
The resistance typically increases by three or more orders of
polymer and conductive particles. At normal temperature,
magnitude. This increased resistance helps protect the
the conductive particles form low-resistance networks in the
equipment in the circuit by reducing the amount of current
polymer (Figure 1). However, if the temperature rises above
that can flow under the fault condition to a low, steady-state
the device’s switching temperature (Tsw) either from high
level. The device remains in its latched (high-resistance)
current through the part or from an increase in the ambient
position until the fault is cleared and power to the circuit is
temperature, the crystallites in the polymer melt and become
cycled – at which time the conductive composite cools and
amorphous. The increase in volume during melting of the
re-crystallizes, restoring the PPTC to a low-resistance state in
crystalline phase separates the conductive particles resulting
the circuit and the affected equipment to normal operating
in a large non-linear increase in the resistance of the device.
conditions.
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Figure 1. PPTC devices protect the circuit by going from a low-resistance state to a high-resistance state in response
to an overcurrent or overtemperature condition.
Because PPTC devices transition to their high-impedance
current to the equipment to very low levels, or as an indicator
state, based on the influence of temperature, they help
to the control system that the equipment is overheating. The
provide protection for two fault conditions – overcurrent and
control system can then determine what action is appropriate
overtemperature. Overcurrent protection is provided when
the PPTC device’s internal temperature rises due to I2R
to protect equipment and personnel.
heating within the device. High current levels through the
PPTC device heat it internally to its switching temperature
causing it to “trip” and go into a high-impedance state.
Tyco Electronics’ PolySwitch PPTC devices are employed as
series elements in a circuit. Their small form factor helps
conserve valuable board space and, in contrast to traditional
fuses
that
require
user-accessibility,
their
resettable
The PPTC device can also be caused to trip by thermally
functionality allows for placement in inaccessible locations.
linking it to a component or equipment that needs to be
Because they are solid-state devices, they are also able to
protected against overtemperature conditions. If the
withstand mechanical shock and vibration.
equipment temperature reaches the PPTC device’s switching
temperature, the PPTC device will transition to its highimpedance state, regardless of the current flowing through it.
In this way, the device can be used either to reduce the
The PolySwitch LVR series of devices includes components
that are rated for line voltages of 120 VAC and 240 VAC, for up
to 2A of operating current at 20°C. They offer low resistance,
fast time-to-trip, and a low-profile form factor.
Overvoltage Protection Considerations
A variety of methods can be used to help protect security
devices remain at the clamping voltage. Therefore, the power
and fire alarm systems from overvoltage conditions caused
dissipated in the foldback device is much lower than in a
by switching or lightning transients. There are two major
clamping device. This allows the use of a much smaller device
categories of overvoltage protection devices – clamping
to conduct the same amount of surge current.
devices and foldback, or crowbar, devices. Clamping devices,
such as metal oxide varistors (MOVs) and diodes, allow
voltages up to a specified clamping level to pass through to
the load during operation. Foldback devices, such as gas
For many applications, Tyco Electronics’ SiBar thyristor surge
suppressor is the preferred solution. The results of recent
testing by Tyco Electronics comparing the behavior of a TVS
diode with that of a SiBar thyristor are shown in Figure 2.
discharge tubes (GDTs) and thyristor surge suppressors,
operate as shunt devices in response to a surge that exceeds
SiBar thyristor surge suppression devices meet the immunity
the breakover voltage.
and test requirements for PoE (Power over Ethernet)
equipment, and their low on-state voltage allows for smaller
In some applications, foldback devices have an advantage
over clamping devices because in the foldback state very
little voltage appears across the load while it conducts
harmful surges away from the load; whereas clamping
form factor devices – as compared with clamping devices of
comparable energy-handling capability. The relatively low
capacitance of SiBar thyristors also makes them useful for
high-data-rate circuits.
TVS: 1.2/50, 8/20ms Combination Waveform,
40 Ohm Source Impedance
1 kVoc, Surge 10
Voltage [SiBar]
Spare
SiBar: 1.2/50, 8/20ms Combination Waveform,
40 Ohm Source Impedance
1 kVoc, Surge 10
Current [Through Circuit]
Voltage [SiBar]
Spare
Current [Through Circuit]
140
25
140
25
120
120
60
10
Voltage [V]
15
Current [A]
Voltage [V]
80
40
100
80
15
60
10
Current [A]
20
20
100
40
5
5
20
20
0
0
50
100
150
0
0
250
200
0
50
100
Time [ µseconds]
150
200
0
250
Time [ µseconds]
TVS Diode Behavior
SiBar Thyristor Behavior
Device
Pk (I)
Pk (V)
Avg (I)
Avg (V)
TVS Diode
23.4
124.8
4.37
74.19
SiBar TVB058SA-L 23.8
89.6
5.41
25.33
Figure 2. Performance comparison of TVS diode and SiBar thyristor.
MOVs
also
offer
high-current-handling
and
energy-
Security and fire alarm equipment must also be protected
absorption capability, fast response, and low cost, making
against damage caused by electrostatic discharge (ESD). To
them suitable for overvoltage protection in line voltage
help shunt ESD away from sensitive circuitry in electronic
applications. Pairing Tyco Electronics’ ROV device with a
devices, Tyco Electronics’ PESD devices offer a very low
PolySwitch PPTC device can help provide a completely
capacitance of 0.2 pF, which is useful in high-frequency
resettable circuit protection solution for power supplies and
applications.
control board transformers.
Hybrid Circuit Protection Options
A generation of polymer-enhanced Zener diodes can also
In operation, the PPTC layer responds to either diode heating
help protect sensitive electronics from damage caused by
or overcurrent events by transitioning from a low to high
inductive voltage spikes, voltage transients, use of incorrect
resistance state. In the event of a sustained high-power
power supplies and reverse bias. As shown in Figure 3, Tyco
overvoltage condition, the tripped PPTC element limits
Electronics’ PolyZen device incorporates a stable Zener diode
current and generates voltage drop to help protect both the
for crisp voltage clamping and a resistively non-linear PPTC
Zener and the follow on electronics – effectively increasing
layer.
the diode’s power-handling capability.
Power Supply
PolyZen Protected Electronics
(External or internal)
VIN
+
2
1
PolyZen
Device
GND
VOUT
3
Regulated
Output
Figure 3. Polymer-enhanced Zener diode helps provide input power protection for portable electronics.
RLOAD
Protected
downstream
electronics
This device is particularly effective at clamping and
Figure 4, Tyco Electronics’ 2Pro device incorporates PPTC
smoothing inductive voltage spikes. In response to an
overcurrent technology with an MOV component into one
inductive spike, the Zener diode element shunts current to
thermally protected device that helps provide current limiting
ground until the voltage is reduced to the normal operating
during overcurrent events and voltage clamping during
range. In the case of a wrong voltage power supply, the
overvoltage events.
device clamps the voltage, shunts excess power to ground,
and eventually locks out the wrong supply.
This hybrid device approach helps reduce component count
and improve equipment reliability by preventing thermal
The relatively flat voltage vs. current response of the
runaway and maintaining varistor surface temperature at less
polymer-enhanced Zener diode helps clamp the output
than 150°C, thereby preventing the device from reaching
voltage, even when input voltage and source currents vary.
unsafe temperatures that may result from overvoltage
Simply put, the polymer-enhanced device helps provide
transients.
The
device’s
small
footprint,
resettable
coordinated protection with a component that protects like a
functionality and coordinated protection capabilities make it
Zener diode, but is capable of withstanding very high-power
suitable for a wide range of telephony and security system
fault conditions without requiring any special heat sinking
applications.
structures beyond normal PCB traces.
Tyco Electronics offers a broad range of devices that help
Integrating PPTC technology with an MOV has resulted in
provide overcurrent and overvoltage protection in security
another circuit protection device that can help equipment
and fire alarm systems. All of the circuit protection devices
manufacturers meet the UL60950, TIA-968-A, and ITU K.20
discussed in this paper are employed in the circuit diagram
and K.21 telecom protection requirements.
shown in Figure 5.
Figure 4. Tyco Electronics’ 2Pro device helps protect against
damage caused by both overcurrent and overvoltage events.
As shown in
Figure 5. Coordinated circuit protection options for security and fire alarm systems.
Raychem Circuit Protection Products
308 Constitution Drive, Building H
Menlo Park, CA USA 94025-1164
Tel : (800) 227-7040, (650) 361-6900
Fax : (650) 361-4600
2Pro, PolySwitch, PolyZen, Raychem, SiBar, TE logo and Tyco Electronics are trademarks. All information,
including illustrations, is believed to be reliable. Users, however, should independently evaluate the suitability
of each product for their application. Tyco Electronics Corporation makes no warranties as to the accuracy
or completeness of the information, and disclaims any liability regarding its use. Tyco Electronics’ only
obligations are those in the Tyco Electronics Standard Terms and Conditions of Sale for this product, and in
no case will Tyco Electronics be liable for any incidental, indirect, or consequential damages arising from the
sale, resale, use, or misuse of the product. Specifications are subject to change without notice. In addition,
Tyco Electronics reserves the right to make changes without notification to Buyer—to materials or
processing that do not affect compliance with any applicable specification.
© 2008 Tyco Electronics Corporation. All rights reserved. RCP0046E.1008
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