PolySwitch Resettable Devices
Fundamentals
How a PolySwitch Device Works
Figure 1.b
Trip Condition
Figure 1.a
Normal Condition
Temperature rises during
short circuit condition
Overview
PolySwitch polymeric positive temperature coefficient (PPTC)
devices help protect against damage caused by harmful
­overcurrent surges and overtemperature faults. Like traditional
fuses, these devices limit the flow of dangerously high current
during fault conditions. The PolySwitch device, however, resets
after the fault is cleared and power to the circuit is removed,
thereby helping to reduce w
­ arranty, s­ ervice and repair costs.
PolySwitch circuit protection devices are made from a composite
of semi-crystalline polymer and conductive particles. At normal
temperature, the conductive particles form low-resistance
­networks in the polymer (Figure 1.a). However, if the temperature
rises, either from high current through the part or from an increase
in the ambient temperature, the crystallites in the polymer melt
and become amorphous. The increase in volume during melting of
the crystalline phase separates the conductive particles, resulting
in a large non-linear increase in the resistance of the device.
How a PolySwitch Device Works
Figure 1.aHow
Normal Condition
a PolySwitch
Works Figure 1.b
TemperatureDevice
drops
Trip Condition
after circuit resets
Figure 1.a
Figure 1.b
Temperature rises during
Normal Condition
Trip Condition
short
circuit
condition
Polymer has a
Polymer expands due
crystalline structure.
to I2R heating.
Temperature rises during
Conductive path breaks down
Conductive path is made of short circuit condition
due to polymer expansion.
carbon black materials.
Temperature drops
after circuit resets
Polymer has a
crystalline structure.
Conductive path is made of
Polymer has a
carbon black materials.
crystalline structure.
Conductive path is made of
carbon black materials.
Temperature drops
after circuit resets
Polymer expands due
to I2R heating.
Conductive path breaks down
Polymer expands due
due to polymer
expansion.
to I2R heating.
Conductive path breaks down
due to polymer expansion.
Typical PPTC Application
I
RS
Overcurrent Protection Using a PPTC Device
The PPTC device is a series element in a circuit. The PPTC device
helps protect the circuit by going from a low-­resistance to a
­high-resistance state in response to an over­current condition, as
shown in Figure 2. This is referred to as “tripping” the device.
In normal operation the device has a resistance that is much lower
than that of the circuit. In response to an overcurrent condition,
the device increases in resistance (trips), reducing the current
in the circuit to a value that can be safely carried by any of the
­circuit elements. This change is the result of a rapid ­increase in
the temperature of the device caused by I2R heating.
Typical PPTC Application
V
RS
I
Typical
PPTC Application
I
RL
RS
V
RL
V
RL
Principles of Operation
PolySwitch PPTC device’s operation is based on an overall
energy ­balance, as shown in Figure 3. Under normal operating
conditions, the heat generated by the device and the heat lost by
the device to the environment are in balance at a relatively low
temperature, as shown between Points 1 and 2.
Resistance Value
Point 4
Example of Operating Curve
for PPTC Device
ExamplePoint
of 2Operating Curve
Point 1
for PPTC Device
Point 4
Point 3
Resistance
Value Value
Resistance
If the current through the device is increased while the
ambient temperature is kept constant, the temperature of the
device ­increases. Further increases in either current, ambient
temperature, or both, will cause the device to reach a temperature
where the ­resistance rapidly increases, as shown in Point 3.
Example of Operating Curve
for PPTC Device
Point 4
Any further increase in current or ambient temperature will
Temperature (˚C)
cause the device to generate heat at a rate greater than the rate
at which heat can be dissipated, thus causing the device to heat
Point 2
Point 1 in temperature, between Points 3 and 4.
up rapidly. At this stage, a very large increase in resistance occurs for a very small change
This is the normal operating region for a device in the tripped state. This large change in resistance
a corresponding decrease
Point 2 causes
Point 3
Point 1
in the current flowing to the circuit. This relation holds until the device resistance reaches the upper knee of the curve (Point 4). As
long as the applied voltage remains at this level, the device will remain in the tripped state (that Temperature
is, the
device
will remain latched in its
Point
3(˚C)
protective state). Once the v
­ oltage decreases, the power is removed and the device cools, the device will reset.
Temperature (˚C)
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TE Connectivity /// Circuit protection product catalog
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11
PolySwitch Resettable Devices
Fundamentals
Example of Hold and Trip Current as a Function of Temperature
Example of Hold and Trip Current as a
Function of Temperature
300
Rated Hold Current (%)
Rated
Hold
Current
(%)
Rated
Hold
Current
(%)
Figure 4 illustrates the hold- and trip-current behavior of a
­PolySwitch PPTC device as a function of temperature. One
such curve can be defined for each available device. Region A
describes the ­combinations of current and temperature at which
the PolySwitch device will trip (go into the high-resistance state)
and protect the circuit. Region B describes the combinations
of current and ­temperature at which the PolySwitch device will
allow for normal operation of the circuit. In Region C, it is possible
for the device to either trip or remain in the low-resistance state
(depending on individual device resistance).
Region
A Current as a
Example of Hold and
Trip
PolySwitch
Device will Tripas a
Example of Hold and
Trip Current
and Protect Circuit
Function
of
Temperature
ITRIP Function of Temperature
300
200
300
Region A
Region
A Device will Trip
PolySwitch
PolySwitch
and ProtectDevice
Circuit will Trip
and Protect Circuit
Region C
ITRIP
IHOLD
I
TRIP
200
100
200
Region C
Region
B
Region
C
PolySwitch Device will Remain
in Low-resistance
IHOLD State
IHOLD
100
0100
-40
Region
-20 B
0
20
40
Region
B Device will Remain
PolySwitch
PolySwitch
DeviceState
will
Remain
in Low-resistance
Temperature
(˚C)
in Low-resistance State
0
0-40
-40
-20
-20
0
0
20
20
60
40
40
80
60
60
80
80
Temperature (˚C)
Temperature (˚C)
Operating Characteristics of a PolySwitch PPTC Device
Example of PolySwitch PPTC Device
Example of PolySwitch PPTC Device
Operating Characteristics
Operating Characteristics
Log Time-to-Trip
Log
Time-to-Trip
Log
Time-to-Trip
Figure 5 shows a typical pair of operating curves for a PolySwitch
device in still air at 0°C and 75°C. The curves are different
­because the heat required to trip the device comes both from
electrical I2R heating and from the device environment. At 75°C
the heat input from the environment is substantially greater
than it is at 0°C, so the additional I2R needed to trip the device
is ­correspondingly less. This results in a lower trip current at a
given trip time (or a faster trip at given trip current).
Example of PolySwitch PPTC Device
Operating Characteristics
0˚C
75˚C
75˚C
75˚C
0˚C
0˚C
Current
Current
Current
Typical Resistance Recovery After a Trip Event
Figure 6 shows typical behavior of a PolySwitch device that is
tripped and then allowed to cool. This figure illustrates how, even
after a number of hours, the device resistance is still greater than
the initial resistance. Over an extended period of time, device
­resistance will continue to fall and will eventually approach initial
resistance.
Cut Off Power Supply
Typical Resistance Recovery After a Trip Event
Typical Resistance Recovery After a Trip Event
Cut Off Power Supply
Cut Off Power Supply
During Trip
0.01
During
Trip
During
Trip
Resistance (Ω)
Resistance
(Ω)(Ω)
Resistance
However, since this time can be measured in days, months, or
years, it is not practical to expect that the device resistance will
reach the ­original value for operational purposes. Therefore,
when PolySwitch devices are chosen, R1MAX should be taken
into consideration when determining hold current. R1MAX is the
resistance of the device one hour after the thermal event.
Typical Resistance Recovery After a Trip Event
Initial
Resistance
Before Trip
0.1
Before Trip
0.01
0.01
Before Trip
Before Trip
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10
100
1,000
10,000
Resetting Time for Resistance (minute)
0.1
0.1
1
1
10
10
100
100
1,000
1,000
Initial
Initial
Resistance
Resistance
Before Trip
Before Trip
10,000
10,000
Resetting Time for Resistance (minute)
Resetting Time for Resistance (minute)
RoHS Compliant, ELV Compliant