W H I T E PA P E R
“Reflowable” Alternative to Traditional Thermal Protection
THERMAL PROTECTION IN POWER ELECTRONICS
In recent years, a variety of innovative technologies have
emerged to help designers of electrical and electronic
applications implement thermal protection. The objective is to
protect the application and the end user from thermal events
by interrupting electrical current flow when a component or
board area is heated to a specific rated temperature.
Most industrial and consumer equipment now incorporates
thermal protection devices to improve reliability and safety
and to prevent damage resulting from overheating. The heat
generated by resistive and inductive loads, power capacitors,
and current drivers to MOSFETs, switches and relays presents
significant challenges to engineers charged with designing in
reliable, safe thermal management.
In addition to MOSFETs, IGBTs have switching characteristics
and are found in applications such as switch mode power
similar to a MOSFET. High-current, high-voltage bipolar junction
supplies (SMPSs), high-voltage power supplies and switching
transistors, or BJTs, are also thermal-generating components
applications for train traction motors and hybrid vehicles.
LIMITS OF TYPICAL SOLUTIONS
Traditional thermal protection devices are available in a variety
be soldered/welded to the leads, a heat sink must be attached
of shapes, sizes and technologies to help protect equipment
to the lead to conduct heat away from the temperature-
from damage caused by thermal events. Two notable devices are
sensitive alloy so as to not activate it or limit the effectiveness
the thermal fuse/thermal cutoff (TCO) and the thermal switch.
of the device before it is to be used in its intended application.
Both provide wide-ranging and specific temperature activation
characteristics in both AC and DC applications and can be
specified as bolt-in, clip-on, pig-tail or lead-type configurations.
However, these devices can complicate design-in
and manufacturing processes. Careful handling
procedures must be adhered to in order to
guarantee that they perform as expected.
Thermal fuses typically contain a component
that is temperature sensitive, such as a lowtemperature alloy or a plastic/wax pellet, and
Another common thermal protection device is the thermal
switch, which is designed for multiple uses and can be configured
to be normally-open or normally-closed. When a specific trip
Traditional thermal
protection devices are
also non-resettable
and must be replaced
after they trip.
which holds a spring contact mechanism. The
device is normally closed and opens when activated at a given
maximum, or trip, temperature. These devices are also nonresettable and must be replaced after they trip.
temperature is reached the thermal switch
activates and opens like a thermal fuse
to stop the current flow. Likewise, when
designed to close during a thermal event,
the thermal switch can be used to activate
a secondary airflow device, such as a fan, to
cool the application. When a pre-determined
temperature is reached the device will revert
to its pre-tripped state, normally-open or normally-closed.
One limitation of traditional TCOs is that they are not surface
mount or reflowable like standard semiconductor products and
Thermal fuses require special handling in the manufacturing
require manual application. Thermal cutoffs also exhibit lower
process. If they are to be soldered, or require wire extensions to
current ratings, limited DC or AC rating, vibration sensitivity
1
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and installation sensitivity. Due to repeated operations at
mounting options, or they may require high-temperature
temperatures close to but below their calibration temperatures,
insulation tape, which loses holding power over time. Lastly, to
or as a result of excessive thermal waves across the case and
improve temperature sensing and response on flat surfaces of
leads of the TCO, nuisance trips may occur because of pellet
the PCB, the use of heat sink compounds between the TCO and
shrinkage.
PCB surface is required for optimal thermal conductivity.
Thermal fuses, unlike electrical fuses, react only to excessive
With applications continually moving towards more compact
temperature and not excessive current – unless the excessive
surface mount designs, the major limitations of traditional
current is sufficient to cause the thermal fuse itself to heat
thermal protection devices are that they are not available in
up to the activating temperature due to I2R effects. Thermal
surface mount configurations, require costly manual application
fuses are also intended as a failsafe or as an added safety back-
processes and, in the case of traditional TCOs, may fail short.
up, which will activate when other electrical safety measures
such as circuit breakers or traditional fuses fail. Thermal fuse
operating
REFLOWABLE THERMAL PROTECTION DEVICE
characteristics
TE Circuit Protection has developed a surface mount thermal
can also change over time
protection device that is pick/place compatible and can be
from self-heating effects or
designed in and reflowed on a PCB, utilizing standard surface
from operating under high
mount lead (Pb)-free reflow manufacturing processes over a
current loads.
broad range of device activation temperatures.
To put things in perspective,
The Reflowable Thermal Protection (RTP) device operates under
working with and installing
the demanding environmental, life, and reliability requirements
traditional
requires
of automotive and industrial applications, including shock,
proper handling techniques
vibration, temperature cycling, and humidity exposure. Once
during installation in order for them to perform properly.
the reflow process is complete an arming procedure is all that
Particular attention should be paid to forming the thermal
is required in order for the device to be ready to activate at its
cutoff leads to prevent the seals from cracking, which may
pre-determined trip temperature.
TE Circuit Protection
has developed
a surface mount
thermal protection
device that is pick/
place compatible
TCOs
result in premature degradation of the pellet. When installing
TCOs, unnecessary bending, twisting, pulling or pushing on the
TCO leads should be avoided. The TCO’s body must maintain
its cylindrical shape to function properly. Excessive clamping
of a TCO may cause denting or crushing of the TCO body,
which may lead to premature failure. When soldering the leads,
The RTP200R060SA device’s maximum electrical specifications
are shown in Table 1. This RTP device has a 200°C activation
temperature and can be reflowed at temperatures up to 260°C,
providing a high degree of flexibility in placement of thermal
protection.
the TCO leads require heat sinking. Lower-temperature-rated
thermal fuses may also require more heat sinking than do
higher-temperature-rated devices.
When designing a circuit and laying out the PCB, design
engineers must deal with all of these limitations and ambiguities
to add or design in thermal protection for a particular area of
interest on the PCB; be it a heat-generating device or the PCB
trace itself.
Mounting
TCOs
in
tight
areas
around
heat-generating
components on a PCB presents a number of issues. TCOs may
have to be fixed with a type of clamp, adding cost and limiting
2
Table 1: RTP200R060SA device’s absolute maximum ratings.
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In addition, the device helps lower assembly costs by eliminating
After arming, the device will open when the calibration
manual and special handling processes. Once it has been
temperature exceeds 200°C. Arming timing is user-determined
reflowed onto the PCB a simple arming procedure is all that
and can occur automatically at first device power up, during
is needed to ready the device so that it will activate at 200°C.
test, or in the field.
Figure 1 shows the reflow profile for the RTP device.
Figure 1. The RTP device can be reflowed at up to 260°C.
Table 2 describes the RTP reflow profile, which is compatible
with standard SMD reflow processes. (For a video describing
the reflow and arming process of the RTP device, go to: http://
www.youtube.com/watch?v=STJkEyDI0V0)
P1:
PTH:
ARM:
Power pin
Power and Thermal Sensing
Electronic Arming Pin
Figure 2. Reflow profile for the RTP device.
Table 2. Reflow profile for the RTP device.
When working with the RTP thermal protection device,
placement, calibration and testing are critical for proper
activation. Figure 2 shows the RTP device’s 3-pin functions.
Figure 3 illustrates the recommended pad layout. After
reflow, the arming procedure can be performed during PCB
inspection or after final inspection of the product/application.
3
Figure 3. Recommended pad layout for the RTP device.
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W H I T E PA P E R
APPLICATIONS FOR THE RTP DEVICE
with the primary thermal pin or heat sink of the FET or other
Applications that can benefit from the use of a surface mount
component’s heat sinking pin(s)/tab(s).
thermal protection device are those in which equipment must
be protected from failures due to thermal events generated by
failed power MOSFETs, power ICs, power capacitors and the
PCB vias and traces.
As shown in Figure 4, the RTP device can be placed in series
and in intimate thermal contact with the power FET to protect
the application against damage resulting from elevated FET
temperatures. The RTP device can also help protect any number
of heat-generating components.
Figure: 5. Thermal coupling of an RTP device.
ARMING THE RTP DEVICE
Figure 6 shows a sample arming option of an RTP device. Figure
7 shows the current path through the device is through pins P1
and PTH. The RTP device is armed by applying current through
the ARM pin either at power up or when performing final QC/
QA inspections on the PCB. Table 3 provides a definition of
terms.
Figure 4. The RTP device helps provide thermal protection in a power FET
application.
In this example the RTP device (model RTP200R060SA) is
rated to activate at 200°C. In service, the RTP device activates
to interrupt the current if FET failure results in overtemperature
conditions, opening before the solder melt point of 220°C.
This device can be used for automotive applications and can
Figure 6. Sample arming option.
be reliably incorporated and reflowed into areas of high-power
components on the PCB wherever thermal protection is needed
to help protect against damage caused by thermal runaway.
Figure 5 shows how proper
The RTP device can
be placed in series
and in intimate
thermal contact with
the power FET.
thermal coupling with the
RTP device can help protect
a
specific
component
or
application. Intimate thermal
contact with the potential
heat
to
source
achieve
is
critical
the
desired
performance. All RTP devices are designed with the expectation
that the PTH pin of the RTP shares a copper mounting pad
4
Figure 7. The current path through an RTP device.
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W H I T E PA P E R
Figures 8a and 8b show infrared tests of the RTP device and
PCB traces at 19A through a 2.0oz 70mil copper trace. By
placing the RTP device in the middle of the longest trace run,
the RTP activates at the calibrated temperature of 200°C.
Figure 8a shows the test before activation with a PCB temp
at 197°C. Figure 8b shows the device activating at 200.3°C,
removing current from the heated trace and allowing it to cool
down.
Table 3. Definition of terms.
PCB PROTECTION
Another area where thermal protection may be desirable is
the PCB trace itself. When PCB traces are electrically stressed,
hot-lines begin forming on the trace causing delamination. If
left unprotected, this can lead to a thermal event that ordinary
electrical fuses and leaded thermal fuses cannot prevent. The
RTP device gives the designer a greater degree of flexibility in
protecting against damage from thermal runaway events in all
types of harsh and vibration-prone applications. When properly
designed and thoroughly tested in the end-application, it helps
protect
traces
PCB
from
board
damage
caused by overheating.
For
instance,
ports
a
power
protected
by
user-accessible,
replaceable fuse can
be
stressed
Figure 8a. PCB temp at 197°C, prior to RTP device activation.
to
the
When properly designed
and thoroughly tested
in the end-application, it
helps protect PCB board
traces from damage
caused by overheating.
point where the trace
may be over-powered and break down thermally. This might
occur if the end user has replaced the electrical fuse with a
higher-current-rated fuse, which would stress the PCB board
trace and feed power through the connector.
TE Circuit Protection testing has shown that to help prevent
PCB trace overtemperature events the optimum location for
the RTP device is centered between two soldered components,
or vias, to the heat sinking pads. The center of a PCB trace
between soldered components is the most vulnerable and
sensitive area. Through thermal conduction, the RTP device
will activate at the desired trip temperature before a thermal
runaway event occurs.
5
Figure 8b. RTP device activates at 200.3°C and removes current from trace.
To reduce temperature rise in PCB traces, they should be
designed to handle specific power requirements. For example, if
the traces are to handle power delivered from an external power
source, the inadvertent use of a higher-rated power supply
could overwhelm the PCB trace characteristics and, through
I2R heating, cause trace temperature to rise significantly. TE
Circuit Protection testing showed that in a scenario where 19A
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was applied to a 2oz.-1.8mm wide trace, at 51mm (2 inches) in
To facilitate safer, more efficient PCB board design, the older
length, it could take up to 40 minutes for failure to occur, with
working standard IPC-D-275 was replaced with ANSI / IPC-
trace temperature approaching 200°C. The test was performed
2221/IPC-2221A design standards for PCB trace width. Also a
at 23°C ambient room temperature. When utilizing an RTP
track width calculator program that provides results based on
device during test, it activated at 200.3°C, before the board
the IPC graphs can be found at www.ultracad.com/calc.htm.
trace could form a hot-line at center-trace and cause the trace
to delaminate. (For a video about protecting PCB traces, please
For ANSI / IPC-2221/IPC-2221A, the trace width formulas
visit: http://www.youtube.com/watch?v=LHpZYVnZQ9s)
are:
In compact power applications, where designs are continually
shrinking, although power requirements remain the same or
Internal traces : I = 0.024 x dT0.44 x A0.725
External traces: I = 0.048 x dT0.44 x A0.725
higher, a critical design issue is to eliminate traditional heat sinks.
where:
With the increasing use of thermally-enhanced semiconductor
I
The RTP device can
help provide secondary
thermal runaway
protection.
= maximum current in Amps
packaging more focus is
dT = temperature rise above ambient in °C
put on the circuit board
A
itself to function as a
heat sink. Even when
designing
mount
in
thermal
surface
relief
pads to act as a heat sink,
the RTP device can help provide secondary thermal runaway
= cross-sectional area in mils2
The subtle difference in equations between IPC-2221(A)
and those in IPC-D-275 is that IPC-2221(A) is slightly more
conservative
and
implicitly
derated
to
compensate
for
manufacturing effects. As a rule of thumb, a 10°C temperature
rise in a trace is a safe limit to use in designing.
protection, if power is delivered by an improper, higher-rated
It should be noted that the RTP device is a thermal cutoff
power supply, or if the driven load goes into a low-impedance
device and is used to detect abnormal rises in temperature and
state, or short.
activate, or open, at the device’s calibrated trip temperature. The
When
laying
out
the
PCB
and
power-handling
traces,
calculating the current-handling capability for the PCB trace
widths is critical not only for managing power but for keeping
RTP device is not a current fuse and does not have consistent
trip times when exposed to overcurrent. However, what the part
can take is quantified in the interrupting current ratings.
thermal conduction and dissipation in check. This is especially
When working with thermal protection devices factors
true in compact, high-power designs where the use of heat
such as mounting location, mounting method or load currents
sinks is limited or impossible. The formulas below provide an
through a thermal protection device, which can cause
approximation for calculating board trace widths according
self-generating heat. The external surrounding temperature
to ANSI/IPC-D-275. Many PCB layout tools can also be set to
must also be accounted for in the design and test phase in
automatically calculate the size and amount of material (usually
order to mitigate nuisance tripping. Evaluating these factors
copper, in oz.) needed for a specific design.
will give the designer a greater degree of safety in applications
such as transformers, motors and other equipment that
For ANSI/IPC-D-275:
can overheat due to excessive load currents; or attributed
I = 0.0150 x dT 0.5453 x A0.7349 for internal traces
abnormal operating conditions such as overloading, locked
I = 0.0647 x dT0.4281 x A0.6732 for external traces
rotor, low operating voltages, overvoltage conditions; or
where:
even due to a loss of phase on a multiphase application such
I = maximum current in Amps
as a power supply or motor. Verification by repeated testing
dT = temperature rise above ambient in °C
of the final design under normal conditions, as well as using
A = cross-secti onal area of trace in mils
conservative estimates of maximum abnormal conditions,
2
is strongly recommended.
6
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W H I T E PA P E R
When working with the RTP device or any surface mount
specification or safety standard the application must meet; then
component, thermal matching to the application and especially
by the characteristics of the intended application (e.g., voltage,
to the PCB trace must also be considered. Side effects of thermal
current or temperature); and finally by the environment in
expansion mismatching between the PCB trace and the RTP device
which the equipment or application will operate (e.g., vibration,
may cause solder joint fracturing or failure when the assembly
humidity, temperature).
is subjected to high operating temperatures, thermal cycling,
thermal shock, and even power cycling. When designing in thermal
protection, it is wise to perform a series of tests not just locally,
where the protection device is located, but with the application
as a whole – especially when different types of enclosures are
employed.
Table 4 compares RTP device characteristics with other
technologies including traditional TCOs and PPTC (polymeric
positive temperature coefficient) devices and standard fuses.
Selecting a protection device should be based first on what
needs to be protected (i.e., the application as a whole or an
area of the application), next by the parameters of a particular
Table 4. Device technology comparison.
SUMMARY
The RTP device is a convenient, cost-effective alternative to traditional thermal protection devices. It allows use of standard surface mount
production methods, obviating the need for special assembly procedures and their associated costs. These characteristics make it suitable
for automotive applications such as HVAC, ABS, power steering modules, DC/DC converters and PTC heaters. The device also helps protect
power components in IT servers, telecom power, LED lighting systems and appliance electronics.
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RCP0108E 10/2014
TE Connectivity, TE connectivity (Logo) and TE (logo) are trademarks. Other logos, product and/or company
names might be trademarks of their respective owners.
While TE has made every reasonable effort to ensure the accuracy of the information in this brochure, TE does not guarantee that it is
error-free, nor does TE make any other representation, warranty or guarantee that the information is accurate, correct, reliable or current. TE
reserves the right to make any adjustments to the information contained herein at any time without notice. TE expressly disclaims all implied
warranties regarding the information contained herein, including, but not limited to, any implied warranties of merchantability or fitness for a
particular purpose. The dimensions in this catalog are for reference purposes only and are subject to change without notice. Specifications
are subject to change without notice. Consult TE for the latest dimensions and design specifications.
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