IEC62133 [LITTELFUSE]
Introduction to Li-ion Battery Technology; 介绍了锂离子电池技术
| 型号： | IEC62133 |
| 厂家： | 
LITTELFUSE |
| 描述： | Introduction to Li-ion Battery Technology |
| 文件： | 总8页 (文件大小：1914K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
This is why LIBs have several levels of fail-safe internal
Introduction to Li-ion Battery Technology
cell level and external protection circuitry, which shuts
down the battery pack when parameters go out of
range. The addition of this protection circuitry takes up
useful space in the battery pack and cell, thereby
reducing the available capacity. It also causes a small
current drain on the pack and contributes to potential
points of failure, which can permanently disable the cell
or pack.
Lithium-ion batteries (LIB) have now become part of
the standard battery pack of choice used in most
notebook, smartphone, e-reader, and tablet designs.
The LIB chemistry produces optimal characteristics
with regard to high energy density, low self-discharge,
light weight, long cycle life, lack of memory effect, and
low maintenance. LIBs are now gaining popularity in
other market segments such as electric vehicles,
power tools, and military/aerospace applications. Since
the technology was developed in the 1970s, LIBs have
improved dramatically in terms of energy density, cost,
durability, and safety.
Internal cell protection consists of a shut-down separator
(for over-temperature), tear-away tab (for internal
pressure), vent (pressure relief), and thermal interrupt
(over-current/over-charging) (Figure 1).
Current
Collector
Sealing Plate
The three main functional components in a lithium-ion
battery cell are the anode (typically graphite), the
cathode (typically lithium cobalt oxide), and a non-aque-
ous electrolyte (typically a lithium salt or organic
solvent containing complexes of lithium ions). The
material choices affect a cell’s voltage, capacity, life,
and safety.
Vent Plate
Positive Cap
Negative Teminal
Gasket
Gas Release Vent
Current Interrupt Device
Insulation Plate
Spacer
Positive
PTC
Tab
Device
Separator
Sealing Cap
Inlet
Gasket
Separator
Positive
Electrode
Insulator
Positive
Electrode
Negative
Electrode
Casing
Negative
Tab
Case
(Positive Polarity)
Li-ion cells are available in a cylindrical solid body,
prismatic semi-hard plastic/metal case, or pouch form,
which is also called Li-polymer. Although pouch cells
and prismatics have the highest energy density, they
require some external means of containment to
prevent an explosion when their State of Charge (SOC)
is high (see Figure 1).
Anode tab
Cathode tab
Negative
Tab
Separator
Top insulator
Anode
Tab Sealant
Positive Tab
Positive
Negative
Electrode
Cathode
Electrode
Tab Sealing
Area
Barcode
Al laminate ꢀlm
Side Folding
Overheating is the main safety concern for lithium-ion
cells. Overheating causes thermal runaway of the
cells, which can lead to cell rupture, ꢀre, or explosion.
A deep discharge event could cause internal shorts in
the cell, which would cause a short circuit upon
charging.
Laminated
Foil
Figure 1. Various Li-ion cell conꢀgurations
Over-charging and deep discharge/short-circuit events
create heat (generated by the anode of the cell) and
oxygen (created by the cathode). Both of these effects
can be dangerous to the cell and cause bloating (in the
case of Li-polymer pouch cells), rupture, ꢀre, or even
an explosion.
Over the last ꢀve years, LIBs have been the subject of
highly publicized recalls of notebook and cell phone
battery packs, as a result of instances of overheating,
ꢀre, and rupture. Several new standards from IEC, UL,
and the DOT/UN have emerged to specify required
safety measures and testing.
©2012 Littelfuse, Inc
1
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
●
Li-ion and Li-ion polymer chemistry has speciꢀc energy
IEC 62133:2002, Secondary cells and batteries
containing alkaline or other non-acid electrolytes—Safety
requirements for portable sealed secondary cells, and
for batteries made from them, for use in portable
applications.
of 400Wh/L at 20°C, which is approximately two times
the speciꢀc energy of NiMH (nickel metal hydride) and
four times that of the old NiCd (nickel cadmium)
chemistry. Li-ion chemistry also operates at higher
voltages of 3.0–4.2V versus 1.0–1.2V for the older
chemistries. The older chemistries had a moderate-to-
high tolerance to over-charging events, whereas the
newer Li-ion chemistry has a very low tolerance to
over-charging
●
IEC 62281, Safety of primary and secondary lithium
cells and batteries during transport—These
requirements cover portable primary
(non-rechargeable) and secondary (rechargeable)
batteries for use as power sources in products.
There are a variety of reasons for battery pack failures:
poorly designed cells, lack of over-current/over-voltage
protection, lack of thermal protection, no tolerance to
swelling, no venting methods for gas, and use in high
temperature environments.
●
UL 2054, Standard for Household and Commercial
Batteries—These requirements are intended to reduce
the risk of ꢀre or explosion when batteries are used in
a product.
●
Over-discharge and over-charge are two externally
created events that can cause problems in LIBs. During
over-discharge, if the cell voltage drops lower than
approximately 1.5V, gas will be produced at the anode.
When voltage drops to less than 1V, copper from the
current collector dissolves, causing internal shorting of
the cell. Therefore, under-voltage protection is required
and is provided by the battery protection IC. Over-charge
creates gassing and heat buildup at the cathode when
cell voltage reaches approximately 4.6V. Although
cylindrical cells have internal protection from pressure,
activated CIDs (current interrupt devices) and internal
PTCs (positive temperature coefꢀcient discs that
increase in resistance when heated), Li-polymer cells
do not have internal CIDs and PTCs. External over-
voltage, over-gas, and over-temperature protection is
especially critical for Li-polymer cells
UN/DOT (Dept of Transportation) Manual of Tests and
Criteria 4th Revised Edition Lithium Battery Testing
Requirements – Sec 38.3.
●
●
●
IEEE 1625 - IEEE Standard for Rechargeable Batteries
for Multi-Cell Mobile Computing Devices
IEEE 1725 - IEEE Standard for Rechargeable Batteries
for Cellular Telephones
IEC/UL 60950-1, Information Technology Equipment
Safety—Limited Power Source, Sec 2.5, Table 2B,
requirements to limit current to less than 8A within
5sec ; this speciꢀcation would apply to most battery
systems used for notebook computers, cell phones,
and tablet devices.
These standards guide manufacturers/suppliers in
planning and implementing the controls for the design
and manufacture of lithium-ion (Li-ion) and lithium-ion
polymer (Li-ion polymer) rechargeable battery packs.
Li-ion Battery Safety Standards
Several safety agency standards apply to lithium-ion
battery packs. These are the key standards that govern
the performance, safety testing, and transportation of
lithium-ion battery packs:
The typical safety-related tests in these standards,
which involve the use of external and internal battery
pack protection, will include the following (standards will
each have their own speciꢀc requirements and this is
just a brief summary of the types of tests conducted):
●
UL 1642-2005, Standard for Lithium
Batteries—Requirements are intended to reduce the
risk of ꢀre or explosion when lithium batteries are
used in a product.
©2012 Littelfuse, Inc
2
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
1. Short-Circuit tests and Forced Discharge tests:
temperature cut-outs (TCO) to sense temperature,
current sense resistors to monitor current, gas gauges
to monitor gas buildup, and fuel gauges to monitor
charge. Upon any unsafe condition, the IC will turn the
FETs off to shut down the pack and stop the fault
event. Because the Li-ion chemistry is so dangerous in
certain conditions, there must be a secondary method
for protection. This secondary protector can be a PPTC
(polymeric positive temperature coefꢀcient) resettable
fuse, thermal fuse, or a controllable battery protector
(see Figure 3).
These tests are conducted by discharging the
battery with a low resistance load and then allowing
the battery to protect itself or fail by ꢀre or
explosion; the latter being a test failure. A test pass
is when battery returns to a safe temperature. Tests
are done at room temperature and elevated
temperatures.
2. Abnormal Charging test, Overcharging test, High
Charging Rate test: These tests are conducted by
subjecting the battery pack to several times more
than the normal charging current or charging at an
abnormally fast rate. When there is a non-resettable
over-current device present, the test is repeated at
a current below which the device activates.
3. Heating and Temperature Cycling tests. These tests
are conducted by raising and cycling the battery
pack to high temperature and then checking to see
if the pack responds safely. Fire, explosion, and
venting would be considered failures.
Battery
cell
The purpose of the safety standards is to ensure the
battery pack and cells have protection mechanisms
designed into the overall system to prevent rapid
thermal runaway, ꢀre, explosion, rupture, venting, or
even gas bloating of the battery packs. All of these
events can create a hazard to the user or any equip-
ment used with the battery pack.
PCM
Figure 2. A typical Battery Management Unit (BMU) design
Typical Li-ion and lithium-polymer battery packs have
several levels of protection in order to meet the
required safety standards and to protect the user and
equipment from battery failure hazards. In addition to
internal cell level protection, external protection
solutions are added to provide further safety mea-
sures. Some battery packs will use what is called a
Battery Management Unit (BMU), which is a small
print circuit board with several protection components
(see Figure 2). The BMU will have a central processing
device, which is usually an IC that controls the battery
charge and monitors the pack for unsafe conditions.
The battery controller IC controls two FETs, which act
as the charge and discharge switches. The battery IC
will turn these FETs off as the primary way to shut
down the battery pack. The IC will use thermistors and
Battery Pack
SMD PTC
+
Switch
Switch
Charge
Discharge
Control IC
Battery
Cell
–
Figure 3. A secondary method of battery protection
©2012 Littelfuse, Inc
3
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
General Safety Standard that Applies to
Introduction to PPTC Technology
Smartphones and Tablets
PTC stands for Positive Temperature Coefꢀcient,
which means the resistance of the device increases as
its temperature goes up. PTCs increase in resistance
as temperature increases due to increased current
ꢁow. Polymer PPTC (PPTC) devices are made of a
polymer plastic material. Unlike a typical “one-time”
fuse, a PPTC device (see Figure 4) will reset when
cooled.
IEC/UL/EN 60950-1 - Information Technology Equip-
ment Safety, Part 1: General Requirement
●
The standard applies to battery operated devices that
can be charged from AC mains supply.
●
Sec 2.5 – Limited Power Source
– Fire enclosure requirements in 4.7.2 are reduced or
not required if the components/connectors are
connected to a Limited Power Source.
– This allows designer to reduce cost, use thinner
materials, etc.
How a PPTC Works
– Limited Power Source spec has two tables:
●
Table 2B – no OC protective device (so using PTC or
electronic fuse)
Carbon
Crystalline Polymer
Carbon
Amorphous
– Must limit current to 8A within 5 sec if using PTC
Under Fault Condition
Under Normal Operation
■
■
Excessive current causes
●
At operating current
Table 2C – OC protective device is used (fuse)
– Fuse rating 5A or less (210% / 120sec overload
gate)
– Limit Short ckt current to less than 1000 / Vmax
and 250VA within 60sec
device to heat up
■
Many conductive paths
■
Fewer conductive paths
■
Very low resistance
■
Result is high resistance
■
Cools down and resets
when fault is removed
●
Where an overcurrent protective device is used, it
shall be a fuse or a non-adjustable, non-autorest,
electromechanical device
Figure 4. How a PPTC works
Agency Approvals: Littelfuse PPTCs are recognized
under the Component Program of Underwriters
Laboratories to UL Standard 1434 for Thermistors. The
devices have also been certiꢀed under the CSA Com-
ponent Acceptance Program.
RPTC
I
Voltage
Source
Load
Resistance
V
RL
Figure 5. PPTC resistors
©2012 Littelfuse, Inc
4
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
PPTC trip times are inꢁuenced by:
●
Resistance of the device
●
Ambient temperature and air currents
●
PCB trace size and copper weight
●
Proximity of other components
Other items that inꢁuence the effective heat transfer
rate from the device to its surroundings can also
impact performance.
Trip Point
Fault
Current
Temperature (°C)
Normal
Operating
Current
Normal
Operating
Current
Figure 6. The effect of temperature on the resistance of a PPTC
PPTC resistors are over-current protection devices. Like
fuses, they have two terminals and are placed in line
with the circuit being protected (see Figure 5). Because
they are ideal for situations where frequent over-current
conditions occur or constant uptime is required, PPTCs
are typically used in Li-ion battery pack applications. In
order to limit unsafe currents while allowing constant
safe current levels, their resistance will “reset” auto-
matically when the fault is removed and temperature
returns to a safe level.
Leakage Power
Current
Down
for
Reset
Time
Figure 7. The effect of changing current levels on a
PPTC’s temperature and resistance
Time Current (TC) curves present the average values
of the trip time at a given current for every part
number (see Figure 8). PPTC trip times will be distrib-
uted above and below the curve. Lower percentage
overloads produce greater variations in trip time.
Customer veriꢀcation tests need to be done for actual
applications to ensure proper component selection.
Under normal conditions, PPTCs act as a low value
resistor – dissipating little power and barely warm.
Under fault conditions, they heat up due to I2R (Ohmic
heating; >100oC) and their resistance increases 1000X
or more, limiting the current to a small value (see Figure
6). When the current is removed, the PPTC will return
to normal temperature and resistance, restoring the
circuit (see Figure 7).
©2012 Littelfuse, Inc
5
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
170%
150%
130%
100
10
110%
90%
70%
50%
30%
10%
1
-40 -30 -20 -10
0
10
20 30
40 50 60 70 80
Temperature (°C)
0.1
Figure 9. A PTC (or PPTC) de-rating chart
Key Considerations in Selecting a PPTC
1. Determine the circuit’s operating parameters:
0.01
0.001
●
Application temperature
●
Hold current requirement
1
10
100
1000
2. Select the PPTC device that will accommodate the
circuit’s maximum ambient temperature and normal
operating current.
Current in Amperes
Figure 8. Time Current (TC) curves
●
Compare the selected device’s maximum
Resistance of the PPTC device changes directly with
temperature. The rating of the PPTC is inꢁuenced by
ambient temperature, as shown at the temperature
de-rating chart (Figure 9). The heat required to trip the
device may come from several sources, such as:
electrical ratings with the circuit’s maximum
operating voltage and interrupt current.
●
Determine time-to-trip.
●
Verify ambient operating conditions.
●
●
Resistive heating from the electrical current
Verify the PPTC device dimensions.
●
Ambient environment
3. PPTC resistances do shift during operation.
●
Adjacent components
●
Repetitive tripping/reset cycles will cause slight
changes in resistance.
©2012 Littelfuse, Inc
6
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
The use of a LoRhoTM SMT PPTC in a Li-ion
polymer battery pack
●
●
Holding the device in a tripped state for long
periods of time will cause an increase in resistance.
Any hand soldering may have a signiꢀcant effect if
not performed properly and therefore is generally
not recommended (particularly for SMT
components)
The SMT form factor is optimized for Li-ion polymer
cell pack construction. Li-ion polymer packs are used
for low proꢀle smartphone and tablet applications. The
SMT PPTC allows for more efꢀcient high volume
manufacturing because the PPTC can be surface
mounted directly on the PCM (Protection Circuit
Module). Also, in low proꢀle designs, the PCM board
is mounted ꢁat in the same plane as the polymer
pouch. This allows for the lowest proꢀle thickness of
the smartphone or tablet design. The low height
capability of the LoRhoTM SMT PPTC makes it a great
choice for this application. The PPTC can be added to
the PCM while maintaining a minimum height proꢀle
of the total assembly.
4. PPTC devices have two distinct resistance ranges:
●
RMIN: the minimum resistance of un-soldered
devices
●
R1MAX: the maximum resistance of a device at
20°C, measured one hour after tripping or reꢁow
soldering at 260°C for 20 seconds.
When measuring resistance:
●
When using LoRhoTM SMT PPTCs, some design and
application testing aspects must be considered:
Always perform the measurement at room
temperature.
●
The overall space allocated for SMT PPTC needs to
●
Perform measurements at least one hour after
be balanced against the total hold current required.
The designer ꢀrst needs to determine the maximum
continuous current (this includes burst use or peak
current use) and the maximum temperature that the
PPTC can experience (ambient temperature in the
PPTC’s vicinity).
any heating process to ensure that the device has
cooled thoroughly (soldering, testing, etc.).
●
Keep in mind that catalog speciꢀcations (trip time,
hold current, etc.) assume the parts have been
mounted on a PCB and the resistance shift has
already occurred.
●
So, the ꢀrst step in selecting the appropriate PPTC is
ꢀnding out how much current the device can hold at
the maximum temperature. The advantage of using a
LoRhoTM PPTC is that the device can hold a large
amount of current in a relatively small form factor
and low proꢀle device.
Other critical considerations for component selection
include:
●
Maximum circuit voltage
●
Maximum available short circuit current
●
Application testing is suggested to verify that the
●
Desired trip current and trip time
selected PPTC can hold the required current at
temperature. Typically, the device is subjected to the
required current for 15 minutes, which is enough
time to reach thermal stability. If device can hold the
current for 15 minutes, then this is one data point
that can be used to verify the correct device has
been selected.
●
Form factor
●
Maximum ambient operating temperature
●
Normal operating current
●
Maximum operating voltage
●
Maximum interrupt current
©2012 Littelfuse, Inc
7
Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
●
It is highly suggested that the application testing be
References
completed on the actual PCM board as the thermal
effects can vary quite signiꢀcantly when compared
to a test board. The printed circuit board, traces,
solder pads, and even adjacent components all have
an effect on the thermal performance of the PPTC.
IEC/UL/EN 60950-1 - Information Technology Equip-
ment Safety, Part 1: General Requirement
●
The next step is to ensure the PPTC will trip or
activate fast enough to meet safety requirements.
Typically, smartphones and smaller tablets will need
to meet the standards in UL/IEC 60950-1, Limited
Power Source Sec 2.5. This standard will require the
PPTC to trip in ꢀve seconds or less during an 8A fault
condition. In other words, the PPTC must limit
current to 8A or less in ꢀve seconds or less.
●
The ꢀnal critical aspect is the impedance of the PPTC
device. The higher the impedance (PPTC are purely
resistive components), the greater the drain on the
battery and the lower the total capacity or battery
“talk-time” energy available. Therefore, minimizing
the impedance is critical. The main advantage of
LoRhoTM PPTCs is their very low resistance compared
to standard PPTCs. However, all PPTCs undergo
what is called “trip jump” after experiencing a
thermal event or short circuit event. Trip jump is a
permanent increase in resistance from initial
resistance as delivered on tape/reel to post-reꢁow
resistance. Therefore, it is very important to
determine the typical and maximum trip jump
associated with a given device and the process that
it undergoes in manufacturing.
●
The true worst-case maximum resistance of the
PPTC needs to be determined. The suggested
application test is for the PPTC to be reꢁowed onto
the actual PCM using the reꢁow oven proꢀle
selected for the assembly. If there are multiple
reꢁow passes and any dwell or cure times, then
these should be applied as well. The PPTC
resistance should then be measured one hour after
the reꢁow process by using the four-wire method.
©2012 Littelfuse, Inc
8
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