Application Note: Use Of Low Resistivity Surface Mount .
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery PacksIntroduction to Li-ion Battery TechnologyLithium-ion batteries (LIB) have now become part ofthe standard battery pack of choice used in mostnotebook, smartphone, e-reader, and tablet designs.The LIB chemistry produces optimal characteristicswith regard to high energy density, low self-discharge,light weight, long cycle life, lack of memory effect, andlow maintenance. LIBs are now gaining popularity inother market segments such as electric vehicles,power tools, and military/aerospace applications. Sincethe technology was developed in the 1970s, LIBs haveimproved dramatically in terms of energy density, cost,durability, and safety.The three main functional components in a lithium-ionbattery cell are the anode (typically graphite), thecathode (typically lithium cobalt oxide), and a non-aqueous electrolyte (typically a lithium salt or organicsolvent containing complexes of lithium ions). Thematerial choices affect a cell’s voltage, capacity, life,and safety.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 cellsand prismatics have the highest energy density, theyrequire some external means of containment toprevent an explosion when their State of Charge (SOC)is high (see Figure 1).This is why LIBs have several levels of fail-safe internalcell level and external protection circuitry, which shutsdown the battery pack when parameters go out ofrange. The addition of this protection circuitry takes upuseful space in the battery pack and cell, therebyreducing the available capacity. It also causes a smallcurrent drain on the pack and contributes to potentialpoints of failure, which can permanently disable the cellor pack.Internal cell protection consists of a shut-down separator(for over-temperature), tear-away tab (for internalpressure), vent (pressure relief), and thermal interrupt(over-current/over-charging) (Figure 1).Positive CapPTCDeviceGasketVent PlateCurrent Interrupt orNegative TeminalSealing PlateGas Release VentInsulation PlateGasketSpacerSealing eCasingNegativeTabCathode tabCurrentCollectorNegativeElectrodeCase(Positive Polarity)Anode tabNegativeTabSeparatorTop insulatorCathodePositiveElectrodeTab SealantNegativeElectrodePositive TabTab SealingAreaAnodeBarcodeAl laminate filmOverheating is the main safety concern for lithium-ioncells. Overheating causes thermal runaway of thecells, which can lead to cell rupture, fire, or explosion.A deep discharge event could cause internal shorts inthe cell, which would cause a short circuit uponcharging.Over-charging and deep discharge/short-circuit eventscreate heat (generated by the anode of the cell) andoxygen (created by the cathode). Both of these effectscan be dangerous to the cell and cause bloating (in thecase of Li-polymer pouch cells), rupture, fire, or evenan explosion. 2012 Littelfuse, IncSide FoldingLaminatedFoilFigure 1. Various Li-ion cell configurationsOver the last five years, LIBs have been the subject ofhighly publicized recalls of notebook and cell phonebattery packs, as a result of instances of overheating,fire, and rupture. Several new standards from IEC, UL,and the DOT/UN have emerged to specify requiredsafety measures and testing.1
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery PacksLi-ion and Li-ion polymer chemistry has specific energyof 400Wh/L at 20 C, which is approximately two timesthe specific energy of NiMH (nickel metal hydride) andfour times that of the old NiCd (nickel cadmium)chemistry. Li-ion chemistry also operates at highervoltages of 3.0–4.2V versus 1.0–1.2V for the olderchemistries. The older chemistries had a moderate-tohigh tolerance to over-charging events, whereas thenewer Li-ion chemistry has a very low tolerance toover-chargingThere are a variety of reasons for battery pack failures:poorly designed cells, lack of over-current/over-voltageprotection, lack of thermal protection, no tolerance toswelling, no venting methods for gas, and use in hightemperature environments.Over-discharge and over-charge are two externallycreated events that can cause problems in LIBs. Duringover-discharge, if the cell voltage drops lower thanapproximately 1.5V, gas will be produced at the anode.When voltage drops to less than 1V, copper from thecurrent collector dissolves, causing internal shorting ofthe cell. Therefore, under-voltage protection is requiredand is provided by the battery protection IC. Over-chargecreates gassing and heat buildup at the cathode whencell voltage reaches approximately 4.6V. Althoughcylindrical cells have internal protection from pressure,activated CIDs (current interrupt devices) and internalPTCs (positive temperature coefficient discs thatincrease in resistance when heated), Li-polymer cellsdo not have internal CIDs and PTCs. External overvoltage, over-gas, and over-temperature protection isespecially critical for Li-polymer cellsLi-ion Battery Safety StandardsSeveral safety agency standards apply to lithium-ionbattery packs. These are the key standards that governthe performance, safety testing, and transportation oflithium-ion battery packs: UL 1642-2005, Standard for LithiumBatteries—Requirements are intended to reduce therisk of fire or explosion when lithium batteries areused in a product. 2012 Littelfuse, Inc IEC 62133:2002, Secondary cells and batteriescontaining alkaline or other non-acid electrolytes—Safetyrequirements for portable sealed secondary cells, andfor batteries made from them, for use in portableapplications.IEC 62281, Safety of primary and secondary lithiumcells and batteries during transport—Theserequirements cover portable primary(non-rechargeable) and secondary (rechargeable)batteries for use as power sources in products.UL 2054, Standard for Household and CommercialBatteries—These requirements are intended to reducethe risk of fire or explosion when batteries are used ina product.UN/DOT (Dept of Transportation) Manual of Tests andCriteria 4th Revised Edition Lithium Battery TestingRequirements – Sec 38.3.IEEE 1625 - IEEE Standard for Rechargeable Batteriesfor Multi-Cell Mobile Computing DevicesIEEE 1725 - IEEE Standard for Rechargeable Batteriesfor Cellular TelephonesIEC/UL 60950-1, Information Technology EquipmentSafety—Limited Power Source, Sec 2.5, Table 2B,requirements to limit current to less than 8A within5sec ; this specification would apply to most batterysystems used for notebook computers, cell phones,and tablet devices.These standards guide manufacturers/suppliers inplanning and implementing the controls for the designand manufacture of lithium-ion (Li-ion) and lithium-ionpolymer (Li-ion polymer) rechargeable battery packs.The typical safety-related tests in these standards,which involve the use of external and internal batterypack protection, will include the following (standards willeach have their own specific requirements and this isjust a brief summary of the types of tests conducted):2
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery Packs1. Short-Circuit tests and Forced Discharge tests:These tests are conducted by discharging thebattery with a low resistance load and then allowingthe battery to protect itself or fail by fire orexplosion; the latter being a test failure. A test passis when battery returns to a safe temperature. Testsare done at room temperature and elevatedtemperatures.2. Abnormal Charging test, Overcharging test, HighCharging Rate test: These tests are conducted bysubjecting the battery pack to several times morethan the normal charging current or charging at anabnormally fast rate. When there is a non-resettableover-current device present, the test is repeated ata current below which the device activates.3. Heating and Temperature Cycling tests. These testsare conducted by raising and cycling the batterypack to high temperature and then checking to seeif the pack responds safely. Fire, explosion, andventing would be considered failures.The purpose of the safety standards is to ensure thebattery pack and cells have protection mechanismsdesigned into the overall system to prevent rapidthermal runaway, fire, explosion, rupture, venting, oreven gas bloating of the battery packs. All of theseevents can create a hazard to the user or any equipment used with the battery pack.Typical Li-ion and lithium-polymer battery packs haveseveral levels of protection in order to meet therequired safety standards and to protect the user andequipment from battery failure hazards. In addition tointernal cell level protection, external protectionsolutions are added to provide further safety measures. Some battery packs will use what is called aBattery Management Unit (BMU), which is a smallprint circuit board with several protection components(see Figure 2). The BMU will have a central processingdevice, which is usually an IC that controls the batterycharge and monitors the pack for unsafe conditions.The battery controller IC controls two FETs, which actas the charge and discharge switches. The battery ICwill turn these FETs off as the primary way to shutdown the battery pack. The IC will use thermistors and 2012 Littelfuse, Inctemperature cut-outs (TCO) to sense temperature,current sense resistors to monitor current, gas gaugesto monitor gas buildup, and fuel gauges to monitorcharge. Upon any unsafe condition, the IC will turn theFETs off to shut down the pack and stop the faultevent. Because the Li-ion chemistry is so dangerous incertain conditions, there must be a secondary methodfor protection. This secondary protector can be a PPTC(polymeric positive temperature coefficient) resettablefuse, thermal fuse, or a controllable battery protector(see Figure 3).BatterycellPCMFigure 2. A typical Battery Management Unit (BMU) designBattery PackSMD PTCSwitchSwitchDischargeCharge Control ICBatteryCell–Figure 3. A secondary method of battery protection3
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery PacksGeneral Safety Standard that Applies toSmartphones and TabletsIEC/UL/EN 60950-1 - Information Technology Equipment Safety, Part 1: General Requirement The standard applies to battery operated devices thatcan be charged from AC mains supply.Sec 2.5 – Limited Power Source– Fire enclosure requirements in 4.7.2 are reduced ornot required if the components/connectors areconnected to a Limited Power Source.– This allows designer to reduce cost, use thinnermaterials, etc.– Limited Power Source spec has two tables:Table 2B – no OC protective device (so using PTC orelectronic fuse)– Must limit current to 8A within 5 sec if using PTCTable 2C – OC protective device is used (fuse)– Fuse rating 5A or less (210% / 120sec overloadgate)– Limit Short ckt current to less than 1000 / Vmaxand 250VA within 60secIntroduction to PPTC TechnologyPTC stands for Positive Temperature Coefficient,which means the resistance of the device increases asits temperature goes up. PTCs increase in resistanceas temperature increases due to increased currentflow. Polymer PPTC (PPTC) devices are made of apolymer plastic material. Unlike a typical “one-time”fuse, a PPTC device (see Figure 4) will reset whencooled.How a PPTC WorksCarbonCarbonAmorphousUnder Fault Condition Excessive current causesdevice to heat up Fewer conductive paths Result is high resistance Cools down and resetswhen fault is removedUnder Normal Operation At operating current Many conductive paths Very low resistanceWhere an overcurrent protective device is used, itshall be a fuse or a non-adjustable, non-autorest,electromechanical deviceAgency Approvals: Littelfuse PPTCs are recognizedunder the Component Program of UnderwritersLaboratories to UL Standard 1434 for Thermistors. Thedevices have also been certified under the CSA Component Acceptance Program.Crystalline PolymerFigure 4. How a PPTC worksIVoltageSourceVRPTCRLLoadResistanceFigure 5. PPTC resistors 2012 Littelfuse, Inc4
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery PacksLog Resistance (Ohms)PPTC trip times are influenced by: Resistance of the device Ambient temperature and air currents PCB trace size and copper weight Proximity of other componentsOther items that influence the effective heat transferrate from the device to its surroundings can alsoimpact performance.Trip PointFaultCurrentFigure 6. The effect of temperature on the resistance of a PPTCPPTC resistors are over-current protection devices. Likefuses, they have two terminals and are placed in linewith the circuit being protected (see Figure 5). Becausethey are ideal for situations where frequent over-currentconditions occur or constant uptime is required, PPTCsare typically used in Li-ion battery pack applications. Inorder to limit unsafe currents while allowing constantsafe current levels, their resistance will “reset” automatically when the fault is removed and temperaturereturns to a safe level.Under normal conditions, PPTCs act as a low valueresistor – dissipating little power and barely warm.Under fault conditions, they heat up due to I2R (Ohmicheating; 100oC) and their resistance increases 1000Xor more, limiting the current to a small value (see Figure6). When the current is removed, the PPTC will returnto normal temperature and resistance, restoring thecircuit (see Figure 7). 2012 Littelfuse, IncCurrentTemperature ( ageCurrentPowerDownforResetTimeFigure 7. The effect of changing current levels on aPPTC’s temperature and resistanceTime Current (TC) curves present the average valuesof the trip time at a given current for every partnumber (see Figure 8). PPTC trip times will be distributed above and below the curve. Lower percentageoverloads produce greater variations in trip time.Customer verification tests need to be done for actualapplications to ensure proper component selection.5
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery 170%150%Percentage of Rated Current100Time in Seconds10130%110%90%70%50%30%110%-400.1-30 -20-1001020 3040Temperature ( C)50607080Figure 9. A PTC (or PPTC) de-rating chartKey Considerations in Selecting a PPTC0.010.0011. Determine the circuit’s operating parameters:1101001000Current in AmperesFigure 8. Time Current (TC) curves Application temperature Hold current requirement2. Select the PPTC device that will accommodate thecircuit’s maximum ambient temperature and normaloperating current. Resistance of the PPTC device changes directly withtemperature. The rating of the PPTC is influenced byambient temperature, as shown at the temperaturede-rating chart (Figure 9). The heat required to trip thedevice may come from several sources, such as: Resistive heating from the electrical current Ambient environment Adjacent components 2012 Littelfuse, IncCompare the selected device’s maximumelectrical ratings with the circuit’s maximumoperating voltage and interrupt current. Determine time-to-trip. Verify ambient operating conditions. Verify the PPTC device dimensions.3. PPTC resistances do shift during operation. Repetitive tripping/reset cycles will cause slightchanges in resistance.6
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery Packs Holding the device in a tripped state for longperiods of time will cause an increase in resistance.The use of a LoRhoTM SMT PPTC in a Li-ionpolymer battery packAny hand soldering may have a significant effect ifnot performed properly and therefore is generallynot recommended (particularly for SMTcomponents)The SMT form factor is optimized for Li-ion polymercell pack construction. Li-ion polymer packs are usedfor low profile smartphone and tablet applications. TheSMT PPTC allows for more efficient high volumemanufacturing because the PPTC can be surfacemounted directly on the PCM (Protection CircuitModule). Also, in low profile designs, the PCM boardis mounted flat in the same plane as the polymerpouch. This allows for the lowest profile thickness ofthe smartphone or tablet design. The low heightcapability of the LoRhoTM SMT PPTC makes it a greatchoice for this application. The PPTC can be added tothe PCM while maintaining a minimum height profileof the total assembly.4. PPTC devices have two distinct resistance ranges: RMIN: the minimum resistance of un-soldereddevicesR1MAX: the maximum resistance of a device at20 C, measured one hour after tripping or reflowsoldering at 260 C for 20 seconds.When measuring resistance: Always perform the measurement at roomtemperature.Perform measurements at least one hour afterany heating process to ensure that the device hascooled thoroughly (soldering, testing, etc.).Keep in mind that catalog specifications (trip time,hold current, etc.) assume the parts have beenmounted on a PCB and the resistance shift hasalready occurred.When using LoRhoTM SMT PPTCs, some design andapplication testing aspects must be considered: Other critical considerations for component selectioninclude: Maximum circuit voltage Maximum available short circuit current Desired trip current and trip time Form factor Maximum ambient operating temperature Normal operating current Maximum operating voltage Maximum interrupt current 2012 Littelfuse, Inc The overall space allocated for SMT PPTC needs tobe balanced against the total hold current required.The designer first needs to determine the maximumcontinuous current (this includes burst use or peakcurrent use) and the maximum temperature that thePPTC can experience (ambient temperature in thePPTC’s vicinity).So, the first step in selecting the appropriate PPTC isfinding out how much current the device can hold atthe maximum temperature. The advantage of using aLoRhoTM PPTC is that the device can hold a largeamount of current in a relatively small form factorand low profile device.Application testing is suggested to verify that theselected PPTC can hold the required current attemperature. Typically, the device is subjected to therequired current for 15 minutes, which is enoughtime to reach thermal stability. If device can hold thecurrent for 15 minutes, then this is one data pointthat can be used to verify the correct device hasbeen selected.7
Application Note:Use of Low Resistivity Surface Mount PPTCin Li-ion Polymer Battery Packs It is highly suggested that the application testing becompleted on the actual PCM board as the thermaleffects can vary quite significantly when comparedto a test board. The printed circuit board, traces,solder pads, and even adjacent components all havean effect on the thermal performance of the PPTC.ReferencesIEC/UL/EN 60950-1 - Information Technology Equipment Safety, Part 1: General RequirementThe next step is to ensure the PPTC will trip oractivate fast enough to meet safety requirements.Typically, smartphones and smaller tablets will needto meet the standards in UL/IEC 60950-1, LimitedPower Source Sec 2.5. This standard will require thePPTC to trip in five seconds or less during an 8A faultcondition. In other words, the PPTC must limitcurrent to 8A or less in five seconds or less.The final critical aspect is the impedance of the PPTCdevice. The higher the impedance (PPTC are purelyresistive components), the greater the drain on thebattery and the lower the total capacity or battery“talk-time” energy available. Therefore, minimizingthe impedance is critical. The main advantage ofLoRhoTM PPTCs is their very low resistance comparedto standard PPTCs. However, all PPTCs undergowhat is called “trip jump” after experiencing athermal event or short circuit event. Trip jump is apermanent increase in resistance from initialresistance as delivered on tap
lithium-ion battery packs: 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. IEC 62133:2002, Secondary cells and batteries
One approach we can use is to play the Am pentatonic scale, but use techniques to alter the C note. For instance, you can bend the C note up to the C# note using a blues bend. In this way, the C note becomes a ‘passing note’ instead of a ‘target note’, since we don’t stay on this C note but use it in a way to get to the C# note.
Application Note SiC Power Devices and Modules Application Note Rev.003 Note: The evaluation data and other information described in this application note are the results of evaluation by ROHM under
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