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Posted on 01 December 2019

MOSFET based Battery Protection Systems

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Board real estate is critical in portable equipment

The proliferation of portable devices within the connected-consumer age places ever-increasing demands on both efficient power management as well as battery protection that guarantees safety through the life-time of the product.

By Ashfaq Afzal, Product Marketing Manager, NXP Semiconductors and Des Beckford, Senior Applications Engineer, NXP Semiconductors

 

Portable products are fuelling the race towards more sophisticated functionality in smaller form factors leading to ever increasing demands on power densities of on board power converters and batteries. Important factors for the battery are system run time on a single charge, the time taken to recharge, low self discharge and the number of charge discharge cycles it is capable of before it comes to the end of its useful life. The proliferation of portable devices has reinforced the need for small batteries with high volumetric and gravimetric energy densities. This has led to lithium-ion and lithiumpolymer becoming the most popular battery chemistries.

The increasing problems concerning the recall of Li Ion battery packs by battery suppliers has highlighted the need for protection measures within the packs. In a correctly designed battery pack there are a number of levels of protection. Some are within the individual cells and the others form part of the battery protection circuit that protects the battery pack as a whole. The requirement for high energy density in mobile computing applications means that parallel/series combinations of Lithium Ion cells are used. The preferred chemistry uses a cobalt based positive electrode to maximise energy density but this is achieved at the expense of safety of the battery.

Within the cell a separator membrane is designed to perform a reset-table over temperature function. Other forms of protection within each cell are pressure relief vents that acts to relieve minor over pressure within the cell, a non reset-table over pressure cut out that permanently open circuits the battery in the case of extreme over pressure and a self reseting thermal interrupt to prevent overcurrent or overcharging. Contamination within the cells when being manfactured can cause some of these safety mechanisms to become inoperative. The recent recalls of Lithium Ion batteries were due contamination issues.

The major difference in the construction of a Lithium Ion Prismatic cell and a Cylindrical Lithium Ion cell is the material used for the positive electrode. The Lithium Ion Prismatic cell has a positive electrode made from manganese dioxide which has a three dimensional spinal structure. The spinal electrode is inherently safer but this is at the expense of a reduction in capacity. The Lithium Ion battery has a cobalt electrode that results in the greater energy density. Over charging a cobalt electrode Lithium Ion cell can cause a safety risk because metallic lithium is produced, where as an overcharge in a spinal cell only causes overheating. Small spinal packs are used in lower power consuming devices such as mobile phones because the lower energy requirement allows lower energy density to be traded against improved battery safety.

The lithium Ion Polymer (Lithium Polymer) cell differs from the other two Lithium Ion batteries in that the electrolyte is in the form of a gel instead of a liquid impregnated in a separator. The similarities in Lithium-ion and lithium-polymer chemistries allow the same charging methods to be used. The batteries are charged by firstly applying a constant current followed by a constant voltage. At the beginning of a full charge cycle when the terminal voltage is low, the battery is charged at a constant current with a value of 0.5C or less until the terminal voltage of the cell reaches a value of 4.20Volts. The charger then switches to constant voltage mode and will maintain this voltage across the battery terminals until the charge current reaches 0.03C at which point the charge is terminated, at this point the battery is fully charged.

Lithium Ion battery cells have an extremely low self-discharge. A contributor to the self discharge of the Lithium Ion battery is the battery monitoring circuit that is needed to provide the second level of protection. The protection circuit prevents the cell voltage from exceeding a predefined level on charge and below a predefined level on discharge. The pack temperature is also monitored and back-toback mosfets in the power line provide a reset-table electronic cut out of the charge or discharge if a fault condition is detected. The thresholds are set to lower levels than those within the cell because all of the electronics protections are designed to self reset once the fault has been cleared. The gate source leakage current (Igss) of the protection mosfet contributes towards the quiescent current of the protection circuit, therefore a mosfet with as low a gate leakage current as is possible should be chosen. The NXP Semiconductors PMK30EP quotes a maximum value of Igss of 100nA although typically values of less than a fifth of this are measured on actual devices.

MOSFETs for Cell Phone Battery Pack

Figure 1 shows a typical protection circuit used within a lithium battery pack containing a safety IC and two back-to-back n-channel protection MOSFETs, such as the common drain PMWD20UN in TSSOP8 package. Because board real estate is critical in portable equipment, this has led to a demand for smaller and smaller footprints. It follows, that as the batteries get smaller, the individual components within the protection boards will have to follow the same trend. The key requirement from battery pack manufacturers is to have the lowest RDSon in the smallest package to maximise battery life. With this in mind, NXP offers a battery pack protection solution with three options: TSSOP8 outline, bare die form, and nanoPAK. The nanoPAK range from NXP Semiconductors reclaims more board space by eliminating the leads whilst enhancing thermal performance to provide an advanced solution in today’s space constrained and power hungry portable applications. The nanoPAK alternative addresses the low thermal impedance requirement as the die attach pad is exposed to provide a direct, low-resistance thermal path to the substrate on which the device is mounted, which means the thermal path is via a large copper pad rather than the leads.

Schematic circuit for a typical Lithium battery pack for a cell phone using either TSSOP8, bare die form, or nanoPAK based protection MOSFETs

MOSFETs for Notebook Battery Pack

Notebook battery packs consist of a number of cells connected parallel/series. The series connection is used to provide a higher voltage, whilst the parallel connection provides higher capacity. The protection devices within a notebook battery pack are two p channel MOSFETS such as the PMK30EP from NXP. Similar to the cell phone battery pack operation, one MOSFET enables the charging of the pack, while the other MOSFET enables discharging. When both MOSFETs are off, the cells are isolated from the external environment to protect the battery.

Schematic circuit for a typical Lithium battery pack for notebook using either SO8 or nanoPAK based protection MOSFETs

 

 

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