Bi-directional FlipFETMOSFETs for Cell Phone Battery Protection Circuits



Category: Telephone, cellular phone and intercom
Manufacture: International Rectifier Corp.
Datasheet: Download this application note


Description:
Bi-directional FlipFET TM MOSFETs for Cell Phone Battery Protection Circuits
As presented at PCIM 2001 Authors: *Mark Pavier, *Hazel Schofield, *Tim Sammon, **Aram Arzumanyan, **Ritu Sodhi, **Dan Kinzer * International Rectifier, Holland Road, Hurst Green, Surrey, RH8 9BB, UK ** International Rectifier, 233 Kansas St., El Segundo, CA, 90245 USA Abstract A bi-directional chip scale power MOSFET device is introduced for use in cell phone battery protection circuits. This device utilises the monolithic integration of two common drain power MOSFETs and solder bump technology to produce an ultra low footprint solution. Battery protection MOSFET packaging technology is reviewed along with cell phone current requirements. Comparisons are drawn between the in circuit performance of the bi-directional device and an industry standard TSSOP8. Device related losses in relation to IC drive strategy and conclusions on the effects of Rdson and package volume on cell phone talk and standby times are presented. Introduction Advances in battery chemistry technology have been one of the key enablers for smaller hand held portable devices. Over the past decade battery technologies have evolved from Nickel based chemistries, such as NiCd and NiMH, to lithium ion chemistries. Whilst lithium ion based chemistries ultimately offer increased gravimetric and volumetric energy densities over their Nickel counterparts, there are potential risks that need to be addressed when adopting Lithium based cells. For example, during overcharge of Lithium ion cells, lithium metal may build up on the cell electrodes. This lithium build-up readily oxidises in air, which is a strongly exothermic reaction and a potential fire risk. Similarly, over-discharge may cause irreparable damage to the cells. The key to preventing these effects occurring is to introduce protection circuitry into the battery pack. These circuits typically contain a control IC, battery protection MOSFETs, and a gas gauge IC to monitor the charge status of the battery p ck or a cell. In this paper we examine battery protection MOSFET technologies and demonstrate a new BiTM directional Nchannel FlipFET device specifically designed for use in battery protection circuits. Typical cell phone power consumption data are presented for talk time and standby mode operating conditions. This data is utilised to compare the in circuit performance of the biTM directional FlipFET device with an industry standard dual N-channel TSSOP8 device. In circuit efficiency data are presented along with conclusions on the effects of MOSFET losses on cell phone operating times. Battery protection circuits A variety of circuit topologies exist for lithium ion cell and battery protection. For battery packs containing 2 or more cells these may consist of N or P-channel MOSFETs switching on the negative or positive rail of the battery output terminals respectively. For cell phones, where space is at a premium and typically only one Li-ion cell is used, N-channel switching topologies are more common. The reason for this being that N-channel power MOSFET devices typically have a higher channel mobility and therefore lower Rdson per unit area for a given blocking voltage. A typical cell phone Li-ion battery protection circuit schematic is shown in figure 1. The circuit consists of a protection IC, gas gauge IC and N-channel power MOSFET devices. Depending upon IC used, additional components may also be present such as a current sense resistor, fuse, and temperature sensing elements.
Figure 1. Li-ion battery protection circuit schematic set internal under-voltage reference. If the battery reaches this point the discharge FET (and charge FET) turn(s) off, effectively open circuiting the load. Charging mode Lithium ion cells are typically charged using a constant current constant voltage strategy. During charge mode the load is replaced or paralleled with a charger. Under normal charge conditions the charge FET is turned on (Vgs2 > Vgth). The discharge FET's internal body diode will be conducting. In most cases the discharge FET will also be switched on to minimize the voltage drop between the charger and cell (Vgs1 > Vgth). As mentioned in the introduction, overcharge can also damage the cell. If the charge voltage increases past that of the IC's built in voltage reference, the charge FET and, if on, the discharge FET, will be turned off.
Where charge and discharge control are required the N channel MOSFET's are normally connected in common drain configuration, as shown in figure 1. The MOSFET whose source is in direct contact to the cells output is referred to as the discharge FET. The FET with its source connected to the load is referred to as the charge FET*.
* Note this terminology also holds when P-channel positive rail switching is adopted.
Discharge mode Under normal discharge operation the discharge FET is turned on (Vgs1 > Vgth). Whilst the charge FET's body diode is forward biased and able to conduct current during discharge, this FET is also turned on in order to reduce the voltage drop between cell output and load (Vgs2 > Vgth). A lithium ion cell will have a lower limit to which the cell voltage must not fall below to prevent damage to the cell. In most cases the IC will have a pre-
Figure 2. Evolution of MOSFET devices for battery protection circuits
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