For portable devices it is important to have an optimized power management system to use the power provided by the battery very efficiently. Typically, two important components for these efficient systems are management IC and MOSFET. In a typical power management system of a portable device, MOSFETs are used for various purposes:
- as protection switches in the battery pack
- as PWM switches in charger circuit and DC/DC converters
- as charging source switch to select adaptor or USB charging
- as charging bypass switch for battery cell balancing
- as load switches to enable/disable different functions
- as level shifters to interface ICs with different I/O levels
This article focuses on the role of the MOSFET in the protection circuit of a battery pack and the MOSFET solutions that Infineon can provide.
BATTERY PACK FOR PORTABLESThe battery pack plays a key role in the performance of portable devices. Battery chemistry and the power management system are crucial to optimize efficiency and maximize battery life.
Li-ion cells are widely used due to the benefit of high energy density, light weight, low self discharge rates, and no memory effect. However, Li-ion batteries are very sensitive to critical operating conditions, and operating them beyond their limits could result in explosion. In order to avoid the over-charge, over-discharge and over-current conditions in the battery pack, a robust protection circuit is necessary. The typical protection circuit used in a Lithium battery pack is shown in Figure 1. The protection system contains a control IC, two back-to-back battery–protection MOSFETs, and a gas gauge IC to monitor the charge status of the battery pack. Two MOSFETs are required because the current has to be blocked in charging and discharging direction.
Notebook battery packs are made of three or four Li-ion cells connected in series, meaning typical nominal voltages in the range of 10.8V to 14.4V per pack. Li-ion batteries are typically charged at currents of 0.7C/h to 0.8C/h, maximum rates vary between 1C/h and 2C/h (“C” stands for the nominal battery capacity in mAh). Typical “C” values for Li-ion notebook batteries are around 4,000mAh to 6,000mAh, therefore a charging with 1C/h will result in a charging current of 4A to 6A.
Smaller portables like cell phones and PDAs use one or two cells connected in series. Nominal voltages are in the range of 3.6V to 7.2V per pack. Typical capacity values are in the range of 1,000mAh. A charging of 1C/h will result in a charging current of 1A. Notebooks and Cell phones may require batteries to supply max. 2C and 3C discharge rates.
REQUIREMENTS FOR MOSFETSThe first selection criterion is based on the maximum blocking voltage required. Typically for notebooks, MOSFETs with a blocking voltage VDSS of 30V are usually chosen considering the safety margin. For smaller portables VDSS is 20V or even less than 20V. Another criterion for p-channel MOSFETs is the VGS rating. In general 30V p-channel MOSFETs are specified with a maximum VGS rating of 20V. As the gate could be exposed to higher voltages, designers very often request a VGS rating of 25V. Moreover, gate threshold voltage is another concern. For a device powered with a single Li-ion battery cell, it is preferable to use a super logic level device (VGS rated at 2.5V or 1.8V).
Since the MOSFETs in the protection circuit do not work in switching mode, the power loss will be solely conduction loss, which is the I2R loss produced by the current through the MOSFET’s channel. Therefore, after selecting VDSS and VGS, the MOSFET’s channel’s “on resistance” is a main selection criterion in battery protection application. Low RDS(on) is especially important for battery pack safety during fault conditions which result in a large discharge current.
The notebook battery protection circuit usually uses p-channel MOSFETs. These MOSFETs allow a simple control circuit: To turn it on, the gate just needs to be pulled to ground, without requirement of a special circuitry like a charge-pump. The main disadvantage of the p-channel MOSFETs is the higher area specific RDS(on), as the mobility of the holes is approximately three times lower compared to the mobility of electrons. In the past two years, IC manufacturers provided a protection IC which supports also n-channel MOSFETs for notebook battery applications. Some notebook’s battery pack makers have already adopted n-channel MOSFETs in their design. For smaller portables, n-channel MOSFETs in small signal packages are also used due to more stringent size constraints.
To cope with the demand for slim-looking mobile devices, the technological trend for battery cells is also to create thinner and lighter batteries. As the batteries get smaller, components with the same layout design rules are needed in the protection circuit boards that are attached to one plane of the battery cells. In an effort to meet the increasing need for power density, many discrete power semiconductor manufacturers have been introducing several new packaging techniques. Main development target is a small footprint and a low profile package.
New packaging technologies, such as CanPAK package, address the increasing demand for ultra-low thermal impedance in a small package that is ideal for space-constrained applications. The bottom of the die attach pad is exposed to provide a direct, low-resistance thermal path to the substrate on which the device is mounted. In other words, the main thermal path is no longer through the leads but through a large area of copper pad. As a result, the thermal resistance is significantly improved.
INFINEON’s MOSFETS PORTFOLIOInfineon offers a broad product portfolio which suits perfectly the requirements for the battery protection application. It comprises both n-channel and p-channel products in the range of 20V and 30V in a wide range of different packages.
Besides the standard SO8 package, Infineon will continue to expand the portfolio of leadless packages like the SuperSO8 (5x6mm footprint, 1mm height) and S3O8 (3.3x3.3x1mm). To help further reduce the dimension of the battery pack, Infineon is also offering dual MOSFETs in small packages such as the dual TSOP-6.
To facilitate heat dissipation in high current applications, Infineon pioneered the use of double-sided cooling package CanPAK for a p-channel MOSFET. It features 2.7mΩ RDS(on), which is the lowest RDS(on) p-channel device in a double sided cooling package.
Besides normal logic level MOSFETs, Infineon also offers MOSFETs with low VGS ratings such as the ultra logic level (1.8V rated VGS) and super logic level (2.5V rated VGS) for low voltage applications. This enables easy interface with latest controllers.
Lowest channel resistance devices are available in n-channel and p-channel (SuperSO8 1.4mΩ n-channel and 3mΩ p-channel; S3O8 3.5mΩ n-channel and 8.6mΩ p-channel). This enables battery cell makers to develop slimmer and high density battery pack without compromising the reliability.
About the authorsDr. Martina Ciacchi is a Marketing Specialist and Wolfgang Peinhopf is a Technical Marketing Engineer for Infineon Technologies AG. Wang Zhong Fang is a Senior Application Engineer, and Lim Choon Kwee is a Senior Manager, both for Infineon Technologies Asia Pacific.
Click here for the illustrations:
Caption
Figure 1. Typical protection circuit in a battery pack.
