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MOSFETs able to balance supercapacitors in industrial applications

Advanced Linear Devices (ALD) has unveiled a family of Supercapacitor Auto Balancing (SAB) MOSFETs which have been designed for industrial applications to regulate and balance leakage currents while at the same time reducing the amount of energy used for balancing supercapacitor cells stacked in series stack of two or more.

The devices are intended for a number of industrial-grade energy storage applications that require an operating temperature range between -40 to 85C and are suitable for a range of applications including: remote monitoring applications, automotive systems, backup power, transportation, automation, or any application that operates in exposure to the elements and needs to endure extreme climate conditions.

In outdoor industrial energy storage, leakage current of supercapacitors used in a stack is balanced by connecting one or more SAB MOSFETs across each cell – this is critical to prevent damage to cells from over voltage that can dramatically shorten supercapacitor operating life.

Each MOSFET contains 26 different products and each product can balance up to 4 supercapacitor cells in a single IC package. Starting with two cells, the devices can balance an unlimited number of supercapacitor cells stacked in a series.

Each device dissipates near zero leakage current to eliminate extra power dissipation and provide a circuit design alternative to passive or active balancing methods by offering automatic active leakage current regulation. As an alternative to op-amp based schemes, the high voltage SAB MOSFET arrays can reduce board space, and lower cost while enhancing system and component lifespan.

SAB MOSFETs connected across these supercapacitors exhibit complementary opposing current levels, resulting in little or no additional leakage currents other than those caused by the supercapacitors themselves.

Leakage current differences can make it difficult to control respective cell voltages as it causes one or more supercapacitors to exceed its rated voltage over time, which reduces lifespan by rupturing materials inside each cell. Eventually this leads to catastrophic failure, which can begin over days, weeks, or months.

The voltage dependent characteristic of the ALD810019xxx/ALD91001xxx on-resistance controls excessive voltage rise of each individual supercapacitor cell. In series-connected stacks, when one supercapacitor voltage rises, the voltage of the other supercapacitors drops. The supercapacitors that have the highest leakage currents also have the lowest voltages. By increasing drain current exponentially when voltages increase, and by decreasing drain current exponentially when voltages decrease, the MOSFET arrays automatically regulate the voltage across each supercapacitor cell.

The products offer different threshold voltages for various supercapacitor operating voltages and leakage current characteristics which diminishes energy spent in balancing the circuits. The new device family covers a wide range of operating voltages from 1.6V to 2.7V and leakage current ranges from <0.3nA to >3000µA.

Neil Tyler

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