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Supercapacitor Auto Balancing FETs Developed by Advanced Linear

November 4, 2013

Advanced Linear Devices, Inc. (ALD) has introduced ALD8100xx and ALD9100xx family of Supercapacitor Auto Balancing (SAB) MOSFETs to eliminate overcharging and regulate load balancing with an elegant solution. Supercapacitors connected in a series of two or more can be balanced with either EPAD precision ALD9100xx dual devices or ALD8100xx quad packages, to cut cost, reduce complexity, eliminate overcharging risk and improve reliability above existing op amp and resistor-based charge balancing. As supercapacitor adoption hits mainstream applications, the key to long-life and reliability is to balance the voltage in a series of devices more precisely to prevent overcharging and power dissipation over the lifetime of continuous charge and discharge cycles. Cost, design complexity and board space are also top concerns for system designers requiring the maximum benefit in mobile, automotive or energy harvesting applications.

The natural threshold characteristics of ALD’s SAB MOSFETS provide superior intrinsic properties over current solutions. The threshold characteristics are the voltages at which a MOSFET turns on and starts to conduct a current. The drain current of the MOSFET is at or below its threshold voltage, is an exponentially non-linear function of its gate voltage. Therefore, making small changes in the MOSFET’s gate voltage, can vary its on-current greatly, by orders of magnitude.

This exponential relationship between the Drain-Gate Source Voltage and the Drain-Source ON Current in EPAD MOSFETS is an important consideration for replacing resistors and op amps in supercap charge balancing applications. These other conventional charge-balancing circuits would continue to dissipate a significant amount of current, even after the voltage across the supercaps had dropped, because the current dissipated is a linear function, rather than an exponential function, of the supercap voltage (I = V/R). For supercap stacks with multiple decices, balancing becomes more onerous. But using ALD SAB MOSFETS can reduce device count by as much as 75 percent in charge balancing applications.

Supercapacitors, also known as ultracapacitors or supercaps, when connected two in series, can be balanced with an ALD9100xx dual package. Supercaps connected two, three or four in series can be balanced with an ALD8100xx quad package. ALD SAB MOSFETs have unique electrical characteristics for active continuous leakage current regulation and self-balancing of stacked series-connected supercaps and, at the same time, dissipate near zero leakage currents, practically eliminating extra power dissipation. For many applications, SAB MOSFET automatic charge balancing offers a simple, economical and effective method to balance and regulate supercap voltages. With SAB MOSFETs, each supercap in a series-connected stack is continuously and automatically controlled for precision effective supercap leakage current and voltage balancing.

SAB MOSFETs offer a superior alternative solution to other passive resistor-based or operational amplifier based balancing schemes, which typically contribute continuous power dissipation due to linear currents at all voltage levels. They are also a preferred alternative to many other active supercap charging and balancing regulator ICs where tradeoffs in cost, efficiency, complexity and power dissipation are important design considerations.

The SAB MOSFET provides regulation of the voltage across a supercap cell by increasing its drain current exponentially across the supercap when supercap voltages increase, and by decreasing its drain current exponentially across the supercap when supercap voltages decrease. When a supercap in a supercap stack is charged to a voltage less than 90% of the desired voltage limit, the SAB MOSFET across the supercap is turned off and there is zero leakage current contribution from the SAB MOSFET. On the other hand, when the voltage across the supercap is over the desired voltage limit, the SAB MOSFET is turned on to increase its drain currents to keep the over-voltage from rising across the supercap.

However, the voltages and leakages of other supercaps in the stack are lowered simultaneously to maintain near-zero net leakage currents. The ALD8100xx/ALD9100xx SAB MOSFET family offers the user a selection of different threshold voltages for various supercap nominal voltage values and desired leakage balancing characteristics. Each SAB MOSFET generally requires connecting its V+ pin to the most positive voltage and its V- and IC pins to the most negative voltage within the package.

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