ALD8100XX/ALD9100XX SUPERCAPACITOR

Advanced Linear Devices, Inc.

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AUTO BALANCING (SAB) MOSFET ARRAY FAMILY

balancing generally contribute additional continuous power dissi-

pation due to linear currents at all supercapacitor voltage levels,

whereas SAB MOSFET leakages decrease exponentially with de-

creased supercapacitor voltages. In many cases, the additional

leakage charge loss is near zero.

UNDERSTANDING SUPERCAPACITOR AUTO BALANCING

USING SAB™ MOSFETS

The principle behind the SAB MOSFET in balancing

supercapacitors is simple. It is based on the natural threshold char-

acteristics of a MOSFET device. The threshold voltage of a

MOSFET is the voltage at which a MOSFET turns on and starts to

conduct a current. The drain current of the MOSFET, at or below

its threshold voltage, is an exponential function of its gate voltage.

Hence, for small changes in the MOSFET’s gate voltage, its drain-

source on-current can vary greatly, by orders of magnitude. ALD’s

SAB MOSFETs are designed to take advantage of this fundamen-

tal device characteristic.

Gate-to-Source and the Drain-to-Source terminals of each MOSFET

and when this joint terminal is connected across a supercapacitor,

SAB MOSFETs can be connected in parallel or in series, to suit the

desired leakage current characteristics, in order to charge-balance

an array of supercapacitors connected in series. The array of com-

bined SAB MOSFETs and supercapacitors would be automatically

self-regulating with various leakage mismatches and environmen-

tal temperature changes. The SAB MOSFETs can also be used

entirely in the subthreshold mode, meaning the SAB MOSFET is

used at min., nominal and max. operating voltages in voltage ranges

below its specified threshold voltage.

of an SAB MOSFET is defined as its drain-gate source voltage at a

specified as xx, where the threshold voltage is in 0.10V increments.

Two examples are: the ALD810025 features a precise threshold

As all ALD8100xx/ALD9100xx devices operate similarly, with lin-

ear voltage shifts, an ALD810025 is used as an illustration of its

characteristics. At voltages below its threshold voltage, the

ALD810025 rapidly turns off at a rate of approximately one decade

controlled and repeatable for different units from various produc-

tion batches.

supercapacitor charge balancing applications currently using fixed

resistors, operational amplifier circuits or other forms of charge bal-

ancing. These other conventional charge-balancing methods con-

tinue to dissipate a significant amount of current, even after the

voltage across the supercapacitors has dropped, because the cur-

rent dissipated is a linear function, rather than an exponential func-

tion, of the supercapacitor voltage (I = V/R). For supercapacitor

series stacks with more than two cells, the challenge of leakage

balancing becomes even more onerous.

With most other passive or active circuits that offer charge balanc-

ing, active power is still being consumed even if the supercapacitor

voltage falls much below its operating voltage. For a four-cell

supercapacitor stack, for example, this translates into a 2.0V x 4

~= 8.0V power supply for an IC charge-balancing circuit. As the

number of cells increase, adding components to the charge bal-

ancing circuit, increased circuit complexity and power dissipation

becomes a greater challenge. A supercapacitor stack using the SAB

MOSFET charge-balancing method, on the other hand, would not

cause extra power dissipation when the number of cells increase.

This method provides an exponentially decreasing amount of charge

loss with time, and helps preserve, by far, the greatest amount of

charge on each of the supercapacitors.

device acts like a voltage limiting regulator with self-adjusting cur-

rent levels. When the SAB MOSFET is connected across a

supercapacitor cell, the total leakage current equals the two in com-

bination automatically compensate and correct for each other.

Consider the case when two supercapacitor cells are connected in

series, each with an SAB MOSFET connected across it, charged

If the top supercapacitor has a higher internal leakage current than

drop lower than that of the bottom supercapacitor. The SAB

drop, drops off rapidly. Meanwhile, the bottom supercapacitor

SAB MOSFET across the bottom supercapacitor. This increased

increase rapidly as well. The excess leakage current of the top

supercapacitor would now leak across the bottom SAB MOSFET,

reducing the voltage rise tendency of the lower supercapacitor. With

this automatic self-regulating mechanism, the top supercapacitor

voltage tends to rise while the bottom supercapacitor voltage tends

to drop, creating simultaneously opposing actions to the

supercapacitor leakage currents.

With appropriate design and selection of a specific SAB MOSFET

device for a given pair of supercapacitors, it is now possible to

regulate and balance two series-connected supercapacitors with

essentially no extra leakage current, since the SAB MOSFET only

conducts the difference in leakage current between the two

supercapacitors.

Likewise, the case of the bottom supercapacitor having a higher

leakage current than that of the top supercapacitor works in similar

fashion, where the bottom supercapacitor voltage tends to drop,

compensated by the tendency of the top supercap voltage to drop

as well, effected by the top SAB MOSFET. This SAB MOSFET

charge balancing scheme also works with four, eight or more

supercapacitors in series by using an equivalent number of SAB

MOSFETs in multiple package(s).

Ambient temperature increases cause supercapacitor leakage cur-

rents to increase. The SAB MOSFET threshold voltage is reduced

ALD8100XX/ALD9100XX FAMILY GENERAL DESCRIPTION (cont.)