Fig. 1.     Series-connected supercapacitors; also called ultracapacitors, or electrochemical double layer capacitors (EDLC). Individual supercapacitors are available from 1Farad to 100s of Farads. Packages vary from credit card size to cylindrical shapes the size of a flashlight. When connected in series their total capacitance, (See below).

Individual supercapacitors have a 2.5 to 2.7 V maximum rating so they must be connected in series to work at higher voltages, which requires the balancing of supercap leakage currents for proper operation (Fig. 1). Now, Advanced Linear Devices (ALD) has developed Supercapacitor Auto Balancing (SAB) MOSFETs that by themselves address regulation and leakage current balancing of series-connected supercaps. Without the proper supercap balancing, overcharging could cause failure or punch through that leads to unreliable performance.


To understand how the SAB MOSFETs work, we first have to review operation of series-connected supercaps, which depends on their material and construction as well as their variable operating characteristics:

·     Initial leakage current

·     Initial supercap voltage

·     Charging voltage

·     Charging current

·     Temperature range

·     Aging

Therefore, problem-free operation of two or more series-connected supercaps must mitigate the impact of these variable characteristics. This requires automatic leakage current and voltage balancing of each supercap because their characteristics can vary from one supercap to another.

There have been passive approaches to voltage balancing that employ high value resistors, even though they waste power and don’t allow for aging and temperature variations. And, active balancing circuits using op amps, zener diodes, MOSFETs and resistors have also been used and they also waste power. These circuit balancing components also increase cost and board space.

Fig. 2.      Quad SAB MOSFET is housed in a 16-pin SOIC package. *IC pins internally connected and connect to V-.

Fig. 2 shows a quad package of one these the SAB MOSFETs; there are also dual MOSFET packages. These MOSFETs have unique electrical characteristics for active continuous leakage current regulation and self-balancing of stacked series-connected supercaps. And, they dissipate near zero leakage currents, practically eliminating extra power dissipation. For most applications, automatic charge balancing with SAB MOSFETs offers a simple, economical and effective method to balance and regulate supercap voltages.

The principle behind the Supercap Auto Balancing MOSFET is basically simple. It is based on the natural threshold characteristics 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 exponentially non-linear function of its gate voltage. Hence, for small changes in the MOSFET’s gate voltage, its on-current can vary greatly, by orders of magnitude.  SAB MOSFETs are designed to take advantage of this fundamental device characteristic.

During the production process, these MOSFETs are trimmed to operate at specific threshold voltages. This produces 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. Each drain pin has an internal reverse biased diode to its source pin, and each gate pin has a reverse biased diode to V-.  All other pins must have voltages within V+ and V- voltage limits. Fig. 3 shows a quad SAB MOSFET connected to four series-connected supercaps.

Fig. 3.     Series connection of four supercaps with a separate SAB MOSFET in parallel with each supercap.

You should use standard ESD handling procedures when working with these sensitive MOSFET devices that are housed in SOIC packages. Use of these packages avoids ESD problems when trimming and also allows access to all three terminals for trimming: drain, source, and gate. If this three-terminal access wasn’t necessary, these MOSFETs could have been mounted in a two-lead package similar to that of a diode.

SAB MOSFETs provide 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 turns off and there is zero leakage current contribution from the MOSFET. On the other hand, when the voltage across the supercap is over the desired voltage limit, the SAB MOSFET turns on to increase its drain currents and keep the over-voltage from rising across the supercap. Also, this simultaneously lowers the voltages and leakages of other supercaps in the stack and maintains near-zero net leakage currents.