SABMB2/SABMB2XX

Advanced Linear Devices, Inc.

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The ALD9100XX SAB MOSFET family offers the user a selection

of different threshold voltages for various supercapacitor nominal

operating 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. Note that each Drain

pin has an internal reverse biased diode to its Source pin, and

each Gate pin has an internal reverse biased diode to V-. All

other pins must have voltages within V+ and V- voltage limits within

the same package unit.

Standard ESD protection facilities and handling procedures for

static sensitive devices must also be used while installing the

ALD9100XX units. Once installed, the connection configuration

will protect the ALD9100XX units from ESD damage. When

connected to a supercapacitor stack, the ALD9100XX is further

protected from virtually any ESD damage due to the large

capacitance of the supercapacitors, which sinks any ESD charge

and thereby reduces any of the terminal voltages to minimal

harmless values.

SABMB2 PRINTED CIRCUIT BOARDS

The SABMB2 Printed Circuit Board is available as a blank PCB

board, made with RoHS compliant FR4 material, ready for

mounting one 8-lead ALD9100XX unit. These units are also

supplied and available with a 2-digit suffix, which denotes the

specific ALD9100XX component mounted and tested on the PCB.

All that is required of the user is to mount the PCB and wire the

appropriate connections from the SABMB2 board to the respective

supercapacitor nodes.

Each SABMB2 Printed Circuit Board has a single 8-lead SOIC

footprint and terminals labeled V+, A, B, C and V-. Each of these

terminals has two wiring holes for easier connection of the same

terminal node to two external connection points. V+ is directly

connected to terminal A, which must be connected to the most

positive voltage for the individual SABMB2 board. V- is directly

connected to terminal C, which must be connected to the most

negative voltage present for the same SABMB2 board. The other

terminal, namely B, must have voltages between V+ and V- for

proper operation of the board.

When two supercapacitors are installed to be balanced by SAB

MOSFETs, a single ALD9100XX unit can be mounted on the

SABMB2. Any number of SABMB2 boards can be daisy-chain

connected in series. For example, three SABMB2 boards, each

with an ALD910025SALI installed, can be connected in series to

a +15V power supply, provided care is taken to insure that each

SABMB2 board V- is connected to the V+ of the next SABMB2

board in series, such that each board would have typical internal

voltages from V+ to V- of +5.0V.

The ALD9100XX is rated for reverse bias diode currents of up to

80mA maximum for each SAB MOSFET on board. Any reverse

bias condition as a result of changing supercapacitor voltages,

especially during fast supercapacitor discharge, could lead to some

internal nodes temporarily reverse biased with surge current in

excess of this limit. The SABMB2 board has additional optional

TO277 footprints for mounting external schottky rectifiers (power

diodes) to clamp such surge current transients. The user is advised

to determine the various power and current limits, including

temperature and heat dissipation considerations, when selecting

a suitable component for such purpose. The appropriate level of

derating and margin allowance must also be added to assure long

term reliability of the PCB board.

SUPERCAPACITORS

Supercapacitors are typically rated with a nominal recommended

working voltage established for long life at their maximum rated

operating temperature. Excessive supercapacitor voltages that

exceed the supercapacitor’s rated voltage for a prolonged time

period will result in reduced operating life and eventual rupture

and catastrophic failure. To prevent such an occurrence, a means

of automatically adjusting (charge-balancing) and monitoring the

maximum voltage is required in most applications having two or

more supercapacitors connected in series, due to the different

internal leakage currents that vary from one supercapacitor to

another.

Each supercapacitor has a tolerance difference in capacitance,

internal resistance and leakage current. These differences create

imbalance in cell voltages, which must be balanced so that any

individual cell voltage does not exceed its rated max. voltage.

Initially, cell voltage imbalance is caused by capacitance value

differences. Supercapacitors selected from the same manufacturer

make and model batch can be measured and matched to deliver

reasonable initial cell voltages. Next, cell voltage imbalance due

to individual cell leakage currents must be compensated.

The supercapacitor leakage current itself is a variable function of

its many parameters such as aging, initial leakage current at zero

input voltage, the material/construction of the supercapacitor, and

the operating bias voltage. Its leakage is also a function of the

charging voltage, the charging current, operating temperature

range and the rate of change of many of these parameters.

Supercapacitor balancing must accommodate these changing

conditions.

By using the appropriate ALD SAB MOSFET and the appropriate

SABMBXX board, users can compensate for all of these causes

of imbalance and automatically balance supercapacitors.

ENERGY HARVESTING APPLICATIONS

Supercapacitors offer an important benefit for energy harvesting

applications using a low energy source, by buffering and storing

such energy to drive a higher power load.

For energy harvesting applications, supercapacitor leakage

currents are a critical factor, as the average energy harvesting

input charge must exceed the average supercapacitor internal

leakage currents in order for any net energy to be harvested and

saved. Often, the input energy is variable, meaning that its input

voltage and current magnitude are not constant and may be

dependent upon a whole set of other parameters such as the

source energy availability, energy sensor conversion efficiency,

changing environmental conditions, etc.

SAB MOSFETs used for charge balancing, due to their high input

threshold voltages, are completely turned off initially, consuming

zero drain current while the supercapacitor is being charged,

SUPERCAPACITOR AUTO BALANCING PCB