GENERAL DESCRIPTION (cont.)
when it is at 2.7V. The primary benefit here is that this process of
leakage balancing is fully automatic and works for a variety of
supercaps, each with a different leakage characteristic profile of its
own.
A second benefit to note is that with ~2.4V and ~2.6V across the
two supercaps, in this example, the actual current level difference
between the top and the bottom SAB MOSFETs is at about a 100:1
ratio (~2 orders of magnitude). The net additional leakage current
contributed by the ALD8110026 in the design example above would,
therefore, be approximately 0.01µA. In this case, the difference in
leakage currents between the two supercaps can have a ratio of
100:1 and could still have charge balancing and voltage regulation.
The dynamic response of a SAB MOSFET circuit is very fast, and
the typical response time is determined by the R C time constant of
the equivalent ON resistance value of the SAB MOSFET and the
capacitance value of the supercap. In many cases the R value is
small initially, responding rapidly to a large voltage transient by
having a smaller R C time constant. As the voltages settle down,
the equivalent R increases. As these R and C values can become
very large, it can take a long time for the voltages across the
supercaps to settle down to steady state leakage current levels.
The direction of the voltage movements across the supercap,
however, would indicate the trend that the supercap voltages are
moving away from the voltage limits.
PARALLEL-CONNECTED AND SERIES-CONNECTED SAB
MOSFETS
In the previous design example, note that the ALD810026 is a quad
pack, with four SAB MOSFETs in a single SOIC package. For a
standard configuration of two supercaps connected in series, the
ALD9100xx dual SAB MOSFET is recommended for charge
balancing. If a two-stack supercap requires charge balancing, then
there is also an option to parallel-connect two SAB MOSFETs of a
quad ALD8100xx for each of the two supercaps. Parallel-connec-
tion generally means that the drain, gate and source terminals of
each of two SAB MOSFETs are connected together to form a
MOSFET with a single drain, a single gate and a single source
terminal with twice the output currents. In this case, at a nominal
operating voltage of 2.50V, the additional leakage current contribu-
tion by the SAB MOSFET is equal to 2 x 0.1µA = 0.2µA. The total
current for the supercaps and the SB MOSFET is = 2.5µA + 0.2µA
~= 2.7µA @ 2.50V operating voltage. At max. voltage of 2.70V
across the SAB MOSFET, V
GS
= V
DS
= 2.70V results in a drain
current of 2 x 10µA = 20µA. So this configuration would be chosen
to increase max. charge balancing leakage current at 2.70V to 20µA,
at the expense of an additional 0.1µA leakage at 2.50V.
This method also extends to four supercaps in series, although this
may require two separate ALD810026 packages, if the maximum
voltage ratings of the SAB MOSFET are exceeded.
For stacks of series-connected supercaps consisting of more than
three or four supercaps, it is possible to use a single SAB MOSFET
array for every three or four supercap stacks connected in series.
Multiple SAB MOSFET arrays can be arrayed across multiple
supercap stacks to operate at higher operating voltages. It is
important to limit the voltage across any two pins within a single
SAB MOSFET array package to be less than its absolute
maximum voltage and current ratings.
ENERGY HARVESTING APPLICATIONS
Supercaps offer an important benefit for energy harvesting appli-
cations from a low energy source, buffering and storing such
energy to drive a higher power load.
For energy harvesting applications, supercap leakage currents are
a critical factor, as the average energy harvesting input charge must
exceed the average supercap internal leakage currents in order for
any net energy to be harvested and saved. Often times the input
energy is variable, meaning that its input voltage and current
magnitude is not constant and may be dependent upon a whole set
of other parameters such as the source energy availability, energy
sensor conversion efficiency, etc.
For these types of applications, it is essential to pick supercaps
with low leakage specifications and to use SAB MOSFETs that
minimize the amount of energy loss due to leakage currents.
For up to 90% of the initial voltages of a supercap used in energy
harvesting applications, supercap charge loss is lower than its
maximum leakage rating, at less than its max. rated voltage. SAB
MOSFETs used for charge balancing, due to their high input thresh-
old voltages, would be completely turned off, consuming zero drain
current while the supercap is being charged, maximizing any
energy harvesting gathering efforts. The SAB MOSFET would not
become active until the supercap is already charged to over 90%
of its max. rated voltage. The trickle charging of supercaps with
energy harvesting techniques tends to work well with SAB MOSFETs
as charge balancing devices, as it is less likely to have high
transient energy spurts resulting in excessive voltage or current
excursions.
If an energy harvesting source only provides a few
µA
of current,
the power budget does not allow wasting any of this current on
capacitor leakage currents and power dissipation of resistor or
operational amplifier based charge-balancing circuits. It may also
be important to reduce long term leakage currents, as energy
harvesting charging at low levels may take up to many days.
In summary, in order for an energy harvesting application to be
successful, the input energy harvested must exceed all the energy
required due to the leakages of the supercaps and the charge-
balancing circuits, plus any load requirements. With their unique
balancing characteristics and near-zero charge loss, SAB MOSFETs
are ideal devices for use in supercap charge-balancing in energy
harvesting applications.
ALD810023, ALD810024, ALD810025,
ALD810026, ALD810027, ALD810028
Advanced Linear Devices, Inc.
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ALD [ ADVANCED LINEAR DEVICES ]
