design application
COMPONENTS
Shield Ultracapacitor Strings From
Overvoltage Yet Maintain Efficiency
Although active cell voltage-equalization circuitry adds complexity,
it has greater energy efficiency than passive techniques.
pressure may build
Switching device
up until the safety
NESS CAPACITOR CO. LTD.
vent on the ultracaS
S
S
pacitor’s package
a ny system applications reopens. Consequentquire that capacitors be conly, more of the elecnected together, in series
trolyte will decomand/or parallel combinations, to form
pose and vaporize
a “bank” with a specific voltage and
until the ultracapacicapacitance rating. The most critical
Load/charge
tor’s effective internal
parameter for all capacitors, including
resistance increases
ultracapacitors, is voltage rating. Suband becomes an 2. For active balancing, a switching device is placed in series with
jecting almost any capacitor to a subopen circuit.
stantially higher voltage than it was
each balancing resistor. The switches are controlled by voltageIn short, impress- detection circuits that only turn a switch “on” when a capacitor’s
designed to withstand usually results in
ing more voltage on voltage approaches its continuous-working-voltage rating.
an irreparably damaged, nonworking
an ultracapacitor
capacitor. This is especially true for
Overall Capacitance Value Of A
than it’s rated to withstand usually
ultracapacitors, so they must be protectSeries-Capacitor String: The net capacinecessitates replacement. Therefore,
ed from overvoltage conditions.
tance of a string of capacitor cells is the
prevention is the most sensible practice,
Capacitors typically have two voltage
reciprocal of the sum of the reciprocals
as it may effectively eliminate ultracaratings. Whatever voltage the capacitor
of every cell’s capacitance. This is most
pacitor repair and maintenance costs. It
can sustain indefinitely, without dameasily understood if all members of the
also eradicates other potential causes of
age or performance degradation, is
string have equivalent capacitance valequipment downtime.
called the continuous-working voltage.
ue. Then, the capacitance of the whole
After voltage rating, the two most sigOn the other hand, the voltage that a
string will equal the individual cell
nificant parameters for all types of
capacitor can handle for just a short
capacitance divided by the number of
capacitors are their capacitance and
period of time, like a few hundred milcells in the string. For example, conequivalent series resistance (ESR).
liseconds, is the momentary peak or
necting 100 cells, each with 1000
When many ultracapacitor cells are
surge rating.
Farads (F) of capacitance in a series
connected together in a series string,
When an ultracapacitor is subjected
string, will produce an overall effective
these three parameters are affected as
to more than a tolerable voltage, the
capacitance of 10 F.
follows:
organic electrolyte within the cell
Overall ESR Of A Series-Capacitor
Overall Voltage Rating Of A Seriesbegins to decompose, producing a
String: The total ESR of the string has
Capacitor String: The total voltage that
gaseous byproduct. If the overvoltage
the same cumulative characteristic as
can be impressed across a string of
condition persists long enough, the
the cell voltage. In other words, it
capacitor cells conequals the sum of all individual ESR
nected in series is the
values. A 100-cell string with 5 mΩ of
sum of each cell’s
dc ESR each will have an overall dc ESR
individual voltage
of 500 mΩ.
rating. UltracapaciCapacitors connected in series are
tors are usually consubject to the “weakest-link” principle.
nected together in
The poorest performer in the string sets
series so that they
the performance “pace” for the rest of
can be subjected to a
Load/charge
the string. So five individual 500-F cells
higher voltage than
in series have 100 F of capacitance. Yet
the available indi1. With passive balancing, a resistive ladder is connected to each
four 500-F cells in series with one 400-F
vidual cells are rated
node in the string of series-connected capacitors.
cell each have only 95 F of capacitance.
to withstand.
Do Yang Jung
M
May 27, 2002 • ELECTRONIC DESIGN
1
ULTRACAPACITOR PROTECTION
ULTRACAPACITOR PROTECTION
2.72
1.4
2.7
Current
Voltage
2.68
0.8
2.66
0.6
R1
Voltage (V)
1.0
C1
R3
R9
R4
IC1
T1
R5
R8
T2
2.64
0.4
R2
0
12
32
52
72
97
117
Time (minutes)
125
134
150
R7
2.6
3. When the capacitor’s voltage approaches the bypass threshold
voltage of 2.68 V (red curve), the active switch turns “on” and diverts
current from the capacitor (blue curve).
Moreover, the failure of any component within the string effectively causes
the unit to “fail” due to the serial connection between the individual string
members. In particular, an open circuit
in any series-connected component
effectively renders the entire string as
open circuited. Plus, ultracapacitors
eventually fail open circuit, so it’s a significant concern when many cells are
connected together in a long string.
That’s because the mean time between
failure (MTBF) of any system is inversely proportional to the number of components in that system.
Need For Voltage Equalization:
Because sustained overvoltage can
cause an ultracapacitor to fail, the voltage across each cell in a series string
must not exceed the maximum continuous-working-voltage rating of the
individual cells in the string. Thus,
preventing the voltage impressed
upon each cell in the string from
exceeding its continuous-workingvoltage rating is the most important
preventive measure for ensuring
trouble-free operation during the
string’s life. The designer must
either reduce the “rate of charge”
being delivered to a cell, or completely stop charging a cell whose
voltage approaches its surge-voltage rating.
The easiest way to reduce
the current that’s charging an
ultracapacitor cell is to divert some
of it around the cell. One such
method employs a passive bypass component. The other, more complicated
procedure uses an active bypass circuit.
Both techniques have advantages and
disadvantages.
2
C2
2.62
0.2
0
R6
To ultracapacitor
1.2
Current (A)
+
ELECTRONIC DESIGN • May 27, 2002
–
5. All individual pc boards in Figure 4 contain this circuitry. Resistor
R9 is the bypass element that diverts charging current from the
ultracapacitor. It is an axial-lead 2.7 Ω, 5-W metal-oxide resistor.
Passive Cell Voltage Equalization: The
simplest implementation of the passive
method involves a resistor “ladder” that
has a “rung” or node connected to each
node where all ultracapacitor cells join.
This places a resistive element in parallel with every ultracapacitor cell (Fig.
1). The value of each resistor in the ladder should be selected so that the current flowing through it is within the
range of two to 10 times the typical initial leakage current of the ultracapacitor
cells in the string. That can be as high as
1 to 3 mA. So with a VC(MAX) of 2.7 V,
the resistance range is:
VC(MAX)/IC(BYPASS) = R
2.7 V/(2 × 1 mA) = 1.35 kΩ to
2.7V/(10 × 3 mA) = 90 Ω
4. Here is an actual series string of 18 1700F ultracapacitors that have active
balancing. A 94-F capacitor with a voltage
rating of 48 V dc results.
But the exact value may need to be
determined by the maximum-possible
charging rate that the string will likely
see. This ensures that enough current
will be bypassed to prevent the cell
from overcharging. The primary benefits of this parallel-resistor ladder circuit
are its low cost and ease of implementation. The main downside is that this circuit is always discharging the string, so
it’s not very energy efficient.
Active Cell Voltage Equalization: The
simplest implementation of the activecircuit method uses a resistor ladder
that’s identical to the one just described
for the passive method. But the active
circuit has an active switching device,
like a bipolar transistor or a MOSFET,
connected in series with each bypass
element of the ladder.
The switches are controlled by voltage-detection circuits that only turn a
switch “on” when the voltage across
that particular cell approaches a value
just slightly below the continuousworking-voltage rating of the cell
(2.68 V in the example to follow).
This is called the bypass threshold
voltage. Figure 2 depicts a typical
block diagram of an active chargingcurrent diversion circuit.
The value and wattage of each resistive element should be sized so that
approximately 1 A of diversion current is siphoned off from each cell
whose voltage exceeds the bypass
threshold voltage. The turning “on”
of one or more charging-current diversion switches could (and should) also
be used as a signal to the charging circuit to terminate the current cell-charging cycle. Figure 3 depicts the current in
the bypass circuit versus cell voltage,
May 27, 2002 • ELECTRONIC DESIGN
2
ULTRACAPACITOR PROTECTION
ULTRACAPACITOR PROTECTION
showing the circuit becoming active at
the bypass threshold voltage.
This circuit is more energy efficient
because the switches are “on” only
when a cell needs to have some of its
charging current diverted. If the voltage
across each cell is under the threshold
set for the detection circuit, the switch is
“off” and the resistor isn’t diverting
charging current from the cell. The
main disadvantage here is that separate
voltage-detection and switch-control
circuits are necessary for every cell in the
string, making it potentially more costly and difficult to deploy. Yet, it provides the most protection for the individual capacitor cells in the string.
Implementing Active Equalization:
Figure 4 shows a practical example of a
series string of capacitor cells that have
active capacitor cell voltage-equalization circuits. Eighteen 1700-F, 2.7-V
NESSCAP ultracapacitor cells are linked
in series. This forms a string with an
overall capacitance of 94 F, a voltage rating of 48 V dc, and an ESR of 12.6 mΩ.
Every ultracapacitor cell in the string
contains a pc board mounted across its
terminals. Each pc board has its own
voltage-detection circuitry, switch, and
bypass element. Figure 5 shows a circuit
diagram of the pc board. The bypass
element, R9, is an axial-lead 2.7-Ω, 5-W
metal-oxide resistor. The switch, T2, is a
BC868 or equivalent, npn SMT bipolar
transistor in an SOT89 package. IC1 is a
precision reference, such as a TL431,
used to set the threshold voltage (2.68
V) at which T2 turns on.
Series ultracapacitor strings can usually be ordered from ultracapacitor
manufacturers. They come as preassembled banks, complete with active capacitor cell voltage-equalization circuits
installed, tested, and guaranteed to
function properly.
Do Yang Jung is chief engineer at Ness
Capacitor Co. Ltd., Kyunggi-Do, Korea
(www.nesscap.com). He received his BS
and MS degrees in material engineering at
Hanyang University, Korea. Jung can be
reached at jungdy@nesscap.com.
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ELECTRONIC DESIGN • May 27, 2002
May 27, 2002 • ELECTRONIC DESIGN
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