Battery Technology - May 2021 - 6
Supercapacitors
V(n001)/lx(U1:Rn)
0.15KΩ
R
(kΩ)
Rn@25C
fixed
Resistor
-2.93Ω
-6.00KΩ
26mA
24mA
22mA
20mA
18mA
16mA
14mA
12mA
10mA
8mA
6mA
4mA
2mA
0mA
0.0Ks
T (°C)
Figure 4. Fixed resistor and NTC thermistor
response vs temperature.
Current
T = 50ºC
T = 40ºC
T = 25ºC
l(C)
0.5Ks
1.0Ks
1.5Ks
2.0Ks
2.5Ks
3.0Ks
3.5Ks
4.0Ks
4.5Ks
5.0Ks
Figure 6. View of supercapacitor's current and NTC resistance in a 10F supercapacitor and
R150@25 °C NTC system.
Time
Easy to implement calculations
Always dissipating energy
Single component solution
Physically large when dealing
with large currents
Lowest-cost solution
Unstable temperatures
Fast
Not suitable for large
temperature ranges
Small
Not suitable for applications
with high number cycle
Inexpensive
Uncontrollable
Sensitivity
Self-heating
Protectionary fast response
time
Heat dissipation
Conclusion
Power
Unprotected equipment
Protected equipment
NTC absorbed power
Time
Figure 7. Overall effect of an NTC upon current inrush.
Fixed Resistor
NTC
are outweighed by control accuracy and
improved system reliability.
IC chipsets used in conjunction with
supercapacitors generally offer features
grouped into:
* Cell Balance Control
* Current Control
* Balance Control & Overvoltage Protection
* Backup & Voltage Regulation
Charge control chipsets use elaborate
and comprehensive active charge control
methods to perform Constant Current and
Constant Voltage (CC/CV) charging, with
programmable input current limits (Figure
9). Many controller ICs come with built-in
voltage regulation, monitoring, and multicell balancing when implementing a stack
of supercapacitors. When implementing a
stack or bank of supercapacitors, it is critical to have a balancing circuit on the supercapacitor stack that is purchased or
an IC that provides active balancing.
Supercapacitor reliability and life are
highly dependent on operating voltage -
derating the voltage by 0.1 V of a 2.7-V rated
part can extend the life of a component by a
factor of 2. Making use of a CC/CV charging
IC with stack voltage regulation and monitoring removes much of the heavy lifting
and board space for an equivalent discrete
solution but comes at a premium and sometimes limits the number of supercapacitors
that can be balanced and monitored.
Figure 5. The effect of NTC body temperature upon current flow through the NTC.
Passive Charge
Control
MOSFETs work very efficiently when
designed with proper caution. Figure 8
shows the fundamental implementation
of a p-channel MOSFET using a low-voltage gate driver controller. The charge circuit is broken up into two sections: a load
switch (Q2; p-channel MOSFET) and a
gate control (Q1; n-channel MOSFET).
The control FET can utilize low-voltage
control signals to effect slew rate of the
load switch. This circuit is based upon the
fact that a small 'control' signal to the
gate of the load switch MOSFET (Q2) can
easily and accurately control the current
delivered to a large supercapacitor. The
disadvantage of this configuration is the
need for an added MOSFET and passive
components. Added components increase
cost, size, and weight to the circuit but in
many cases, any perceived disadvantages
Pro
Con
A high-level comparison of passive
and active supercapacitor charging con-
Table 2. Pros and cons of a fixed resistor and an NTC.
6
Battery Technology, May 2021
Cov
ToC
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Battery Technology - May 2021
Table of Contents for the Digital Edition of Battery Technology - May 2021
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