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.

Battery Technology, May 2021

Cov

ToC

http://www.abpi.net/ntbpdfclicks/l.php?202105TBNAV

Battery Technology - May 2021

Table of Contents for the Digital Edition of Battery Technology - May 2021