Super-capacitors are emerging as a possible alternative to batteries
for energy-storage in some applications. However, the major advantages
that super-capacitors offer must be balanced against some significant
disadvantages.
On the plus side, super-capacitors have a virtually unlimited
lifetime of around 10,000,000 charge/discharge cycles and they can
charge and discharge at phenomenal currents in excess of 1,000 Amperes.
They are also largely immune to temperature variations. However, they
cannot compete with batteries in energy density or cost: typically
super-capacitors offer just 3-5% of the energy density of Li-Ion
batteries and cost 10 to 15 times more.
There are, however, some applications for which the advantages
outweigh even these limitations. But super-capacitors also present two
significant design challenges in how they charge and retrieve energy.
With charging, the challenge is to transfer energy to the capacitor
when it is completely discharged (effectively presenting a short
circuit), while retrieving energy also becomes progressively more
difficult as the capacitor voltage approaches 0V. Overcoming these two
challenges is the main hurdle for the efficient use of super-capacitors
as replacements to battery storage.
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| Figure
1: SMPS-based constant-current charger |
The charging challenge
Linear chargers dissipate a large percentage of energy when charging a
capacitor which is completely discharged. Then, as the capacitor
charges, a smaller percentage of the energy is lost, and more energy
makes it into the capacitor. Adding the power absorbed by the capacitor
and the power dissipated in the charger, the charger will actually
dissipate more than half of the available energy as heat, over a full
charge cycle. In fact, a linear charger throws away almost 58% of the
available charging energy as heat.
The other charging option is to use a system based on a Switch-Mode
Power Supply (SMPS), where the difference between the output capacitor
voltage and the source voltage is dropped across an inductor. In a
voltage-regulated SMPS design (Figure
1, above) the inductor current is driven by the difference
between the voltage across the output capacitor and a fixed reference
voltage. This difference voltage is then amplified, integrated, and
phase-shifted, before it is fed back into the Pulse-Width-Modulation
(PWM) comparator.
The PWM comparator then uses that voltage to determine how much
current to pump through the inductor on the next cycle. Often, SMPS
circuits can achieve conversion efficiencies of greater than 80-90%,
with careful design.
In the charger circuit, very little time is spent operating with a
constant output voltage. By definition, the charger circuit is designed
to do most of its work while ramping up the capacitor voltage from zero
to the final voltage. It is during this charge-up period that energy
transfer needs to be optimised.
The charging circuit requires a system that will regulate the
charging current of the capacitor, independent of the output voltage,
and only use the voltage feedback as the means of determining when the
charge is complete. Figure 1 shows how this can be accomplished using a
variation on the typical SMPS design. Here, the current in the inductor
is regulated by comparing the current in the inductor against two fixed
levels; one at the maximum desired current, and the other at the
minimum.
Initially, it will take the inductor very little time to ramp up
from the minimum to maximum current, as the voltage across the inductor
is at its maximum. The discharge time will be correspondingly longer,
as the inductor has to discharge into a relatively small voltage. As
the charge in the capacitor increases, however, the voltage difference
will drop, increasing the ramp-up time, and the capacitor voltage will
rise, shortening the discharge time.
While something similar can be implemented with a traditional
time-base driven PWM, the selection of the inductor becomes critical to
maintaining the minimum current level. Additionally, instability can
occur when the duty cycle is greater than 50%. A simple solution to
avoid this instability is to use a relaxation-oscillator, 555
Timer-style system, using two comparators and a SR flip-flop, so that
the inductor component values set the frequency.
