[Technology Report]
Get The Lowdown On Ultracapacitors
Now that you're armed with more capacitance than earlier generations of engineers ever dreamed of, what do you do with it?
A designer could just use a series resistor to limit current, but that would result in an unacceptably long time for charging the capacitor. DeLurio describes a PC Card application in which sizing the resistor for PC Card host/card negotiation current limits would yield a charge time on the order of seven minutes.
Allowing a higher current to flow after host/card negotiation would reduce charging time. In fact, that concept could be extended to providing a means for switching in a succession of resistors as the capacitor charged up.
Yet this approach “requires that the timing of the switching points be closely controlled, which would demand very accurate and expensive resistors, or monitoring by additional voltage detectors,” DeLurio says. “Furthermore, when the capacitor is fully charged and the PC card is removed, the energy stored in the capacitor would be sufficient to damage the connector pin.”
Instead, DeLurio introduces a new Analogic Tech “smart switch.” The AAT4620 current-limited P-channel MOSFET power switch is designed expressly for wireless-card ultracappower applications. It has two independent, resistor-programmable current limits and a power loop controlled by the AAT4620’s die temperature.
Moving up the power scale, “Super-Capacitor Power Storage” by Keith Curtis of Microchip starts by noting the inefficiency of charging an ultracap using a linear charger. He then goes on to propose a modified dc-dc buck regulator (Fig. 6a) as the appropriate charging circuit because it can “regulate the charging current of the capacitor, independent of the output voltage... using the voltage feedback as the means of determining when the charge is complete.”
The effect is somewhat like what DeLurio described, but more general. Explaining the circuit’s operation, “Current... 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,” Curtis says.
“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,” he notes. “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.”
Curtis says that switching frequency is based on a “relaxation-oscillator, 555-timer-style system, using two comparators and a SR flip flop,” so that the inductor component values will set the frequency.
He then uses similar logic to arrive at a switched-mode boost circuit for converting the capacitor output voltage into a reasonably constant load voltage. The upshot is that Curtis arrives at a combined buck/boost charge-discharge circuit in which a switching MOSFET replaces the flyback diode in the charging circuit (Fig. 6b). A PIC microcontroller integrates control and most of the necessary peripherals.
Microchip worked with AMSAT-NA, the not-for-profit private organization that develops amateur-radio satellites. AMSAT’s next big project, the Eagle satellite, is slated for launch in March 2009. To make Eagle function for decades, it will have a power system based on this work that combines solar panels, lithium-ion batteries, and ultracaps in an integrated power system that will optimize the use of each of those components.
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