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Examining a bi-directional supercap charger

Posted: 11 Aug 2015 ?? ?Print Version ?Bookmark and Share

Keywords:supercapacitor? capacitor? switching algorithm? charger? regulator?

A supercapacitor (also known as an SCAP, supercap, double-layer capacitor, or ultracapacitor) is more than just a capacitor with a very high level of capacitance. Compared to standard ceramic, tantalum or electrolytic capacitors, supercaps offer higher energy density and higher capacitance in a similar form factor and weight. As the cost to produce them continues to decline and as the market gets exposed to their capabilities, supercapacitors are carving out an ever-growing niche between conventional capacitors and batteries.

Although supercaps require some "care and feeding," they are augmenting (as a complementary power source to reduce strain on the primary source, thereby extending its life) C or even replacingbatteries in data storage applications requiring high current/short duration backup power. Furthermore, they are also finding use in a variety of high peak power and portable applications in need of high current bursts or momentary battery backup, such as UPS (uninterruptible power supply) systems. Compared to batteries, supercaps provide higher peak power bursts in smaller form factors and feature longer charge cycle life over a wider operating temperature range. Supercap lifetime can be maximised by reducing the capacitor's top-off voltage and avoiding high temperatures (>50C). See figure 1 for energy density capability.

Figure 1: Storage Element Energy Density vs. Power Density.

The supercap design dilemma
Supercaps have many advantages; however, when faced with charging energy storage devices in series, the end product designer may be faced with such problems as cell balancing, cell over-voltage damage while charging, excessive current draw and a large footprint/solution when space is critical.

Cell balancing of series-connected capacitors ensures that the voltage across each cell is approximately equal; a lack of cell balancing in a supercapacitor may lead to over-voltage damage. External circuitry with one balancing resistor per cell is one solution to this problem. The balancing resistor value will depend on the supercap operating temperature and its charge/discharge profile. In order to limit the impact of the current drain due to the balancing resistors on supercap energy storage, designers can alternatively use a very low current active balancing circuit. Another source of cell mismatch is differences in leakage current. Leakage current in the capacitor cells starts off quite high and then decays to lower values over time. But if the leakage is mismatched between series cells, the cells may become over-voltaged upon recharge unless the designer swamps out the leakage with the balance resistors. However, balancing resistors burden the application circuit with unwanted components and load current.

Supercap charger IC design challenges
Some of the tougher issues a designer must consider in the beginning of a supercapacitor charging design are the needs for:
???Backup capability. The supercapacitor storage capacitors ultimately provide the stored energy to back up the main power rail should it fail. As a result, two separate power converters are generally required: the first is for charging the supercapacitors and the second is for holding up the main power rail from the stored energy in the supercapacitors. A single converter to service both of these functions is ideal; however it must operate bi-directionally, sense when the main power is absent and seamlessly transition between backup and charging modes while having a wide operating range to ensure that all of the available backup energy is used.
???High efficiency and high charge current. A high efficiency, high current buck-boost supercapacitor charger/balancer can include all the features and functionality required to exploit the benefits of super capacitors. Whereas discrete solutions, while possible, are complicated, larger, lower efficiency and less accurate.
???High accuracy and load sharing capability. Input current limit with 2% accuracy and input load sharing enables multiple loads to share the full capability of the same power source with minimal derating/margin. This functionality is impractical to achieve with a discrete solution.
???Active balancing. Most supercapacitor systems utilise dissipative (resistor) balancing. Active balancing efficiently shuttles charge between the capacitors, eliminating the power losses and required subsequent recharge cycles with dissipative methods.

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