Bi-Directional Supercap Charger Integrates Backup & Balancing

Bi-Directional Supercap Charger Integrates Backup & Balancing

Technology News |
By eeNews Europe

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) – or even replacing – batteries 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 maximized by reducing the capacitor’s top-off voltage and avoiding high temperatures (>50°C). See Figure 1 for energy density capability and Table 1 for a comparison to alternatives.

Figure 1. Storage Element Energy Density vs. Power Density

Summary of Supercaps vs. Batteries:
• Batteries:

• Good energy density
• Reasonable power density
• High ESR at cold temperatures

• Supercaps:

• Reasonable energy density
• Good power density
• Low ESR – even at cold temperatures (~2x increase at –20°C vs. 25°C)

• Supercap Limitations:

• Limited to 2.5V or 2.75V maximum termination voltages
• Insertion inrush current too high
• No reverse current protection in hold-up applications

• Series Supercap Benefits:

• Allow for better energy utilization due to E = 1/2 CV2
• Simplifies “dying gasp” / backup circuits

• Step-down vs. boost for 3.3V backup

• Good for high power backup, industrial temperatures

• Potential Problems for Series Supercaps:

• SCAPs can have mismatched capacitances
• SCAP leakage mismatches can cause overvoltage over time – cells need continuous balancing
• SCAP capacitance and ESR degrades over time and not always at the same rate
• SCAP degradation accelerated by overvoltage and high temperatures

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 overvoltage 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 overvoltage 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 utilize dissipative (resistor) balancing. Active balancing efficiently shuttles charge between the capacitors, eliminating the power losses and required subsequent recharge cycles with dissipative methods.

A buck-boost IC supercapacitor charging solution that solves these problems already outlined herein needs to possess all of the following performance characteristics:

• Provide backup power as well as charge the supercapacitors
• Flexibility – it must operate efficiently in step-up or step-down modes
• Perform active charge balancing with programmable maximum capacitor voltage
• Provide high charge current capability
• Have accurate programmable average input current limit
• Have a small, low profile solution footprint
• Possess advanced packaging for improved thermal performance and space efficiency

A New IC Does it All

The LTC3110 is a bi-directional, programmable input current buck-boost supercapacitor charger with active charge balancing for 1 or 2 series supercapacitors. Its proprietary low noise buck-boost topology does the work of two separate switching regulators – saving size, cost, and complexity. The LTC3110 operates in 2 modes, backup and charge mode. In backup mode, the IC maintains a system voltage, VSYS, of 1.71V to 5.25V, powered from the supercapacitors’ stored energy. Further, the supercapacitor storage input, VCAP, features a wide practical operating range from 5.5V down to 0.5V. This ensures that virtually all the stored supercapacitor energy is utilized, extending backup times or shrinking the storage capacitors. Alternatively, in charge mode, when the main power system is active, the LTC3110 can autonomously (or through user command), seamlessly reverse the direction of power flow using the regulated system voltage to charge and balance the supercapacitors. VCAP is efficiently charged to above or below VSYS by the buck-boost converter. The device also features a charge mode average input current limit that can be programmed up to 2A with +/-2% accuracy, preventing system power source overload while minimizing capacitor recharge time. See Figure 2 for a typical application circuit.

Figure 2. LTC3110 Typical Application Circuit

The LTC3110’s active charge balancing eliminates the constant drain of dissipative external ballast resistors, ensuring balanced operation and charging even with mismatched capacitors and less frequent recharge cycles. Programmable maximum capacitor voltage regulation actively balances and limits the voltage across each capacitor in the series stack to 1/2 of the programmed value, ensuring reliable operation as capacitors age and develop mismatched capacities. The low RDS(ON), low gate charge synchronous switches provide high efficiency conversion to minimize the charging time of storage elements. As a result, the LTC3110 is ideal for safely charging and protecting large capacitors in backup power applications such as servers and RAID systems and RF systems with battery/capacitor backup.

The LTC3110’s input current limit and maximum capacitor voltage are resistor programmable. Average input current is accurately controlled over a 0.125A to 2A programming range. Pin-selectable Burst Mode operation improves light-load efficiency and reduces standby current to only 45µA, with a shutdown current less than 1µA. Other features of the LTC3110 include high 1.2MHz switching frequency to minimize external component size, thermal overload protection, two voltage supervisors for direction control and end of charge and one general purpose comparator with an open-collector output for interfacing with a μC or µP. The LTC3110 is housed in compact, thermally enhanced 24-lead 4mm x 4mm QFN and TSSOP packages, both featuring H-grade operation from -40°C to +150°C.

To summarize, the LTC3110’s key features are:

  • • VCAP Operating Range: 0.1V to 5.5V
  • • VSYS Operating Range: 1.71V to 5.25V
  • • Automatic Switching from Charge to Backup Mode
  • • Programmable ±2% Accurate Charge Input Current Limit from 125mA to 2A
  • • ±1% Backup Voltage Accuracy
  • • Automatic Capacitor Balancing
  • • Fixed 1.2MHz Frequency Switching
  • • Burst Mode® Operation: 45μA IQ
  • • Extra Programmable Multipurpose Comparator with Open-Collector Output
  • • Open-Collector Outputs to Indicate Direction of Operation and End of Charge
  • • Low Profile TSSOP-24 and 4mm × 4mm QFN-24 Packages

Efficient Charging
A proprietary switching algorithm provides seamless transitions between operating modes and eliminates discontinuities in the average inductor current, inductor current ripple, and loop transfer function throughout all regions of operation. These advantages result in increased efficiency, improved loop stability, and lower VSYS voltage ripple in comparison to the traditional 4-switch buck-boost converter. The switch topology for the buck-boost charger is shown in Figure 3.

Figure 3. LTC3110’s Charger Buck-Boost Switch Topology

Two switches (C and D) connect SW2 to VSYS to provide high efficiency over the entire output voltage range. The LTC3110 has very high efficiency, almost 95% as shown in the graph of Figure 4.

Figure 4. LTC3110 Efficiency vs. VOUT Characteristics

The LTC3110 is a bi-directional average input current controlled buck-boost DC/DC supercapacitor charger/regulator that utilizes a proprietary switching algorithm, enabling its outputs to be regulated above, below, or equal to the input voltage. The device is composed of a 1-chip, compact, powerful and flexible solution. It integrates high efficiency supercapacitor charging, backup regulation and balancing/protection functions in a versatile format making it easily adaptable to multiple system configurations. This significantly simplifies what was historically a very difficult design task.

About the authors:

John Bazinet is Staff Scientist, Power Products, at Linear Technology Corporation.
Steve Knoth is Senior Product Marketing Engineer, Power Products at Linear Technology Corporation.
Sam Nork is Director, Boston Design Center, Power Products, at Linear Technology Corporation.

If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :    eeNews on Google News


Linked Articles