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Supercap Backup Circuit Provides Reliable Uninterrupted Power

Sam Nork - Boston Design Center Manager Sep 29th 2014

Temporary backup power is a common requirement for a wide range of applications whenever the main power source is suddenly unavailable. Examples include data backup applications ranging from servers to solid-state drives, power fail alarms in industrial or medical applications, and a host of other “dying gasp” functions where orderly power-down must be assured and system status communicated to a powered host. In the past, these types of high reliability systems used batteries to provide an uninterrupted power source whenever the main supply of power was inadequate or unavailable. However, many trade-offs accompany battery backup, including long charge times, limited battery lifetime and cycle life, safety and reliability concerns, and large physical size. With the advent of high value electric double layer capacitors, better known as supercapacitors, alternate backup architectures may be employed which eliminate many of these trade-offs.

Batteries vs. Capacitors

Systems relying on batteries for backup power require that a fully charged battery is available at all times with suitable capacity to keep volatile memory alive or alarms sounding until power is restored. Typically, systems employing battery backup enter a low power standby state whenever the main power fails, and only the critical volatile memory or alarm sections of the systems remain powered. Since power failure duration is impossible to predict, such systems require oversized batteries to avoid the possibility of data loss during a lengthy outage.

Capacitor based backup systems use a different methodology. Unlike battery based systems which provide continuous power during the entire backup time, capacitor based systems require only short-term backup power in order to transfer volatile data into flash memory or provide “dying gasp” alarm operation for a minimum necessary amount of time. Once the required data has been saved and the power fail alarms have been properly issued, the power restoration time is unimportant.

There are several advantages to this approach. First of all, the numerous trade-offs associated with batteries can be avoided altogether. There is also no longer a need to oversize the energy storage elements for a worst case backup duration. While the backup power requirements of a capacitor based system are typically much higher than those of a battery based system, the backup energy requirements are generally much lower. Since the cost and size of a backup solution is usually dominated by the storage element, capacitor solutions are often smaller and cheaper. With the emergence of small, relatively inexpensive supercapacitors capable of storing numerous Joules of energy, the number of backup applications that can be satisfied with capacitors instead of batteries has grown considerably.

Backup System Requirements

All capacitor based backup systems share many common elements. Power Path™ control and power fail detection are required to supply power to the load from the proper source and to alert the system when transitioning from normal operation into backup mode. The storage capacitor needs to be charged, and ideally this is done in a fast, efficient manner. Since proper backup is not possible unless an adequate number of Joules are stored on the backup capacitor, many applications require that charging is completed by the time the system boots up and is ready for operation. Hence, high charge currents are generally desireable, and since supercapacitors typically have a max operating voltage of 2.7V, it is common and often necessary for several to be stacked in series. In such cases, provision must be made for balancing and protecting the capacitors as they charge to prevent damage and lifetime degradation due to overvoltage.

Figure 1 shows a simplified schematic for the LTC3350, a capacitor charger and backup controller IC designed specifically to address capacitor backup applications. The LTC3350 includes all of the features necessary to provide a complete, standalone backup controller for applications needing capacitor based backup. The device can charge, balance and protect up to four capacitors in series. Input power fail threshold, capacitor charge voltage and regulated minimum backup voltage can all be programmed with external resistors. In addition, the device contains a very accurate 14-bit internal measurement ADC which monitors input, output and capacitor voltage and current. The internal measurement system also monitors parameters associated with the backup capacitors themselves including capacitor stack voltage, capacitance and stack ESR (Equivalent Series Resistance). All system parameters and fault status can be read back over a two-wire I2C bus, and alarm levels can be set to alert the system to a sudden change in any of these measured parameters.

High Current Supercapacitor Charger and Backup SupplyFigure 1. High Current Supercapacitor Charger and Backup Controller

Supercapacitor Charging Basics

Charging a supercap is similar to charging a battery except for a couple of key points. The first is that a completely discharged capacitor can be charged at full current for the whole charge cycle, whereas a battery needs to be trickle charged until the battery reaches a specified minimum voltage. A second point is that no termination timer is required for capacitors. Once the final “float” voltage is reached, no additional charge can be stored by the capacitor and charging must stop. If two or more supercaps are charged in series, any mismatch in capacitance from cell to cell will result different rates of voltage increase across each capacitor as the stack is charged. Additional safety features need to be in place to assure that none of the capacitors exceeds its maximum voltage rating during the charging cycle. In addition, a balancing system must be used to assure that once the stack is charged, all of the cells are forced to the same voltage and do not drift apart over time due to self-discharge differences. Such cell to cell balancing ensures maximum capacitor lifetime.

The charging circuitry in the LTC3350 consists of a high current, synchronous buck controller with a resistor programmable max charge current and max stack voltage (Figure 2). Since the charger is powered from the same supply that is powering the load, the LTC3350 also contains a separate programmable input current limit which automatically reduces charge current to the capacitors under heavy VOUT load conditions. Internal, low current balancers (not shown in Figure 2) force all cells to within 10mV of each other up to a max voltage of 5V per cell. Internal protection shunts (also not shown) will automatically reduce charging current and shunt the remaining charge current around any capacitor that has reached the 2.7V default or a user-programmed max cell voltage. In addition, the stack charge voltage may be reduced under software control in order to optimize capacitor lifetime for a given backup energy requirement. More on this topic follows below.

Power Path Block Diagram—Power Available from VINFigure 2. High Current Supercapacitor Charger and Backup Controller

Backup Mode

Once the backup capacitor stack is charged, the system is now able to provide backup power. Charge mode and backup mode are determined by the voltage on the PFI (Power Fail Input) pin. If the VIN voltage drops such that the PFI comparator trips low, the part immediately enters backup mode (see Figure 3). VOUT will drop as VIN drops, and once the VOUT voltage falls below the capacitor stack voltage, the OUTFET ideal diode conducts to prevent VOUT from falling further. Once VOUT falls to a voltage programmed by a resistor divider on the OUTFB pin, the capacitor charger operates in the opposite direction as a synchronous boost backup DC/DC converter using the VCAP stack as its input source and VOUT as its regulated output. The boost backup converter will continue to run until it can no longer support the VOUT load conditions and the voltage on VOUT falls below the 4.5V UVLO point. This allows virtually all of the usable energy in the supercap stack to be transferred to the load during backup since the boost will continue to run when the stack voltage is well below 4.5V. A typical backup scenario is also shown in Figure 3. In this example, a stack of four series capacitors is charged to 10V, and during backup mode VOUT is regulated to a minimum of 8V until all energy is depleted from the backup capacitors.

Power—Path Operation in Backup ModeFigure 3. Power Path Operation in Backup Mode

“Health” Monitoring Assures Reliability and Optimizes Performance

In high reliability systems requiring short-term backup power, adequate energy must be stored and available in order to perform critical functions immediately following a main power failure. It is essential that the backup energy source is able to deliver the necessary backup power. Supercapacitors are an excellent choice for such applications due to their extremely high capacitance per unit volume and very low ESR. However, like batteries, their performance will degrade over time. Capacitor lifetime is commonly (and somewhat arbitrarily) defined as the time required for capacitance to drop by 30% and/or ESR to increase by 100%. As shown in Figure 4, capacitor degradation is accelerated by either high operating voltages or elevated temperatures. Since both capacitance and capacitor ESR are critical for ensuring that the system can perform a reliable back-up, it is important that the system is able to monitor and report the “health” of the backup capacitors as they age.

Typical Supercapacitor Lifetime VS. Temperature and VoltageFigure 4. Typical Supercapacitor Lifetime vs. Temperature and Voltage

The LTC3350 automatically monitors both stack capacitance and stack ESR at a time frequency chosen by the user once the capacitor stack is fully charged. The part employs a precision current source, precision timing circuit and its internal 14-bit ADC to accurately monitor the stack capacitance. A precise, programmed current is pulled from the top of capacitor stack while the charger is forced off. The time required for the capacitor stack to drop by 200mV is precisely measured, and the stack capacitance is calculated from these parameters. Once the capacitance test is completed, the ESR test is done by measuring the stack voltage with and without the high current charger running to re-charge the stack. Using the charger to perform this test eliminates the need for an external high power test load. The instantaneous increase in stack voltage once the charger is enabled corresponds to the measured charge current * stack ESR. The most recent values for capacitance and cap ESR may be read back at any time over I2C.

Once the stack capacitance and ESR values are known, it is straightforward to compute the minimum stack voltage necessary to assure a reliable backup for a given application. Since most backup systems are designed with built-in margin, it is often safe to reduce the stack voltage from its nominal value, thereby maximizing the lifetime of the capacitors. This is easily accomplished through software control of the LTC3350 VCAP feedback DAC voltage.

Conclusion

The combination of very high capacitance and very low ESR have enabled supercapacitors to provide new methods for solving common problems such as backup power solutions. However, big leaps in performance rarely come without trade-offs. Making effective use of supercaps often requires series-connected cells, which in turn require protection and balancing circuits. While the cycle life and lifetime in general of a supercapacitor may far exceed that of a competing battery technology, small changes in cap voltage and temperature may lead to dramatic changes in the capabilities of the system over time. For this reason, “health” monitoring is often a required feature in any capacitor-based backup system. New products, such as the LTC3350, aim to address issues like these that pertain specifically to supercapacitor backup applications, and provide the simplest means possible for developing a reliable, flexible, high performance backup solution.