Short-term Supercapitor Benefits

Supercapacitors are emerging as an alternative to batteries in applications where the importance of power delivery trumps that of total energy storage. Supercapacitors have a number of advantages over batteries that make them a superior solution when short term, high power is needed, such as in power ride-through applications. These advantages include lower effective series resistance (ESR) and enhanced durability in the face of repeated charging.



Like batteries, supercapacitors have some specialised application needs that make using a dedicated IC desirable. Supercapacitor technology can now offer capacitors as large as 100F, but the maximum working voltage on these capacitors is 2.7V or less. Because most systems require operating voltages higher than this, many supercapacitors are supplied as a pair of capacitors within a single, centre-tapped package. The LTC4425 is designed to charge two stacked supercapacitors and provide a regulated output voltage for the system load.



Ride-through systems



Many electronics systems require a short-term power backup system that allows them to ride through brief interruptions in power. In a similar vein, some systems need time to save states, or empty volatile memory or perform other housekeeping tasks when power is abruptly removed. For example, a hard drive may need to park the heads, so that they don’t land on the media surface. This is an electromechanical system that requires 20ms–100ms of continuous power before it can completely shut down.



Another example involves the effect of large electrical machines on power systems. If a large electric motor is started, such as a commercial building air conditioner or elevator, the mains supply may collapse for several line cycles. Usually the input supply stores only enough energy for between half a cycle and one cycle. Devices powered by the input supply need a way to operate normally until the mains recovers.



Ride-through applications can certainly be implemented with battery backup, but in many cases it requires a very large battery array to satisfy the ride-through power requirements. Although batteries can store a lot of energy, they cannot supply much power per volume due to their significant source impedance. Batteries also have relatively short lives, 2~3 years, and their care and feeding requirements are substantial.



Supercapacitors, on the other hand, are well suited to short-power-burst, ride-through applications. Their low source impedance allows them to supply significant power for a relatively short time, and they are considerably more robust than batteries.



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Figure 1: Complete supercapacitor-based power ride-through system



Figure 1 shows a complete power interruption ride-through system using the LTC4425, LTC4416, LTC3539 and LTC3606. Figure 2 shows the layout. This design can hold up a 3.3V rail at 200mA for almost eight seconds.



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Figure 2: Front and back board layout used to test the circuit in Figure 1



The LTC3606 is a micropower buck regulator that produces 3.3V. The LTC4416 provides a dual ideal diode-OR function to ensure maximum efficiency when switching from the regular input to the supercap. The LTC3539 is a micropower boost regulator with output disconnect. This boost regulator operates down to 0.5V, and can support loads of 1.3A × VOUT/VIN at its output. The supercapacitor is a CAP-XX HS206F, 0.55F, 5.5V capacitor.



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Figure 3: If the boost regulator is disabled in the circuit of Figure 1, the ride-through applications can support a 0.67W load for about 4.68s.



Figure 3 shows the waveforms if the LTC3539 boost circuit is disabled. Run time, from input power off to output regulator voltage dropping to 3V, is 4.68s. Figure 4 shows the waveforms if the LTC3539 boost circuit is operational. Run time, from input power off to output regulator dropping to 3V, is 7.92s.



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Figure 4: With boost regulator enabled in the circuit of Figure 1, the ride-through applications can support a 0.67W load for about 7.92s.



When the LTC3539 boost regulator is disabled, as soon as input power falls, the LTC4416 based ideal diodes switch the input energy supply for the LTC3539 buck regulator to the supercap. In Figure 3, the voltage across the supercap (VSC) linearly decreases due to the constant power load of 200mA at 3.3V on the buck regulator (VOUT).



When the input voltage to the LTC3539 reaches the dropout voltage of the regulator, the output voltage is seen to track the input voltage. At 4.68s after input power removal, the voltage on the supercap reaches 3.0V plus the dropout voltage, and VOUT drops below 3V. The buck regulator continues to track the supercap voltage down until it reaches 2V, whereupon the buck regulator shuts off.



In Figure 4, the voltage across the supercap (VSC) linearly decreases due to the constant power load of 200mA at 3.3V on the buck regulator. When VSC reaches 3.4V, the regulation point of the boost regulator, the boost regulator begins switching. This shuts off the ideal diode and disconnects the buck regulator from the supercapacitor. The energy input to the buck regulator is now the boost regulator’s output of 3.4V. VSC remains at 3.4V, but the supercap begins to discharge exponentially, because as the input voltage of the boost regulator drops, it must draw higher and higher current to sustain its output at 3.4V.



Because the input of the buck regulator remains at 3.4V, its output remains in regulation. When the boost regulator reaches its input UVLO it shuts off, and its output immediately collapses. Since its input voltage has now collapsed, the buck regulator shuts off.



Energy scavenging



What voltage should the boost output be set to? Clearly, operation is identical, with or without the boost circuit enabled until the input dropout of the buck regulator is reached. One goal in the design is to minimise the amount of time that the boost regulator is used in the power chain, because each additional regulation step lowers the overall efficiency. Here, we set the boost regulator output voltage as close to the buck regulator input dropout voltage as possible, or 3.4V.



The boost regulator must have a synchronous output to maximise efficiency once the boost regulator engages. This implies a boost regulator with a ‘blocking’ output. This in turn necessitates the second ideal diode to allow the supercapacitor to power the buck regulator until the boost regulator engages. The boost regulator must operate to as low a voltage as possible to ensure that the maximum amount of energy is scavenged from the supercapacitor.



If the supercapacitor is initially charged to 5V, then the energy in the supercapacitor is:

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The output power is 3.33V at 0.2A = 0.67W, so the percentage of the energy stored in the supercap that is extracted with a buck-only circuit is:

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The percentage of the energy stored in the supercap, extracted when the boost regulator is enabled, is:

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The percentage of energy stored in the supercapacitor that is recovered increases from 45.1% to 77%. This allows use of a smaller, less expensive supercapacitor.



The power ride-through system shown here uses a 0.55F supercap to hold up power long enough for a microcontroller to complete some last gasp housekeeping tasks. One way to extend the ride-through time for a given supercapacitor is to add a boost regulator to the system, which allows for energy scavenging. The run time of a given supercapacitor can be extended by >30% if energy scavenging is used. This is particularly relevant if the supercapacitor operating voltage is reduced to ensure high temperature reliability.



In addition, the shape of the output voltage is considerably improved as the input voltage to the output regulator is now square in shape. This results in a steady 3.3V output voltage with a sharp cutoff, instead of a ramped voltage drop as the supercap drains.