How to achieve complete depletion of electric energy in supercapacitors

When charging a supercapacitor for a backup power system such as a solid state drive (SSD) or a portable medical system, the value, size, and cost of the supercapacitor are directly proportional to the required hold time. Once the user removes the system from the input power and the operation is switched to the supercapacitor, your system requires minimal runtime. The goal is to make the super capacitor just large enough to power the system in the time it takes for your system to write critical information to non-volatile memory and shut down. But sizing the super capacitor is not your only challenge.

Because the voltage of the super capacitor changes, power is required between the super capacitor and the system supply voltage rail. Since the rated voltage of a supercapacitor is usually only a few volts, a step-up converter is required to raise the voltage to a 3.3V, 5V, or 12V system rail. If you just want to discharge your own supercapacitor to 2.5V, then this bi-directional backup power solution will fit perfectly between the supercapacitor and the system rail.

However, if the supercapacitor is only discharged to 2.5V, a considerable amount of power will remain in the supercapacitor when the system is shut down. The electric energy stored in the supercapacitor is equal to 1⁄2 CV2. For example, charge your 1F supercapacitor to 5V with your input power so that when it is discharged to 2.5V, the energy drawn from the capacitor is approximately 9.4J. However, if the just-mentioned supercapacitor is used to power the system to reduce its voltage to 0.7V, then the energy drawn from the capacitor is about 12.2J - more than 30% of the power drawn!

The TI Designs reference design with a low input voltage, high current boost converter with TPS61088 shows a simple design to extract more from ultracapacitors or other extremely low input voltage sources such as single cell alkaline or nickel cells. Electric energy. The high-power TPS61088 step-up converter completes the task of converting the supercapacitor's energy into the system's voltage rails. The tiny low-power TLV61220 step-down converter provides a bias voltage to power the TPS61088. This small, low-cost add-on is key to extracting almost all of your power from your supercapacitors.