DESIGN SOLUTION 7565

Keep Your System Up and Running with a Single Supercapacitor

By: David Andeen

Abstract:

This design solution presents a novel solution for backing up system power in both battery and line-powered systems. The elegant architecture runs from a single supercapacitor, provides a tightly regulated 5V output, at up to 3A, and features 94% efficiency.


Introduction

Modern life now requires continuous operation and constant uptime. Once limited to mission-critical installations like hospitals and data centers using uninterruptable power supplies (UPS), many systems, from homes to utility meters to scanners, now utilize backup power. Advances in supercapacitor technology and analog power supplies now make the addition of backup power to a system more compact and convenient than ever.

While many types of backup power options exist, supercapacitors present an elegant solution for board-level electronics. With a much greater energy per unit volume or mass than conventional capacitors, supercapacitors can drive a system for much longer, providing more than “last gasp” operation. In contrast to rechargeable batteries, supercapacitors charge much faster and tolerate many more charge cycles. Most effective in short-term burst modes, supercapacitors serve as an excellent option for system backup during short outages such as power outages and battery exchanges.

Implementing a backup power-supply circuit used to be a significant undertaking. The image in Figure 1 shows a hypothetical system in which two or more supercaps provide backup for a 5V system. A typical supercap charges to 2.7V. To provide a 5V rail, the 2.7V must either get boosted or multiple supercapacitors must be used to create a 5V output. In Figure 1, two or more supercapacitors provide assurance that a reliable 5V rail will be available. The additional cost and complexity come with the cell balancing (either active or passive), which is a requirement when using multiple supercapacitors.

This block diagram shows a multiple supercap backup system with required cell balancing and a buck converter to provide the 5V output. Figure 1. This block diagram shows a multiple supercap backup system with required cell balancing and a buck converter to provide the 5V output.

Figure 2 shows a system using a single supercapacitor. A single supercapacitor architecture eliminates the complexity and cost of cell balancing. In this simpler system, the output of the main system buck or boost would be around or above 5V, enough to overcome the voltage drop across the diode and provide 5V to the system. The supercapacitor gets charged by a charging device, and discharges through a boost converter, when needed. Diodes allow either the primary power source or the supercapacitor to power the system.

This block diagram shows a single supercap backup system, which eliminates the need for cell balancing and uses a boost converter to provide the 5V output. Figure 2. This block diagram shows a single supercap backup system, which eliminates the need for cell balancing and uses a boost converter to provide the 5V output.

In Figure 3, the MAX38889 Continua from Maxim Integrated® showcases the simplest circuit. The MAX38889 is a reversible buck-boost. Running off the main system power rail, the MAX38889 charges the supercapacitor by stepping down the voltage. When system power gets removed, the MAX38889 senses the lack of power and reverses, to boost the supercapacitor to the appropriate system voltage. The duration of backup depends on the system power draw. The features of the MAX38889 allow for maximum power from a single, 2.7V supercapacitor, while minimizing extra circuitry, reducing overall BOM cost and simplifying the overall implementation.

This block diagram shows a system using the Continua MAX38889. This simple architecture manages the supercap charge and discharge, while providing a consistent 5V output. Figure 3. This block diagram shows a system using the Continua MAX38889. This simple architecture manages the supercap charge and discharge, while providing a consistent 5V output.

Building power architectures for continuous operation requires additional circuitry and a power source. Supercapacitors uniquely fit the application, especially for 5V and below applications with battery changes or power outages. The MAX38889 reduces circuit complexity and cost, while providing a highly reliable, single supercapacitor solution up to 5.5V and 3A output.

Maxim Integrated is a registered trademark and Continua is a trademark of Maxim Integrated Products, Inc.