Due to their high capacitance compared to conventional capacitors, supercapacitors (SK) are ideal as electrical energy storage devices that can supplement or replace batteries in power supply applications. While batteries only release their electrical power rather slowly, supercapacitors can handle short charge and discharge cycles and power peaks particularly well.

The typical voltage of a supercapacitor of about 2.7 V is not sufficient as an operating voltage in many applications, so that a cascade of series-connected SK cells has then to be built up for this purpose.

Due to fluctuations in capacitance and insulation resistance caused by manufacturing or aging, the voltage drop across individual capacitors can deviate significantly from their rated voltage. Therefore, voltage leveling (balancing) is required to prevent accelerated aging of the capacitor cell. In AppNote ANP090, the effect of unbalanced voltage division in such series circuits is explained in principle. For better comprehensibility, the phenomenon is illustrated by a series circuit of two capacitors. In addition to the theoretical derivation, some measurements as well as discussions based on practical examples show which approaches result for passive or active balancing. The aim is to give an overview of possible balancing strategies as well as an understanding of the different concepts. Depending on the application requirements, the developer can then select a strategy and adapt it accordingly to his own constraints.

Content:

• Voltage scattering and imbalances in capacitor series circuits.

• Balancing current and balancing speed

• Strategies for voltage balancing

    - Passive balancing with resistors

    - Passive balancing with Zener diodes

    - Passive balancing with MOSFETs

    - Active balancing with an operational amplifier

    - Active balancing with a DC/DC converter

• Measurements and component selection

Download: ANP090