POWER ELECTRONICS HANDBOOK
Applying large banks of supercapacitors It pays to know techniques for mitigating leakage current and overvoltages in uses where several supercapacitors work in parallel.
MICHELE KINMAN
ADVANCED LINEAR DEVICES INC.
IN APPLICATIONS REQUIRING rapid charge/ discharge cycles or short-term energy storage, you’ll
Precision dual SAB overt voltage protection PCB SABMBOVP2XX schematic diagram V+
often find supercapacitors connected in series or parallel. However, there’s a potential problem when supercaps are wired together in banks: No two
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supercaps are identical, a fact that may lead to a
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slight voltage imbalance between them. More specifically, these imbalances arise because the individual capacitors in capacitor assemblies each have minuscule variances in their make-up that contribute to slight differences in their electrical properties such as capacitance, internal resistance, and leakage current. In particular, supercapacitor leakage current depends on parameters such as aging, the material/construction of the supercapacitor, and the operating bias voltage. Leakage current is also a function of the charging voltage, the charging current, operating temperature range, and the rate-ofchange of many of these parameters. The usual way of accommodating these changing conditions is with a balancing circuit. Here, added balancing circuitry ensures weaker capacitors don’t drain stronger units during discharge and that individual capacitors don’t see overvoltages during charging. Large-cell supercapacitors, in particular, require over-voltage balancing because these cells can incur large energy flows. The simplest supercapacitor balancing circuit consists of a resistor put in parallel with the capacitor terminals. Resistors with the same value in parallel with all cells allow cells with higher voltages to discharge through the external resistor at a higher rate than the cells with lower voltages, thus distributing the total capacitor bank voltage evenly across the capacitors.
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DESIGN WORLD — EE NETWORK
2 • 2020
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The SABMBOVP2XX schematic. The circuit can be viewed as a precision voltage clamp that functions like a Zener diode. Typically, the clamp current changes from a few nanoamps to over 100 mA (about 1,000,000 times higher) at the clamp voltage within a 100-mV transition.
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