Environmental Engineering Reference
In-Depth Information
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Figure 10.36 Flyback converter cell equalization
The second technique involves a cascaded set of buck-boost converters that
are overlapping and daisy chained across the entire ultra-capacitor string as shown
in Figure 10.37. This is a form of current diverter operating in discontinuous con-
duction mode of charge transfer for highest efficiency. A design challenge with the
buck-boost converter is the high component count, particularly active switching
devices and magnetic components. Each ultra-capacitor in the buck-boost equal-
izer requires 3/2 active switch and 1/2 magnetic component of an inductor.
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Figure 10.37 Buck-boost converter method of cell balancing
An advantage, if it can be called so, is that the voltage rating of the individual
active switches in the daisy-chained buck-boost converter method may be very
low, on the order of twice the maximum cell voltage or 5 V. This voltage re-
quirement is beneficial to trench MOSFET technology, or perhaps some other very
low voltage power electronic component. In operation the buck-boost stages
shuffle charge amongst pairs of ultra-capacitors until the entire string is balanced.
When all the cell voltages are equal, the buck-boost converters shut down until the
cell voltages drift apart due to loading or self-discharge.
