Environmental Engineering Reference
In-Depth Information
The last of the non-dissipative cell balancing techniques to be described is the
forward converter. This is a dual of the flyback converter. Rather than charge
transfer to the lowest voltage cell after the transformer magnetizing inductance is
charged, the forward converter transfers charge during the magnetizing phase.
Figure 10.38 illustrates the circuit configuration of a forward converter.
N
p
1
C
1
i
(
t
)
N
p
2
N
s
C
2
N
p
n
C
N
Figure 10.38 Forward converter method of cell balancing
There are other, more proprietary, methods of non-dissipative equalization
in use. The main concept is that charge is either diverted to the lowest voltage cell
(i.e. cell with highest capacitance) at the sacrifice of the highest voltage cell. For a
given amount of capacitance dispersion amongst the series connected cells, an
equalized string holds the optimum energy for the amount of capacitance available.
Multi-winding transformer balancers have the advantage of being amenable to
module-to-module balance. At cell level these can be viewed as either one-for-all
3
in the case of one cell being at higher voltage than its neighbours in which case its
charge gets shared with all its neighbours, or an all-for-one mode in which a low
voltage cell has its charge brought up via the balancer, extracting some small
charge from all its neighbours. Top balancing in fact is to transfer charge from a
cell with overcharge and share that with all its neighbours in the module string.
Bottom balancing is the converse, to extract some charge from (
N
1) cells in a
module string and transfer this to one cell that is at low charge.
One could also envision a method of charge transfer from cells that are in
danger of being in overvoltage by an elaborate network of semiconductor switching
components that would connect every capacitor in the string momentarily in par-
allel to equalize the voltages and then switch back to the full series connection for
further charging or discharging. At this time such a technique would appear
unfeasible but, as power electronics technology matures, this may in fact turn out to
be a practical solution because no magnetic components and minimal, if not zero,
sensors
would
be
necessary.
Such
a
technique
could
operate
autonomously
3
A convenient way to think of this is to recall the motto of the three Musketeers.
