Environmental Engineering Reference
In-Depth Information
I
+
U
c1
C
1
V
z
i
(
t
)
Vz
V
+
U
c2
C
2
V
z
dL
/
dV
= 1/
R
z
Figure 10.35 Zener diode cell balancing method
from the lower valued capacitor would simply be
d
r
I
where
I
is the charge current.
Since operating currents can be several hundred amperes, this current diverter
approach will require large components in the equalization network [23].
In the case of Zener diodes, this means the need for high current semi-
conductors and some means to dissipate Joule heating during clamping. These
disadvantages have led to the development of some very novel non-dissipative cell
equalization networks. An alternative approach is to replace the Zener diodes in
Figure 10.35 with three silicon diodes in forward conduction plus a positive tem-
perature coefficient (PTC) current limiter in series placed across each ultra-
capacitor cell. This network will begin to conduct when the cell voltage is near 2.7 V.
In effect, this diode plus PTC network represents a low end voltage management
system that shunts cell current when a cell voltage threshold is crossed.
10.2.5.2 Non-dissipative cell equalization
Non-dissipative cell equalization networks are almost entirely derived from active
switching devices and magnetic components such as inductors and transformers.
The motivation for non-dissipative equalizers is a quest for highest possible charge/
discharge efficiency. Three types of non-dissipative equalizers are treated here:
(1) flyback dc/dc converter with distributed secondaries, (2) cascaded buck-boost
converters and (3) forward converter with distributed primaries. Each of these
approaches has relative merits and disadvantages that will be discussed.
Figure 10.36 is the centralized flyback converter with distributed secondaries.
In this topology the main switch transistor is gated ON when an undervoltage is
detected on one cell relative to the others. Primary current charges the magnetizing
inductance of the transformer at which point the main switch is gated OFF. At turn-
OFF, stored energy in the transformer magnetizing inductance is transferred to the
secondaries, specifically the one having lowest voltage at the steering diodes
cathode. When all the cells in a series string are equalized, the secondary diode
conduction times will balance. A design challenge with this equalization technique
is the need to match leakage inductance on all the secondary windings. This is a
design difficulty because the proximity of each secondary to the magnetic core will
be different, as will its location relative to the primary, making such balancing
difficult. More details on the application of multi-winding transformers and dc/dc
converters as cell equalization means can be found in References 25 and 26.
