Environmental Engineering Reference
In-Depth Information
+3 V
i
ac
PMOS1
PMOS2
V
dc
Z
s
g
1
g
2
ν
s
R
sense
ν
1
C
filter
R
sense
R
load
ν
2
NMOS2
NMOS1
g
3
g
4
V
gnd
3 V
3 V
+
-
+
-
FIGURE 2.5
Schematic diagram of a full-bridge active MOSFET rectifier.
necessary in this configuration because the current has an alternative path
through the body diode of the MOSFET whenever the MOSFET switch is not
activated.
The main function of the second stage of the rectifier is to control the on and
off switching sequences of the MOSFETs residing in the first stage of the active
rectifier to facilitate the rectification process. According to Lam et al. [64] and
Seeman et al. [65], the proposed active diode stage uses a fast and low-power
comparator circuit that continuously samples the voltage across each of the
MOSFET switches. The problem with this kind of voltage-sensing approach
is that if the voltage difference between the input AC voltage
v
1
or
v
2
and the
output DC voltage
V
dc
is small or the comparator is too slow, then there are
excessive oscillations occurring during the on and off switching transitions
as illustrated in the zoomed waveforms in
Figure 2.6
. Due to these unwanted
oscillations, some part of the harvested electrical energy is lost, resulting in
the reduction in the overall efficiency.
To overcome the problem associated with very small sensing voltage across
the respective gating signals for the MOSFET switches
g
1
to
g
4
in the active
rectifier circuit. The proposed current-sensing circuit is specially designed
to be simple and power saving using two precise zero-crossing comparators
(LMC7215) with very low overhead power consumption of 90
W. The source
voltage of the comparators of 3 V is taken from the storage capacitor as shown
in
Figure 2.5
.
Whenever the peak value of the input AC voltage during either the posi-
tive or negative half-cycles
v
1
or
v
2
,respectively, is higher than the output
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