Environmental Engineering Reference
In-Depth Information
PMOS
a
c
L
+
+
R
s
,TEG
C
+
-
v
in
v
o
R
L
V
oc
D
Switch Cell
-
-
p
hermal Energy
Harvester
FIGURE 3.5
Buck converter.
energy harvester with very little control circuit overhead. A buck converter
topology is selected for the power converter due to the high output voltage
of 5 to 35 V (see
Figure 3.4
) generated by the thermal energy harvester (input
voltage
V
in
to the buck converter). The main purpose of the buck converter
is to match the optimal resistance of the thermal energy harvester, that is,
R
s,TEG
=
R
opt
=82k
at the converter input port, and efficiently transfer the
energy to its output port based on the voltage and charge characteristics of
the energy storage element. A previous approach [70] has shown that over a
certain range of input power level, operation of a boost (step-up) converter
in discontinuous conduction mode (DCM) with a fixed duty cycle results in
maximum output power. The results as reported by Paing et al. [70] and Paing
and Zane [94] can be related to this research work by showing that the DC-DC
converters (e.g., buck, boost, and buck-boost) in DCM acts as a near-constant
resistance at its input port for large step-up/step-down conversion ratio.
To fully understand how the buck converter, when operating in DCM, emu-
lates the source resistance of the thermal energy harvester (
R
s,TEG
)to
achieve MPPT, the electrical model of the buck converter shown in
Figure 3.5
has been modelled into an averaged equivalent circuit model as shown in
Figure 3.6.
The modelling process is based on the analysis made by Erickson
et al. [71] that shows the average voltage and current of the semiconduc-
tor switch are proportional, thus obeying Ohm's law, and the switch can be
=
82 k
I
in
I
o
a
c
L
+
+
R
s
,TEG
R
e
(
d
)
+
-
V
oc
V
in
R
L
V
o
P
-
-
hermal Energy
Harvester
p
FIGURE 3.6
An averaged equivalent circuit of a buck converter.
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