Electrical Power Transfer with “No Wires” - Energy Harvesting Autonomous Sensor Systems - page 187

Power (mW) vs. Resistance

20
18
16
14
12
10
8
6
4
2
0

4 A
3 A
2 A
1 A

270

0 500 1000 1500 2000 2500 3000 3500 4000

Resistance

FIGURE 6.6
Electrical power harvested over a range of load resistances for different input currents.

From the I-V curve shown in Figure 6.5 , it can be seen that the obtainable
open-circuit voltage for different currents flowing in the power line increased
by around three times, ranging from 0.7 to 3.5 V. Although the output voltage
of the harvester had already been increased by series connecting three fer-
rite coils, the induced voltage at some operating points, especially when the
magnetic field was weak due to low current flows in the AC power line, was
considerably low. As such, the magnetic energy harvester may not be able
to drive the electronic output load. This low output voltage generated from
the magnetic energy harvester would pose a challenge for the design of the
power management circuit. Another analysis carried out was on the power
curve shown in Figure 6.6 , where maximum power was attainable at a load
resistance of 270 . Referring to Figure 6.6 , with the source current of 1 to 4

A flowing in the AC power line, the maximum electrical power available for
harvesting ranged from 1 to 18 mW. The challenge here was that when the
source current was low, say 1 A, the radiated magnetic field became weak,
so the maximum power available for harvesting dropped tremendously to
around 1 mW or so and may not be sufficient to power the RF transmitter
load continuously. Hence, a power management circuit designed to address
the low-voltage and low-power challenges of the magnetic energy harvester
has been proposed.

6.1.2 Power Management Circuit

Based on the analysis and characterization performed on the designed mag-
netic energy harvester, the concept of harvesting stray magnetic energy via
ICPT was found to be a viable solution for powering the low-power wireless
sensor nodes. The block diagram in Figure 6.7 illustrates the energy harvesting
scheme and its application for wireless sensor nodes. Since the voltage source