Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 44

Main : 2.5 k 5 ms/div

V dc V 1

I ac

V 2

Z1 250.00 500 us/div

Zoomed

V dc

I ac

Excessive
Oscillations

V 1

FIGURE 2.6
Experimental waveforms of an active MOSFET full-bridge rectifier using a voltage sense ap-
proach resulting in excessive oscillations.

DC voltage V dc , current i ac flows through the sensing resistor R sense , and
the zero-crossing comparators sense the flow of the AC current by measur-
ing the voltage across R sense . The comparators generate the control signals to
turn the respective combinations of the MOSFET switches on or off, that is,
(1) positive half AC cycle: PMOS1 and NMOS2 on, PMOS2 and NMOS1 off;
and (2) negative half AC cycle: PMOS2 and NMOS1 on, PMOS1 and NMOS2
off. During the switching transition of the MOSFET switches between on and
off states, it can be clearly observed from the zoomed waveforms of Figures
2.6 and 2.7 that using the voltage sense approach introduces more unwanted
oscillations ( Figure 2.6 ) than using the current sense approach ( Figure 2.7 ).
The use of the voltage sense approach leads to more energy loss incurred
in the active rectifier, thus reducing its overall power conversion efficiency.
Hence, the current sense approach, which yields better performance than its
counterpart, is employed in the active rectifier to control the switching of the
MOSFET switches in the bridge rectifier bridge.
The performance of the MOSFET-based active rectifier using the proposed
current sense approach is compared with the conventional diode-based pas-
sive rectifier to seek a better solution for rectifying low-amplitude AC voltages
generated by a micro wind turbine, especially at low wind speeds. The per-
formance comparison is based on the voltage drops measured across the two
diodes and the two MOSFETs of the rectifiers, which are plotted over a range
of the load resistances as shown in Figure 2.8 . At optimal load of 150 ,it

is observed that the voltage drop in the passive rectifier at a wind speed of
3.62 m/s is around 0.6 V, which is four times more than the voltage loss in
it of around 0.15 V. With less voltage drop in the active rectifier, the active
rectifier, it is able to rectify much lower amplitude input AC voltages than