## 4 AC/DC Converter  
Harvested power is usually need in a DC regime. Consequently, a converter that changes an AC regime into DC must be included [42; 43].  
In this research the chosen conversion mode is the AC-DC direct conversion. Nevertheless, a basic architecture of direct rectification of an AC signal into DC will be analyzed too for comparison purposes: three phase diode bridge (Point 4.4.1).  
Figure 20 shows the block diagram AC system with the harvester (mini wind harvester), power management (AC/DC converter) and the storage device (supercapacitor).  
The main objective of this subsection is to research, simulate and verify the feasibility of the development and implementation of an AC/DC converter for the mini wind harvester for voltage and frequency input ranges with more than a 50% efficiency.  
Figure 20: AC complete system block diagram.  
However, the input operation principles are set by the mini wind harvester, which imposes several design requirements on the converter, such as:  
* A low-value coil: only 2 \(\mu\)H. The harvester's own coil is used in order to avoid the losses of including another one.
* A low input AC voltage for energy harvesting the usual converters (voltage levels are between 0.3 V and 1.5 V, commercial diodes do not work properly in the rectification step with such low voltage levels due to the internal losses or the forward voltage, VF).
* An 8 kHz range frequency input: switching frequency changing.
* The influence of the reactive power of the generator must be considered.  
On the other hand, electronic devices usually operate in the range of 3 V to 5 V. If the minimum voltage level is not reached, the voltage must be raised by a step-up device. On the contrary, if the voltage level exceeds the maximum, the power storage system will be damaged. Thus, decreasing the voltage level with a step-down device is required. In this research, these situations are not valid because an extra element is needed, not fulfilling the predefined requirements. Moreover, the converter must maintain its performance within the whole range of voltage and generator operating conditions. These aforementioned design requirements aim for a correct and efficient integration between the harvester and the converter.  
The best AC/DC converter must be designed selecting the appropriate architecture for the usage described in this work; thus the architecture selection is shown after prior evaluation of diode and metal-oxide-semiconductor field-effect transistor (MOSFET) selection under different ambient conditions. All the analyses were carried out under two extreme test conditions, in order to guarantee that, in such conditions, the minimum and maximum points of energy and work conditions are achieved. As the system is mostly linear, the intermediate values are supposed to be linear and within the defined range:  
1. A minimum wind velocity condition of 3 m/s, i.e., when available input power is the minimum possible, because at lower wind velocity the generator rotatory movement does not generate enough power to start working the electronic system.
2. A maximum wind velocity condition of 10 m/s, i.e., when available input power is the maximum possible, basically to ensure the mechanical integrity of the bearings. At this wind velocity the achieved angular velocity is 32.87 krpm and the harvester bearing is ensured until 35 krpm of angular velocity.  
The selection of the diode, MOSFET and architecture was performed and verified with different simulation tools, LTSpice(r) [44] and (Power Electronics Simulation) PSIM(r) [45], however other simulation packages could be used. PSIM is mainly used for modelling basic architectures and LTSpice for structural and behavioral models.