## 3 Results
### Active Rectifier  
The use of diodes leads to two complex drawbacks: (1) the voltage drop of nearly 3 V in the pair of conducting diodes is not affordable without affecting the system's overall performance, and (2) power losses in the diodes can reach values above 450 W for an output current of 150 A. Moreover, the weight of a three-phase diode rectifier is not negligible. For all the above reasons, the use of active rectifiers built with parallelized surface-mounted devices (SMDs) with extremely low channel resistances reduces the total voltage drop to below 0.5 V and dissipates the power to less than 1/10 th of the power losses of conventional diodes. Figure 14 shows a diagram in which the voltage drop in conductive diodes is presented. The replacement of diodes by low channel resistance MOSFETs, parallelized in three branches, yields a final resistance of 1 m\(\Omega\) and a voltage drop of 0.3 V in our prototype, with transistors conducting at 150 A. Active or synchronous rectifying (SR) is a technique that is widely used to reduce the losses in the rectification process. As is well known, the diodes are replaced by MOSFETs in order to reduce the losses. This might seem to be an old-fashioned technique if it were applied as it was originally developed; the control signals were obtained by means of a transformer, and thus they are delayed with respect to the optimal commutation point. This problem arises because they begin conduction when the current in the rectifier is already flowing. In our prototype, taking advantage of the FPGA, a specific algorithm has been developed in such a way that the control signals are generated in advance. Therefore, the approach to SR is essentially different, and the previous drawbacks are overcome. The algorithm implemented in the FPGA measures the time at every single cycle and produces the control waveform accordingly. Considering the size and weight of the FPGA, its integration in the prototype even reduces the weight and volume while adding more control blocks for the other functions of the system. The use of microcontrollers and microprocessors in power electronics is quite usual. FPGAs are now replacing these components, since they are faster, and several control systems can be provided with a single chip [30]. In hybrid generators, the output voltage is low, while the current is medium to high. For instance, a small 2.4 kW unit operating at 50 V must deliver nearly 50 A. Therefore, if a DC/DC converter is used to control throttle, the converter requires active/synchronous rectification as well [31]. It must be underlined that this strategy is used in different topologies for different applications [32], but always with the goal of improving the efficiency. On the other hand, the replacement of diodes by MOSFETs implies the generation of gate-driving signals; these can be obtained in most cases by means of additional windings [33], which is  
Figure 14: Active or synchronous rectification. only possible by adding magnetic elements. In some applications, the adjustment of the gate-signals is necessary only to prevent short-circuit situations and ensure adequate behaviour under different operating conditions [34]. Generating gate-driving signals without magnetic components is a complex process. However, when correctly done, losses during rectification or conversion are negligible, and almost no voltage drop is exhibited. Figure 15 shows the selected device for the prototype. The FPGA is much smaller than the board presented in the figure, which includes a development system. Control is programmed using Cadence(tm) 9.2.3 software. Gate-driving signals and control algorithms for both the rectifier and DC/DC converter have been programmed and tested. The rectifier and converter boards have been prototyped. The former is pending manufacturing, and the latter needs additional tests and a redesign of the inductor. Photographs of the prototyped PCBs are shown in the next figures. The management of the SC array has not been addressed yet.