## 1 Introduction  
Most electrical loads use direct current (DC) electrical energy, which implies the use of a rectifier that converts alternating current (AC) electrical energy to DC, which is also known as a static rectifier. The rectifier was widely developed thanks to the emergence of semiconductor devices as diodes (1900s) and transistors (1950s) a story told already in [1]. There are several types of AC-DC converters (rectifiers) prepared for various loads. The character of the load depends on the respective need and the cost associated with the rectifier. Briefly, rectifiers are classified into two groups, bidirectional and unidirectional. The bidirectional rectifiers are usually formed by a Graetz bridge (single-phase or three-phase) with transistors (MOSFETs, IGBTs, etc. ), thus requiring a control circuit to drive the transistors. They are often used as a first stage for an AC-DC-AC converter, when after the rectifier the goal is to inject energy to the grid. As an example, some wind turbines apply this double conversion for producing the final AC power [2, 3, 4] or in aeronautical applications [5]. The unidirectional rectifiers use diodes as a switching device in most of the cases. There are other unidirectional rectifiers, implemented as switching converters, but they own the possibility to be reformatted into bidirectional ones. The diodes and thyristors are naturally commutated by the periodic changes of the polarity of the AC supply voltage. These rectifiers are also commonly constructed as Graetz bridges of diodes, single-phase or three-phase ones. The diodes Graetz bridges are more attractive, as they are simple, robust, reliable, and low-cost rectifiers when compared to the to the bidirectional rectifiers or any sophisticated AC-DC switching converters. Single-phase rectifiers are used for loads that demand lower power, and some of them are coupled to a converter in order to reduce the disturbances that the rectifier can inject into the electrical network. The three-phase rectifiers, on the other hand, cause great disturbance in the electrical network, due to the large amount of power demanded by the load and its nature (usually inductive, sometimes capacitive and nonlinear loads), compromising the quality of the energy in the electrical network [1]. Therefore, in order to dimension the three-phase rectifiers, some important parameters must be analyzed, being the rectifier and the interface between the grid and the load [6]. Briefly, the following criteria are most important:  
Criteria for connecting correctly the rectifier to the grid:  
* Voltage level and its variation;
* Nominal frequency and its variation;
* Minimum power factor (PF) value allowed by the energy supplier;
* Harmonic distortion level that the energy supplier allows. Criteria for connecting the rectifier to the load:
* Nominal voltage and respective operating current variation;
* Maximum allowed ripple of voltage and current;
* Accurate and reliable regulation of the current and voltage, as the load requires. Additional criteria:
* Efficiency;
* Guarantee of reliability;
* Acceptable physical and mechanical parameters (weight, volume, temperature). The drawback of the unidirectional diode or thyristor rectifiers is the high total harmonic distortion (THD) that it injects into the electric grid and the low total power factor (PF) value. With this, the rectifier compromises the quality of energy in the electrical network. As a way of safeguarding and control of the quality of energy in the electric grid, some standards are emerging. Examples of international standards used on energy quality are IEC61000-3-2 and IEC61000-3-4, and for the THD of the current injected into the electrical network, IEEE Std 519-2014 is used. As a way to use the more sophisticated unidirectional rectifiers in the electric networks, some versions of them are aimed to improve the total harmonic distortion (THD).