## 2 LLC Resonant Converter Structure  
The general structure of the LLC resonant converter is shown in Figure 1. It consists of square generator (also called square wave generator), resonant tank, transformer, and rectifier. An EMI filter can also be added in the output port optionally. By switching the square wave generator, either half-bridge or full-bridge topology, in a variable frequency with the fixed duty cycle, the DC gain of the resonant converter is varied to regulate the output voltage. Both square wave generator in the primary side and rectifier in the secondary one can be half-bridge or full-bridge, depending on the application. Full-bridge topology has several advantages when compared with the half-bridge, like the two-fold voltage and the half current, less reverse voltage across secondary switches and better use of the transformer by using more power switches. Regarding the transformer, although a half-bridge requires half of the primary number of turns for the same voltage gain and magnetic flux swing and, thus, half of the primary winding resistance, the primary copper losses are still twice when compared to the full-bridge because the squared RMS current in the half-bridge case is four times more [2]. The output low-pass filter is commonly made up by a capacitor in parallel to the output rectifier instead of a LC filter, because the series filter inductance would be connected in series with the resonant inductance, affecting the resonance and, thus, the behavior of the converter. Normally, Pulse-Frequency Modulation (PFM) is used to regulate the output power of the LLC converter. Beside the above-mentioned advantages, if the switching frequency drifts away from the resonant frequency, the circulation current will be increased; then the efficiency and performance would be degraded. A large resonant inductor is necessary to support a wide voltage range. Such a large resonant inductor ends in a high RMS current, flux, and peak voltage [26; 42; 43; 44]. Although a higher frequency is preferred because it equals higher power density and efficiency, there is always a trade-off between ensuring the maximum frequency and covering a wide input voltage range. Since they have a narrow input voltage range, a large capacitor is required in the input to support the required holdup time that degrades the power density [2]. It is suggested to add a small capacitor in series with the resonant inductor to maintain the same holdup time with a capacitor with half size [2]. There have been several revisions in the LLC resonant converter structure to expand the output voltage range. [PERSON] and [PERSON], for instance, added an extra capacitor in parallel with an inductor to form an anti-resonant LC circuit [45; 46]. By using this technique, the voltage gain becomes zero at the resonant frequency of the anti-resonant circuit and the input impedance will be infinite. In a reduced frequency range, wide range voltage regulation would be available. It should be noted that this converter is very sensitive to the switching frequency for the low output voltage levels. In [47; 48], an additional switch is added to the primary side of the LLC converter to generate various AC input voltage levels. In the proposed structure, various voltage gains are achieved in the predefined frequency range. However, this technique suffers from more cost, conduction loss, and switching loss because of an additional switch and, also, the uneven current distribution between switches. Using the cascaded rectifier in the secondary side of the LLC resonant converter has been proposed in some researches [49; 50; 51]. However, the need for complex coding for the controller and uneven current sharing among switches are the disadvantages of this technique. Using semi-active reconfigurable rectifiers with reduced frequency range or even fixed switching frequency has also been proposed to expand the voltage range [52; 53; 54; 55]. The  
Figure 1: General circuit diagram of the LLC resonant converter.