Small-Signal and Large-Signal Analysis of the Two-Transformer Asymmetrical Half-Bridge Converter Operating in Continuous Conduction Mode Manuel Arias, Marcos Fernández, Diego González Lamar, Francisco Fernández Linera, Javier Sebastián Abstract.—The Asymmetrical Half-Bridge converter (AHBC) has many advantages over other PWM converters. The possibility of soft switching in primary switches and reduced switching losses in the secondary ones implies that the AHBC is a suitable topology for many high-performance applications. Besides, the lack of dead times, except those needed for achieving soft switching, is a very interesting feature to implement self-driven synchronous rectification. Moreover, the small size of its output filter is also a remarkable advantage in some fields (e.g., LED lighting). On the other hand, it also has some disadvantages. One of them is the short range of the duty cycle (lower than 0.5) and the other one is the difficult regulation due to a complex transfer function. The Two-Transformer AHBC (TTAHBC) solves the first problem as it enlarges the duty cycle range making its top limit higher than 0.5. Nevertheless, the regulation of this converter is still very complex and, besides, the transfer functions of the standard AHBC are not valid for the TTAHBC. As a consequence, the small- and large-signal models have yet to be studied. In this paper, the complete small-signal and large- signal analysis of the TTAHBC operating in Continuous Conduction Mode is provided. The large-signal and small- signal models are developed taking into account the main parasitic components that affect the transient response of this converter. The validation of the resulting model is carried out by means of both, simulation and experimental results. The prototype is a 60-W TTAHBC designed for an input voltage of 400 V and an output voltage of 48 V. Keywords: Asymmetrical Half Bridge, Half Bridge with Complementary Control, Small signal, large signal. I. INTRODUCTION Due to its many advantages, the use of the Asymmetrical Half Bridge (AHBC) [1], [2] has expanded over many fields of application, such as lighting [3], [4], PC power supplies [5], Power Factor Correction [6], [7], telecommunication and computer server applications [8], [9] and, in general, low-to-medium power applications [10]. One of its advantages is that the voltage withstood by the primary switches is limited to the input voltage. Besides, Zero Voltage Switching (ZVS) [11]-[15] can be achieved in these switches thanks to the energy stored in the leakage inductance of the transformer and to the right selection of short dead times, strongly reducing switching losses [16]- [18]. Moreover, the achievement of ZVS and a deep analysis of the switching process [19]-[21] also reduce the voltage and current spikes in the rectifier diodes (boosting efficiency as well). The output filter of the AHBC, for applications with a narrow output voltage range, can be very small, reducing cost and size and allowing the development of topologies without electrolytic capacitor, something very important in, for instance, long-lifespam lighting applications [3], [22]-[23]. Besides, the aforementioned dead times used in the driving signals are very short and, as a consequence, energy is transferred from input to output almost all the time, boosting the power-size ratio [24]. Finally, the AHBC is a perfect candidate for Self-Driven (SD) Synchronous Rectification (SR) technique [25] in low- output-voltage applications, boosting efficiency while the driver of the secondary MOSFETs can be strongly simplified [11], [26]-[28]. It should be mentioned that, due to its many advantages, some topologies derived from the AHBC have been also presented in literature: the AHBC based on the flyback topology [8], [28], the resonant AHBC [13], [29], the AHBC with tapped inductor [10], the AHBC with unbalanced turns ratios in the transformer [30], the two- transformer forward-flyback converter [9], or the Two- Transformer AHBC (TTAHBC) [31]-[32]. Each of them has some advantages and disadvantages in comparison to the standard AHBC (lower number of components, extended duty cycle range, low-profile magnetics, etc.). Obviously, the AHBC also has some disadvantages. First of all, the maximum duty cycle is 0.5. Therefore, if a wide output voltage and/or input voltage range is desired, the limitation in the maximum duty cycle implies that the converter will have to work with low duty cycles under certain conditions, with the associate problems of current ripple, losses, etc. Another problem is that it is a converter difficult to control [33]-[38]. The transfer function between the output voltage and the control variable is strongly conditioned by the resonance of the magnetizing inductance with the input capacitors [34]. This resonance adds complexity to the task of designing a stable controller [37], [38] and makes virtually impossible to achieve high bandwidths. Therefore, the AHBC is normally relegated to applications with relaxed requirements regarding time response. Regarding the stability problems and the design of the closed-loop controllers, [35]-[38] present the small-signal analysis of the standard AHBC while [33] presents the same analysis but when a voltage doubler is used. Moreover, [34] explains the advantages of including a feedforward loop in the standard AHBC. These small-signal models allow the designers to adjust the phase margin of the AHBC in close loop without instability risks (even though the controller still cannot be very fast). References [39]-[41] present the large- signal and small-signal models of the current-mode control of the standard AHBC while [30] presents the same model but with unbalanced turns ratios in the transformer. Although bandwidth can be boosted, the inner loop may lead to fast variations of the duty cycle and, as a consequence, to