Due to their high reliability and luminous efficacy, HB-LEDs are being widely used in lighting applications and, therefore, their power supplies are required to have also high reliability and efficiency. A very common approach for achieving this in ac-dc applications is using a two-stage topology. The Power Factor Corrector Boost converter operating in Boundary Conduction Mode is a very common converter used as first stage. It is normally designed without electrolytic capacitors, improving reliability but also increasing the low-frequency ripple of the output voltage. The Asymmetrical Half-Bridge (AHB) is a perfect option for the second stage as it has very high efficiency, it operates at constant switching frequency and its output filter is small (i.e., it can be also easily implemented without electrolytic capacitors). Moreover, the AHB is an excellent candidate for Self-Driven Synchronous Rectification (SD-SR) as its transformer does not have dead times. However, the standard configuration of the SD-SR must be modified in this case in order to deal with the transformer voltage variations due to the input voltage ripple and, more important, due to the LED dimming state. This modification is presented in this paper. Another important issue regarding the AHB is that its closed loop controller cannot be very fast and it cannot easily cancel the previously-mentioned low-frequency ripple. In this paper, a feed-forward technique, specifically designed to overcome this problem, is also presented.The experimental results obtained with a 60-W topology show that efficiency of the AHB may be very high (94.5%) while the inherent control problems related to the AHB can be overcome by the proposed feed-forward technique.

Abstract.-The Asymmetrical Half Bridge converter (AHBC) has proven to be a promising candidate for LED lighting applications. It provides high efficiency, galvanic isolation and it can be easily built without electrolytic capacitor. On the other hand, its main drawback is its poor attainable bandwidth. In any two-stage ac-dc LED driver based on the AHBC, the first stage is a Power Factor Corrector (PFC) converter which has to be also implemented without electrolytic capacitor. As a consequence, its output voltage (input voltage of the AHBC) presents a low-frequency ripple. Due to the poor bandwidth of the AHBC, this voltage ripple will be transferred to the converter output voltage, leading to flickering. Due to the complex and non-linear transfer function of the AHBC, any analog feedforward loop has to be tuned for a given operating point, leading to a poor performance when the AHBC moves away from that point. In this paper, a digital feedforward loop is proposed in order to solve this problem. The digital implementation allows the feedforward loop to perfectly cancel the ripple under any condition (e.g., output voltage variation due to dimming). Besides, this digital feedforward loop has been designed and simplified considering the specific purpose of cancelling flickering in the emitted light. In this way, it can be easily implemented in small-size microcontrollers. Experimental results with a 40-W prototype prove the usefulness of the proposed feedforward loop.

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 largesignal analysis of the TTAHBC operating in Continuous Conduction Mode is provided. The large-signal and smallsignal 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.

Silicon Carbide (SiC) devices are becoming increasingly available on the market due to the mature stage of development fact of their manufacturing process. Their numerous advantages compared to silicon (Si) devices, such as, for example, higher blocking capability, lower conduction voltage drop and faster transitions make them more suitable for high-power and high-frequency converters. The aim of this paper is to study the switching behavior of the two most-widely studied configurations of SiC devices in the literature: the normally-on SiC JFET and the cascode using a normally-on SiC JFET and a low-voltage Si MOSFET. A detailed comparison of the turn-on and turn-off losses of both configurations is provided and the results are verified against experimental efficiency results obtained in a boost converter operating in both Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). Furthermore, special attention will be paid to the switching behavior of the cascode configuration, analyzing the effect of its low-voltage Si MOSFET and comparing different devices. The study carried out will confirm that the overall switching losses of the JFET are lower, making it more suitable for operating in CCM in terms of the overall converter efficiency. However, the lower turn-off losses of the cascode show this device to be more suitable for DCM when ZVS is achieved at the turn-on of the main switch. Finally, all the theoretical results have been verified in an experimental 600W boost converter.

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