~ A Piecewise Digital Control: method is presented in this paper to improve the converter characteristics especially the efficiency. By combining more than one control method orland compensators in a single converter controller, improved performance may be achieved. Piecewise control opens the possibility of combining advantages of different control schemes and preventing or minimizing their disadvantages. In this paper, by combining the asymmetric control and the duty-cycle-shifted control in a new piecewise control scheme, the performance and efficiency of half-bridge DC-DC converter with high switching frequency and wide input voltage range can be improved, without adding additional components. The proposed control is theoretically analyzed and its implementation considerations are discussed. Experimental results are also presented.

Asymmetrical half-bridge dc-dc converter has favorable features, which allow the converter to operate at higher frequencies with higher power density. However, asymmetrical half bridge dc-dc converter displays nonlinear dc gain variation with input voltage and duty cycle. The conventional compensator design is based on a small-signal transfer function of power stage and worst case has to be considered in the closed-loop design. With the variation of input line, the system loop gain deviates from the nominal designed trajectory, and correspondingly, the system bandwidth, phase margin and gain margin are going to vary with input voltage line. As a result, the system steady-state and dynamic performance are affected as well. This paper proposes an adaptive nonlinear compensation approach to adjust the system loop gain dynamically and thus achieve unified system loop gain, bandwidth and performance. The adaptive digital compensation algorithm is simulated and verified based on PSIM simulation software. I.INTRODTUCTION In asymmetric controlled half bridge dc-dc converter, the dead-time between the two switches is minimized and ZeroVoltage-Switching (ZVS) can be achieved and the primaryside ringing is eliminated. These features become more attractive in high switching frequency applications [1][2][3][4][5][6].Average small-signal modeling is the state-of-the-art technique for control design of a power supply system [4][5][6]. For conventional buck-derived dc-dc topologies such as forward, push pull, full bridge and symmetric half bridge dc-dc converters, the conversion DC gain can be described as:where n is transformer turn ratio; K is a constant depending on topologies; and D is the steay-state duty cycle value. According to small-signal modeling, the corresponding output-to-control transfer function is linear in term of duty cycle.AHB converter's small signal model is distinct from conventional buck-type topologies due to the asymmetry of the converter operation. First, the transformer becomes an energy-storage element and is included in the state variable vector. Second, the dc gain of the power train's output-tocontrol transfer function in the small-signal model becomes depending on steady-state duty cycle. Generally, an AHB converter's DC voltage conversion gain can be described as:which is nonlinear in term of the duty cycle D. Correspondingly, AHB's small-signal output-to-control transfer function can be derived as:Where the DC gain of the transfer function varies with the steady-state duty cycle as well as the input voltage. That is to say, with the changing of the duty cycle due to input voltage or load variation, the system frequency response is affected resulting in the variation of the system performance.To ensure the converter operate at various input line and load, the worst case has to be considered in the closed-loop compensation design. In this paper, the average small-signal model of a AHB converter is derived using the average state-space smallsignal modeling method. Based on the model, the nonlinear...

AbsIracI -Current doubler rectifier (CDR) is one of the hest candidates for high output current DC-DC converters. In this paper, several conventional CDR snubber circuits are evaluated, and two passive lossless snubber circuits for the CDR are proposed to reduce the reverse-recovery-related losses and clamp the voltage spikes across the rectifiers. The snubber circuits are tested and verified experimentally, and higher efficiency is achieved when compared to the conventional resistive snubbers.

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