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1 Introduction
In recent years, a variety of high-frequency pulse-width-modulation (PWM) resonant tank inverter-fed dc-ac-dc converter configurations have been studied and evaluated for many specified power supply applications. The main component of these converters is an inverter operating at high frequency. High frequency operation enables the whole system to adapt itself more easily to new conditions and parameter changes. In addition, high-frequency operation of the converters decreases the size and weight of converters and increases their efficiency ([7] Sun et al. , 1996). An important drawback of high-frequency operation is its high switching losses that can be removed by operating the system around the resonance frequency and at lagging power factor mode.
The operation of converters at lagging power factor mode removes the disadvantages of the parallel resonant converters operating below resonance frequency, such as use of lossy RC snubbers and di/dt limiting inductances, need for fast recovery diodes across the switches ([1] Bhat and Swamny, 1990).
Resonant converters may be classified as resonant and non-resonant couplings according to the methods of coupling between the resonant and output circuits of the converter. The converter presented in this paper is a parallel resonant converter with capacitive output filter called the non-resonant coupled ([3] Sooksatra and Lee, 1989; [5] Steigerwald, 1985). In the resonant converter operation, the response of the converter is determined from a sequence of different topological circuits called circuit modes. The resonant operation of a dc-ac-dc converter or the operation of a dc-ac-dc converter at unity power factor depends on both the operating frequency and the load value ([8] Takano et al. , 1995; [5] Steigerwald, 1985). The converter presented in this paper, is not operating exactly at the resonance mode (unity power factor) but is operating near resonance at lagging power mode due to the above mentioned advantages.
In high dc-voltage applications like X-ray generators, a high-turns-ratio transformer is used to increase the output voltage to the required voltage level and this implies a very large reflected secondary capacitance. Using the leakage inductance and the secondary layer capacitance of a transformer as the resonant elements of a parallel resonant converter is desirable for higher output voltage converters ([6] Steigerwald, 1988). An important property of parallel resonant converters is that the output voltage gain...