I. INTRODUCTION
The growing demand for higher power density and low profile in power converter designs has forced designers to
increase switching frequencies. Operation at higher frequencies considerably reduces the size of passive
components such as transformers and filters. However,switching losses have been an obstacle to high frequency
operation. In order to reduce switching losses, allowing high frequency operation, resonant switching techniques have been developed . These techniques process power in a sinusoidal manner and the switching devices are softly
commutated. Therefore, the switching losses and noise can be dramatically reduced. Conventional resonant converters use an inductor in series with a capacitor as a resonant network. Two basic configurations are possible for the load connection;series connection and parallel connections. For the series resonant converter (SRC), the rectifier-load
network is placed in series with the L-C resonant network as depicted in Fig.1 From this configuration, the resonant
network and the load act as a voltage divider. By changing the frequency of driving voltage Vd, the impedance of the
resonant network changes. The input voltage will be split between this impedance and the reflected load. Since it is a
voltage divider, the DC gain of an SRC is always lower than 1.At light load condition, the impedance of the load will be very large compared to the impedance of the resonant network; all the input voltage will be imposed on the load. This makes it difficult to regulate the output at light load. Theoretically,frequency should be infinite to regulate the output at no load. For parallel resonant converter, the rectifier-load network is placed in parallel with the resonant capacitor as depicted in Fig. 2 Since the load is connected in parallel with the resonant network, there inevitably exists large amount of
circulating current. This makes it difficult to apply parallel resonant topologies in high power applications.