With great advances of power semiconductor switching devices such as **MOSFETs**, **IGBTs**, and ESBTs as well as high-frequency passive circuit components, the leading development of the high frequency resonant pulse inverter type switching mode DC-DC power conversion circuits and systems have attracted special interest for high voltage DC power applications

The “hard - switching” **dc – dc converter** suffer from high switching loss and reduced reliability. Even increasing power densities has been limited by the size of both reactive elements and the isolation transformer. While component sizes tend to decrease with an increase in the switching frequency, device switching losses are proportional to frequency attainable in a given circuit. The high frequencies are a key to realizing multiple benefits of high power density and good transient response.

The use of soft-switching techniques, alleviate switching loss problems and allow a significant increase in the converter switching frequency. [2]. Further, the proposed topology features device stress, reduced EMI, high power density, improved power factor, etc. **Resonant converters** (**RC**s) eliminate much of the switching losses encountered in pulse width modulation (**PWM**) converters. The active device is switched with either zero current (ZCS) or zero voltage (ZVS) at its terminals. Included in the specification of any dc/dc converters are criteria for line regulation, load regulation, response time and stability.

Normally, the supply voltage and the load regulation have a wide range of variation, so the controller must be designed to give suitable behavior in any working condition of the converter. The design of a controller for resonant converters is often done with classical control methods. If there can be a large change in the operating point, changes in the linear model must also be considered. One way to fulfill all the specifications is to study the design of the controller for worst conditions.

Traditionally rectifiers used diode or thyristor line commutated circuits to produce either uncontrolled or controlled DC outputs. However, these circuits have poor input current waveforms and power factor, and as power quality standards have been tightened such as the IEEE Standard 519 or the aircraft current harmonic limits standard, the use of these circuits has become unacceptable in many applications.

Among the various kinds of **resonant converters**, the simplest and most popular resonant converter is the LC series resonant converter. Where the rectifier load network is placed in series with the LC resonant network. While much research has been done on the LLC resonant converter topology ever since its introduction in the 1990s. LLC resonant converters display many advantages over the conventional LC series resonant converter such as narrow frequency variation over wide range of load and input variation and zero voltage switching even under no load conditions. Switched mode power supplies based on resonant operating converter topologies find increasing interest for all power levels today. Resonant topologies are typically applied when low EMI is needed or when the switching losses have to be reduced in order to allow higher frequencies for miniaturization.

In addition, resonant operation enables high frequency power transfer via a transformer [7]. For a high efficiency dc-dc converter, the LLC series-resonant half-bridge converter is gaining its popularity. A half bridge parallel resonant converter running above resonance in the constant output voltage mode was analyzed by M. Emsermann (1991). Secondary-side control of a constant frequency series resonant converter using dual-edge PWM was given by Darryl J. Tschirhart and Praveen K. Jain (2010). Constant Switching Frequency Series Resonant Three-port Bi-directional DC-DC Converter was given by H. Krishnaswami and N. Mohan, (2008). Design of the Half-Bridge, Series Resonant Converter for Induction Cooking was given by Henry W. Koertzent et al, (1995). In this work they dealt with the forced commutated, half-bridge, series resonant converter is well suited for induction cooking.

Analysis and Design of a Double-Output **Series-Resonant DC–DC Converter** was presented by Yu-Kang Lo et al, (2007). In the proposed work high frequency half bridge series resonant dc-dc converter is modeled and hardware implementation results are presented.

The** Metal Oxide Semiconductor Field Effect Transistor** (**MOSFET**) is a device used to amplify or switch electronic signals. It is a unipolar, unidirectional, voltage controlled device. The symbol of **MOSFET** is shown below.

**Symbol of MOSFET**

The circuit consists of a half bridge **MOSFET** inverter having a high frequency (HF) resonant circuit. This is the high frequency link. A HF transformer provides voltage transformation and isolation between the dc source and the load.

The resonant link circuit is driven with either square waves of voltage or current in the inverter. The voltage or current in the resonant components becomes maximum at the resonant frequency and by altering the frequency around the resonant point, the voltage on the resonant components can be adjusted to any desired value. By rectifying the voltage across the inductor or capacitor, a DC voltage is obtained. The DC voltage can either be increased or decreased. Thus this converter be operated as step-up or step-down converter. In the present work, step-down converter is used.

Here the driving pulse for **MOSFET** is shown above the first mosfet (M1) on state at the same time second mosfet (M2) is off state.

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