INTRODUCTION

        The full-bridge converter is widely used in medium-to-high power dc–dc conversions because it can achieve soft-switching without adding any auxiliary switches. The soft-switching techniques for PWM full bridge converter can be classified into two kinds: one is zero-voltage-switching (ZVS) and the other is zero-voltage and zero-current-switching (ZVZCS). The   leakage inductance of the transformer and the intrinsic capacitors of the switches are used to achieve ZVS for the switches. The ZVS characteristics are load dependent and will be lost at light load. In ZVZCS PWM full-bridge converters, one leg achieves ZVS, and the other leg achieves ZCS. However, there is serious voltage oscillation across the rectifier diodes caused by the reverse   recovery no matter ZVS or ZVZCS is realized for the switches. In order to overcome this problem resonant inductance and two clamping diodes into the primary side of transformer. The solution eliminates the voltage ringing and overshoot, thus the voltage stress of the rectifier diodes is reduced and introducing losses or an additional controlled power device. The difference between the two locations of the resonant inductance and the transformer was analyzed and an optimal position was presented. Rue net al analyzed the issue in detail and also observed the effects of the blocking capacitor in different positions, and a best scheme was determined. No matter what the positions of the transformer and the resonant inductance are, the resonant inductance is clamped and its current keeps constant when the clamping diodes conduct. The output filter inductance must had enough current ripple so that the clamping diodes turn off naturally, otherwise the clamping diodes will be forced to be turned off, resulting in serious reverse recovery.  

 In this paper, an auxiliary transformer winding is introduced to the ZVS PWM full-bridge converter to be in series with the resonant inductance. The introduced winding not only makes the clamping diode current decay rapidly and reduces the primary side conduction losses, but also can makes the current ripple of the output filter be smaller; hence the output filter capacitor can be reduced. The winding plays the role of forcing the clamping diode current to decay to zero, so it is called reset winding. The operation principle of the proposed converter gives the comparisons between the full-bridge converters with/without reset winding. The experimental results are presented in Section IV to verify the validity of the proposed converter. In recent years, the soft-switching PWM full bridge converters have attracted more anmoreattentions and there are various topologies and modulation strategies were proposed. Phase-shifted zero-voltage-switching (ZVS) PWM full bridge converter realizes ZVS for both leading leg and lagging leg with the use of leakage inductance of the main transformer and the output capacitors of the power switches. Phase-shifted zero-voltage and zero-current-switching(ZVZCS) PWM full bridge converter realizes the ZVS for leading leg and Z lagging leg and  proposed two kinds of PWM full bridge converters which realize ZVS for one leg and ZCS for the other leg It is meaningful to reveal the relationship among these topologies and modulation strategies.

       Quite complex, they are very small and so is the additional cost. Two major techniques are generally employed to achieve soft switching:

·         Zero-Current-Switching (ZCS)

·         Zero-Voltage-Switching (ZVS).

        In this thesis A Novel Zero-Voltage-Switching PWM Full Bridge Converter with a reset winding and  auxiliary LC circuit  is implemented in this converter the  transformer divided in to two parts one is called reset winding anther one is called primary winding  the capacitors are connected  parallel to the  all IGBT switches zero voltage achieved due to the thanks to the  capacitor before  using reset winding  the resonant inductor is blocked so clamping diode  are hard turned off  output filter  inductor is  relatively larger due to effect  reverse recovery current appear power losses  are more efficiency of the converter decreased so that problems  avoided  by using of reset   winding  .   This paper improves the full-bridge converter by introducing a reset winding in series with the resonant inductance to make the clamping diode current decay rapidly when it conducts. The reset winding not only reduces the Conduction losses, but also makes the clamping diodes naturally turn-off and avoids the reverse recovery, improved efficiency of the proposed converter compared with the conventional techniques with using of reset winding1.2 Block Diagram and Its Description

                This Is shown below. It is two stages DC-DC converter one is inversion and the other is rectification. The basic block diagram of A Novel Zero-Voltage-Switching PWM Full Bridge Converter

ZERO VOLTAGE SWITCHING (ZVS)

Zero voltage switching means that the power to the load is switched on or off only when the output voltage is zero volts. Converter circuits which employ zero voltage and zero current switching are called as resonant converters. Technique in which power switch in a converter turns on and off when there is a zero voltage across it is called as zero voltage switching. Zero voltage switching can extend the life of a controller and of the load being controlled.

ZVS CIRCUIT DIAGRAM

Zero voltage switching circuit diagram

It consists of a diode D1 and capacitor C connected across switch S.  L,C are the resonant circuit components and  L1, C1 are the filter circuit components. The function of resonant capacitor C is to produce zero voltage across the switch S. Diode D2 provides freewheeling path to load current Io.The working of this converter is divided into five modes with equivalent circuits shown in fig   .The time origin t=0 is redefined as the beginning of each mode. Load current Io is assumed constant and filter inductance i0 is also taken to remain level at Io. Initially switch S is on and conducting I0.Therefore inductor current iL=Io and initial voltage across capacitor is zero.

MODE 1(0<=t<=t1)

Initially switch S is off. Therefore the current flows through Vs-C-L.As a result voltage across S or C  builds up linearly from 0 to Vs at t=t1.Diode D is off . Capacitor gets charged from 0 to Vs. At t=0, Vc=0 therefore switch is turned off at zero voltage as required.

MODE 2 (0<=t<=t2)

At. t=t1 capacitor gets overcharged Vc>Vs. Therefore diode D2 becomes forward biased. Now a resonant current iL is setup in series circuit Vs-C-L and D2.  The capacitor voltage Vc is given by Vc=Vs+VmSinWot.

Fig 4.11(a) Modes of operation  for ZVS resonant converter

MODE3 (0<=t<=t3)

Initially at t=0,Vc=Vs and iL=i0.With time t reckoned zero from the beginning of this mode, capacitor voltage is given by Vc=Vs-Vm SinWot and iL= -Io CosWot. At the end of this mode, at t=t3, Vc=0: as a result reverse bias across D1 vanishes and iL begins to flow through D1.

Waveforms for ZVS converter

MODE4 (0<=t<=t4):

During this mode, capacitor voltage is clamped to zero by diode D1 conducting negative current IL. As soon as ant parallel diode D1 begins to conduct at t=0, gate drive is applied to switch S. The inductor current I_L rises linearly from –I_L3 to zero. At this instant reverse bias of D1 vanishes and already gated switch S turns on. This shows that switch S turns on at zero voltage and zero current. After this current rises linearly to Io in the circuit formed by Vs, C, L and D2.

MODE5 (0<=t<=t5):

At the end of mode 4 or in the beginning of mode5 at t=0, I_L reaches Io. And therefore diode D2 turns off. Switch S continues conducting Io. Mode 5 ends at t=t5 when switch is turned off again at zero voltage. The cycle repeats as before.

IGBT’S

The IGBT has a threshold voltage of around 0.7V; a voltage drop lower than this value is not possible. The “resistive part” of the output characteristics of an IGBT is very low, and so it can conduct large currents with a low voltage drop. It is thus most suitable for use at high current densities. An IGBT can be simply modeled as a pnp-transistor driven by a MOSFET. The disadvantage of this structure is the turn off. If a pnp transistor is to be turned off quickly, a positive base current must be supplied, to force the carriers in the base to recombine and stop the device conducting. In the IGBT, the base of the pnp stage cannot be accessed directly, and so this current cannot be delivered at turn off, meaning that the device continues to conduct while the carriers recombine "naturally". During this time, a current tail appears.

Zero Voltage Switch

In a Zero Voltage Switch, the IGBT must turn off a current. Even if the voltage across the switch rises with a limited dV/dt (sinusoidal waveform), the current tail phenomenon means that turn off losses will be important. Therefore the IGBT is not very suitable for zero voltage switching.

Zero Current Switch

In a Zero Current Switch, the external circuit defines the current in the switch. This current tends to zero, and hence the IGBT does not turn off current, so no tail appears. Another problem that can occur with the IGBT, latching, does not occur in this mode. Even if the IGBT latches at the maximum current, it can turn off later because the current is defined by the external circuit. The carriers that remained in the base of the pnp-transistor can be recovered by a positive current into the base. In a Zero Current Switch, the negative half wave of the resonant current flows through the ant parallel diode. During that time, a negative voltage is applied to the IGBT. Current flows through the body diode of the internal MOSFET into the base of the pnptransistorResonant converter topologies can be used to increase circuit switching speeds, allowing the cost of circuit magnetic to be reduced, while still keeping switching losses to a minimum. Full wave rather than half wave topologies are generally used, as they generate less EMI. Capacitive switching losses when turning on with a high drain-source voltage means that MOSFETs are more suitable for Zero -Voltage than Zero-Current switches, while its poor turn-off characteristics mean that the IGBT is more suited to Zero-Current topologies.

RECTIFIER

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. A device which performs the opposite function (converting DC to AC) is known as an inverter. When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". Rectification may occasionally serve in roles other than to generate direct current per se. For example, in gas heating systems flame rectification is used to detect presence of flame. Two metal electrodes in the outer layer of the flame provide a current path, and rectification of an applied alternating voltage will happen in the plasma, but only while the flame is present to generate it.