Research on a Novel Soft-Switching Buck Converter

Based on classical zero voltage transition buck pwm converter, an ideal buck converter with pwm- controlled soft-switching circuit is proposed. The proposed auxiliary circuit allows the main switch to operate with zero-voltage switching. Besides, all of the semiconductor devices operate under soft-switching conditions. Thus, losses were reduced. It was analyzed in detail to demonstrate the operating principle of the novel circuit. Finally, simulation results are given analysis and the simulation results are provided to verify the performance of the proposed buck Converter.


INTRODUCTION
Recent years, as the interest increased by the industry in power converters with high frequency and small size, lots of novel topologies have been proposed in literatures (Martins et al., 2005;Lin et al., 2010).With the switching frequency increase, high power density can be achieved.But under the hard switching condition, the dt di and dt du is large during the communication process.So soft-switching technique is developed and some was employed to the power converters, usually auxiliary components are used (Wu et al., 2008;Li et al., 2010).In the last years, researchers paid more attention to Zero Voltage Transition (ZVT), which is the voltage-mode soft-switching method (Seyed et al., 2008).The operation of ZVT is similar to PWM converters and its additional conductions have low losses (Adib and Farzanehfard, 2009;Iannuzzo, 2005).The location of auxiliary circuit causes these remarkable features and the auxiliary circuit is placed in parallel with the main power converter.Before the main switch is turned on, the auxiliary circuit works, when the main switch is turned off, the auxiliary circuit ceases (Martins et al., 2005).In that way, the additional conduction losses are mostly reduced.Moreover, the operation characteristics have few influences.Previously proposed ZVT-PWM converters have at least one of the following drawbacks (Panda et al., 2008): • The auxiliary switch is turned off while it is conducting current.That causes the switching losses • The main converter switch operates with a higher peak current stress and/or voltage stress • The auxiliary circuit components have high voltage and/or current stress • Some circuits are complex, difficult to control and so on.
This study presents a novel ZVT buck converter by using a resonant auxiliary network.And the proposed converter achieves zero-voltage switching for the main switching and zero-current switching for the auxiliary switching.

THE NOVEL SOFT-SWITVHING BUCK CONVERTER
Circuit description and assumption: The proposed novel soft-switching buck converter is show in Fig. 1.It is the combination of the conventional PWM buck converter and the proposed auxiliary snubber circuit.The auxiliary circuit consists of a resonant inductor L r , resonant capacitor C r , C 1 , C 2 , three auxiliary diodes VD 1 , VD 2 , VD 3 and auxiliary switching VT 2 .
The following assumptions are made during one switching cycle in order to analyze the steady-state operations of the proposed circuit.
Fig. 2: Key theoretical waveforms of the proposed converter

Reverse recovery time of all diodes is ignored
Operation principles and analysis: Based on the assumptions above, circuit operation in one switching cycle can be divided into nine stages.The key waveforms are shown in Fig. 2 and the equivalent circuit of the operation stages is shown in Fig. 3.The detailed analysis of every stage will be discussed in the following paragraphs.
Mode 1( 1 0 ~t t ): Prior to 0 t , the main diode VD is conducting, while the main switching VT 1 and the auxiliary switching VT 2 were off.At the beginning of this stage: At t = t 0 , the auxiliary switching VT 2 is turned on, which realizes zero-current turn-on as it is in series with the resonant inductor L r .During this stage, r L i rises and the current of the main diode fall simultaneously at the same rate linearly.This mode ends at t = t 1 when r L i reaches I o and i D becomes zero.The main diode VD is turned off with ZVS because of C 1 and C 2 being existent.In this state, ( ) (1) ( ) The diode VD 2 starts conducting with ZVS at the instant when the main diode VD is turned off.At In this interval, resonance occurs with the inductor L r and the capacitor C r , r L i increases on and r C U falls, at the end of this mode, the voltage of the capacitor C r falls to zero: is conducting, it clamps the voltage of the main switching with zero.At this moment: The current of the resonance inductor flows through the diode which reverse paralleled VT 1 and the main switching VT 1 turns on with ZVS.And the time when VT 1 turns on is later than the time VT 2 turns on.The delayed time: In this interval, resonance occurs with the inductor L r and capacitor C 1 and C 2 .This mode ends with C 2 is charged up to the input voltage U i : where, U becomes U i , the diode VD 1 is turned on with ZVS.In this state: .When 1 VD turns on, new resonance starts with r L and 1 C , and 1 C is charged up: ( ) ( ) ( ) where, This mode ends when C 1 is charged up to its maximum voltage 2 1 C U .Diodes VD 1 and VD 2 are turned off with ZCS due to the existence of L r .And at the same time, auxiliary switching VT 2 turns off with ZCS.In this state:

~t t
): There is no resonance in this mode and the circuit operation is identical to that of a conventional PWM buck converter: . The main switching VT 1 is turned off under ZVS because the voltage of the capacitor r C cannot rise suddenly.C 1 is clamped to zero.Resonance occurs with L r and C 2 .The voltage and current equations for this mode are: This mode ends when the voltage across 2 C becomes zero.

Mode 7(
The diode VD 2 turns on under ZVS.Resonance starts with inductor L r and C 1 capacitor C 1 .And C 1 is charged up.For this state, the equations are: i becomes zero, this mode comes to an end.The time interval for this mode is given as: The stored energy of capacitor C1 is now transferred to load. Mode 9( 9 8 ~t t ): At t = t 8 , the main diode VD is conducting under ZVS, the load current will flow through the main diode VD.During this mode, the converter operates like a conventional PWM buck converter until the auxiliary switching VT 2 is turned on in the next switching cycle.In this mode:

DESIGN PROCEDURE
The proposed converter operates with an input voltage , load current of 20A and the switching frequency of 20kHz..The following design procedure is developed considering procedure of literatures (Zhang et al., 2007).
• Selection of the resonant inductor L r .From the analysis of mode 1, the speed of the current flowing through L r increases fast if the resonant inductor L r is too small, thus it can't limit the reverse current of the main diode VD.And from the analysis of mode 2, if the resonant inductor r L is too small, r L I ∆ will be very large, which may increase the turn-on losses of the auxiliary switching VT 2 .In the designations, it is considered as followings: . In this case, from (1): • Selection of C r , C 1 , C 2 .In order to provide ZVS turn-on for the main switching VT 1 , the energy of The capacitor C 2 will transform the energy to L r at least the time period t f during the turn off of the main switching, according to formula (22): With the energy balance defined as: The capacitor C 1 can be calculated.

SIMULATION RESULTS
The converter is simulated using simulation software OrCad, version 10.5.The major parameters are given as follows: depicts the drive waveforms of main and auxiliary switching when the main switching's duty cycles is 0.4 and Fig. 5 depicts the drive waveforms of main and auxiliary switching with 0.8 duty cycles of the main switching.The waveforms of current and voltage on main switching with 0.4 duty cycles of the main switching is shown in Fig. 6, when the voltage of main switching falls to zero, the current then increases.And before the voltage increases, the current falls to zero.So the power loss is nearly to zero, the main switching achieves zero-voltage switching.Figure 7 describes the waveforms of current and voltage on auxiliary

CONCLUSION
As the simulation results shown, the main switching turns on and off with ZVS.And the auxiliary switching ZCS turns on and turns off.All the diodes operate under soft-switching conditions.Switching losses are reduced.And the additional voltage and current stresses on the main devices do not take place.The proposed circuit can also be used in the vehicle voltage converter, which the author will develop further research next step.

Fig. 3 :
Fig. 3: The equivalent circuit of the operation stages the fall time of the main switching.

Fig. 4 :
Fig. 4: The drive waveforms of main and auxiliary switching at 0.4 duty cycle of the main switching

Fig. 10 :
Fig. 10: The waveforms of current and voltage of auxiliary switching at 0.8 duty cycle