INTRODUCTION
Nowadays the kinds of control type soft switching circuits are few. Phase shifted fullbridge converter, asymmetrical halfbridge converterpushpull converter and LLC series resonant converters and so on are the typical topologies.
The FullBridge (FB) ZeroVoltageSwitched (ZVS) converter is one of the most
attractive techniques. It is the most widely used softswitched circuit in highpower
applications (Redl et al., 1991; Sabate
et al., 1991; Chen et al., 1995).
This constantfrequency converter employs PhaseShift (PS) control and features
ZVS of the primary switches with relatively small circulating energy. In this
technique a control circuit serves to supply pulsed control signals to the switching
transistors of the converter for maintaining the output voltage at its desired
level using phase shift control in known manner. Though the snubber approaches
in (Redl et al., 1991; Sabate
et al., 1991) offer practical and efficient solutions to the secondaryside
ringing problem, they do not offer any improvement of the secondaryside dutycycle
loss.
Several techniques have been proposed to extend the ZVS range of FB ZVS converters
without the loss of duty cycle and secondaryside ringing (Jain
et al., 2002; Ayyanar and Mohan, 2001; Mason
and Jain, 2005; Jang and Jovanovic, 2004). Generallythese
circuits utilize energy stored in the inductive components of an auxiliary circuit
to achieve ZVS for all primary switches in an extended load and input voltage
range. Ideally, the auxiliary circuit needs to provide very little energy, if
any, at full load because the fullload current stores enough energy in the
converter’s inductive components to achieve complete ZVS for all switches.
As the load current decreases the energy provided by the auxiliary circuit must
increase to maintain ZVS with the maximum energy required at no load. The energy
stored for ZVS is independent of load as described by Jain
et al. (2002) and Ayyanar and Mohan, (2001).
Adaptive energy storage in the auxiliary circuit has been given by Mason
and Jain (2005) and Jang and Jovanovic (2004). However,
these converters have to use large inductors so; high circulating energy is
needed to achieve noload ZVS. ZVS full bridge DC to DC converter with ZVS over
the entire range is given by Borage et al. (2008).
High power density multikilowatt DC to DC converter with galvanic isolation
is given by Pavlovsky et al. (2009).
Conventional pushpull converter is used in low and medium power systems for
the reason of less conduction loss than that of half bridge or full bridge converters
and low side driving of both primary switches. As featuring in ZVS for primary
switches over entire load ZCS for rectifier diodeswide input voltage range capability
and high efficiency under all input voltages the LLC series resonant converter
gains popularity in recent years (Lee et al., 2002;
Wei et al., 2007).
Pushpull type LLCSRC combines characteristics of both conventional pushpull
converter and LLC series resonant converter. An LCL resonant pushpull topology
operating under ZVS condition was presented by Ryan et
al. (1998). The circuit exhibits ZVS for the MOSFET switches and the
resonant capacitor snubs the reverse recovery transient of rectifier diodes.
Since the load of this topology is series connected with the resonant inductor
the output current will swing corresponding to the resonant current and it’s
difficult to reduce the ripple of output voltage. An LC resonant pushpull topology
was presented in which the primary switches and secondary rectifiers turn on
and turn off under zerovoltage and zerocurrent switching conditions respectively
Boonyaroonate and Mori (2002). With most of the resonant
current flowing through the output capacitorit’s easy to control the ripple
of output voltage. But for resonant converters the large current or voltage
stress on switching elements and higher conduction loss are difficult to control.
Detail stage equations for each stage and design process for a conventional
LLC series resonant converter have been proposed by Liu
et al. (2006). High dc gain stepup pushpull type LLC series resonant
DCDC converter is proposed by Chen et al. (2008).The
analysis of seriesparallel resonant DCtoDC converter has been illustrated
by Padmanabhan et al. (2007). The above literature
does not deal with comparison of pushpull LLC and PWM ZVS FB converters.
In this study, a FB ZVS converter with adaptive energy storage that offers ZVS of the primary switches over a wide load range with greatly reduced noload circulating energy and with significantly reduced secondaryside duty cycle loss is considered for comparison with pushpull LLC series resonant converter.
OPERATIONAL PRINCIPLE OF FB ZVS CONVERTER
The circuit diagram of the FB ZVS converter is shown in Fig. 1. In the circuitsince the ZVS energy stored in the primary inductor is dependent on its inductance value and the voltsecond product of the secondary of auxiliary transformer TRA the size of the primary inductor can be minimized by properly selecting the turn ratio of auxiliary transformer TRA. As a result the size of the primary inductor is very much reduced compared to that of the conventional PS FB converter. In addition since the auxiliary transformer does not need to store energy, its size can be small. Finally because the energy used to create the ZVS condition at light loads is not stored in the leakage inductances of transformer Tr the transformer’s leakage inductances can also be minimized. As a result of the reduced total primary inductance i.e. the inductance of the primary inductor used for ZVS energy storage and the leakage inductance of the power transformer the modified converter exhibits a relatively small dutycycle loss. This minimizes both the conduction loss of the primary switches and the voltage stress on the components on the secondary side of the transformer which improves the conversion efficiency. Moreover, because of the reduced total primary inductance the secondary side parasitic ringing is also reduced and is effectively controlled by primary side diodes D and D_{1} as shown in Fig. 1.
Technical specifications of Fig. 1: DC input voltage = 48VL_{p }= 0.01 μHR = 25ΩL_{1 }= 28μ HC_{7 }= 120μ FC_{5 }= 0.5μ FC_{6 }= 12μ F, Operating frequency = 20 KHz.
To achieve ZVS energy stored in the primary inductor L_{p}(E_{LP}) must be higher than total energy required to charge C_{1} and discharge C_{2}:
where, C_{1 } = C_{2} = C. = Capacitance across Q_{1} and Q_{2} respectively:
where, f_{s }is the switching frequency and T is the duty cycle of
switch:
n 
= 
Main transformer turn ratio 
n_{i} 
= 
Auxiliary transformer turn ratio. 
OPERATIONAL PRINCIPLE OF PUSHPULL LLC SERIES RESONANT CONVERTER
Power stage of the modified pushpull LLC series resonant converter is shown in Fig. 2. The main components of the converter are: two main switches Q_{1} and Q_{2} constitute the two pushpull branches respectively. The DC to DC converter further includes a resonant tank connected to the secondary of transformer comprising a series capacitor connected to a series inductor and a parallel inductor. The components of resonant tank are L_{ss}C_{r} and L_{sr}. C_{1} and C_{2} represent parasitic capacitor of Q_{1} and Q_{2} respectively. The output rectifier consists of four diodes D_{1}D_{4}.
Technical specifications of Fig. 2: DC input voltage
= 48V, Ls1 = 0.11 μH, Ls2 = 0.13 μH C_{1 }= C_{2 }=
0.01*10^{12}F, Lss = 19.8 μH, Lsr = 6.32 mH C_{r }= 3.06μ
F,C_{0 }= 300μ F, R = 25Ω Operating frequency = 20 KHz.
The control strategy is similar to a conventional LLC series resonant converter i.e., Q_{1} and Q_{2} conduct alternately in a switching cycle under variable frequency modulation. It is assumed that the converter is under steady operation and the output capacitor C_{0} is large enough to be considered as a voltage source. The series capacitor functioning with the series inductor provide a first characteristic resonant frequency represented by f_{s}and the series capacitor functioning with the series inductor and the parallel inductor to provide a second characteristic resonant frequency represented by f_{m }where f_{s}>f_{m}:
When the operation frequency is between first and second resonant frequency i.e. f_{m}<f<f_{s }the switches operate under zerovoltageswitching condition and the rectifier circuit operate under zerocurrentswitching condition. Fig. 4 has drawn when f_{m}<f<f_{s.}
The boundary conditions for ZVS can be obtained according to energy balance:
where, n is the transformer turns ratio. I_{Lsr} is the current across L_{sr}
SIMULATION RESULTS OF FB ZVS CONVERTER
The ZVS DC to DC converter is simulated using matlab simulink and the results are presented here.

Fig. 2: 
PushPull LLC eries resonant converter 

Fig. 3: 
Simulink model of FB ZVS DC to DC converter 

Fig. 6: 
Output voltage across Q_{1} and Q_{2} 

Fig. 7: 
Output voltage across Q_{3} and Q_{4} 

Fig. 8: 
Voltage across the secondary 

Fig. 9: 
DC output current and voltage 

Fig. 10: 
Simulink model of modified LLC SRC 
Simulink model of FB ZVS DC to DC converter is shown in Fig. 3. Driving pulses are shown in Fig. 4. DC input voltage is shown in Fig. 5. Output voltage across Q_{1} and Q_{2} is shown in Fig. 6. Voltage across Q_{3} and Q_{4} are shown in Fig. 7. Secondary voltage is shown in Fig. 8. DC output current and voltage are shown in Fig. 9. DC output voltage is 12V and the current 1A. It can be seen that the DC output is free from ripple.
For constantfrequency variable duty cycle control of the proposed converter switches Q_{1} and Q_{2} always operate with approximately 50% duty cycle whereas switches Q_{3} and Q_{4} have a duty cycle in the range from 0 to 50% as shown in Fig. 5.
SIMULATION RESULTS OF PUSHPULL LLC SERIES RESONANT CONVERTER
The ZVS pushpull LLC series resonant converter is simulated using matlab simulink and the results are presented here.
Simulink model of LLC series resonant converter is shown in Fig. 10. Driving pulses are shown in Fig. 11. DC input voltage is shown in Fig. 12. Drain to source voltage across switch Q1 is shown in Fig. 13. Secondary voltage is shown in Fig. 14. Voltage across L_{sr} is shown in Fig. 15. DC output current and voltage is shown in Fig. 16. DC output voltage is 12V and the current is 2.46A. It can be shown that DC output voltage is free from ripple.

Fig. 12: 
DC input voltage 

Fig. 13: 
Drain to source voltage across switch Q_{1} 

Fig. 14: 
Secondary voltage of transformer 

Fig. 15: 
Voltage across Lsr 

Fig. 16: 
DC output current and voltage 

Fig. 17: 
Efficiency versus load resistance 

Fig. 18: 
Load current versus efficiency from simulation 

Fig. 19: 
Load current versus switching frequency from simulation 

Fig. 20: 
Switch stress for pushpull LLC 
Fig. 17 shows the variation of efficiency with load resistance. Fig. 18 shows load current versus efficiency from simulation. Fig. 19 shows load current versus switching frequency from simulation. Fig. 20 shows switch stress for pushpull LLC converter and Fig. 21 shows switch stress for FB ZVS converter.

Fig. 21: 
Switch stress for FB ZVS 

Fig. 22: 
Voltage across the primary X axis 1 div = 0.02 m sec Y axis
1 div = 20 V 

Fig. 23: 
Voltage across the secondary X axis 1 div = 0.02 m sec, Y
axis 1 div = 30 V 
EXPERIMENTAL VERIFICATION OF FB ZVS CONVERTER
The DC to DC converter was built and tested at 48 V DC. The circuit parameters are as follows.

Fig. 24: 
Oscillogram of load voltage X axis 1 div = 0.02 m sec, Y axis
1 div = 10 V 

Fig. 25: 
Voltage across the primary X axis 1 div = 0.02 m sec, Y axis
1 div = 30 V 

Fig. 26: 
Voltage across secondary X axis 1 div = 0.02 m sec, Y axis
1 div = 10 V 

Fig. 27: 
Oscillogram of load voltage 

Fig. 28: 
Load current versus efficiency from experiment 

Fig. 29: 
Load current versus switching frequency from experiment 
R = 25ΩC_{7 }= 100μ FL_{1 }= 28 mHL_{p }= 0.02 mHand the switching frequency is 20 kHz. Experimental waveform of voltage across the primary is shown in Fig. 22 voltage across the secondary is shown in Fig. 23 and Oscillogram of load voltage is shown in Fig. 24.
EXPERIMENTAL VERIFICATION OF PUSHPULL LLC SERIES RESONANT CONVERTER
The DC to DC converter was built and tested for pushpull LLC series resonant converter at 48V DC. The circuit parameters are as follows.
R = 22Ω C_{0 }= 1000 μFL_{sr }= 6 mHC_{r }= 2 μFand the switching frequency is 20 kHz. Experimental waveform of voltage across the primary is shown in Fig. 25 voltage across the secondary is shown in Fig. 26 and Oscillogram of load voltage is shown in Fig. 27.
COMPARISON
From Fig. 9 and 24, output voltage of
FB ZVS converter is nearly same as 12 V from simulation and experiment. From
Fig. 17, efficiency of pushpull LLC converter is better
than FB ZVS converter. From Fig. 20 and 21,
switching stress of pushpull LLC converter is less than FB ZVS converter. From
Fig. 16 and 27, output voltage of pushpull
LLC is nearly same as 12V from simulation and experiment. From Fig.
18 and 28, load current versus efficiency of both converters
are nearly same from simulation and experiment. From Fig. 19
and 29, load current versus switching frequency of both converters
are nearly same from simulation and experiment. From Fig. 28,
the efficiency of the FB ZVS converter is less than pushpull LLC converter
and decreases at light load but keep the efficiency curve flat over a wide load
current range. From Fig. 28 and 29, the
switching frequency operating range varies from 20 to 300 kHz which matches
to the simulation results diagram of Fig. 18 and 19,
to some extent and keep the frequency curve flat over a wide load range for
both converters. At low power and high efficiency requirement pushpull LLC
series resonant converter is better than FB ZVS converter.
CONCLUSIONS
Soft switched ZVS DC to DC converters are analysed, simulated, tested and results are presented using full bridge and pushpull LLC series resonant converter. Conversion of 48V to 12V is done using two methods and results are compared. At low power applications like battery charging high efficiency and low switching stress must be maintained from no load to full load. It appears that pushpull LLC series resonant converter is better than modified FB converter. The experimental results coincide with the simulation results.