INTRODUCTION
Recently, there is an increased interest in the area of resonant converters due to their advantages, i.e., higher frequency of operation, higher efficiency, small size, light weight, reduced EMI, lowcomponent stresses, etc. Recent research has shown that SeriesParallel Resonant Converters (SPRCs) (also called using LCCtype commutation or LCCtype parallel resonant converter) have a number of desirable features compared to series resonant or parallel resonant converters. Operation of such converters above resonance (lagging pf mode) results in a number of advantages: Elimination of di/dt inductors and loss snubbers, use of slow recovery diodes internal to MOSFET, reduced size of magnetic components, etc. An approximate analysis of seriesparallel resonant converter operating above resonance using complex circuit analysis has been presented Bhat and Dewan (1987). It is verified using simulation and experimental results.
MATERIALS AND METHODS
Analysis of series parallel resonant converter: The principle of resonant converter can be explained with its operating modes. The resonant converter can operate in five different modes. However the broad classification of modes can be made depending on the duty ratio. n the duty ratio is 1, the resonant converter can operate in two modes via mode 1 and mode 2. The other modes are obtained when duty ratio is less than 1, depending upon the frequency of operation, circuit impedance across terminal AB and duty ratio. When pulse width is maximum (Duty ratio D = 1) the resonant converter can operate in two different modes and under reduced pulse width (D<1), the converter can operate in three different modes. The relative polarity of the parallel capacitor voltage and the voltage across point A and B decides the interval in which the resonant converter can operate (Bhat, 1993).
Mode of operation: Controlling the pulse width of the input voltage
to the tank circuit regulates the output voltage of the converter. Here the
switching frequency is more than the series resonant frequency and the load
is such that equivalent impedance across the terminal AB is inductive, therefore
in this mode the converter operates with lagging power factor. The operating
waveforms for this mode are shown in Fig. 1. At t = t_{0}
I(t) is negative therefore D_{1} conducts (Fig. 1a,
b, c). During the interval t_{0}t_{1}
the current freewheels through S_{4}, resonant tank, D_{1} and
back to S_{4}. This interval is called freewheeling interval (FI). At
t = t_{1}, D_{1} and D_{2} conduct transferring the
energy stored in the inductor back to the source. As a result I(t) reduces to
zero and S_{1} and S_{2} starts conducting. This marks the beginning
of power interval. At t = t_{3}, ib (t) changes fromI_{0}t_{0}
+I_{0} and converter operation changes. The power interval ends at t
= t_{4} i.e., (t_{0}+T/2). It may be noted that in this mode
also all switches turned on at zero voltages, facilitating the use of loss less
snubber.
Mathematical analysis of converter: Following assumptions are used in
the analysis of the seriesparallel resonant converter.
• 
The switches, diodes, inductors, capacitors and snubber components
used are ideal. 

Fig. 1: 
Typical waveform in mode of operation; (a) Gate pulse (b)
V_{AB} (t) (c) I (t) and I(_{b}(t) 

Fig. 2: 
Output circuit of bridge rectifier and filter component to
resonant converter 
• 
The effects of snubber are neglected. 
• 
The inductance I_{0} is large enough to keep the load current
constant. 
• 
The high frequency transformer is ideal and has unity turns ratio. 
From Fig. 2 V_{cp} and I_{b} represent the
rms fundamental component of V_{cp} (t) and I_{b} (t), respectively.
Because of Diode Bridge rectifier and inductive filter present in the output
circuit.
The D.C. output Voltage is obtained as the average of A.C. input voltage, Vcp
(Rasid, 1998).

Fig. 3: 
PSPICE simulation of series parallel resonant converter 
ω 
= 
2πf and f is the switching frequency. 
The Fundamental Component of Diode Bridge current is calculated as (using Fourier
Analysis).
Using Eq. 1 and 2 the equivalent A.C. resistance
as seen at the I/P of the Rectifier Bridge is given by:
δ and D are related by:
The RMS fundamental component of inverter output voltage at terminal AB can
be found using the waveform shown in (Fig. 3). The duty ratio
D is defined as the ratio of the time duration for which the switch S_{1}
and S_{2} or S_{3} and S_{4} are switched on simultaneously
i.e., ton to the half of the switching period (T/2) D = ton/T/2.
The RMS fundamental voltage across AB is given by:
Design of seriesparallel resonant converter: Following criteria has
been taken into account in order to obtain optimum design of seriesparallel
resonant converters.

Fig. 4: 
PSPICE Simulation results for series parallel resonant converter
at full load with m = 1 : (a) V_{AB}, V_{0} (b) I_{L2}
(c) V_{cs}, V_{cp} 
• 
Normalized switching frequency y, such that maintains the
lagging power factor conditions. 
• 
Minimum inverter output peak current for small rating and losses. 
• 
Minimum stress in series and parallel capacitor. 
• 
Minimum variation of Duty ratio from full load to no load i.e., good voltage
regulation. 
Design example:
Minimum input voltage E_{in} = 50 volts.
Output power p = 100 watts.
Switching frequency = 50 kHz.
m = Cs/Cp = 1, Q = 5, y = 8.
The converter gain is given E_{o}/E_{1} = 0.8 by full load resistance:
Resonant frequency F_{o} is given by:
The values of L and C from (Fig. 3 and 4)
are L = 204 μH
C 
= 
0.0318 μ F 
Since Cs 
= 
Cp has been chosen 
Cs 
= 
Cp = 2C = 0.0636 μF 
RESULTS AND DISCUSSION
Simulation of seriesparallel resonant converter: The proposed resonant
DCtoDC converter is simulated using PSPICE software package. It is a generalpurpose
circuit simulation program, which can be used to obtain the waveforms of different
circuit variables both in transient and steady state. However in present study,
the simulation is aimed to determine the various voltages and current waveform
in steady state (Fig. 3).
Simulation with variation load: Figure 4 and 5
present some of the simulated waveforms obtained for variation of load from
full load to 25% of rated load. These waveforms were obtained after a number
of simulation runs by varying δ to obtain the approximately the rated load
voltage, when load was changed.
Block diagram for series parallel resonant converter with dsp control:
The DC input is given to high frequency bridge inverter circuit in series with
a resonant circuit, which produces alternating voltage. The switchers are turned
on at zero voltage. This circuit is in series with a high frequency transformer.
The output of the transformer is rectified using bridge diode rectifier and
then filtered.

Fig. 5: 
PSPICE Simulation results for series parallel resonant converter
at 25% load with m = 1: (a) V_{AB}, V_{0} (b) I_{L2}
(c) V_{cs}, V_{cp} 
The filtered output is given as input to the load. Depending on the pulse given
by the MOSFET firing circuit controlled by DSP controller, the output voltage
is varied (Fig. 6).
Algorithm:
• 
START 
• 
Disable all interrupts. 
• 
Clear all interrupt flags. 
• 
Stop Timer flag. 
• 
Initialize memory to zero. 
• 
Read memory fro angle between 1 and 4 and 2 and 3. 
• 
Angle in the form of time. 
• 
Make output 0001. 
• 
Run timer for time in the memory 
• 
Make output 1001. 
• 
After time out, make output 0100 
• 
Read the time for angle 
• 
Run timer. 
• 
Make out put 0101 (ABCD) 
• 
Jump to step 6. 
Experimental results: Some testing results are presented here to verify
the theoretical predictions. An experimental prototype has been implemented
for a resistive load of 100W and 16Ω. The resonant inductor is 0.261 mH
and the series resonant capacitor is 0.047 μF. The switching frequency
is 50 KHz (Fig. 78 and Table
1).

Fig. 7: 
Experimental results of series parallel resonant converter
across V_{AB} and series inductor current at(a) 25% of load and
(b)100% of load 
Table 1: 
Comparison of results between simulated calculated and experimental
series parallel resonant converter for I/PDC supply voltage = 50 V and
switching frequency = 50 kHz 


Fig. 8: 
Output across the load at 25% load 
CONCLUSIONS
In this dissertation seriesparallel resonant DCtoDC converter has been proposed.
This study has led to identification of mode of operation of the converter.
For the mode of operation type of switches, diodes and snubber needed have been
discussed thoroughly. Converter is analyzed using complex ac circuit analysis
method. The analysis presented was used to obtain the design curves. A simple
design procedure has been illustrated using a design example of 100 W resistive
load. Detailed PSPICE simulation results have been presented to evaluate the
performance the converter. Experimental output is taken. The calculated values,
simulated values and the experimental values show very less deviation. So series
parallel resonant converter can be used for very large load variation to maintain
constant DC output voltage with less loss.