JOURNAL OF RESEARCH INOVATION IN ENGINEERING & TECHNOLOGY
VOL. 1
No. 2
MODELLING AND ANALYSIS OF FULL BRIDGE SERIES –PARALLEL
RESONANT CONVERTER FOR WIDE LOAD VARIATIONS
Chinju.S G
Department Of Electrical & Electronics
Rajadhani Institute Of Engineering & Technology
Nagaroor, Trivandrum
cgs299@gmail.com
ABSTRACT: LLC Resonant converters are widely used
in dc-dc converters. In this paper, the full bridge series
parallel resonant converter is operated under phase
shifted modulation and switching frequency modulation
to get the maximum efficiency for different loads(from
light load to full load). The full bridge series parallel
resonant converter is operated under switching
frequency modulation for most of the loads to achieve
ZVS. For light loads, the full bridge series parallel
resonant converter is operated under phase shift
modulation. The Design procedure is verified through
the simulation output of Full bridge series- parallel
resonant converter.
Keywords: LLC Converter, SRC, PRC, SPRC, Zero
voltage switching.
I.
INTRODUCTION
In recent years, dc power supplies are widely use in most
of an electrical and electronic appliances, such as high
power loads and low power loads. Resonant converters are
desirable for power conversion due to their comparatively
smaller size and low power losses resulting from high
frequency operation. Resonant converters uses a resonant
circuit for switching the transistors, when they are at zero
current or zero voltage point, this reduces the stress on the
switching transistors and radio interference [1]. Resonant
converters operate pushing the semi-conductor devices to
turn on and/or turn off at the instants of extinction of
current or voltage, in order to reduce the switching
losses, facilitating the increase of the frequency without
the corresponding increase of stress and degrading of the
efficiency.
Resonant converter SRC, PRC, SPRC is investigated
in [2]-[3], which can operate at high
switching frequency due to very low switching losses. But
each of them has its disadvantage. Light load regulation is
the major problem of SRC, high circulating energy sending
back to the input source is the major drawback of PRC, and
some high circulating energy problem will occur in high
input voltage for SPRC. LLC Converter doesn’t exist in the
above problem, they posses many advantages such as ZVS
in full load range; low turn off current, high efficiency at
high input voltage, low voltage stress on secondary
rectifier and so on. The control circuits for this project used
low cost components easily available yet giving excellent
performance and satisfactory results.
A. Comparison of Resonant Converter Topologies
Series-Resonant Converter
In series resonant converter, the resonant
inductor (Lr) and the resonant capacitor (Cr) are in series.
The resonant capacitor is in series with the load. The
impedance of the resonant tank can be changed by varying
the frequency of the driving voltage. The DC gain is
always lower than 1(maximum gain happens at the
resonant frequency).The series resonant converter has the
advantage of reduced switching loss and electromagnetic
interference through ZVS and getting improved efficiency.
Another advantage is that it has reduced magnetic
components size by high frequency operation. The seriesresonant converter (Fig 1) has the main disadvantage that
the output voltage cannot be regulated for the no-load case.
This means that this converter would only be used ―as is‖
in applications where no-load regulation was not
required.[1-3]. Another disadvantage of this converter is
that the output dc filter capacitor must carry high ripple
current (equal in magnitude to 48 percent of the dc output
current). This is a significant disadvantage for applications
with low output voltage and high current. For this reason
the series resonant converter is not considered suitable for
low-output-voltage high-output-current converters but
rather is more suitable for high-output-voltage low-outputcurrent converters. For the high-output-voltage case no
magnetic components are needed on the high-voltage side
of the converter. It cannot regulate the output at no load
condition.
The main advantage of the converter is that the series
resonant capacitors on the primary side act as a dc blocking
capacitor. Because of this fact converter can easily be used
in full bridge arrangements without any additional control
to control unbalance in power FET switching times or
forward voltage drops(i.e.,dc current kept out of the
transformer).For this reason the series-resonant converter
is suitable for high-power applications where full bridge
converter is desirable.
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Fig.1. series resonant converter
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output voltages because the capacitor would have to carry
too much ac current. However, for higher output voltage
converters this placement of the resonant capacitor may be
desirable. Also, the resonant capacitor can be placed on a
tertiary transformer winding. The parallel-resonant
converter is naturally short circuit proof. This property can
be seen by applying a short directly across the resonant
capacitor. For that case, the entire square wave voltage
applied by the inverter is directly across the resonant
inductor and, therefore, the current is limited by this
impedance. This property makes the parallel- resonant
converter extremely desirable for applications with severe
short circuit requirements
C. Combination Series-Parallel Converter
Another advantage of the series-resonant converter is that
the currents in the power devices decrease as the load
decreases. This advantage allows the power device
conduction losses (as well as other circuit losses) to
decrease as the load decreases, thus maintaining high part
load efficiency. As will be seen in the next section, this is
not the case for the parallel-resonant converter. Note that if
the converter is operating near resonance (i.e., at heavy
load) and a short circuit is applied to the converter output,
the current will rise to high values. To control the output
current under such conditions, the frequency of the
converter is raised by the control.
B. Parallel-Resonant Converter
In this, the resonant inductor (Lr) and the resonant
capacitor (Cr) are in series. The resonant capacitor is in
parallel with the load. The impedance of the resonant tank
can be changed by varying the frequency of the driving
voltage. The advantage of parallel resonant converter is
that there is no problem in output regulation at no load
condition. The main disadvantage of the parallel-resonant
converter is that the current carried by the power FETs and
resonant components is relatively independent of load..
Conversely, the converter is better suited to applications
which run from a relatively narrow input voltage range
(e.g., plus or minus 15 percent) and which present a more
or less constant load to the converter near the maximum
design power (e.g., 75 percent of maximum design power).
Of course, the power converter must be designed thermally
for the maximum power and, therefore, has no problem
running at reduced power thermally-only the part-load
efficiency is less than the full-load efficiency.
. The inductor limits the ripple current carried by the output
capacitor. Note also that the transformer leakage
inductance could be used as the resonant inductance by
placing the resonant capacitor across the total span of the
secondary winding.[4] This is normally not ideal for low
The converter combining the series and parallel
configurations, called a series-parallel resonant converter
(Fig 2), has been proposed. One version of this structure
uses one inductor and two capacitors, or an LCC
configuration. Although this combination overcomes the
drawbacks of a simple SRC or PRC by embedding more
resonant frequencies, it requires two independent physical
capacitors that are both large and expensive because of the
high AC currents. An advantage of the LLC over the LCC
topology is that the two physical inductors can often be
integrated into one physical component, including both the
series resonant inductance, Lr, and the transformer’s
magnetizing inductance, Lm.
The series-parallel converter can operate and
regulate at no load provided that the parallel-resonant
capacitor C, is not too small (if C, is zero, then the circuit
reverts to the series-resonant converter). It is seen that the
smaller C, is, the less ―selectivity‖ is available in the
resonant curves. That is, the converter resembles a series
converter more and more as C gets smaller and smaller. [46]However, for reasonable values of C, the converter will
clearly operate with no load, which removes the main
disadvantage of the series-resonant converter. The LLC
resonant converter has many additional benefits over
conventional resonant converters. For example, it can
regulate the output over wide line and load variations with
a relatively small variation of switching frequency, while
maintaining excellent efficiency. It can also achieve zero
voltage switching (ZVS) over the entire operating range.
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metalized polypropylene film. These capacitors present
very low DF and are capable of handling high-frequency
current. Before a capacitor is selected, its voltage rating has
to be derated with regard to the switching frequency in use.
Fig.3:.Characteristics
converter
of
combination
series-parallel
Features of an LLC Converter:

Reduced switching loss through ZVS: Improved
efficiency.

Narrow frequency variation range over wide load
range.

Zero Voltage switching even at no load
condition.
II. PROPOSED METHOD
A. CIRCUIT DIAGRAM OF PROPOSED METHOD
Fig 4: circuit diagram of full bridge LLC converter
The circuit has three passive components: r L, m L and rC,
the secondary side is centre-taped rectifier followed by
bulk capacitor without output filter inductor. The resonant
capacitor (Cr) must have a low dissipation factor (DF) due
to its high-frequency, high-magnitude current. Capacitors
such as electrolytic and multilayer X7R ceramic types
usually have high DF and therefore are not preferred
.Capacitors often used for LLC converters are made with
In an LLC converter, the output filter may consist of
capacitors alone instead of the LC filter seen in most pulsewidth-modulated converters, although a small second-stage
LC filter can be an option. If the filter has only capacitors,
they should be chosen to allow conduction of the rectifier
current through all AC components. Usually a single
capacitor will not allow such a high RMS current, so
several capacitors connected in parallel are often used and
may offer a lower profile.
The converter configuration in has three main parts:
[1]. Power switches Q1 and Q2, which are usually
MOSFETs, are configured to form a square wave
generator. This generator produces a unipolar square-wave
voltage V12 by driving switches Q1and Q2, with alternating
50% duty cycles for each switch. Similarly Q3 and Q4
generate the square wave. A small dead time is needed
between the consecutive transitions, both to prevent the
possibility of cross conduction and to allow time for ZVS
to be achieved.
[2] The resonant circuit, also called a resonant network,
consists of the resonant capacitance, Cr, and the series
resonant inductance, Lr and magnetizing inductance Lm.
The transformer turns ratio is n: 1:1. The resonant network
circulates the electric current and, as a result, the energy is
circulated and delivered to the load through the
transformer. The transformer’s primary winding receives a
bipolar square-wave voltage, Vso. This voltage is
transferred to the secondary side, with the transformer
providing both electrical isolation and the turn’s ratio to
deliver the required voltage level to the output.
[3].On the converter’s secondary side, two diodes
constitute a full-wave rectifier to convert AC input to DC
output and supply the load RL. The output capacitors
smooth the rectified voltage and current. The rectifier
network can be implemented as a full-wave bridge or
center tapped configuration, with a capacitive output filter.
The rectifiers can also be implemented with MOSFETs
forming synchronous rectification to reduce conduction
losses, especially beneficial in low-voltage and high
current applications.
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B. ZERO VOLTAGE SWITCHING
To achieve ZVS, a MOSFET is turned on only after its
source voltage, Vds, has been reduced to zero by external
means. One way of ensuring this is to force a reversal of
the current flowing through the MOSFET’s body diode
while a gate-drive turn-on signal is applied.
The junction capacitance of power MOSFETs to be
conducted is supposed to discharge to zero by the energy
stored in resonant inductance and magnetizing inductance,
the degree junction capacitance discharged of MOSFET to
be conducted depends on the resonant inductance current
value, the current at the moment the leading leg Q3 turns on
is greater than that the lagging leg Q2 turns on, so the
leading leg Q3 is easier to achieve the ZVS than the lagging
leg Q2 , the current at the moment the MOSFET Q2 to be
conducted is Im, before the lagging leg Q2 turns on, the
energy stored in r L and m L will discharge the Q2 junction
capacitance and charge the Q4 junction capacitance, the
discharge current is m I.
Fig 6: voltage between transistors 1& 2
III. SIMULATION RESULT
The Full bridge series resonant converter is supplied
with 100V DC supply ZVS Technique is adopted .It
operate at the switching frequency of100KHZ.
Fig 7: current through resonant inductor
fig 5: Duty cycle modulation
Fig 8: output voltage
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IV. CONCLUSION
This paper has presented design consideration for the
full bridge series resonant converter utilizing the
leakage inductance and magnetizing inductance of the
transformer as resonant components. The FBSRC is
operated under switching frequency modulation for
most of the load range to achieve ZVS and low
switching noises. For the lighter loads, the FBSRC is
operated under Phase shifted duty cycle modulation to
regulate the output voltage and maintain the ZVS
feature. The new topologies have been verified by
simulation by using MATLAB. Analysis and
Simulations are carried out with the step by step
procedure and the results show the better performance
compared with that of the conventional control
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