International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
Analysis On Closed Loop Center Tap Rectifier
Voltage Oscillation Of
LLC Resonant Converter
Mr.N.Soundiraraj, M.E.,
Mr.Amal Arockia Raj.S
Assistant Professor, Department of EEE
PSNA College of Engineering & Technology
Dindigul – 624 622, Tamilnadu, India.
PG scholar, Department of EEE
PSNA College of Engineering & Technology
Dindigul – 624 622, Tamilnadu, India.
Abstract -The increasing requirements of lighter, smaller and
more efficient electronic products demand the power supply
designers to develop DC/DC converter with high power density
and efficiency. The LLC resonant converter employing a
center-tap rectifier can suffer from a high voltage oscillation
across rectifier diodes owing to a leakage inductance of a
transformer secondary. The amplitude of this voltage
oscillation is varied according to design parameters, parasitic
components, and operation regions, i.e., below-resonant region
and above-resonant region. To reduce the diode voltage stress,
this paper analyzes the voltage oscillation mechanism and
presents the design consideration. The major aim of this work
is to analyze on center –tap rectifier voltage oscillation of LLC
resonant converter and reduce the losses using closed loop with
PID controller and through simulation
Keywords - LLC resonant converter and rectifier voltage
oscillation
I.INTRODUCTION
Increasing the frequency of operation of power
converters is desirable, as it allows the size of circuit
magnetics and capacitors to be reduced, leading to cheaper
and more compact circuits. However, increasing the
frequency of operation also increases switching losses and
hence reduces system efficiency. One solution to this
problem is to replace the "chopper" switch of a standard
SMPS topology (Buck, Boost etc.) with a "resonant" switch,
which uses the resonances of circuit capacitances and
inductances to shape the waveform of either the current or
the voltage across the switching element, such that when
switching takes place, there is no current through or voltage
across it, and hence no power dissipation. A circuit
employing this technique is known as a resonant converter.
Resonant converter, which were been investigated
intensively in the 80's ,can achieve very low switching loss
thus enable resonant topologies to operate at high switching
frequency. In resonant topologies, Series Resonant
Converter (SRC), Parallel Resonant Converter (PRC) and
Series Parallel Resonant Converter (SPRC, also called LCC
resonant converter) are the three most popular topologies.
The analysis and design of these topologies have been
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studied thoroughly. these three topologies will be
investigated for front-end application.
A Zero Current Switching (ZCS) circuit shapes the current
waveform, while a Zero Voltage Switching (ZVS) circuit
shapes the voltage waveform.
II. RESONANT SWITCH
Prior to the availability of fully controllable power
switches, thyristors were the major power devices used in
power electronic circuits. Each thyristor requires a
commutation circuit, which usually consists of a LC
resonant circuit, for forcing the current to zero in the turnoff process.
This mechanism is in fact a type of zero-current
turn-off process. With the recent advancement in
semiconductor technology, the voltage and current handling
capability, and the switching speed of fully controllable
switches have significantly been improved. In many high
power applications, controllable switches such as GTOs
and IGBTs have replaced thyristors.
However, the use of resonant circuit for achieving
zero-current-switching
(ZCS)
and/or
zero-voltageswitching (ZVS) has also emerged as a new technology for
power converters. The concept of resonant switch that
replaces conventional power switch is introduced in this
section.
A resonant switch is a sub-circuit comprising a
semiconductor switch S and resonant elements, Lr and Cr.
The switch S can be implemented by a unidirectional or
bidirectional switch, which determines the operation mode
of the resonant switch.
Two types of resonant switches, including zerocurrent (ZC) resonant switch and zero-voltage (ZV)
resonant switches, are shown in Fig. 1 and Fig. 2,
respectively.
.Zero current switch
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A typical Zero Current Switch consists of a switch
S, in series with the resonant inductor LRES, and the
resonant capacitor CRES connected in parallel. Energy is
supplied by a current source.
Lr
Cr
S
Lr
Lr
S
(a)
Lr
Cr
(b)
Fig 2: Zero-voltage (ZV) resonant switch
Cr
S
S
(a)
Cr
(b)
Fig 1: Zero-current (ZC) resonant switch
If an output transformer is used, in certain cases its parasitic
inductance can be used as the resonant inductance (in both
this and the zero voltage topology). However, as its value is
generally not known, the resonant frequency will not be
fixed, which may cause
problems in the circuit design. When the switch S is off, the
resonant capcitor is charged up with a more or less constant
current, and so the voltage across it rises linearly. When the
switch
is turned on, the energy stored in the capacitor is transferred
to the inductor, causing a sinusoidal current to flow in the
switch. During the negative half wave, the current flows
through the anti-paralleled diode, and so in this period there
is no current through or voltage across the switch; and it can
be turned off without losses.
A voltage source connected in parallel injects the
energy into this system. When the switch is turned on, a
linear current flows through the inductor. When the switch
turns off, the energy that is stored in the inductor flows into
the resonant capacitor. The resulting voltage across the
capacitor and the switch is sinusoidal. The negative halfwave of the voltage is blocked by the diode. During this
negative half wave, the current and voltage in the switch are
zero, and so it can be turned on without losses.
III. LLC RESONANT CONVERTER WITH
CENTER- TAP RECTIFIER
The LLC resonant converter shown in Fig. 3 is
one of the most popular topologies for its simple structure,
zero- voltage switching (ZVS) of primary switches,
zero-current switching (ZCS) of secondary rectifier diodes.
A large number of literature deal with a design guideline
considering magnetic components, switching frequency FS
variation range, efficiency, and size However, till now, the
rectifier voltage oscillation problem across center-tap
rectifier has rarely been discussed
This type of switching is also known as thyristor
mode, as it is one of the more suitable ways of using
thyristors; these devices will only turn off if the current
through them is forced to zero, which occurs naturally in
this topology.
In general, isolated-type converters employing an
inductive output filter suffer from a voltage ringing across a
rectifier stage since a leakage inductance of a transformer
and a junction ca- pacitance of rectifier diodes are
interacted after a
Zero voltage switch
This voltage oscillation increases a voltage stress on the
secondary diodes. A snubber is generally required to
suppress this additional voltage stress, however, it could
degrade the efficiency.
A typical Zero Voltage Switch consists of a switch
in series with a diode. The resonant capacitor is connected
in parallel, and the resonant inductor is connected in series
with this configuration.
In the LLC resonant converter, three types of output
stages are commonly adopted in the secondary side according
to applications, i.e., voltage-doubler rectifier, full-bridge
rectifier, center-tap rectifier, as shown in Fig. 4. In case of the
full-bridge rectifier and the voltage-doubler rectifier, the diode
voltage stress is clamped to the output voltage VO , which
makes them suitable for high output voltage applications
On the other hand, in case of the center-tap
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rectifier, which is normally adopted for low output voltage
applications, the diode voltage stress in steady state is
approximately twice the output voltage, 2VO
voltage stress on the diodes, a rather high voltage
oscillation could be occurred at switching transitions.
However, since there is no clamping path for the
diode junction capacitance: CJ . For the sake of the voltage
oscillation analysis in the secondary side, the equivalent
circuit reflected to the secondary side is used where
VSs = VS /n,
VCRs = VCR/n,
COSSs = n2COSS,
LMs = LM /n2 ,
LRs = LR/n2 , CRs = n2CR,
and ILMs = nILM .
Fig 3: Circuit diagram of LLC resonant converter
with Center- tap rectifier
Since VO can be considered as a voltage source during
switching transitions, each side of the center-tap rectifier
can be separated with its own VO and repositioned.
Fig 4: Rectifiers for LLC resonant converter. (a) Full-bridge rectifier.(b)
Voltage-doubler rectifier. (c) Center-tap rectifier.
By simply adopting snubbers, the diode voltage
stress can be suppressed. but a loss occurs. To reduce the
voltage stress on the center-tap rectifier without snubbers.
a rectifier voltage oscillation of the LLC resonant converter
and provides a design consideration for a small voltage
oscillation.
Fig 5:. PSIM simulation waveform of LLC resonant converter.
IV. ANALYSIS OF RECTIFIER VOLTAGE
OSCILLATION
The voltage oscillation across rectifier diode is
caused by an interaction between a transformer leakage
inductance and parasitic capacitances at switching
transitions. the LLC resonant converter including parasitic
components, i.e., the switch output capacitance: COSS; the
trans former primary leakage inductance: LKp; the
secondary leakage inductance: LKs1 and LKs2 ; and the
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BR Region [FR > FS]
The key waveforms and equivalent circuits for the
BR region are presented in Figs. 6 and 7, respectively.
During t1–t2, the resonant operation transfers the power to
the output through the rectifier diode D1. At t2 , ID 1
reaches zero and some voltage oscillation is occurred
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across D1 and is expressed as in (1). This oscillation is
proportional to VCRs(t2 ), i.e., the ripple of VCR, and is
small enough not to affect the peak voltage stress of the
diode. Although the ripple of VCR could be increased as Q
(= _LR/CR/RO ) is increased, the upcoming oscillation is
more dominant with respect to the voltage stress on
rectifier diodes.
During t2–t3 , only ILM flows in the primary and
both D1 and D2 are OFF-state. At t3 , Q1 is turned OFF
and the equivalent circuit shown in Fig. 7(a) is constructed.
During this switching transition, all the parasitic
capacitances, i.e., COSS and CJ , take part in the operation.
VCRs can be considered as a constant voltage source since
CRs has a sufficiently large capacitance compared to
COSSs or CJ . Similarly, ILM can be considered as a
constant current source.
In this mode, ILMs flows through COSSs and CJ ,
as presented by the dotted line, i.e., both COSSs and CJ are
charged simultaneously. Therefore, VQ1 and VD 1 are
increased linearly by the current source. ILKs 1 (t4 ) and
ILKs2 (t4 )can be determined by the capacitance ratio of
COSSs to CJ At t4 , both the antiparallel diode of Q2 and
D2 conduct, and the equivalent circuit is changed,
The voltage oscillation is excited by the initial
currents of leakage inductors presented in (2) and by the
ripple of VCRs,ΔVCRs, as well.
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Fig 6: Experimental waveform of LLC resonant converter with center-tap
rectifie
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The key waveforms and equivalent circuits for the AR
region are presented in Fig. 8 and 10, respectively The
most noticeable operation of switching transition in the AR
region compared to the BR region is that Q1 is turned OFF
while D1 is still conducting,
.
Fig 7: Key waveform in BR region
Fig 9: Key waveform in AR region
which lead to different oscillation factors. After Q1 is
turned OFF at t4 , ILR discharges COSS and VQ2 is
decreased to zero.
Fig 8: Equivalent circuit for BR region.
AR Region [FR < FS ]
Fig 10: Equivalent circuit for AR region.
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Then, ILR and ID 1 are decreased. When ID 1
reach zero at t6 , the equivalent circuit is constructed,
where ILMs(t6 ) can cancel out the initial value of ILRs.
Unlike the BR region, COSS does not take part in the
oscillation. VD 1 (t) and VD 2 (t) can be expressed as in (4)
and (5), respectively. At t7 , VD 2 (t) reaches zero and the
equivalent circuit is changed,
The voltage oscillation is excited by ILKs1 (t7 )
and ILKs2 (t7 ) presented in (6) and by ΔVCRs as well. If
the reverse-recovery current of the diode IRR is considered,
it would cause an additional oscillation term
V. DESIGN CONSIDERATION
Half bridge LLC resonant converter
LLC resonant converters display many advantages
over the conventional LC series resonant converter such as
narrow frequency variation over wide range of load and
input variation and zero voltage switching even under no
load conditions.
The diode voltage oscillation mechanisms
between two resonant regions are different and their
voltage oscillation factors can be obtained .Even though
these might not be precise values since they are
approximated, they can predict the tendency of the rectifier
voltage oscillation. It is noted the main oscillation sources
in BR region are ILMs and ΔVCRs at the switching
transition in the case of AR region, VO , ΔVCRs at the
switching transition, and IRR are the main sources.
KL , i.e., LKs/(LRs + LKs), should be small.
Since ΔVCRs is proportional to a load condition, a larger
oscillation may occur as load increases.
AR Region
In AR region, Vosc AR1 and Vosc AR2 have the
same effect with Vosc BR2 ofBRregion, i.e., small values
forΔVCRs and K are desirable. As for Vosc BR3, which is
caused by the reverserecovery phenomenon, IRR itself or
Llkg /CJ should be small. In order to reduce IRR, di/dt of
the diode at the switching transition should be reduced.
Therefore, if this term affects the voltage
oscillation severely, larger LR will be beneficial by
alleviating di/dt.
Common Solution
Among the voltage oscillation factors in Table I,
only LKs is the common factor. Therefore, minimizing LKs
is the foremost choice to reduce the voltage oscillation
across rectifier diodes over a wide operation range.
In other words, the transformer should have a
small leakage inductance and the required resonant
inductance LR should be adjusted by the additional LEXT
in the primary for a small diode voltage oscillation. Smaller
ΔVCRs is also beneficial over both AR and BR regions,
however, it is determined primarily by the resonant tank
design.
Table1. RECTIFIER VOLTAGE OSCILLATION
FACTORS OF LLC RESONANT CONVERTER
BR Region
The first term ILMs(t3 ), which is mainly
determined by LM , directly affects the oscillation, it
should be small, i.e., LM should be large. However, a large
LM compared with LR would increase the inductor ratio K,
i.e., LM /LR, which leads to an increase in a FS variation
range according to an input voltage or a load change [1]–
[3]. In addition, lowering ILMs(t3 ) gives a negative effect
on ZVS, i.e., a longer dead time between switches is
required to ensure ZVS.
Regarding the second term, which consists of
parasitic capacitances, smaller CJ is preferred to a small
oscillation. However, it is determined by the diode
selection. Instead, COSS can be increased by paralleling
additional capacitor to the switches; however, a larger
COSS gives detrimental effect on ZVS. In case of the third
term, which consists of inductances, LKs should be small.
In order to reduce the diode voltage oscillation caused by
Vosc BR2, ΔVCRs or the secondary leakage inductance
ratio
ISSN: 2231-5381
adjusting the voltage oscillation factors on top of
minimizing LKs to reduce the voltage oscillation further
sometimes requires a change of the resonant tank design.
So to analyze the voltage oscillation of LLC resonant
converter and reduce the losses using closed loop control
with PID controller through simulation.
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Table 2. Input parameters for converter
VII. RESULT OF ANALYSIS ON CLOSED LOOP
CENTER
TAP RECTIFIER VOLTAGE
OSCILLATION OF LLC RESONANT CONVERTER
VI. SIMULATION AND EXPERIMENTS
It is done in MATLAB Simulink. Simulink is a software
package for modeling, Simulink, and analyzing Dynamic
system
Fig 12: Simulation Result
VIII. CONCLUSION
The LLC resonant converter employing a centertap rectifier can suffer from a high voltage oscillation
across rectifier diodes owing to a leakage inductance of a
transformer secondary. This voltage oscillation increases a
voltage stress on the secondary diodes. A snubber is
generally required to suppress this additional voltage stress,
however, it could degrade the efficiency. To reduce the
diode voltage stress, this paper analyzes the voltage
oscillation mechanism and presents the design
consideration. Which could reduce the stress on secondary
diodes.
To analyze on center tap rectifier
voltage
oscillation of LLC resonant converter and reduce the losses
using closed loop control with PID controller through
simulation..
ACKNOWLEDGMENT
Fig 11: Simulink model for Circuit Diagram
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I would like to acknowledge the sincere
support provided by my guide , Mr.N.SOUNDIRARAJ,
M.E.,(PhD) Assistant Professor in Electrical and
Electronics Department PSNACET DINDIGUL for his
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valuable guidance, encouragement, constructive criticism
and unreserved co-operation extended at each stage to
complete this project successfully. Also, I am extremely
grateful to all the faculty members of EEE department,
for their constant encouragement and moral support
throughout my venture.
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