1. Research Explorer January - June 20137
Vol . II : Issue. 6 ISSN:2250 - 1940
TWOFOLD MODE SERIES ECHOING DC-DC CONVERTER FOR AMPLE LOAD
VARIATIONS
Harine Knagaraj
M.Tech Candidate, Software Engineering
Institute of System Sciences, National University of Singapore (NUS), Singapore.
ABSTRACT
In order to satisfy demands like higher conversion efficiency and power density, many topologies and control
methods are proposed. Among them, dc to dc series resonant converters with zero voltage switching features
are getting more attention. This paper presents the design of a dual mode full-bridge series resonant converter
(FB-SRC). It is operated in series resonant mode at normal loads or higher loads. The switching frequency is
varied to regulate the output voltage. The fixed frequency phase shifted pulse width modulation , on the
other hand, is used to adjust the effective duty cycle and regulate the output voltage at light loads . The
proposed converter exhibits high conversion efficiency for wide range load conditions.
Keywords - Conversion efficiency, phase-shifted full-bridge converter, series resonant converter (SRC), Zero volatge
switching (ZVS), pulse width modulation (PWM)
Introduction
The switching devices in converters with a
pulse width modulation (PWM) control can be gated
to synthesize the desired shape of the output voltage
or current. However, the devices are turned on and
off at the load current with a high di/dt value. The
switches are subjected to a high voltage stress and
the switching power losses increases [2]. The turn
on and turn off losses could be a significant portion
of the total power loss. The electromagnetic
interference is also produced due to the high di/dt and
dv/dt in the converter waveforms. The disadvantages
of the pwm control can be eliminated if the switching
devices are turned on and turned off when the voltage
and current are forced to pass through zero crossing
by creating an LC-resonant circuit , thereby called a
resonant pulse converter [4].
The primary design feature of ZVS PWM
power converters is the addition of an auxiliary switch
in the quasi-resonant circuit. Resonance is dominated
by the auxiliary switch, which generates resonance
and temporarily stops a period that can be regulated,
there by overcoming the disadvantages of fixed
conduction or cutoff time in a quasi-resonant power
converter. Themain design feature of ZVT soft-switching
power converters is the installation of resonant
components that reduce conduction losses [6].
The main benefit of the converter is the extension
of resonant time using two clamp diodes. The
improvement in the voltage and current stress over
those obtained using traditional resonant components
implies in reduction of switching losses and the
elimination of parasitic effect.
Due to its high current gain, series resonant
converters are mainly used for applications like arc
welding, electronic ballast, induction heating and
fluorescent lighting involving wide range load
variations.
Series Resonant Convertor
A. Principle of operation
The series resonant converter shown in fig.1
converts dc voltage into ac through full bridge inverter
and then converts ac voltage again to dc. It works on
the basis of resonant current oscillation. The
resonating components and switching devices are
placed in series with the load to form an underdamped
circuit. The size of resonating components is small
Available online at www.selptrust.org
Research Explorer
ISSN : 2250 - 1940
Vol II : Issue. 6
January - June 2013

2. Research Explorer January - June 20138
Vol . II : Issue. 6 ISSN:2250 - 1940
due to the high switching frequency. The operating
frequency is generally close to the resonant frequency
of the tank. Operation with switching frequency
lesser than resonant frequency is called sub resonant
frequency operation. The input voltage sees a net
capacitive tank circuit and facilitates ZCS. When
switching frequency is greater than resonant
frequency, the operation is termed as super resonant
frequency and the tank presents a net inductive circuit
which facilitates
Fig 1 Operating principle of SRC
B. Dual mode condition
For a series resonant converter, the output
voltage is regulated by changing the switching
frequency. However, it is impractical to raise the
switching frequency at lighter loads due to the
limitation of semiconductor switch device. Several
schemes are proposed to solve this problem such as
burst mode control [7] , turn off time modulation,etc.
The penalty is that the ZVS feature is no longer kept.
In this paper, the phase-shifted duty cycle control with
ZVS at a fixed highest switching frequency is
proposed to regulate the output voltage at light loads.
Although the phase-shifted modulation features the
constant switching frequency and ZVS function over
wide input voltage and output load ranges, its
efficiency at heavy load is lower than that of an SRC
due to the high duty cycle loss. Therefore, the
proposed control scheme adopts the frequency
modulation with heavy-load efficiency and the phase-
shifted modulation [5] with a better output voltage
regulation and ZVS function at light loads. Through
this dual mode operation higher conversion efficiency
is fulfilled for wide-range load variations.
C. Zero voltage switching
When the PM-SRC is operated such that
its switching frequency is greater than the resonant
frequency of the tank, zero-voltage turn-on of the
inverter devices is possible because the effective
impedance offered by the resonant tank is inductive.
Tank current lags the input voltage. ZVS ensures the
inherent output capacitance in the switching devices
is discharged prior to switch turn-on, thus prevent
turn-on losses and generated EMI.
D. Basic requirements of ZVS
· The device should turn off with a positive current
flowing through it
· The delay time and turn off current have to be large
enough to completely charge/discharge the
snubber capacitors and subsequent turn on of the
reverse diode for conduction
· The delay time has to be small enough to prevent
the tank current from reversing before the switch
turns on.
The Resonance Concept
· From a circuit standpoint, a dc-to-dc resonant
converter can be described by three major circuit
blocks as shown in the figure 2
· The dc-to-ac input inversion circuit, the resonant
energy buffer tank circuit, and the ac-to-dc output
rectifying circuit
· The resonant tank serves as an energy buffer
between the input and the output is normally
synthesized by using a lossless frequency selective
network
· The ac-to-dc conversion is achieved by
incorporating rectifier circuits at the output section
of the converter
Fig 2 Typical block diagram of soft-switching dc-
to-dc converter
Proposed Circuit and its Operation
It consists of a full bridge inverter consisting
of four MOSFETS fed by a dc source. The next
section is the resonant tank section formed by a
resonant inductor and capacitor connected in series.
Finally a diode rectifier along with filter and load
circuit is used. Here Dc to Ac and again to Dc

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Vol . II : Issue. 6 ISSN:2250 - 1940
conversion is carried out. Figure 3 shows the circuit
diagram of the proposed series resonant converter.
Fig 3 Proposed series resonant converter
The proposed FB-SRC has 4 MOSFET
switches Q A ~ Q D with the output parasitic
capacitors Coss, A ~ Coss, B. Lr and Cr forms the
series resonant circuit. A centre tapped transformer
of turn ratio n: 1:1 is used. Two rectifying diodes D1
and D2 are employed. The filter capacitor is Co. RL
is the load resistance. The control signals of QA/QD
and QB/QC are complementary. Dead times
preventing the simultaneous conduction of switches
are inserted to delay the turn-ons of the switches.
When QA/QD or QB/QC conducts, the input power
is transferred to the output load. Zero voltage
switchings are achieved by the resonance of Lr and
the equivalent capacitor formed by the parallel
connection of Cr and the output parasitic capacitors
of the switches during dead times.
The gate pulses applied to the above converter
is shown in fig 3. The leading leg switches are given
by S1 and S1’ and lagging leg switches are given by
S2 and S2’. The tank current i(t) is rectified by a
diode bridge rectifier and filtered by a capacitive filter
to get required output voltage. The magnitude and
wave shape of the resonant current depends on fs, D
and the load factor (Q) of the converter. Q is defined
as the ratio of resonant tank characteristic impedance
and the resistive load as seen from the resonant tank.
For phase modulation full bridge inverter with
fully controlled devices is required as shown in fig 3
each device is switched at 50% duty ratio with the
switching of the devices on the same leg being
complementary. As shown in fig 4, conduction of
switches on the same leg of the inverter (S1 and S1’)
is phase shifted with respect to the conduction of
switches on the lagging leg (S2 and S2’) , resulting in
the quasi-square input voltage.
Fig 4 Gate waveforms of series resonant
converter
Modes of Operation
There are two modes of operation in the
proposed FB-SRC.
They are:
· Frequency Modulation keeping duty ratio constant
· Phase Shift Modulation keeping switching frequency
constant
A. Switching frequency modulation mode
The gate signals for switching frequency
modulation mode is shown in fig 5.
Fig 5 The gate pulses for switching
frequency modulation mode
This mode of operation can be explained under 3
states.
They are:
· First energy transfer state (t0 < t < t1)
· First resonance state (t1 < t < t2)

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Vol . II : Issue. 6 ISSN:2250 - 1940
· First commutation state (t2 < t < t3)
· First Energy Transfer State (t0 < t < t1) :
In this state, QB and QC are turned on, and QA
and QD are turned off. D1 conducts and energy is
transferred to the secondary through the transformer.
· First Resonance State (t1 < t < t2)
All the switches are turned off during this state.
Since the inductor current iLr must be continuous, it
discharges Coss, A and Coss, D to zero voltage, and
charges Coss, B and Coss, C to VI. Then zero-
voltage turn-ons of QA and QD can be achieved. As
long as iLr is larger than the reflected secondary load
current, D1 is still conducting. The load power is
supplied by Lr.
· First Commutation State (t2 < t < t3) :
In this state, QA and QD are turned on, and
QB and QC are turned off. iLr flows through body
diodes DA and DD initially. Since the energy at the
primary side is insufficient, the load power is supplied
by C0.
B. Phase shift modulation mode
The gate signals for phase shift modulation
scheme are presented in fig 6.. For the PS PWM, it
can be observed that dead times. During which ZVS
is accomplished, are inserted before turning on
switches . It can also be noticed that before ZVS
takes place, there are two resonance states (t1 ~ t2
and t3 ~ t4) .
Fig 6 The gate signals for phase shift
modulation scheme
Here there are 5 operating states
They are :
· Energy transfer state ( t0 < t < t1)
· First resonance state (t1 < t < t2)
· Linear Discharge state (t2 < t < t3)
· Second resonance state (t3 < t < t4)
· Commutation state (t4 < t < t5)
· Energy Transfer State ( t0 < t < t1) :
In this state, QB and QC are turned on, and
D1 conducts. The input energy is transferred to the
secondary through the transformer, and C0 is charged.
· First resonance state (t1 < t < t2) :
At t1, QC turns off. iLr stops increasing ,
then charges Coss, C to VI and discharges Coss, D
to zero voltage. DD conducts at t = t2 . The equivalent
resonant inductor (Lr) and the equivalent resonant
capacitor ( Cr + Coss) starts resonanting. Since the
primary current is larger than the reflected load
current , D1 still conducts and D2 carries no current.
· Linear Discharge state (t2 < t < t3) :
DD conducts at the end of the last state.
Therefore QD can be turned on at zero voltage .
The primary voltage is zero. The energy stored in Lr
is transferred through the transformer to the
secondary.
· Second resonance state (t3 < t < t4) :
This state starts when QB is turned off. iLr
charges Coss, B to VI and discharges Coss, A to zero
voltage. Then DA conducts and the resonance stops.
During this state, Lr is not capable to supply the
required energy. The transformer is in free-wheeling
state. A short circuit appears at the transformer
secondary . To achieve ZVS, the energy stored in
the equivalent resonant inductor must be larger than
that in the equivalent resonant capacitor.
· Commutation State (t4 < t < t5) :
During this state, the transformer primary is
short-circuited. A voltage of -VI is across the Lr –
Cr combination. Therefore, iLr decreases linearly until
its magnitude is larger than the reflected load current.
Then, the transformer starts to transfer energy and
the other half switching cycle begins. D1 is turned
off, and D2 conducts. Co is also charged.
Defining Terms and Assumption
The resonant tank has a natural frequency
determined by the resonant capacitor and resonant
inductor.
fs = switching frequency
fr = resonant frequency
Pin = Input power

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Vol . II : Issue. 6 ISSN:2250 - 1940
Pout = Output power
D = Duty ratio
= Efficiency
where Lr and Cr are resonant tank elements
D = Ton / Ts/2
Where Ts = switching period
VI = Input voltage
Vo = Output voltage short-circuited.
A voltage of M = gain = Vo/ VI
The parameter Zc called the characteristic
impedance of the tank is defined as
Capacitor Cr can be found by the following relation
Cr = 1/ωrZc
Inductor Lr can be given by
Lr = ωr/Zc
Current is given by
i = VI / Zc
Simulated Results
VDC(v)
Time (s) Fig 7 Dc supply voltage
VAB (v)
Time (s) Fig 8 Bridge voltage
0.4471 0.44711 0.44712 0.44713
T ime (s)
0
-500
500
VP9
Vout (v)
Time (s)
Fig 9 Output Voltage of both frequency
modulation and phase shift modulation
VDS,VGS(V)
Time (s) -- Fig 10 VDS and VGS
I(Lr) A
Time (s) -- Fig 11 Inductor current

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VQA,VQD and VQB,VQC
Time (s) Fig 12 Gate pulses for switching
frequency modulation
VQA,VQB,VQC,VQD
Time (s) -- Fig 13 Gate pulses for Phase shift
modulation
Iout (A)
Time (s) -- Fig 14 Output current
Conclusion
To avoid poor output voltage regulation and low
conversion efficiency at light loads, a dual-mode control
strategy is presented in this paper. The FB SRC is
operated under switching frequency modulation for most
of the load range to achieve ZVS and low switching
noises. For the lighter loads, the FB SRC is operated
under phase-shifted duty cycle modulation to regulate
the output voltage and maintain the ZVS feature. The
proposed two-mode control scheme for a FB SRC is
especially suitablefor applications with wideinput voltage
and load variations.
References
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