Download International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.3, pp. 1822-1827

International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.3, pp. 1822-1827
ISSN 2078-2365
http://www.ieejournal.com/
A Novel Phase Shifted DC-DC Converter
with Adaptive Soft Switching to Improve
Efficiency under wide Load Range
Sudha Bansal, Lalit Mohan Saini
[email protected]

Abstract—This paper presents a novel dc-dc converter with
adaptive soft switching as a means to achieve ZVS operation for
all the switches. It adopts phase shift modulation features for
constant frequency operation and adaptive soft switching at low
load. To accomplish this task two loops are employed; inner
current control loop is to check the loading condition and turn
on of auxiliary switch accordingly. Outer control loop is used to
improve the dynamic performance of DC-DC converter by
achieving a robust output voltage against load disturbances.
This paper also presents the performance of various controllers.
A 1kW/100KHz dc/dc converter is simulated and analyzed.
Performance of the proposed topology is evaluated at different
loads i.e. static and dynamic load (DC motor). An efficiency
comparison of the converter with a reported topology has also
been carried out.
Index Terms— DC-DC converter; phase-shifted; resonant
tank; reverse recovery; resonant converter; Soft switching.
I. INTRODUCTION
DC-DC conversion technology has been developing very
rapidly, and DC-DC converters have been widely used in
industrial applications such as dc motor drives, computer
systems and communication equipment. The output voltage of
pulse width modulation (PWM) based DC-DC converters can
be controlled by changing the duty cycle. In general, to
minimize the size and weight of pulse width modulated
(PWM) converters, it is required that the switching frequency
must be increased [1]- [2]. However, increasing the switching
frequency leads to substantial switching losses, which causes
deterioration in system efficiency. Therefore, the switching
losses should be reduced in the case of high switching
frequency operation. Various types of resonant converters
have been reported to decrease the switching losses during the
transient states [3] - [5]. Numerous soft switching techniques
for the switching power converters have been proposed [6] [9]. These techniques reduce the switching losses, thus
enabling high frequency operation and also reduce the overall
system size.
Among many new techniques proposed for high
frequency power conversion to reduce the switching loss in
traditional PWM converters, the phase- shifted zero-voltage
full-bridge pulse width modulation (ZVS FB- PWM)
converters are most desirable since they reduce switching loss
considerably without the penalty of a significant increase in
conduction loss [10] - [12]. For power levels up to 3 kW, the
full-bridge converters employ MOSFET switches and use
Phase-Shift Modulation (PSM) to regulate the output voltage.
The FB ZVS phase-shift DC-DC converter is preferred due to
its remarkable features. In most of these converters, zero
voltage switching (ZVS) is achieved by placing a snubber
capacitor across each of the switches and either by inserting
an inductor in series with the transformer or by inserting an
inductor in parallel to the power transformer. These benefits
can be summarized as: ZVS for all the bridge transistors,
reduction of the conduction losses as compared with
quasi-resonant converters, reduction of the electromagnetic
noise, utilization of the device output capacitance and
transformer
leakage
inductance,
fixed-frequency
operation[13] - [14]. However, there are some drawbacks as
high circulating currents , loss of duty cycle , ZVS is lost for
light loads i.e. narrow ZVS range and load-dependent dc
characteristics, also, it undergoes various influences by
ringing between the parasitic capacitor of rectifying diodes
and the leakage inductance, which cause switching losses and
switching noise. Since LLC resonant converter can offer
wider zero voltage switching region and higher conversion
efficiency, it recently has been widely adopted in the design of
front end power supply where output voltage is relatively high
and output load current is relatively low.
The problem of narrow ZVS range at light load current is
addressed in this paper. For this an L-C- L resonant circuit,
with an auxiliary active switch is employed. The main
contribution of this paper is to propose a simple adaptive soft
switching converter to achieve ZVS operation for all the
switches. It adopts phase shift modulation features for
constant frequency operation and adaptive soft switching at
low load. ZVS of active switches in the lagging leg can be
achieved from zero- to full-load condition. In this paper the
operation of proposed full bridge phase-shifted converter and
then steady state analysis of the converter is discussed in
1822
Sudha and Lalit
A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.3, pp. 1822-1827
ISSN 2078-2365
http://www.ieejournal.com/
D1
Cs2
Cs3 II. Chapter III explains the design of controller. The
chapter
Cs4
performance
at different loading condition (static and dc
D5
motor load) is analyzed in chapter IV.
.
Q1
C1
D1
I0
Q3
C3
D3
Lr
Vi
Cr
C0
D2
Vg1,2
C2
D4
Q1
Vg3,4
D R2
Qa
Q4
C4
Q2
Q1
Q3
Q4
Qa
R0
La
Ca
Q2
L0
DR1
Q4
Qa
Vab
i1
ia
t0 t1 t2 t3 t4
t5 t6 t7 t8
t9
Fig. 1. An adaptive soft switching converter and its Steady state waveform
II. INSIGHT INTO THE OPERATING PRINCIPLE AND
CIRCUIT DESCRIPTION
Fig.1 shows the schematic of full bridge adaptive soft
switching converter and its steady state waveforms. The
primary size contains five switches Q1 ̴ Q4 & Qa . The phase
shifted full bridge is formed by the switches Q1 ̴ Q4 with their
output parasitic capacitors C1 ̴ C4, anti- parallel diodes D1 ̴ D4.
Resonant circuit is formed by resonant inductor Lr, resonant
capacitor Cr, parallel inductor La in series with an active
switch Qa. Dr1 & Dr2 are the output rectifier diodes, Lo is the
output filter inductor and Co is output filter capacitor. Vo’ is
the output voltage reflected to primary, Vi is the input voltage,
Qr = (Lr/Cr)1/2/RL’, Ro’ = Vo’2/Po, ω = ωs/ωr, ωr is the angular
resonant frequency, ωr = 2πfr = (Lr/Cr)1/2, ωs is the angular
switching frequency of converter, ωs = 2πfs. The duty cycle of
auxiliary switch Qa is kept low and control pulse given to the
auxiliary switch depends upon the load current. The details of
this current controller are discussed in chapter III.
The operation of the converter for heavy load attains ZVS
for all switches but under light load, the load current is low,
the ZVS of the lagging-leg switches is lost as the energy
stored in the leakage inductance of the transformer is
insufficient to discharge the switch and transformer
capacitances. Thus, the operation of the converter for light
load is discussed here. The pulse width given to switch Qa
depends upon the load current when the current reduces, pulse
duration increases. Inductance in the circuit increases with
turn on of Qa, so as to obtain sufficient energy to discharge the
switch and transformer capacitances and hence ZVS
condition at low load is obtained. Under steady state
operation, eight operating states in each half switching cycle
are discussed. To simplify the analysis, few assumptions
made are as follows:
 All active switches and diodes are ideal.
 Inductors, capacitors and transformers used in the
circuit are ideal.
 The output filter inductor, Lo is large enough so that
the ripple current is neglected.
 The output filter capacitance, Co is large enough so
that the output voltage is constant
The operation is divided into ten intervals in one cycle.
Mode 1(t0-t1) : Prior to t0 switches Q1 & Q4 are
conducting and power is transferred from input to the load
through switches, and rectifier diode DR1. At t= t0, switch Q1
turns off with ZVS as the capacitor C1 blocks the sudden rise
of the voltage across the switch. Now in this mode flow of
primary current charges the output capacitance C1 of Q1 and
discharges capacitor C2 of Q2. At t=t1, the voltage of C2
decreases to zero and the body diode D2 conducts naturally.
This mode consists of only charge and discharge operations,
it lasts a short time interval (from t0 to t1). Hence the dead
time should be sufficient enough to enable this operation. In
this interval the load current flows through the transformer
secondary winding. Hence inductive energy provided to
charge/ discharge the capacitor comes from the leakage and
output inductor.
VC1
VC2
Vi
td
t0
t1
Fig. 2: Charge/discharge Operation of Capacitor
t1  t 0  t d  (C1  C 4 )
Vi
ip
(1)
Mode2 (t1- t2): At t= t1, capacitor C2 is completely
discharged hence diode D2 starts conducting. Now primary
current freewheels through D2, Q4. The primary current
decreases from IP-PK to IP2 with slope – Vo’/Lo’. Auxiliary
switch Qa starts conducting in this duration and auxiliary
inductor La energies by the auxiliary current. Since La << Lo,
hence most of the current passes through La and stores the
1823
Sudha and Lalit
A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.3, pp. 1822-1827
ISSN 2078-2365
http://www.ieejournal.com/
energy in the inductor. The auxiliary current flowing through
the switch is given by
V
(2)
i a (t )  i a (t1 )  i (t  t1 )
Le
Where, ia (t1 ) is the initial current flowing at t1 . The voltage
across the switch is
(3)
vds1  Vi cos[r (t  t1 )]
Where, r  1 / Ce ( La  Lr ) is the resonant frequency,
and Ce = C3 + Cr
Mode 3 (t2- t3) : This mode starts with the turn off of the
switch Q4, primary current flows through C3 & C4. The
capacitor C4 begins to charge by the primary current and when
voltage across C4 becomes higher than the total voltage drop
value of the inductances, rectifier diode DR2 turns on and
starts conducting and hence the secondary winding of the
transformers is short circuited. Primary voltage changes from
0 to –Vin. The energy required to charge the capacitor C4 and
discharge the capacitor C3 is provided by leakage and
auxiliary inductor.
1
1
2
2
( L r  La ) I p  (C 4  C3 )Vi
2
2
Where,
Mode 4 (t3- t4) : At the end of the mode 3, C3 gets
completely discharged, hence diode D3 starts conducting.
Primary current falls rapidly with the slope of –Vi/Le. Diode
D3 and Q2 conducts during the time interval t3-t4 and Q3, Q2
conduct during the time interval t4-t5. At the end of this mode
the primary current reaches to a magnitude –IP1.
(5)
Where, Le = Llk+ Lr
Mode 5 (t4 – t5): Switches Q3 & Q2 conduct. The power is
transferred from input to the output circuit by the rectifier
diode DR2. The current rises in the circuit and it reaches to
I P  PK at the end of this mode.
di p
dV '
V'
(6)
( Llk  L'o )
 Vi  Vo' & Co 0  i p  0
dt
dt
R0
From t6 – t10: same cycle repeats for the next half cycle.
In summary since the stored energy in the auxiliary
inductor is utilized additionally in lagging leg switching
transition, all switching devices in the full bridge inverter of
the proposed converter turn on & off with ZVS condition
from no load to full load.
In order to analyze the performance of the converter in
switching transition period can be expressed as:
( Lr  La )
di p (t )
dt

1
Cr
i
p
(t ).dt  0
Refer equation 3, at t=t3, vds3= Vi and td = (t3-t2)
(7)
i1
.sin[r (t  t2 )]
Ce .r
Hence,
 V .C .
1
. tan 1  i e r

Ce
ip

td =




(8)
A. Zero Voltage Condition
Due to the influence of energy stored in the output filter
inductor, ZVS is achieved by the leading leg switches Q1 & Q4
for wider range of load. For the lagging leg switches the
critical current is required to attain ZVS depends on the
energy stored in the inductive circuit. For the particular value
of inductance the critical current required to supply sufficient
inductive energy to ensure ZVS can be determined. The
critical current required for the converter to attain ZVS
without having any auxiliary inductance is given by,
Ct
I p , min 
.Vi
Llk
With the incorporation of auxiliary inductor the required
current will be,
I p , min, a 
(4)
vds3 is the voltage across the switch Q3.
 di1 (t ) Vi & dia (t  t3 )  vds2 (t  t3 )


dt
Le
dt
Le
Vi  Vi . cos[r (t  t2 )]  Vi 
Ct
.Vi
Le
On comparing,
I
Llk or

Le   p , min, a
 I
I p , min
Le
 p , min
I p , min, a
2

 .Llk


Since, Le= Llk+ La, hence the value of auxiliary inductor
required to increase the ZVS range depends on the value of ‘a’


La  a 2  1 .Llk ; Where, a =
I p , min,a
(9)
I p ,min
A. The Current Ripple
The effective duty cycle deff is the ratio of the voltage pulse
duration from +Vi or –Vi appearing at the transformer
secondary winding in a switching cycle. The DC voltage
conversion ratio of the converter directly depends upon the
effective duty cycle.
V
N
  o  s .d , the effective duty cycle is given by,
Vi
Np
eff
d eff = do - d , where, do is the original duty cycle of the
converter & d is duty cycle loss and is given by,
d 
I p1  I p 2
Vi TS
.
Llk 2
B. Dead Time
The dead-time between the lagging leg switches is one
fourth of the resonant period (T R/4), which is
calculated by utilizing the transformer leakage inductance and
the total output parasitic capacitance of the lagging leg
switches. The dead-time requirement for MOSFETs is short
while IGBTs require larger dead-time due to the tail current.
1824
Sudha and Lalit
A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.3, pp. 1822-1827
ISSN 2078-2365
http://www.ieejournal.com/
Hence, the dead-time requirement of the circuit must be
determined based on the switching device characteristics.
III. CONTROLLERS DESIGN
A dual loop controller is designed for the system.
A. Inner Loop
For adaptive control of switch Qa a current controller is
designed. The principle behind the operation of this controller
is that it will direct the switch only in the case of current falls
to the 20% of rated current. This switch remains inoperative
in the case of high load current. It will help in getting zero
voltage condition from zero to the rated load condition.
DC/DC
DC/DC
CONVERTER
CONVERTER
Vi
Switching
Pulse to
Qa
Load
Io
+
+
-
H(s)
IratedController
Vref
+
(a)
Vref
PI/ Fuzzy Φ
Controller
+
+Switching
Pulse
To Sa
Control
Circuit
Switching
Pulse
+-
Power Vo
Circuit
Voltage
Sensor
to Qa
(b)
Fig. 3. (a) Inner Current Control loop & (b) Outer Voltage controlled loop
B. Outer Loop
To obtain the output voltage regulation various
controllers can be used. Here in this paper two types of
controllers’ i.e. PI and fuzzy controllers are discussed and
later on the performance of the proposed converter using
these controllers are analyzed.
+ -
i) PI controller: In order to design a PI controller, the
linearized model of the converter has to be
determined. In this, the output voltage is compared
with the reference to generate the error signal and
this signal is given to the PI controller to obtain the
desired phase- shift for the active switches. These
switches are given phase – shifted pulse to control
the output voltage. The value of proportional
controller, kp and integral controller, ki are taken as
0.1 and 20 and are finely tuned thereafter.
ii) Fuzzy logic controller : PI controller is simple to
implement and easy to design, but its performance
generally depend on the working point, so that the
presence of parasitic elements, time-varying loads
and variable supply voltages can make selection of
the control parameters difficult, which ensure a
proper behavior in any operating conditions.
Achieving large-signal stability often calls for a
reduction of the useful bandwidth, so affecting
converter performance. Fuzzy control is applied to
control dc–dc converters because of its simplicity,
ease of design and ease of implementation. Fuzzy
controllers are well suited to nonlinear time-variant
systems and do not need an exact mathematical
model for the system being controlled. The fuzzy
logic controller determines the operating condition
from the measured values and selects the
appropriate control actions using the rule base
created from the expert knowledge. The control will
work based on two input sets: the output voltage
error,
e(k) = Vref - Vdc (k)
(10)
and the change in error variations
de(k) = e(k) - e(k-l))
(11)
Which are sampled at every Tsv= 5µs. The k is the actual
sampling sequence. The basic control structure of a fuzzy
logic system and its membership function for error, change in
error and output are shown in Figure 4(a)-(d). The FLC has
three functional blocks for calculation and two databases. The
functional blocks in FLC are: 1) fuzzifier; 2) rule evaluator;
and 3) defuzzifier. The two databases are Rule base and
Database. Fuzzy logic uses linguistic variables instead of
numerical variables. The process of converting a numerical
variable (real number) into a linguistic variable (fuzzy
number) is called fuzzification. For a given crisp input,
fuzzifier finds the degree of membership in every linguistic
variable. Since, there are only two overlapping memberships
in this specific case, all linguistic variables except two will
have zero membership. In FLC, the equivalent term is rules
and they are linguistic in nature. A fuzzy logic controller so
designed is used to control the converter by sending the
desired control signal to the PWM signal generator.
(a)
(b)
1825
Sudha and Lalit
A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.3, pp. 1822-1827
ISSN 2078-2365
http://www.ieejournal.com/
zero
ps
psm
psl
pm
pml
pl
pvl
pel
1
Degree of membership
0.8
0.6
0.4
0.2
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
output
(c)
(d)
Fig. 4. (a) Internal structure of FLC (b) Membership function plot for
error, (c) Membership function plot for change in error and (d) Membership
function plot for output
IV. RESULTS AND DISCUSSION
The simulation of the 1kW, 100/50 Volt, 100 kHz full bridge
phase- shifted DC/DC converter circuit is carried out. The
systems transient performance is evaluated on applying
sudden load changes and the results so obtained are analysed
in terms of steady state error, peak overshoot, settling time.
Here two types of controllers’ i.e. PI and fuzzy controllers
are discussed and the performance of the proposed converter
using these controllers are analyzed.
(c)
Fig.6.(a) With PI Controller,(b) With Fuzzy Controller &(c) Current of the
DC/DC converter at 10% of load, when sudden load is increased to 10% of
full load
B. At full load and 50% of load
The converter is initially full loaded and then the load is
reduced suddenly to 50% of full load at t = 0.06 sec. The
output voltage and current are measured with this sudden
change at t = 0.12 sec load is again increased suddenly to full
load.
Fig. 6 (d). Output Voltage and Current of the DC/DC converter at 100%
load, When Sudden load is reduced to 50% load at t =0.06 Sec & again
increased at t=0.12 sec.
Fig. 5. Transformer voltage and current waveform at the
primary of the Inverter
A. At 10% of full -load to 20% of full -load:
In this converter is initially loaded with 20% of the full load
and then load is suddenly decreased to 10% of full -load at
t=0.06 sec by step decrease in load. The output voltage and
current waveform for PI Controller and Fuzzy controller are
shown in fig.6.
C. At different values of Load Inductance, keeping resistive
load constant:
The behavior of converter circuit with different values of
inductive load is observed. The effect of inductance variation
on output voltage is shown in fig.7.
(a)
Fig. 7. The output Voltage at different load conditions
(b)
D. Zero voltage Switching:
All the active switches in the converter turns on and off at
zero voltage, so as to minimize switching losses. The voltage
across drain to source at 5% of load and at full load is shown
in fig. 8 (a) & (b) respectively.
1826
Sudha and Lalit
A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.3, pp. 1822-1827
ISSN 2078-2365
http://www.ieejournal.com/
Hua G., Lee F. C., Jovanovic M. M., “An Improved Full-Bridge
Zero- Voltage- Switched PWM Converter Using a Saturable Inductor,”
IEEE Transaction on Power Electronics 1993, vol. 8, issue 4, pp.
530-534.
[6].
Sun J., Hamada S., Yoshitsugn J., Guo B., Nakaaka M. , “Zero
Voltage Soft Commutation PWM DC-DC Converter with Saturable
Reactor Switch-Cascaded Diode Rectifier,” IEEE Transaction on
Circuits System 1998, vol. 45, issue 4, pp.348-354.
[7]. Redl R., Balogh L., Edwards D. W., “Optimum ZVS Full-Bridge
DC/DC Convener with PWM Phase-Shift Control: Analysis, Design
Considerations, and Experimental Results,” Proceedings of IEEE
APEC conference 1994, pp.159 – 165.
[8].
Valtchev S., Barges B.V., “1KW/250 kHz Full Bridge Zero Voltage
Switched Phase Shift DC-DC Converter with Improved Efficiency,”
Proceedings of INTELEC conference 1995, pp.803-807.
[9]. Koo G., Moon G., and Youn M.Y., “New zero-voltage switching
phase-shift full-bridge converter with low conduction losses”, IEEE
Transaction of Industrial Electronics 2005, vol. 52, pp. 228–235.
[10]. A. K. Rathore, A. K. S. Bhat and R. Oruganti, “A comparison of
soft switched dc-dc converters for fuel cell to utility interface
application,” IEEJ Transactions on Industry Applications, vol. 128,
no. 4, pp. 450-458, 2008.
[11]. A. K. Rathore, A. K. S. Bhat and R. Oruganti “Small signal
modeling and closed loop control design of ZVS current-fed
half-bridge L-L type dc/dc converter with active-clamp," International
Journal on Computer and Electrical Engineering, vol. 2, no. 6, Dec
2010, pp. 951-959.
[12]. Wu X., Zhang J., Xie X., and Qian Z., “Analysis and optimal design
considerations for an improved full bridge ZVS dc-dc converter with
high efficiency”, IEEE Transactions on Power Electronics 2006, vol.
21, issue 5, pp.1225-1234.
[13]. Song T., Wang H., Chung H., Tapuhi S., and Ioinovici A., “ A
high-voltage ZVZCS dc–dc converter with low voltage stress”, IEEE
Transactions on Power Electronics 2008, vol. 23, issue 6, pp.
2630–2647.
[14]. Joseph X.F., Islam N., Kumar S.P., Dominic D.A. and Prabha D.M.M.
S.R., “Design and Simulation of a PI controlled Soft Switched Front
end Converter for Switched Reluctance Motor,” International Journal
of Computer Applications 2011, vol. 34, issue 10.
[15]. S. Sunisith Lizi Joseph and M. Saritha, “Comparison of Fuzzy PID
Controller with Conventional PID Controller in Controlling the Speed
of a Brushless DC Motor”, International Electrical Engineering Journal
(IEEJ),VOL_5 NO_12, 25 JAN, 2015
[5].
(a)
(b)
Fig. 8(a) & (b) Vds and Gate pulse at 5% of load & at 100% of load
TABLE I.
PERFORMANCE COMPARISON BETWEEN FUZZY AND PI
CONTROLLER
S. No.
Peak
overshoot
Settling time
(Sec)
%
Steady
state error
PI controller
0- 3.6
0- 0.08
0 - 0.3
Fuzzy Controller
1.1 - 1.2
0.02
0.02
V. CONCLUSION
This paper has presented two different controllers PI and
fuzzy controller to improve the dynamic response of the
DC/DC boost converter against the load variation. It has
been observed that the system is stable from 5% of load to
full load. The system shows excellent performance when DC
motor load is applied. The system is also verified on
frequent change of load. The simulation result shows that
the performance of converter with fuzzy controller ensures
the tight regulation of output voltage effectively under the
load uncertainty. The efficiency of the converter at different
loading conditions is also measured and improvement in
efficiency is found.
.
REFERENCES
[1]. A. K. Rathore, A. K. S. Bhat and R. Oruganti “Analysis, design and
experimental results of wide range ZVS active-clamped L-L type
current-fed dc-dc converter for fuel cell to utility interface
application,” IEEE Transactions on Industrial Electronics, vol 59,
issue 1, Jan 2012, pp 473-485.
[2].
A. K. Rathore, "Interleaved soft-switched active-clamped L-L type
current-fed half-bridge dc-dc converter," International Journal on
Hydrogen Energy(Elsevier), vol. 34, issue 24, Dec 2009, pp.
9802-9815.
Hua G.and Lee F. C., “Soft-switching techniques in PWM
converters,” IEEE Transaction of Industrial Electronics 1995; vol. 42,
issue 6, pp. 595 – 603.
[4]. Watson R.and Lee F. C.,”A soft-switched, full-bridge boost converter
employing an active- clamp circuit”, Proceedings of IEEE PESC
conference 1996 pp. 1948-1954.
[3].
1827
Sudha and Lalit
A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range