Highly efficient discontinuous mode interleaved dc-dc

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International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Highly efficient discontinuous mode interleaved
dc-dc converter
Praful V Nandankar
Dr. (Mrs.) Jyoti P Rothe
Electrical Engineering Department
St.Vincent Pallotti College of Engg. & Tech.
Nagpur, India
[email protected]
Electrical Engineering Department
St.Vincent Pallotti College of Engg. & Tech.
Nagpur, India
[email protected]
Abstract—The bidirectional dc-dc converters are inefficient
due to switching loss, conduction loss and passive component loss.
The ripple reduction is possible in the load current by
interleaving of inductor currents in multiphase bidirectional dcdc converter. The bidirectional dc-dc converter is operated in
discontinuous conduction mode in order to minimize the size of
passive inductor. The snubber capacitance is designed to reduce
the turn-off loss which occurs due to discontinuous mode of
operation and hence it improves the efficiency. The design
procedure is presented and an algorithm is devised to optimize
the size of inductor and capacitor in order to achieve high
efficiency operation from low to full load operating conditions.
The performance of this converter is presented with single legged
and three legged converter in simulation. The prototype of
proposed converter is designed and its implementation is given
with operational results. The efficiency curves are also plotted to
validate the results.
Keywords—Bidirectional power flow; zero voltage switching;
snubbers.
I. INTRODUCTION
Bidirectional DC/DC converter has gradually gained
interests in both industry and academic world of power
electronics, which can perform as the transaction platform of
different voltage levels and make management of the power of
two voltage level. It has promising prospects in application of
automation electronics, solar photovoltaic technology and
wind power generation etc. Bidirectional dc-dc converters can
be non-isolated [2]-[4] or isolated [5]-[7], depending on the
application. Buck and Boost type dc converter are usually
chosen for non-isolated power conversion systems.
The high frequency transformer is an ideal candidate to
achieve isolation between source side and load side. But in
order to improve efficiency and to reduce size, weight and
cost, non-isolated dc-dc converters are attractive. The basic
non- isolated bidirectional dc-dc converter is the combination
of step-up stage together with step-down stage and these two
stages are connected in antiparallel. The dc-dc converter stepup stage is used to boost the battery voltage and to control the
inverter input in motor drive operations. The dc-dc converter
step-down stage provides a vehicle regenerative braking. The
step-down stage offers a path for the braking current and it
recovers vehicle energy in the battery. The converter is
operated in DCM to achieve a high power density. By
operating the converter in DCM, the size of an inductor can be
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
minimized. But DCM operation increases the high-frequency
switching current ripple which can be reduced by interleaving
multiple phases [8], [9]. Zero conduction loss and minimum
diode reverse recovery loss are two major advantages of DCM
operation. But, the DCM operation increases turn-off loss as
the main switch is turned off at twice the load current or
higher. This is the main drawback of reduction in inductor
size. The reduction in inductor size increases inductor current
parasitic ringing [8] because the inductor tends to oscillate
with the device output capacitance during device turn-off
period. The efficiency can be hampered due to all side-effects
produced by DCM. Although the lossless capacitor snubber
can be added across the switch for soft turn off, it requires
certain amount of energy stored in the inductor to discharge
the capacitor energy before device is turned on. Thus, both
soft-switching turn on and turnoff are achieved.
In this paper, the design of a high-efficiency non isolated
buck and boost dc-dc converter is proposed to achieve high
efficiency and high power density. Zero voltage switching,
low diode reverse recovery loss and ripple current
minimization are the main advantages of this proposed
converter. As the converter input current can be shared among
the phases, therefore the converter is most desirable for heat
dissipation which improves the efficiency and reliability. An
interleaved bidirectional dc-dc converter is an ideal candidate
for current sharing when there is a need to handle high
currents.
II. CIRCUIT TOPOLOGY WITH ITS OPERATING PRINCIPLE
A. Circuit Topology
Non-isolated bidirectional dc-dc converters are based on a
half bridge configuration where step-down and step-up stages
are combined. Fig.1 shows the non-isolated single phase
bidirectional dc-dc converter. With smaller inductance, the
single phase converter can operate with an inductor current
that flows in both directions during each switching period. The
ZVS operation of single phase converter is also achieved by
operating the converter in discontinuous mode. In Fig.1, Su
operates as the main switch and Sd as the auxiliary switch for
buck mode operation. The proposed bidirectional dc-dc
converter topology is shown in Fig.2. The bidirectional dc-dc
converters can be classified into buck and boost type
depending on the placement of auxiliary energy storage. The
energy storage is placed on high voltage side in buck type and
it is placed on low voltage side in boost type. In order to have
power flow in both the directions, the switch should have
antiparallel diode and it should carry current in opposite
direction.
DTs
(1-D)Ts
S1u
S1d
Ts/2
S2u
Su
S2d
iL
L
Chigh
Vdc
Ts
S3u
Sd
Clow
Vo
S3d
Fig. 3.
Fig. 1.
Circuit diagram
bidirectional dc-dc converter
S1u
of
S2u
zero-voltage
switching
single
phase
S3u
Ld1
Ld2
Vdc
Chigh
Ld3
S1d
S2d
S3d
Clow
Load
Fig. 2.
Circuit diagram of zero-voltage switching three phases interleaved
bidirectional dc-dc converter.
In Fig.2, MOSFET switches S1u-S3u and S1d-S3d serve as
the main switches for either buck mode or boost mode. Each
switch has its own antiparallel diode which is carrying current
during freewheeling period. Each switch is paralleled with a
lossless snubber capacitor. Three inductors Ld1-Ld3 can be
used as the boost inductor under boost-mode operation or lowpass filter inductor under buck-mode operation. Capacitors
Clow and Chigh serve as the smoothing energy buffer. With
interleaved inductor currents, the ripple current going into
these capacitors is minimized. When the top three switches are
actively switching, the power is transferred from high voltage
side to low voltage side in buck mode. When bottom three
switches are conducting, then the power is transferred from
low voltage side to high voltage side in boost mode. ZVS can
be achieved by simply utilizing the existing switches and
without using any extra components.
Fig.3. shows the timing diagram of the 3-phase
bidirectional dc–dc converter with duty cycle defined in buck
mode. Each phase has 1200 phase shift. Two active switches in
the same leg of half-bridge configuration are complementary
to each other.
Timing diagram for a duty cycle (D=0.5) in buck mode operation.
B. Circuit Topology
The inductor stores energy and when the current through
the inductor stops, it releases its energy in another circuit. Too
low of an inductance will cause the switches to go into
discontinuous mode. Too high of an inductance will cause
excess resistance on the core and lower the efficiency of
circuit. The ripple current in the inductor is determined by the
choice of inductance for the inductor. The smaller the
inductance, the bigger is the ripple current. The inductor
design improves the system performance by realizing ZVRT
soft switching and by reducing switching loss, system size and
inductor loss. The inductor value in a buck converter is usually
kept at a high value. The high value of inductance reduces the
inductor ripple current (ILp-p). This is done to minimize output
ripple voltage and maximize output load current in a dc-dc
converter. The large inductor values allow the converter to
operate in discontinuous mode only at light loads. Therefore
all the design considerations are required to optimize the
inductance value. Typically a minimum inductance can be
obtained at the boundary of CCM-DCM condition.
The equations (1)-(5) provides the relationship between
inductor peak current Ipeak, minimum current Imin, and inductor
root-mean-square (rms) current Irms where Iload is the load
current, Ts is the switching period, ∆I is the inductor current
ripple and P is the load power.
ΔI =
1 Vin − V o V o
.
.
.T
s
2
L
V
in
I
load
=
P
V
o
I peak = I load + ΔI
(1)
(2)
(3)
I min = I load − ΔI
2
I rms = I load
+
(4)
ΔI 2
3
calculated. The inductor value in a buck configuration is
usually kept large to minimize the ripple current and to reduce
the ripple in output voltage. In such cases, the discontinuous
mode will be related with critical value of the resistance.
(5)
Rcrit =
The critical value of inductance is obtained by making Imin
to zero value. This critical value of inductance makes the
converter to operate at the boundary condition between CCM
and DCM.
1 (V − Vo ) Vo2
Lcrit = . in
. .Ts
P
Vin
2
Lcrit
R(1 − D).Ts
=
2
2L
(1 − D)Ts
(10)
START
Enter the
Pomax,Vo,Vlmax,Vlmin,Vlnom, fs
(6)
Compute maximum load current(I omax) and
minimum load resistance(RLmin)
(7)
An optimized value of inductor should satisfy both the
parameters i.e. zero voltage switching condition and lowest
volume.
Compute dc voltage transfer functions
MVDCmin , MVDCnom and M VDCmax
In case of light loads, a high value of inductor is
selected for discontinuous conduction. DCM operation for
light loads is always associated with large inductor values.
Therefore in order to select an optimum value of inductor, a
tradeoff should be established between buck and boost mode
of operation. Hence, the DCM operation solely depends upon
critical value of load i.e. Rcrit .
Enter the desired efficiency
Compute the maximum duty cycle at the
CCM/DCM boundary at full load
Compute the maximum inductance required
for DCM operation, Lmax
R>Rcrit for DCM
R<Rcrit for CCM
C. Snubber Capacitor Design
A large capacitance decreases turn-off loss
but it may increase turn-on loss. Thus, the design tradeoff is to
minimize the total turn-on and turn-off losses. The design of
snubber is obtained by charge balance of Cv2 and 1/2Li2.
Cv
2
=
1
Li
2
2
Whether continue
for multiphase
No
(8)
where i is inductor current and v is capacitor voltage.
D. Algorithm for design of inductor
The algorithm is devised to compute the critical value
of inductance at the boundary of CCM/DCM condition for
single phase and multiphase.
Lmax =
RL min (1 − DB max )
2 fs
(9)
After obtaining critical value of inductance from algorithm
shown in Fig.4, the critical value of resistance can also be
Yes
For each phase
inductance
Ln=Lmax*n
Display the result
STOP
Fig. 4.
Flowchart of generalized algorithm
III. SIMULATION RESULTS
The simulation of single and three phase bidirectional dcdc converter has been performed. The simulation has been
done in order to check the operating principle of single phase
and three phase bidirectional dc-dc converter.
MODELING OF BIDIRECTIONAL CONVERTER
Value
250 µH
0.1 Ω
4700 µF
220 µF
0
Vds (V)
0
0.1502
0.1504
0.1506
0.1508
0.1502
0.1504
0.1506
0.1508
0.151
0.1512
0.1514
0.1516
0.1518
0.152
0.151 0.1512
Time (sec)
0.1514
0.1516
0.1518
0.152
0.1512
0.1514
0.1516
0.1518
0.152
0.151 0.1512
Time (sec)
0.1514
0.1516
0.1518
0.152
IL (A)
10
5
(a)
Vds (V)
40
20
0
0.1502
0.1504
0.1506
0.1508
0.1502
0.1504
0.1506
0.1508
0.151
IL (A)
4
2
0
-2
0.15
(b)
Vds (V)
40
20
0
0.15
0.1502
0.1504
0.1506
0.1508
0.151
0.1512
0.1514
0.1516
0.1518
0.152
4
IL (A)
0.1502
0.1503
0.1504
0.1505
0.1506
0.1507
0.1508
0.1505
0.1506
0.1507
0.1508
ZVS
2
0
-2
0.15
0.1501
0.1502
0.1503
0.1504
Time (sec)
Fig. 6.
Drain-source voltage and current through switch (including
antiparallel diode).
20
0.15
0.1501
4
40
0
0.15
20
0.15
A. Single phase Converter Performance
The simulated inductor current and drain-source voltage
are shown for continuous mode, boundary of continuousdiscontinuous mode and discontinuous mode in Fig. 5(a),
Fig.5 (b) and Fig.5(c) respectively.
0.15
40
Vds (V)
Parameters
Inductor Inductance
Inductor Resistance
Input Capacitance
Output Capacitance
Is (A)
TABLE I.
The single phase dc-dc converter operates at the boundary
of CCM-DCM condition for the critical value of load
resiatance. If the load resitance is increased above the critical
value, then the circuit enters into discontinuous mode of
operation. In DCM condition, the inductor current increases
from positive direction to negative direction and then it again
swings back to positive value. ZVS condition is achieved in
single phase dc-dc converter when it operates in DCM
condition . ZVS condition is shown in Fig. 6. In DCM
condition, the switch current flows only when the drain-source
voltage across switch is zero. Before that ZVS instant, the
current was flowing through a diode connected antiparallel to
the switch. This antiparallel diode current makes drain –source
voltage to a zero value. When the switch is gated on under
zero voltage condition, then switching losses are minimized
and it improves the efficiency. Both the switches of a single
leg ie. upper and lower switches are gated on at zero voltages.
The current gets transferred naturally from antiparallel diode
to the switch. Hence, the diode also turns off naturally without
any occurrence of reverse recovery loss.
When the gate-source voltage is applied to the switch, then
drain-source voltage across switch becomes zero. The inductor
starts to increase from zero value after a certain time interval
and it is clearly indicated fin Fig.6. The switching losses
become zero due to operation of single phase dc-dc converter
in DCM condition. The switching losses increase with
increase in switching frequency. These losses reach a high
value at higher switching frequencies. ZVS condition is
observed only under DCM condition and at the boundary of
CCM-DCM condition.
2
0
-2
0.15
0.1502
0.1504
0.1506
0.1508
0.151 0.1512
Time (sec)
0.1514
0.1516
0.1518
0.152
(c)
Fig. 5.
Drain source voltage across switch and inductor current under
CCM operation, (b) Drain source voltage across switch and inductor current at
the boundary of CCM-DCM operation, (c) Drain source voltage across switch
and inductor current under DCM operation.
B. Three phase Converter Performance
In Fig 7(a), the three inductor currents are 1200 separated
having magnitude of 1.2 A peak to peak. The total current is
averaged at 3.7A with only 0.4A peak to peak ripple, or 1/3rd
of the individual phase current ripple. Similarly in Fig. 7(b),
three inductor currents are having magnitude of 1.2A peak to
peak with only 0.4 A peak to peak ripple and the total current
is averaged at 1.8A. In Fig 7(c), the total averaged current is at
0.9A and peak to peak ripple is 1/3rd of the individual phase
current ripple.
iL1
iL2
3
iL3
iL (A)
IL (A)
2
1
-3
0.065
0
0.15
0.1501
0.1502
0.1502
0.1502
0.1503
0.1504
0.1504
0.067
0.068
0.069
0.07
0.071
Time (sec)
0.072
0.073
0.074
0.075
0.066
0.067
0.068
0.069
0.07
0.071
Time (sec)
0.072
0.073
0.074
0.075
9
7
0.065
3.5
0.066
11
Vo (V)
0.15
4
IL-all (A)
1
-1
iL-all
3
0.15
0.15
0.1501
0.1502
0.1502
Time (sec)
0.1502
0.1503
0.1504
0.1504
Fig. 8.
Transient response of the converter under buck mode operation
from no load to loading condition.
(a)
IL (A )
2
iL1
iL2
iL3
1
0
-1
0.15
0.15
0.1501
0.1502
0.1502
0.1502
0.1503
0.1504
0.1504
0.1502
Time (sec)
0.1502
0.1503
0.1504
0.1504
IL-all (A )
2.5
2
1.5
C. Efficiency curve obtained through simulation results
The efficiency curve of 3-Ф and 1-Ф dc-dc converter
shows that 3-Ф dc-dc converter has higher efficiency as
compared to 1-Ф dc-dc converter. The % improvement in
efficiency is found to be 2%. The reason of higher efficiency
for 3-Ф dc-dc converter is due to ripple cancellation and low
switching losses. The efficiency is plotted against critical
values of load resistance corresponding to their respective
critical frequencies.
iL-all
1
0.15
0.15
0.1501
0.1502
(b)
2
IL (A)
iL1
iL2
1
iL3
0
-1
0.15
0.15
0.1501
0.1502
0.1502
0.1502
0.1503
0.1504
0.1504
IL-all (A)
1.5
Fig. 9.
Efficiency curve at different values of critical resistances
corresponding to respective frequencies.
1
0.5
0.15
IV. EXPERIMENTAL RESULTS
iL-all
0.15
0.1501
0.1502
0.1502
Time (sec)
0.1502
0.1503
0.1504
0.1504
(c)
Fig. 7.
Three phase inductor currents, total current, output voltage and
load current in continuous mode of operation, (b) Three phase inductor
currents, total current, output voltage and load current at the boundary of
continuous-discontinuous mode, (c) Three phase inductor currents, total
current, output voltage and load current in discontinuous mode
The output voltage and load current is shown for all the
modes of operation. The converter operation from no-load to
full-load is shown in Fig. 8. The output voltage is maintained
constant. The inductor current flows continuously under all the
loading conditions and hence the transient response of this
converter is smooth.
The specifications of DC-DC converter are tabulated in
Table II. The performance of the DC-DC converter is
evaluated with R load.
TABLE II.
CIRCUIT PARAMETERS FOR SINGLE PHASE
CONVERTER ANALYSIS
Parameters
Inductor (L)
Inductor Resistance
Input Capacitance (Cin)
Output Capacitance (Cout)
Values
250 µH
0.1 Ω
4700 µF
220 µF
To evaluate the performance of the converter, the
efficiency of DC-DC converter under different operating
conditions is evaluated. The experimental inductor current is
shown in Fig. 10 (a), (b) and (c) for CCM, boundary of CCMDCM and DCM at 10 kHz switching frequency, 40 to 20V,
100W buck mode operation. The CCM and DCM mode of
operation depends upon the critical value of load.
(a)
(b)
Fig. 12. Efficiency curve of three phase bidirectional dc-dc converter.
V. CONCLUSION
(c)
(d)
Fig. 10. Experimental waveforms of inductor current under (a) continuous
mode, (b) boundary of continuous-discontinuous mode, (c) discontinuous
mode, (d) Experimental waveforms of gate-source voltage and inductor
current indicating ZVS operation.
Zero voltage switching of switches obtained
experimentally is shown in Fig. 10(d) and it is observed that
ZVS occurs at discontinuous mode of operation.
The design algorithm of inductor and snubber capacitance
is presented for bidirectional dc-dc converter which operates
at the boundary of CCM-DCM condition. The design
algorithm gives an optimized value of inductor at a fixed duty
cycle in order to increase the efficiency. The ripples in load
current are reduced by interleaving of three phase inductor
currents. The micro-controller is used to provide switching
pulses to bidirectional dc-dc converter. The ZVS operation is
indicated in the simulation of three phase bidirectional dc-dc
converter. The ZVS operation improves the efficiency of dcdc converter up to 98%. The prototype of dc-dc converter is
tested and its hardware results are obtained to validate the
operation of bidirectional dc-dc converter.
References
[1]
(a)
(b)
Fig. 11. Three phase inductor currents at 10 kHz switching frequency, (b)
Gate-source voltage, Vgs and resultant current at 10 kHz.
The circuit of three phase bi-directional DC-DC converter
is realized through a prototype. The prototype has been
implemented for a 100W system. The three phase inductor
currents at 10 kHz switching frequency is shown in Fig. 11(a).
The three phase inductor currents are 1200 separated having a
magnitude of 1A. The resultant current is 1/3rd of individual
phase current ripple which is shown in Fig. 11(b).
The comparison between single phase and three phases is
plotted graphically and shown in Fig.12. It is seen that three
phase is having higher efficiency because of ripple
cancellation. The regulation of three phase bidirectional DCDC converter is better as compared to single phase DC-DC
converter. The experimental efficiency of single phase DC-DC
converter attains maximum value only at one particular load
whereas in three phase DC-DC converter, the efficiency
remains constant for all loads. The experimental results
closely follows the simulated results shown in Fig. 5 and
Fig.6.
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