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235
Multi-Output Buck-Boost Converter with Enhanced Dynamic
Response to Load and Input Voltage Changes
Arash A. Boora, Firuz Zare, Senior member IEEE, Arindam Ghosh, Fellow IEEE
Queensland University of Technology
2 George St. GPO. Box 2434, Brisbane, QLD 4001, Australia
Abstract
This paper presents a new multi-output DC/DC converter topology that has
step-up and step-down conversion capabilities. In this topology, several output
voltages can be generated which can be used in different applications such as
multilevel converters with diode-clamped topology or power supplies with
several voltage levels. Steady state and dynamic equations of the proposed
multi-output converter have been developed, that can be used for steady state
and transient analysis. Two control techniques have been proposed for this
topology based on constant and dynamic hysteresis band height control to
address different applications. Simulations have been performed for different
operating modes and load conditions to verify the proposed topology and its
control technique. Additionally, a laboratory prototype is designed and
implemented to verify the simulation results.
1. Introduction
Multi-output DC/DC converters have a range of applications [1-4]. Some of these
converters employ transformers to supply separate loads, which increases the size,
weight and cost of the total system. The other category of multi-output DC/DC
converters is single-inductor multi-output DC-DC converters. Outputs may be
connected in parallel or series.
Multi-output converters with outputs connected in series, are promising as the
supplier of multi-level inverters to reduce the dependency of DC-link voltage
balancing and power factor of the load. Voltage balancing of diode-clamped multi-
level inverter DC-links is limited by the power factor of their load [5]. Diode-
clamped inverters supplying loads with high power factors cannot balance their
upper DC-link capacitors. Therefore, they fail to apply all possible voltage levels and
their modulation index falls to around 0.5 for nearly resistive loads. Reference [6]
has suggested a voltage balancing circuit for symmetrical DC-links. However, it
236
cannot step-up or step-down input voltage and cannot regulate DC-link voltages
asymmetrically.
Besides, there is a possibility to improve the quality of a multi-level inverter by
charging the DC-link capacitors asymmetrically [7,8]. Multi-output DC/DC
converters can facilitate the utilization of quality advantage of asymmetrically
supplied DC-links.
As another application, Multi-voltage DC-networks [9] have challenges, which can
be resolved by MOBB converters. For instance, instability in the auxiliary power of
E-cars [10] or electric trains [11], when there is a widely variable primal voltage
source (the main battery of the electric vehicle or catenary voltage in presence of
several travelling trains) cause interruptions to the operation of auxiliary loads.
Alternatively, when there are sensitive loads sharing the voltage terminal with step-
changing heavy loads, the sensitive load may malfunction due to terminal voltage
fluctuation. A multi-output converter with outputs connected in series, may generate
different voltage levels, and prioritize them to secure the voltage across the sensitive
load when there is a disturbance.
Additionally, DC-networks may supply disturbing loads such as uncertain loads [12],
constant-power loads [13], nonlinear loads [14] step loads [15], or switching loads
[16]. In this paper, a step-changing load is considered as the load disturbance [17,18].
This paper presents a novel single-inductor multi-output DC/DC converter topology
named Multi-Output Buck-Boost (MOBB) converter, which is capable of step-up
and step-down conversions Fig. 1. In this topology, several series-connected output
voltages can be generated, which may be useful in a variety of applications such as
diode-clamped multi-level inverters and multi-voltage DC-networks supplying loads
with different requirements.
Fig. 1(a) shows the proposed new multi-output topology, which can perform both
step-up and step-down conversions. MOBB is versatile due to its capabilities to
improve dynamic response when there are input voltage and load disturbances.
Furthermore, for applications, where there is a pre-knowledge or predictability of
load or input voltage disturbance, it has the capacity to remove the effect of these
disturbances from the output voltages. Additionally, the proposed topology may
prioritize the output voltages to achieve better dynamic performance where sensitive
loads are supplied along with frequently and highly varying loads. These capabilities
are explained and mathematically proven in this paper. The proposed converter is
237
simulated and experimented for two outputs (across the capacitors C1 and C2 of Fig.
1(b)) in this paper. The suggested control strategy is developed to realize above
mentioned advantages.
SBoost
S1
S2
Sn-1
Dn
Vn
Vn-1
V2
V1
SBuck
Vin
Rn
Rn-1
R2
R1
RkVk
Sk
SBoost
S1
D2V2
V1
SBuck
Vin
R2
R1
C1
C2
LL
C2
C1
Ck
Cn-1
Cn
(a) (b)
Z
V3
V2
V1
Vin
(c)
Load
with
different
voltageV2
V1
Sensitive
load
Not
sensitive
load
Step
changing
load
V1
Vin
(d)
Fig. 1: a) MOBB converter b) double-output-Buck-Boost converter
c) supplying a multi-level single-phase diode-clamped inverter with a three-output-Buck-Boost
d) Supplying a multi-voltage DC-network with a three-output-Buck-Boost
Fig. 1(c) shows a three-output Buck-Boost converter supplying a four-level single-
phase inverter. The DC-link voltages are named differently (V1,V2, and V3) to
highlight the possibility of asymmetrical DC-link voltages. The main input voltage is
variable (example: PV units for domestic application in different shading condition)
and therefore, both step-up and step-down conversions are essential.
238
Fig. 1(d) illustrates a multi-voltage DC-network supplied by a three-output Buck-
Boost converter. The “sensitive load” has a devoted terminal to avoid the
disturbances caused by the “step changing load”. Additionally, another voltage level
is generated to supply the “load with different voltage”. The main input voltage is
variable (for example, main battery of electric car, catenary voltage), and hence, both
step-up and step-down conversions are required.
The MOBB converter has the capacity of inductor pre-charging. Non-inverting
Buck-Boost converter [19-21] and Tri-state Boost converter introduced in [22,23]
have the same capacity. Therefore, when a load rise or input voltage drop is known a
priori (for example, new load demand through the main controller of the E-
car/Hybrid car, or when the main supply of a hybrid car changes from PV/fuel-cell to
battery), or anticipated, [24,25] (for example, when two electric trains are
approaching a particular power station, catenary voltage drops), inductor pre-
charging may be utilized to remove disturbance from all output voltages.
The rest of the paper is organized as follows. Section 2 describes the topology of
proposed converter with two outputs. The dynamic and steady state equations of the
proposed converter are derived in Section 3. The equations are presented to
mathematically explain the capabilities of the presented topology. Section 4 presents
the applied control strategy of a double-output Buck-Boost converter and includes
the simulation results. Section 5 presents the experimental results obtained on a
laboratory prototype. The paper concludes in Section 6.
2. Topology
The topology of double-output Buck-Boost converter is presented in Fig. 2(a), where
the function of each switch in steady state mode is identified. According to the steady
state behaviour of switches of Fig. 2(a), the equivalent circuits of the proposed
topology for steady state step-down (double-output-Buck) and steady state step-up
(double-output-Boost) conversions are presented in Fig. 2(b) and 2(c) respectively.
Possible switching states of a double-output-Buck and double-output-Boost
equivalent circuits are illustrated in Fig. 2(d) and 2(e).
239
SBoost
S1
D2V2
V1
SBuck
Vin
R2
R1
C1
C2
L
Continusly turned on in step up
(Idling switch)
Switching in step down
Switching in step up
Continusly turned off in step down
(Idling switch)
Operating to perform voltage sharing
(a)
S1
V2
V1
SBuck
Vin
+ -
R2
R1
(b)
SBoost
S1
D2V2
V1
Vin
R2
R1
+ -
(c)
000
001
010
100
101
V2
V2
V2
V2
V2
V1
V1V1
V1
V1
R2
R2
R2
R2
R2
R1R1
R1R1
R1
Vin
Vin
Used only
in transient
Step down steady state
iL iL
iL iL
iL
(d)
100
101
110
V2
V2
V2
V1
V1
V1
R2
R2
R2
R1
R1
R1Vin
Vin
Vin
010
V2
V1
Used only
in transient
Step up steady state
iL
iL
iLiL
(e)
Fig. 2: a) double-output Buck-Boost converter
b) double-output-Buck equivalent circuit in step-down c) double-output-Boost equivalent circuit in step-up
d) switching configurations of double-output-Buck e) switching configurations of double-output-Boost
240
To explain the operation of MOBB converters, the switches are classified as “Buck
Switch” (SBuck), “Boost Switch” (SBoost), and “Current Sharing Switches” (Sj;
j=1,2,…,n). To minimize switching loss in steady state step-up conversion, SBuck is
always on and in steady state step-down conversion, SBoost is always off.
A. Steady state step-down conversion mode
When Vin is more than V1ref (the reference of V1), the MOBB converter operates in
step-down mode. During this mode, SBoost is turned off, SBuck is switched to chop
input voltage and S1 operates to share inductor current between output capacitors and
to regulate the output voltages. The switching configurations applied in steady state
step-down mode are 000,001,100,101, as indicated in Fig. 2(d) in the enclosed box.
B. Steady state step-up conversion mode
When Vin is less than V1ref, the MOBB converter operates in step-up mode. During
this mode, SBuck is continuously turned on, SBoost operates to step-up input voltage
and S1 operates to share inductor current between output capacitors and regulates
output voltages. The switching configurations applied in steady state step-up mode
are 100,101,110 as shown in Fig. 2(e) in the enclosed box.
In Fig 2(d) and 2(e), states 001 and 101 are conducting current to C1 to increase V1.
States 000 and 100, conduct the inductor current to both C1 and C2 and increase both
V1 and V2. Additionally, state 110 connects the inductor to input voltage and lets the
inductor current increase linearly. These states and their effect on output voltages and
the inductor current are summarized in Table 1.
Table 1: charging state of C1, C2, L for each switching configuration of double-output Buck-
Boost
SBuck SBoost S1 C1 C2 L
0 0 0 Charging Charging Discharging
0 0 1 Charging Discharging Discharging
0 1 0 Discharging Discharging No Change
1 0 1 Charging Charging Discharging
1 0 0 Charging Discharging Discharging
1 1 0 Discharging Discharging Charging
As can be observed in Fig. 2(d), 2(e) the state (010) is not included in any of step-up
and step-down steady state equivalent circuits (in boxes). However, this switching
state is utilised to enhance the dynamic response to load or input voltage
241
disturbances. Moreover when input voltage changes suddenly, the (010) state can be
utilized to smooth the transient from step-down to step-up conversion and vice versa.
Furthermore, for applications with pre-known or predictable load rise or input
voltage drop, the switching state (010) is used to pre-charge the inductor and remove
the disturbance from DC-link voltages. This concept has been introduced in [26,27]
and is explained in the control strategy section.
3. Steady state and dynamic equations
In this section, the steady state and dynamic equations are developed for double-
output Buck-Boost converter. The equations for an n-output MOBB converter are
presented in Appendix.
Let us assume that for any variable of x(t), x is a small perturbation around a DC
value of X. We denote the following duty cycles: the duty cycle of SBuck as dBuck(t),
the duty cycle of SBoost as dBoost(t), the duty cycles of current sharing switch S1 as
dS1(t), and the duty cycle of diode D2 as dD2(t) (the switches are shown in Fig. 1(b)).
vin(t) is the input voltage, and C1,C2, and L are shown in Fig. 1(b).
Constrains on duty cycles are:.
1
10
10
10
10
21
1
2
tdtdtd
td
td
td
td
DSBoost
Buck
Boost
S
D
(1)
Dynamic equations of a double-output Buck-Boost converter are:
tvtvtdtvtdtvtddt
tdiL DSinBuck
L21211
(2)
1121
11 Rtvtdtdti
dt
tdvC DSL
(3)
222
22 Rtvtdti
dt
tdvC DL
(4)
The State space form for double-output Buck-Boost converter in (5) is extracted after
linearization of dynamic equations (2-4).
242
2
1
211
2
1
22
121
221
2
1
2
1
000
00
10
01
0
00
00
00
D
S
Buck
in
L
LL
inBuckL
D
DS
DDSL
d
d
d
v
I
II
VVVVD
v
v
i
RD
RDD
DDD
v
v
i
C
C
L
(5)
Transfer functions of a double-output Buck-Boost converter are:
11 22
2
2
11
2
211 2
sCR
DR
sCR
DDRLs
D
sv
si
DDS
Buck
in
L
(6)
111
2111
sCR
DDR
si
sv DS
L
(7)
122
222
sCR
DR
si
sv D
L
(8)
The steady state equations are derived by substituting s = 0 in the transfer functions
of equations (6-8);
in
DDS
BuckL V
DRDDR
DI
2
22
2
211
(9)
in
DDS
DSBuck VDRDDR
DDRDV
2
22
2
211
2111
(10)
in
DDS
DBuck VDRDDR
DRDV
2
22
2
211
222
(11)
As has been mentioned in circuit analysis and is depicted in Fig. 3, SBuck and SBoost do
not switch simultaneously when the MOBB converter is operating in the steady state.
The graphical view of Equations (9-11) for variation in the values of R1, R2, DS1, DD2,
DBuck is presented in Fig. 3. The input voltage Vin is assumed to be constantly equal to
200V. However, form Equations (9-11) Vin has a linear relationship with IL, V1, and
V2.
243
Inductor current IL output voltage V1 output voltage V2
Changing
R1
(10,20,30)
DS1
(0.2-0.7)
DBuck
(1-0.2)
Changing
R2
(10,20,30)
DS1
(0.2-0.7)
DBuck
(1-0.2)
Changing
R1
(10,20,30)
DD2
(0.2-0.7)
DBuck
(1-0.2)
Changing
R2
(10,20,30)
DD2
(0.2-0.7)
DBuck
(1-0.2)
Fig. 3: variation of IL, V1, and V2 as functions of parameters DBuck, DS1, DD2, R1, R2
Examining equations (9-11), the capacities of MOBB converter, which may be
utilized to improve its dynamic response, can be observed. When the input voltage
rises, DBuck may be reduced immediately to compensate for this rise. Additionally,
when R1 and/or R2 are increased and the load is reduced, DS1 and/or DD2 may be
244
reduced immediately to compensate for the reduction of the load and avoid over-
voltage effectively.
Furthermore, the inductor pre-charging capacity of MOBB converters may be
utilized to avoid output voltage drops caused by input voltage drop or load rise. In
(9-11), decreasing duty cycles DBuck, DS1, and DD2 by multiplying them in the factor
of k (k<1), the inductor is over-charged by the factor of 1/k while V1 and V2 are kept
unchanged, as given in (12-14).
LLin
DDS
Buckin
DDS
Buckk_L II
kV
DRDDRk
kDV
kDRkDkDR
kDI
12
22
2
211
22
22
2
211
(12)
12
22
2
211
2
211
2
2
22
2
211
2111 VV
DRDDRk
DDRDkV
kDRkDkDR
kDkDRkDV in
DDS
DSBuckin
DDS
DSBuckk_
(13)
22
22
2
211
2
22
2
2
22
2
211
222 VV
DRDDRk
DRDkV
kDRkDkDR
kDRkDV in
DDS
DBuckin
DDS
DBuckk_
(14)
Therefore, the MOBB converters may store some extra current in the inductor while
their output voltages are constant.
If there is a rise in load current or a drop in input voltage, the extra current stored in
the inductor may be supplied to the load to remove or attenuate output voltage
fluctuations. Therefore, this extra current improves the dynamic response of MOBB
converters to disturbances caused by load or input voltage sudden changes. However,
the extra current causes extra switching loss [19]. Therefore, the time of operating
with over-charged inductor and the level of extra current must be minimized to
reduce overall switching and conduction loss. The strategy applied in this paper for
pre-known disturbances, is to pre-charge the inductor a few ten microseconds before
the disturbance happens. Therefore, extra loss is limited to occasions of load rises or
input voltage drops. For sudden unexpected disturbances, a Dynamic Hysteresis
Band (DHB) strategy is adopted to improve the dynamic response.
245
4. Control strategy and simulation results
The control strategy is developed to utilize the 010 switching state and current
storage capability of the topology to enhance the dynamic response of the converter
when load or input voltage disturbances are applied. For the cases of input voltage
rise or load current drop, which may cause output over-voltage, there is an “over-
voltage reduction” unit which senses over-voltages and activates idling switch (SBuck
in step-up conversion and SBoost in step-down conversion) to divert the inductor
current from the load and to remove over-voltage. For the cases of input voltage drop
or load current rise, two immediate response control techniques for different
applications are suggested. They utilize extra current storage capacity to remove or
attenuate output voltage drops.
The control strategy is based on hysteresis method. The block diagram of the control
system is presented in Fig. 4(a), where the function of each block is explained along
with the description of each signal conducted to or generated by that block. The
flowchart of the program applied in experiments is illustrated in Fig. 4(b). The
flowchart is divided into partitions, which perform functions mentioned in Block
diagram in Fig. 4(a). Switching of all switches, output voltages, and inductor current
are shown in Fig. 4(c) and Fig. 4(d) for step-down and step-up conversions
respectively. The switching states resulted by switching are also mentioned.
4.1. Current reference unit:
The “Current reference unit” determines the level of inductor current. This block
examines output voltages (lines 1a,2a in Fig. 4(a)), compares them with reference
voltages (line 3a) and decides to increase or decrease the level of inductor reference
current (line 4) according to output voltage errors. The function of this unit is
detailed in the control flowchart. To limit the speed of current reference change, the
parameter N is used. Each time the flowchart runs the parameter N increments.
Before modifying the current reference, the parameter N is checked. If it is over 100,
the current reference will be changed; otherwise the current reference change block
in the flowchart will be skipped. This way, the rate of current reference change is
limited to avoid unstable reference change. The maximum value of N depends on the
speed of the controller. To have constant rate of current reference change, the
maximum value of N should be increased for faster controllers.
246
Fig. 4: a) Block diagram of control system b) Control flowchart
Inductor current and output voltages c) step-down d) step-up
4.2. Current hysteresis unit:
This block senses the inductor current (line 5 in Fig. 4(a)) and generates a logical
signal (line 6). The signal is “1” if the inductor current is less than the lower
hysteresis band and it is ”0” if the inductor current is more than the hysteresis upper
band. “Signal rotating unit” directs these signals to operating switches to perform
hysteresis current control.
4.3. Signal rotating unit:
The first partition of the flowchart shows that “Signal rotating unit” compares Vin
(line 8 in Fig. 4(a)) with V1ref (line 3b) and selects the operation mode as step-down
SBoost
S1
D2
V2
V1
SBuck
Vin
R2
R1C1
C2
L
Current
Sharing
Unit
Current
Reference
Unit
DHB
Hysteresis
Unit
V1ref,V2ref
IRef
IL
Over Voltage
Reduction Unit
SLC
+
-
+
-
SUSU SD
Signal
Rotating
Unit
+
-
upper hysteresis
band
413
5
8
11 6
9
10a12
2a
2b
1a
1c
10b
3a
1b
3b
12
7
(a)
SU: SShare=0
SU: SBoost = 1
SD: SBuck = 0
Iref=Iref-e
iL,V1,V2,Vin
iL> Iref+Bup
iL< Iref-BlowSU: SBoost = 1
SD: SBuck = 1
SU: SBoost = 0
SD: SBuck =0
V2<V2refSU: SShare = 1
SD: SShare = 0
SU: SShare = 0
SD: SShare= 1
Iref=Iref+k1(V1-V1ref)+k2(V2-V2ref)
N=0
N<100 N=N+1
SU: V2>V2ref
SD: V1>V1ref
y
n
y
y
n
n
y
yn
Over Voltage Reduction
n
Current Hystersis
Current Sharing
Current Reference
Vin< V1ref SU
SD
Signal Rotating
y
n
V1out<V1ref Bup=Imax
ySLC Iref=Iref+c
y
n
n
SLC Routine
Dynamic hysteresis
band
(b)
V1
V2
SShare
iL
SBuck
100 101 001000 010
OV
SBoost
Over
Voltage
Removal
(c)
V1
V2
SShare
iL
SBoost
100101110 010
OV
SBuck
Over
Voltage
Removal
(d)
247
(SD) if Vin>V1ref. Otherwise, the operation mode is step-up (SU). In some condition
and decision blocks of the flowchart, the examined condition or executed assignment
varies according to the operation mode. Additionally, this block senses Vin (line 8)
and decides to send the signal of “Current hysteresis unit” (line 6) to either SBuck (by
line 9) (step-down) or SBoost (by line 10a) (step-up). Furthermore, this block decides
to send the signal of “Over-voltage reduction unit” (line 11) to the idling switch
(SBuck in step-up and SBoost in step-down).
4.4. Current Sharing unit:
The inductor current is shared between outputs by “Current sharing unit”. This block
prioritizes output voltages. V2 is prioritized; the “Current sharing unit” examines V2
at first (line 2b), if V2 is less than V2ref, “Current sharing unit” turns off S1 (line 12)
and charges V1 and V2. When V2 has reached V2ref, “Current sharing unit” turns S1 on
and only V1 continues to charge. Therefore, (I1 =V1/R1) can be more than (I2 =V2/R2).
In Fig. 4(a), the “Current sharing unit” monitors the condition of SBoost (line 10b),
because it cannot turn on S1 when SBoost is turned on.
4.5. Over-voltage and under-voltage reduction
The “Over-voltage reduction unit” utilizes the idling switch (SBoost in step-down and
SBuck in step-up) to handle disturbances including load drop and input voltage rise
(sensed from line 1c), which cause over-voltage. In such disturbances, there is too
much current flowing in the inductor. Therefore, if over-voltage happens, the
reference current would be decreased gradually by the signal sent to “current
reference” block (line 13). In Fig. 4(b) this reduction of Iref has happened by constant
of e.
In step-up operation, “Over-voltage Reduction” block turns off the SBuck to let the
SBoost turn on without increasing the inductor current. In step-down SBoost turns on to
avoid over-voltage and SBuck operates to control inductor current. Fig. 4(c), 4(d)
shows the operation of the idling switch to control over-voltage for step-down and
step-up conversions.
Two under-voltage reduction strategies are developed for loads with different
natures. For load or input voltage changes that happen suddenly without any warning
or predictability, DHB is developed. However, there is a possibility to enhance
248
dynamic response to load or input voltage changes, which are pre-known or
predictable. For these cases, a Smart Load Controller (SLC) is suggested (line 12).
4.5.1. Dynamic Hysteresis Band (DHB)
To reduce under-voltages when the load or input voltage change is not pre-known or
predictable, the controller senses voltage drop and increases the upper hysteresis
band (by line 7) to maximum current that the inductor can conduct. Therefore, the
average level of inductor current will be increased immediately and under-voltage
will be limited. Since the “Over-voltage reduction unit” is operating, the temporary
extra current of the inductor does not lead to over-voltage or instability of the
converter. Besides, this extra current does not cause extra switching loss even
temporarily because the switching frequency is reduced because of higher upper
hysteresis band. The logical presentation of this unit is shown in Fig. 4(b).
Simulation results illustrating the functionality of “Over-voltage reduction unit” and
DHB strategy for step-down and step-up conversions are presented in Fig. 5. Fig.
5(a) and 5(b) illustrate the performance of “Over-voltage reduction unit” and DHB
strategy in step-down and step-up conversions respectively. The input voltage is
400V and 100V in step-down and step-up respectively. Output voltages are V1=200V
and V2=150V. The output capacitors are 1mF and the inductor is 2mH.
In both cases of step-down and step-up, the load currents (I1,I2) have increased at
0.05sec and 0.25sec. At 0.05sec, R2=20Ω is changed to R2=10Ω and at 0.25sec,
R1=10Ω is changed to R1=6.7Ω. The load currents decrease at 0.15sec and 0.35sec
when R2 and R1 are changed to their initial values. The last traces of Fig 5(a) and 5(b)
show the switching of all switches. SBuck in step-up and SBoost in step-down have been
switched in transients caused by load current drop to handle the extra current of the
inductor and avoid over-voltages. Alternatively, when there has been an under-
voltage, upper hysteresis band has increased to reduce voltage drop.
Fig. 5(c) illustrates the performance of the system when input voltage changes
suddenly. The input voltage has changed enough to force the transition from step-up
conversion to step-down conversion (at 0.05sec) and reverse (at 0.15sec). “Over-
voltage reduction” unit has performed to control over-voltage when input voltage
raises form 100V to 400V and DHB strategy has been applied to reduce under-
voltage when input voltage has dropped to 100V from 400V.
249
(a)
(b)
(c)
Fig. 5: Response of double-output-Buck-Boost converter with DHB to a) load change in step-
down b) load change in step-up c) input voltage change and transition from step-up to step-
down conversion and reverse.
4.5.2. Smart Load Controller (SLC)
Under-voltages caused by load rises or input voltage drops, which are pre-known or
predictable may be avoided. To perform under-voltage removal, the inductor should
be pre-charged sufficiently and prior to occurrence of the disturbance.
To utilize this capacity, a SLC has been presented in Fig. 6(a). The SLC receives the
request for load change (example: in an E-car, when any new equipment is ordered to
turn on.) or the warning of input voltage sudden drop (example: in electric railway)
(Signal 1 in Fig. 6(a), 6(b)). The SLC orders the controller to increase inductor
250
current (Signal 2). The controller increases the inductor current (by constant of c in
Fig. 4(b)) and acknowledges the SLC (Signal 3). The SLC adds the new load (Signal
4). The controller reduces the reference of the inductor current to decrease the time-
share of “010” state to zero. At this point, the MOBB converter is working as multi-
output-Buck or multi-output-Boost and the loss is minimized.
Fig. 6(c) and Fig. 6(d) illustrate under-voltage reduction by the “SLC” block in cases
of load current sudden rise at 0.05sec (R2: 20Ω→10Ω) and 0.25sec (R1: 10Ω→6.7Ω)
for step-down and step-up respectively. The SLC has been informed of upcoming
load current rise and has signalled (included in the first part of Fig. 6(c), 6(d)) the
controller to pre-charge the inductor. The inductor current has reached the sufficient
level to avoid undershoots caused by sudden load rises.
Current changes in Fig. 6(c), 6(d), marked by dashed circles, are handled by “Over-
voltage reduction unit” and current rises marked by solid circles are handled by the
“SLC unit”.
Fig. 6(e) illustrates the performance of the system when input voltage changes
suddenly. The input voltage has changed enough to force the transition from step-up
conversion to step-down conversion (at 0.05sec) and reverse (at 0.15sec). “Over-
voltage reduction unit” has performed to control over-voltage when input voltage
raises form 100V to 400V and “SLC” unit has been informed of upcoming input
voltage drop to pre-charge inductor and avoid under-voltage.
251
Controller
Source
Smart
Load
Controller
MOPBB
1
2
3
45
Loads
Load
chnge
demandSource
Voltage
chage
signal
1
(a)
12
345
Vout
ILoad
Iinductor
Sig
na
ls
1
2
(b)
(c)
(d)
(e)
Fig. 6: a) Block diagram of SLC b) signalling of SLC, Response of double-output-BB converter
with SLC c) to load change in step-down d) to load change in step-up e) To input voltage change
and transition from step-up to step-down conversion and reverse.
252
5. Experimental results
A laboratory prototype is designed to implement the proposed double-output
topology and validate presented control strategies. The controller has been developed
utilising the 32-bit 64MHz microcontroller NEC-V850/IG3. The inductor is 7mH
and the capacitors C1=C2=3.2mF.
Fig. 7 illustrates the steady state switching of the double-output Buck-Boost
converter in the step-down (Fig. 7(a)) and step-up (Fig. 7(b)) conversions. In the
step-down conversion, SBoost is turned off and SBuck operates to control the inductor
current. In the step-up conversion, SBuck continuously conducts and SBoost switches to
control inductor current. SShare operates to regulate V2 in step-up and step-down
conversions. Fig 7(a) shows the switching of SBuck and SShare and their functioning to
control their related variables. Fig. 7(b) shows the switching of SBoost and SShare.
Fig.7: experimental results: a) switching of SBuck to control inductor current and the switching of
SShare in step-down conversion. Ripple of V2 is shown as well b) switching of SBoost to control
inductor current and the switching of SShare. Ripple of V1 is also shown.
Both of two suggested strategies (DHB and SLC) are applied to step-up and step-
down operations. The loads in the cases of step-down and step-up conversions are
different.
Load change experiments results are shown in Fig 8(a-f). Fig. 8(a) and 8(c) illustrate
the step-down conversion and Fig. 8(b), and 8(d) show the step-up conversion. In
Fig. 8(a) and 8(b), the DHB has been applied and there has been no prior knowledge
of load rise. In Fig. 8(c) and 8(d), the SLC has been utilized with the prior knowledge
of load rise.
(a)
(b)
253
Fig.8: experimental results
Response of double-output-Buck-Boost converter with DHB and over-voltage reduction a) step-down b) step-up
Response of double-output-Buck-Boost converter with SLC and over-voltage reduction c) step-down d) step-up
Response of e) double-output-Buck f) double-output-Boost to same load disturbances
Transition between step-up and step-down conversions with over-voltage reduction and g) DHB h) SLC
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
254
Fig 8(e) and 8(f) show the performances of the double-output Buck-Boost converter
when the idling switch is not utilized and DHB or SLC strategies can not apply. In
other words, these figures show the performance of double-output-Buck (Fig. 2 (b))
for step-down conversion and double-output-Boost (Fig. 2(c)) for step-up
conversion. These load change experiments have been directed so the results may be
compared with the performance of double-output Buck-Boost topology and its
control strategies illustrated in Fig. 8(a-d). Additionally, to highlight the
improvement achieved by the DHB applied in Fig. 8(a), 8(b), this strategy has not
been applied in Fig. 8(e) and 8(f). The change of load current I2 has been shown in
these figures to illustrate the stability of V2 guaranteed by ”Current sharing unit”.
In applications, which there are no prior knowledge of load change, the under-
voltage may not be removed by “SLC unit”. Therefore, DHB strategy has been
realized. However, over-voltage reduction is always functional. Input voltages, load
changes, output voltages, and their fluctuations in Fig. 8(a-f) are summarized in
Table 2.
Table 2: overshoots and undershoots caused by load changes in Fig. 8
Step-down
Vin=30V
Output
voltages
R1
(10Ω→5Ω)
R1
(5Ω→10Ω)
R2
(10Ω→5Ω)
R2
(5Ω→10Ω)
Fig. 8a V1=13.5V
V2=7V
-3V
0V
+1V
0V
0V
0V
0V
0V
Fig. 8c V1=13.5V
V2=7V
0V
0V
+1V
0V
0V
0V
0V
0V
Fig. 8e
V1=13.5V
V2=7V
-5V
0V
+8V
0V
0V
0V
0V
0V
Step-up
Vin=15V
Output
voltages
R1
(50Ω→25Ω)
R1
(25Ω→50Ω)
R2
(50Ω→25Ω)
R2
(25Ω→50Ω)
Fig. 8b V1=20V
V2=10V
-2V
0V
+ 1V
0V
0V
0V
0V
0V
Fig. 8d
V1=20V
V2=10V
0V
0V
+1V
0V
0V
0V
0V
0V
Fig. 8f V1=20V
V2=10V
-4V
0V
+6V
0V
-1V
0V
+1V
0V
To compare the transient of the double-output Buck-Boost converter from step-up to
step-down conversion and reverse, two experiment results are presented in Fig 8(g,h)
for R1=25Ω and R2=50Ω. The output voltages are controlled to be V1=17V and
V2=8.5V. Input voltage is 30V at the start and the converter performs step-down
conversion. Input voltage drops to 15V to necessitate step-up conversion (at about
1.5sec). Input voltage rises to 30V to switch back to step-down conversion (at about
4.5sec). Fig. 8(g) shows the dynamics of mentioned transition when there is no prior
knowledge of input voltage drop, which necessitates the application of DHB. The
255
voltage V1 drops to about 3V, while V2 has not been disturbed. Fig. 8(h) shows the
transition when there is prior knowledge of the input voltage drop. The inductor
current has been increased before input voltage drop happens and the under-voltage
of V1 has limited to 1V while V2 has not been disturbed because the controller
prioritizes it. In both cases of Fig. 8(g) and 8(h), the over-voltage reduction has
performed satisfactorily to keep over-voltage caused by input voltage rises within
1V.
6. Conclusion
A new single-inductor multi-output DC/DC converter with capability of step-up and
step-down conversions is presented. The circuit has been analysed and multi-level
inverters and multi-voltage DC-networks are suggested as suitable applications.
Comparing with the strategy of using separate DC/DC converters to produce required
voltages, the number of needed inductors increases to the number of outputs with
several DC/DC converters. However, the presented topology requires only one
inductor.
The steady state and dynamic equations are developed and the functionality of the
purposed topology is explained based on these equations. Since the elements are
considered ideal, there is no limit for step-up and step-down conversion ratios.
However, in practice, the nonlinearities like inductor resistance limit step-up
conversion ratios. The control strategy of double-output Buck-Boost converter is
presented. Some simulation results are included to show properties of introduced
topology with presented control strategy. To validate the new topology and related
control strategy a laboratory prototype is developed and experiments are carried out.
Some experimental results are presented to illustrate the performance of presented
topology for different applications. Experiments confirm the advantage of load
prioritization. Furthermore, experiments show that the utilization of idling switch and
inductor pre-charging capacities of MOBB converters may improve the dynamic
response remarkably.
The proposed converter is suggested to supply a symmetrical/asymmetrical multi-
level diode-clamped inverter to perform DC-link voltage balancing in presence of
resistive loads. Furthermore, for the applications of electric/hybrid cars where there
is pre-knowledge or predictability of load change or input voltage disturbance, there
is even more capacity to avoid output voltage drop/rise by inductor pre-charging and
256
utilization of the idling switch. For the application of electric train, in addition to pre-
known load changes, there is a predictable tendency of catenary voltage rise/drop
caused by travelling trains and the operation (accelerating, decelerating, or running
steadily). Therefore, the suggested topology and its control strategy can enhance the
performance of the electric system of the train. Additionally, for both applications of
electric/hybrid cars and electric trains, there is a variety of loads with different
ratings and sensitivities. The MOBB converter with its multiple prioritized outputs
may preserve the quality of power provided to sensitive loads while other loads apply
disturbances to the system.
When no priori information is available about load or input voltage disturbance, this
paper has suggested the fast response control strategy of DHB to enhance dynamic
response of the converter to disturbances in input voltage and load.
Nevertheless, both control strategies (DHB and SLC) are developed with
consideration of switching and conduction loss. They limit the utilization of idling
switch and extra inductor current to transient of load or input voltage disturbance to
minimize loss and preserve the efficiency of the circuit along with its dynamic
quality.
Appendix
Dynamic and steady state equations of an n-output MOBB (Fig. 1(a)):
Constrain of the duty cycles for MOBB:
10101
Bu
n
j
Boostj d&djdd (15)
The dynamic equations:
n
k
n
kj
jkinBuL dtvtvddt
tdiL
1
(16)
k
k
n
kj
jLRk
n
kj
jLk
kR
tvdtiidti
dt
tdvC
(17)
The state space equation:
257
n
j
in
Bu
L
L
LL
LL
LLL
n
j
j
k
j
j
j
jBuin
n
k
L
nn
k
n
kj
j
n
j
j
n
n
kj
j
n
j
j
n
k
L
n
k
d
...
d
...
d
v
d
I
I.........
I...I
I...I.........
I...I...I
V...V...VDV
v
...
v
...
v
i
RD
......
RD
......
RD
D...D...D
v
...
v
...
v
i
C
......
C
......
C
......L
1
11
1
1
11
1
1
11
000000
000
0000
0
00
10000
0000
00100
0000
00001
0
00000
0000
00000
0000
00000
000
(18)
The transfer functions:
1sCR
dR
)s(i
)s(v
kk
n
kj
jk
L
k (19)
n
k kk
n
kj
jk
Bu
in
L
sCR
dR
Ls
d
)s(v
)s(i
1
2
1
(20)
Steady state equations:
inn
k
n
kj
jk
n
kj
jkBu
k V
DR
DRD
V
1
2
(21)
inn
k
n
kj
jk
BuL V
DR
DI
1
2
(22)
ACKNOLAGEMENT
The authors thank the Australian Research Council (ARC) for the financial support
for this project through the ARC Linkage Grant LP0774899.
References
[1] Yilei Gu; Lijun Hang; Huiming Chen; Zhengyu Lu; Zhaoming Qian; Jun Li;“A simple
structure of LLC resonant DC-DC converter for multi-output applications” Applied Power
Electronics Conference and Exposition, APEC-2005. Twentieth Annual IEEE Volume 3,6-10
March 2005 Page(s):1485-1490 Vol. 3
[2] I. Harada, N. Hara, F. Ueno, I. Oota, “Multi-output SC type DC-DC converter using a
flexible capacitor ring operation” Telecommunications Energy Conference, 1999.
258
INTELEC'99. The 21st International 6-9 June-1999 Page(s):4 pp.
[3] A. Parayandeh, A. Stupar, A. Prodic, “Programmable Digital Controller for Multi-Output
DC-DC Converters with a Time-Shared Inductor”; Power Electronics Specialists Conference,
PESC'06. 37th IEEE 18-22 June 2006 Page(s):1–6
[4] J.A. Oliver, R. Prieto, ; V. Romero, J.A. Cobos, ,“Behavioral Modelling of Multi-Output
DC-DC Converters for Large-Signal Simulation of Distributed Power Systems”.; Power
Electronics Specialists Conference PESC'06. 37th IEEE 18-22-June- 2006 Page(s):1–6
[5] J. Pou, R. Pindado, D. Boroyevich, (2005)“Voltage-balance limits in four-level diode-
clamped converters with passive front ends”. IEEE Transactions on Industrial Electronics,
52,190-196.
[6] C. Newton and M. Sumner “A Novel Arrangement for Balancing the Capacitor Voltages of a
Five-level Diode-Clamped Inverter” Seventh International Conference on Power Electronics
and Variable Speed Drives, 1998. On page(s): 465-470
[7] A. Nami, F. Zare, G. Ledwich, A. Ghosh“A New Configuration for Multi level converters
with diode clamp topology”, IPEC2007, page.661-665
[8] S. Mariethoz, A. Rufer, “New configurations for the three-phase asymmetrical multilevel
inverter.” Conference Record of the 2004 IEEE Industry Applications Conference, 39th IAS
Annual Meeting..
[9] D. Deaconu, A. Chirila, M. Albu, L. Toma, “Studies on a LV DC network” European
Conference on Power Electronics and Applications, 2-5 Sept.2007 Page(s):1–7
[10] A.M.; Rahimi, A. Emadi, “An Analytical Investigation of DC/DC Power Electronic
Converters With Constant Power Loads in Vehicular Power Systems” IEEE Transactions on
Vehicular Technology, Volume 58, Issue 6, July 2009 Page(s):2689-2702
[11] Xiangzheng Xu, Baichao Chen, "Study on Control State and Development of Power Quality
for Railway Traction Power Supply System" paccs, pp.310-313, Pacific-Asia Conference on
Circuits, Communications and Systems, 2009
[12] Fei-Hu Hsieh; Yen, N.-Z.; Juang, Y.-T.“Optimal controller of a buck DC-DC converter using
the uncertain load as stochastic noise” IEEE Transactions on Circuits and Systems II:
Express Briefs, Volume 52, Issue 2, Feb 2005 Page(s):77-81
[13] A. Khaligh, A.M. Rahimi, A. Emadi, “Modified Pulse-Adjustment Technique to Control
DC/DC Converters Driving Variable Constant-Power Loads” IEEE Transactions on
Industrial Electronics, Volume-55, Issue-3, March 2008 Page(s):1133-1146
[14] Shi Wenqing; Xu Haiping; Wen Xuhui; Wen Wei; “One-cycle controlled DC-DC converters
operating with nonlinear power load” Electrical Machines and Systems, 2005. ICEMS-2005.
Proceedings of the Eighth International Conference on Volume 2,29-29 Sept.2005
Page(s):1361-1365 Vol.2
[15] S. Samanta, P. Patra, S. Mukhopadhyay, A. Patra, “Optimal slope compensation for step load
in peak current controlled dc-dc buck converter” Power Electronics and Motion Control
Conference, 2008. EPE-PEMC 13th 1-3 Sept.2008 Page(s):485-489
[16] Dongbo Zhao; Yonggang Guan “Energy-Based Switching Control for DC-DC Buck
Converters with Switching Loads” 2nd IEEE Conference on Industrial Electronics and
Applications, ICIEA-2007. 23-25 May-2007 Page(s):938-942
[17] G. Garcera, E. Figueres, M. Pascual, J.M. Benavent, “Analysis and design of a robust
average current mode control loop for parallel buck DC-DC converters to reduce line and
load disturbance” Electric Power Applications, IEE Proceedings–Volume-151, Issue-4,7 July
2004 Page(s):414-424
[18] S.R.H. Amrei, Dian Guo Xu; Y.Q. Lang,“High power DC-DC converters under large load
and input voltage Variations: a new approach” Power Engineering Conference, 2005.
IPEC2005. The 7th International Nov.29 2005-Dec.2 Page(s):815-820 Vol.2
[19] Arash A. Boora, Firuz Zare, Gerard Ledwich, Arindam Ghosh, “A General Approach to
Control a Positive Buck-Boost Converter to Achieve Robustness against Input Voltage
Fluctuations and Load Changes” Power Electronics Specialists Conference, PESC-2008.
IEEE 15-19 June 2008 Page(s):2011-2017
[20] Arindam Chakraborty, Alireza Khaligh, Ali Emadi, “Combination of Buck and Boost Modes
to Minimize Transients in the Output of a Positive Buck-Boost Converter” IECON-2006 -
32nd Annual Conference on IEEE Industrial Electronics, Nov.2006 Page(s):2372–2377
[21] A. Chakraborty, A. Khaligh, A. Emadi, A. Pfaelzer, “Digital Combination of Buck and
Boost Converters to Control a Positive Buck-Boost Converter”; Power Electronics
Specialists Conference, 2006. PESC'06. 37th IEEE 18-22 June-2006 Page(s):1–6
[22] K. Viswanathan, R. Oruganti, D. Srinivasan, “Tri-state boost converter with no right half
259
plane zero”2001 4th IEEE International Conference on Power Electronics and Drive
Systems, Proceedings., Volume 2, 22-25 Oct.2001 Page(s):687-693 vol.2
[23] Kapat Santanu; A. Patra, S. Banerjee, “A novel current controlled tri-state boost converter
with superior dynamic performance” IEEE International Symposium on Circuits and
Systems, 2008. ISCAS2008. 18-21 May 2008 Page(s):2194-2197
[24] A. Khaligh, P. Chapman, “Reduction of output capacitance in dc-dc converters using
anticipated load transients” Applied Power Electronics Conference and Exposition,
APEC2008. Twenty-Third Annual IEEE 24-28 Feb-2008 Page(s):818-823
[25] C. Gezgin, “Predicting load transient response of output voltage in DC-DC converters”
Applied Power Electronics Conference and Exposition, 2004.APEC-'04. Nineteenth Annual
IEEE Volume 2,2004 Page(s):1339-1344 vol.2
[26] T. Senanayake, T. Ninomiya, “High-current clamp for fast-response load transitions of DC-
DC converter” Proceedings of the 2003 International Symposium on Circuits and
Systems,2003. ISCAS '03. Volume 1, 25-28 May 2003 Page(s):I-653-I-656 vol.1
[27] R. Mahadevan, S. El-Hamamsy, W.M. Polivka and S. Cuk, “A converter with three
swithched-networks improves regulation, dynamics and control” pp E1.1 E1.19 March-1983
