A NONINVERTING BUCK-BOOST CONVERTER WITH REDUCED
COMPONENTS USING A MICROCONTROLLER
Kevin M. Torres, Member, IEEE
Robert S . Weissbach, Member, IEEE
rsw7@psu.edu
kevin-torres@psu.edu
The Pennsylvania State University at Erie
School of Engineering & Engineering Technology
Erie, PA 16563-1701
the output voltage relative to the input
voltage [l], according to the equation:
ABSTRACT
*
This paper presents a simple method for
microcontroller implementation of a
noninverting buck-boost converter. The
converter requires two switches, but only
one set of controls. As compared to a
cascaded buck converterhoost converter
topology, the noninverting buck-boost
converter requires one less capacitor and
inductor. The system operates as a buck
converter for duty cycles between 0 - 50% in
the microcontroller, and as a boost converter
(up to a gain of twice the input voltage) for
duty cycles between 50 - 100%. The system
is therefore also advantageous because the
microcontroller algorithm prevents a dead
short from occurring during operation in the
boost converter mode. Experimental and
Pspice results prove that both buck and
boost operation are possible using this
topology. The effect of the load resistance
on the results is discussed.
Vout = Vin-d
where: Vout is the dc output voltage
Vin is the dc input voltage
d is the duty cycle
The boost converter raises the output voltage
relative to the input voltage via the equation:
-
1
Vout = Vin 1-d
(2)
The buck-boost converter allows for dc
voltage at one level to be either raised or
lower, depending on the switch duty cycle.
The equation for this converter is:
Vout = -Vin-
1-d
(3)
Another common buck-boost converter
is the Cuk converter. This converter has the
same voltage ratio as the buck-boost
converter, but has the advantage that the
input and output inductors create a smooth
current at both sides of the converter while
the buck-boost has at least one side with
pulsed current. However, both buck-boost
configurations result in the output voltage
being inverted in magnitude relative to the
input voltage. In some situations, this is not
desirable. Huang, et. al. [2] presented a
multilevel noninverting Cuk converter. This
converter is based on the work by
Middlebrook [3]. The author in [3] uses
multiple stages to reduce component
stresses. The authors in [2] modi@ this
work by rearranging the direction of current
flow to obtain their noninverting converter.
KEYWORDS
Noninverting buck-boost converter, duty
cycle, microcontroller, Pspice
I. INTRODUCTION
With the use of power electronics
devices, high efficiency conversion of
energy may be accomplished. One area
where these devices are used is in dc-dc
converters. These converters act as “dc
transformers” due to their ability to change a
dc voltage from one level to another with
high efficiency. The buck converter reduces
0-7803-6748-0/01/$10.00 02001 IEEE
(1)
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The equation for this noninverting buckboost converter is:
.cJ
out = V iN(n1-d)d (4)
12
-
10
-
8
:":
where N is the number of stages. The
authors in [2] state that this converter is
good for high frequency switching at low
duty cycles. As an example, for a 3-stage
converter (N=3), the ratio of Vout to Vin can
be seen as a function of duty cycle in Fig. 1.
As seen in Fig. 1, the system is highly
nonlinear. Operation in the buck mode (low
duty cycle) allows for accurate control,
while accurate control in the boost mode is
more difficult. The duty cycle for this 3stage system to achieve a voltage gain of
unity is 75%, while to achieve a gain of two
requires an 86% duty cycle .
In this paper, a noninverting buck-boost
converter is provided which reduces the
number of components and attempts to
linearize the gain as a function of duty cycle.
Although two switches are required versus
only one in the inverting or Cuk converters,
only one capacitor and inductor are
necessary (versus two for each in the Cuk
converter). In addition, only one set of
controls is required to achieve either a
bucking or boosting of the output voltage
relative to the input voltage. Almost linear
control can be theoretically achieved over a
gain range of 0 - 200% of the input voltage.
c I 4
2 -
0-
I
,
I
I
I
,
a
,
I
I
-
Fig. 1. PIot of dc gain versus duty cycle for the
noninverting Cuk converter
closed and switch S2 is controlled, the
system looks like a boost converter with the
exception of the freewheeling diode D1. If
switch S1 remains closed while switch S2
remains open, then the output voltage will
equal the input voltage (minus the voltage
drop across diode D2, assuming an ideal
inductor). With this knowledge, a table can
be developed which covers all of the
possible switching permutations for S1 and
S2. This is provided in Table 1.
Table 1 shows that if both switches are
closed, then switch S2 will short across the
load. This is the condition encountered
during boost converter operation when the
inductor is storing energy. If both switches
are open, then no power is provided to the
load. This is the condition encountered
during buck converter operation when the
energy stored in the inductor is freewheeled
through the load and diodes D1 and D2.
When switch $1 is closed and switch S2 is
open, then the supply voltage will be
provided directly to the load. This is the
condition encountered in the buck converter
when energy is flowing from the supply to
the load. It is also the condition encountered
during boost converter operation when both
11. SYSTEM DESIGN
The noninverting buck-boost converter is
shown in Fig. 2 . A principal motivation in
this design is to minimize the number of
required components. Hence, this design
requires two controlled switches, two
diodes, one inductor and one capacitor. One
can see from this design that if switch S2
remains open and switch SI is controlled,
the system looks like a buck converter with
the exception of the loss across series diode
D2. Conversely, if switch S1 remains
I
I
I
-
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s1
s2
Open
Open
Closed
Closed
Open
Closed
Open
Closed
OPERATING MODE
Buck
NIA
Buck-Boost
Boost
The gating signal logic was provided at a
frequency of 1 kHz by an Infineon C504 8bit CMOS microcontroller. Two different
load resistors were used to illustrate the
effect of the load on the system. The results
for the two different load resistors are
provided in Fig. 5, with comparison to
theoretical values, based on [l]. An output
voltage of between 13 and 14 volts was
measured at the transition from buck to
boost mode, due primarily to the voltage
drop across diode D2.
Figure 5a indicates that in the buck
converter mode, the output voltage rises
faster than the theoretical output value, until
the microcontroller reaches a DC control of
around 40% (corresponding to a buck
converter duty cycle of 80%). Above this
duty cycle, and into the boost converter
mode of operation, the output voltage tends
to saturate as the DC control increases.
In Fig. 5b, the output voltage in the buck
mode with the 1.5kSZ load resistor rises
quickly towards approximately 14 volts as
the DC control increases. In the boost
mode, the output voltage rises to a much
higher level in Fig. 5b with the larger load
resistance. Clearly, Fig. 5a and 5b indicate
that the load resistance will have a
significant effect on the output voltage for
this converter design.
To validate the experimental results, the
system was simulated in Pspice As seen in
Fig. 6, the output voltage for the two
different load resistances tends to follow the
curves measured experimentally, rather than
the theoretical values.
J
the supply energy and inductor energy are
provided to the load, boosting the load
voltage. Finally, it is seen from Table 1 that
neither the buck converter nor boost
converter includes a condition where switch
S1 is open and switch S2 is closed.
The flowchart for microcontroller
implementation of this system is provided in
Fig. 3. A counter is incremented from 0 100% for each period of a pulse width
modulation (PWM) wavefonn. At each
increment, the counter is compared with a
duty cycle (DC) control to determine
whether switches S1 and S2 should be open
or closed. The microcontroller algorithm
allows for a seamless transition from the
buck mode to the boost mode at 50% duty
cycle. This requires buck operation from
0% - 100% of the output voltage to occur
using only 0 - 50% duty cycle of the control
system. This is achieved by comparing the
counter to twice the DC control level. If the
counter is less than twice that level, then
switch S1 in Fig. 2 will be on. Switch S2
tums on if the counter exceeds 50% of its
maximum value and still remains below the
value of the DC control level.
The
switching of both S1 and S2 for varying
duty cycle controls is provided in Fig. 3.
Figures 4a and 4b illustrate the gating of the
two switches for buck and boost operation,
respectively.
Table 2. Parameter values used for
experimental testing.
Parameter
Value
s1 3 2
MOSFET TP8N08
15 Vdc
18.71mH, R=32.6R
C
12.35uF
1N914DI,D~
Load Resistance
3792, 1.5kQ
111. EXPERIMENTAL AND
SIMULATION RESULTS
The non-inverting buck-boost converter
of Figure 2 was constructed using the
measured parameters provided in Table 2.
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0
START
YES
SWITCH 1
SWITCH 1
ON
OFF
NO
SWITCH 2
OFF
9
SWITCH 2
Fig. 3. Flowchart for switch control of the noninverting buck-boost converter
s1
G
T
Voltage
Counter (“Aduty cycle)
Counter (% duty cycle)
s2
Gate
Voltage 5
(v)
T
s2
Gate
Voltage 5
v-l
I
I
I
I
I
I
I
I
I
L
20 40 BO 80 100
Counter (% duty cycle)
I
20 40 40 80 100
Counter (% duty cycle)
Fig. 4b. Gate voltages for a microcontroller
duty cycle control of 70%.
Fig. 4a. Gate voltages for a microcontroller
duty cycle control of 30%.
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,
//I
Vo (R=375 ohm)
-Theoretical
vo
I
I
1
1
0
-.
0
20
40
60
80
100
DC Control (%)
Fig. 5a. Output voltage as a function of DC control for a load resistance of 37552.
25
20
15
10
(R=1500 ohm) i
ii -Theoretical
-Vo
vo
I
5
0
0
20
40
60
DC Control (%)
80
100
Fig. 5b. Output voltage as a function of DC control for a load resistance of 1.5k52.
40 ,
I
35
30
25
20
I
--
__.__
-
Vo (R=375 ohm)
+Vo (R=l500 ohm)
15
10
5
0
-Theoretical
0
20
40
60
80
Vo
I
~
100
DC Control (%)
Fig. 6. Pspice simulation of noninverting buck-boost converter with different load resistors.
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IV.
Robert S . Weissbach received his B.S.E.E.
from the University of Florida in 1987, his
SUMMARY
M.S.E.E. from Rensselaer Polytechnic Institute
in 1990, and his Ph.D. in electrical engineering
from Arizona State University in 1998. From
1988 - 1994 he was employed by General
Dynamics Electric Boat Division in Groton, CT,
where he worked on the design, construction
and testing of submarine turbine-generator sets.
He is currently an assistant professor of
engineering at Penn State Erie and a co-director
of an applied research center located at the
campus. His research interests are in flywheel
energy storage systems and power electronics.
A noninverting buck-boost converter has
been developed. The system requires two
switches but only one algorithm for their
control. A microcontroller has been used to
This
implement the control scheme.
topology prevents operation in a dead short
condition, since the boost converter is
limited to at most a gain of twice the input
due to the maximum duty cycle provided by
the microcontroller. Both buck and boost
operation has been achieved. Test and
Pspice results indicate that the value of load
resistance has a direct influence on the
output voltage. Future work in this area
includes optimizing the components used in
the system, studying the theoretical effect of
the load resistance on the output voltage,
and implementing feedback control into the
system to correct for changes in the load
resistance.
Kevin M. Torres received the B.E.E. degree
(Cum Laude) from Auburn University in 1990,
and the M.S.E.E. degree from Georgia Institute
of Technology in 1996. From 1990 - 1998 he
was employed by the Georgia Tech Research
Institute (GTRI) as a research engineer. While
at GTRI Mr. Torres was involved in the design
and upgrade of military EW systems. He is
currently a lecturer of engineering at Penn State
Erie, where
he
teaches
electronics,
microcontrollers and programming.
His
interests include high-density electronic
packaging
specializing
in
hybrid
microelectronics engineering for DC, analog and
RF systems. He is a member of IEEE, IMAPS,
V. REFERENCES:
[ 13 M. H. Rashid, “PowerElectronics: Circuits,
Devices, & Applications“, Englewood
Cliffs: Prentice Hall, 1992
E23 W. Huang, K. Yen, G. Roig and E. Lee,
and ASEE.
“Voltage Divided Noninverting
Cuk
Converter with Large Conversion Ratios,”
IEEE Proceedings of Southeastcon 1991,
Williamsburg, VA, April 7 - 10, 1991
[3] R. D. Middlebrook, “Transformerless Dcto-Dc Converters with Large Conversion
Ratios,” IEEE Transactions on Power
Electronics, Vol. 3, No. 4, pp. 484 - 488,
October 1988
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