2013 XXIV International Conference on
Information, Communication and Automation Technologies (ICAT)
October 30 – November 01, 2013, Sarajevo, Bosnia and Herzegovina
An Algorithm for Boost Converter Efficiency
Optimization
Zeljko Ivanovic, Branko Blanusa, Mladen Knezic
Faculty of Electrical Engineering
Banja Luka, Bosnia and Herzegovina
zeljko.ivanovic@etfbl.net
Abstract—In this paper, an algorithm based on the technique of
variable switching frequency is applied, so that working point of
boost converter is at the boundary between continuous and
discontinuous working mode aiming at achieving maximum
efficiency of the converter. Controller is based on variable
switching frequency and measuring the voltage on the main
converter switch. The proposed algorithm is verified by the
simulations and experimental measurements on a converter
prototype.
Keywords—efficiency, boost converter, frequency controller
I.
INTRODUCTION
The share of renewable energy sources in production of
electrical energy is constantly growing. DC/DC converters are
becoming essential part of the system for the production of
electrical energy from renewable energy sources [1], [2]. In
these applications, due to variability of operating conditions
and limitations imposed by primary energy sources, efficiency
of the used converter is very important. Also, efficiency is an
indicator of success of new topologies and/or new control
algorithms. Highly-efficient converters are small, have smaller
temperature changes, and also higher reliability [3].
In the systems with wind turbines and synchronous
generator, photovoltaic sources and fuel cells, basic topology
of DC/DC boost converter (Fig. 1) is often used [1], [4] - [7].
In the papers [4], [5], [8], it was shown that the maximum
efficiency of the boost converter in the wind power plant with a
permanent magnet generator (PMG) can be achieved if it
works at the border between the continuous (CCM) and the
discontinuous current mode (DCM).
In this paper, an algorithm that uses a variable switching
frequency and keeps converter operating point at the border
between the DCM and the CCM is applied. Change of the
switching frequency is based on the fact that converter often
works with the input power less than rated. This is a very
common case for the converter in renewable energy sources, as
well as battery-operated devices. Range of switching frequency
is determined by the specified operating conditions (output
voltage ripple, maximum flux density in the inductor core,
etc.). The proposed algorithm is verified by the simulations and
experimental measurements on the converter. Rated power of
used converter is 500W.
This paper work consists of five sections. Efficiency of the
boost converter as a function of switching frequency is resented
in the second section.
978-1-4799-0431-0/13/$31.00 ©2013 IEEE
iL
L
D iD
.
iLOAD
.
+
+
-
iSW
Tr
VIN
iC
C
RL
VOUT
VG
+
-
.
.
.
-
Figure 1. Boost converter.
Proposed algorithm, which maintains the converter working
point at the border between the DCM and the CCM and
simulation results are given in the third section. The
experimental set up and the results of measurements are given
in the fourth section. Results are summarized in the conclusion.
II.
BOOST CONVERTER EFFICIENCY
Conduction and dynamic losses are main part of boost
converter losses. Total power losses PLOSS are equal to [9], [10]:
PLOSS = PCOND + WT OT ⋅ f sw ,
(1)
where: PCOND – conduction losses, WTOT – total energy of
dynamic losses during one switching period, fsw – switching
frequency. Term PDYN = WTOT⋅fsw represents the average
dynamic power losses. Conduction losses are directly
proportional to the load, and very little dependent on the
switching frequency. Dynamic losses are directly proportional
to the swithing frequency, and very little dependent on the
load.
The power losses in the boost converter obtained by
simulations (based on the model given in [8]) are presented in
Fig. 2. Work of boost converter is simulated in system of wind
power plant with a permanent magnet (PM) generator. In this
simulation converter input power is 150 W. The frequency fb is
boundary between the DCM and CCM. At frequencies lower
than fb, converter operates in DCM mode. By decreasing the
switching frequency, current ripple grows up and,
consequently, effective value of current through the inductor.
Increase of the effective value of the current through the
elements results in the increase of conductive losses in the
converter. Increase of the switching frequency results in lower
effective value of current through the elements and thus
reduces the conduction losses (dashed line). However, with
increase of switching frequency, dynamic losses grow up
(dotted line), which at certain switching frequency become
larger than the conduction losses.
15A
DCM
Rectifier
current
CCM
10A
Inductor
current
5A
0A
200ms
I(D2)
205ms
I(D3)
I(L1)
210ms
215ms
220ms
Time
In DCM, current through the inductor falls to zero, so that soft
switching is realized when switch is turning on. This is a
reason why the characteristic of dynamic losses is refracted at
frequency fb, as can be seen in Fig. 2. Total losses (solid line)
have the minimum around the boundary between the DCM and
the CCM, i.e. in this case converter has the highest efficiency.
III.
CIRCUIT FOR DETECTING THE BOOST CONVERTER
OPERATION MODE
To achieve maximum efficiency of boost converter when
its input power changes, converter working point should be
around the border between the CCM and DCM. Converter
controller and algorithm based on the measurement of the
minimum inductor current aiming at determining operation
mode (CCM or DCM) are proposed in [5]. Switching
frequency of the converter changes so that operating point
leads to boundary between the DCM and the CCM. However,
the boost converter is powered through the diode rectifier, so
that inductor current has the low frequency component (Fig. 3).
Topology with diode rectifier and boost converter is used in
low power wind power plant with PM generator and in some
power factor correction circuits [6], [7], [11].
Waveforms of the current through the single phase rectifier
and the inductor in boost converter are shown in Fig. 3. As it
can be seen, inductor current, besides a high frequency, has low
frequency AC component. This is due to low impedance of
inductor in boost converter for AC voltage on its input. In this
case, the capacitor as an output filter in boost converter acts as
a part of rectifier circuit. When the converter works at the
border between the CCM and DCM, and rectifier diodes
conduct, current through the inductor is not close to zero,
which might be a problem for the controller based on
measuring the current through the inductor (Fig. 3). Also,
depending on the metod of current measurement, efficiency of
the converter can be decreased.
Voltage across the transistor in boost converter without RC
snubber circuit, when it works in DCM and CCM, is shown in
Fig. 4. Waveforms of the boost converter, which is powered
through the single-phase rectifier (Fig. 4), are obtained by
PSPICE simulations.
As opposed to the current through the inductor, the voltage
across the rectifier output has no affect to waveform of voltage
across the boost converter transistor. Oscillations in the DCM
are result of the energy exchange between the inductor and the
parasitic capacitances in the transistor.
15V
Control pulse
Figure 2. Power losses of boost converter in wind turbine as a function of
switching frequency when input power is 150 W.
Figure 3. Current through the rectifier and the inductor in boost converter.
10V
5V
SEL>>
0V
V(V2:+)
400V
VDS
200V
0V
125.00ms
125.02ms
V(L1:2)
125.04ms
125.06ms
125.08ms
125.10ms
Time
a)
15V
Control pulse
fb
10V
5V
0V
V(R12:1)
400V
VDS
200V
SEL>>
0V
125.000ms 125.005ms 125.010ms 125.015ms 125.020ms 125.025ms
V(L1:2)
Time
b)
Figure 4. Control signal and voltage across the transistor for a) DCM and b)
CCM.
Proposed circuit for detecting falling edge at the switch is
shown in Fig. 5. Voltage across the switch is lowered by the
voltage divider (resistors R1 and R2) and then is led to the delay
circuit, which consists of resistor R3 and capacitor C2.
Comparator compares the original and delayed signal and gives
positive pulse at the output when voltage across the switch
drops. Capacitor C1 is used to filter out the high frequency
components.
Control pulses
15V
10V
5V
SEL>>
0V
V(R12:1)
400V
300V
VDS
200V
100V
0V
V(L1:2)
5.0V
Comparator
output
As can be seen in Fig. 4, the transistor voltage in
discontinuous mode drops before the rising edge of the next
control pulse. In CCM the voltage across the switch drops after
the control pulse. The fact that, in DCM, voltage across the
transistor drops before the rising edge of the next control pulse,
can be used to keep the converter operating point at the
boundary between the CCM and the DCM.
2.5V
Vds
R1
R3
1k
470k
8
3
C1
C2
1n
470p
15k
R2
2
R4
2.2k
V+
+
U1A
LM393OUT
-
4
1
Vcc
5V
V
V-
125.01ms
125.02ms
b)
125.03ms
Time
Figure 7. Simulation results of the boost converter in CCM.
When voltage VDS is zero, than voltage across the capacitor C2
is greater than the voltage across the capacitor C1, and output
of the comparator is positive.
0
Figure 5. Circuit for detection of falling edge of voltage across the transistor.
By comparing the positive pulses at the output of the
comparator with control pulses, it can be concluded in which
mode converter works. If the coverter operates in
discontinuous mode, a positive pulse at comparator output will
appear before the control pulse. Greater time between pulses at
comparator output and control pulses means that converter
works deeper in DCM. When the converter operates in CCM,
pulse at comparator output appears after the control pulse.
Simulation results of circuit from Fig. 5 in DCM and CCM are
shown in Fig. 6 and Fig. 7, respectively.
Control pulses
0V
125.00ms
V(R4:1)
15V
10V
5V
0V
V(V2:+)
Proposed boost converter controller is shown in Fig. 8. It
consists of two independent controllers, voltage controller and
frequency controller. Algorithm of frequency controller based
on measuring the voltage on the switch is shown in Fig. 9.
Pulses at the comparator output and control pulses are
compared and the time between the rising edges of these two
signals is calculated (Diff). This time is compared with two
thresholds Up and Down, which are positive and Up > Down.
If Diff is greater than the threshold Up, converter is deeper in
DCM, and it is necessary to increase the switching frequency
in order to return operating point back to the border between
the DCM and the CCM. When Diff is less than Down,
converter is (or will be) in the CCM, so it is necessary to
reduce switching frequency in order to maintain operating
point around the boundary between the DCM and the CCM.
When working point is around the border between the DCM
and the CCM, Diff would be between the thresholds Up and
Down.
400V
VDS
200V
0V
Comparator
output
V(L1:2)
5.0V
2.5V
SEL>>
0V
125.00ms
V(R4:1)
125.04ms
a)
125.08ms
125.11ms
Time
Figure 6. Simulation results of the boost converter in DCM.
Figure 8. Block diagram of proposed boost converter controller.
In this case, the switching frequency remains constant, until the
value of operating point is changed, for example due to
changes of input power etc. Switching frequency is changed
with the step ∆f.
Value of the switching frequency is bounded from below
and above with constants fmin and fmax, respectively. Converter
must enter the steady-state before the new value of switching
frequency is generated. Choice of constants Up and Down
defines sensitivity of frequency controller, as well as the value
of the converter working point in a steady state. By choosing
appropriate thresholds (Up and Down) it is possible to stop the
change of switching frequency when working point of
converter is close to border between the DCM and the CCM.
Clearly, in this case and in the steady state, converter works in
DCM, but very close to border with CCM. In the proposed
algorithm, the frequency controller can work with only one
threshold with which Diff time is compared, but in that case
switching frequency would always change.
IV.
EXPERIMENTAL RESULTS
To verify the proposed algorithm, measurements were
performed on a prototype whose characteristics are: input
voltage VIN=100-300 V; output voltage VOUT=300 V (0-100%
of the load); maximum output power POUT=500 W; maximum
switching frequency fsw=100 kHz and maximum ripple of
output voltage ∆VOUT < 3V.
Block diagram of the experimental set up is shown in
Fig. 10. Instead synchronous generator, autotransformer was
used. Frequency controller in Fig. 8 and measurement of
converter efficiency are implemented in HUMUSOFT MF624
card and Real-Time Windows Target of MATLAB-Simulink.
Resistor 75 Ω and MOSFET were used as variable electronic
load. Oscilloscope HP Infinium 500MHz was used for
recording characteristic waveforms.
Boost inductor
VDS
VPULSE
Sw
+
-
Yes
fSW(n-1) < fMAX - Df
Diff < Down
Yes
Time stamp
fSW(n-1) > fMIN + Df
Yes
Yes
fSW(n) = fSW(n-1) + Df
fSW(n) = fSW(n-1) - Df
Figure 9. Algorithm applied in frequency controller.
L
.
D
VDS
.
VOUT ,iLOAD
+
Tr
CF
220V
50Hz
ELECTRONIC
LOAD
C
+
-
.
.
.
PIC24FJ
+Dfsw
-Dfsw
PERSONAL COMPUTER
MATLAB
SIMULINK
+
REAL TIME WINDOWS TARGET
MF624 I/O CARD
HUMUSOFT
Figure 10. Block-diagram of experimental set up.
Due to the relatively high switching frequency, acquisition
card cannot compare the control pulses and pulses at the
comparator output (Fig. 5). Therefore, microcontroller
PIC24FJ64GA002 was used. Depending on which output is
active (+∆fsw or -∆fsw), switching frequency increases or
decreases. It was assumed that the microcontroller outputs are
updated every second (Time stamp). Switching frequency was
changed with the step of 1 kHz, with the range 20kHz-100kHz.
Waveform of the drain-source voltage and pulses at the
comparator output (Fig. 5) in DCM are shown in Fig. 11. The
results of measurements shown in Fig. 11 verify the simulation
results, and the correctness of the proposed circuits for
detecting falling edge of drain-source voltage across the switch.
Control pulses and pulses at comparator output, in both
converter operating modes are shown in Fig. 12. Dependence
of the converter efficiency as a function of its input power is
shown in Fig. 13. It can be seen that by using the proposed
algorithm, in comparison with the standard approach with a
fixed switching frequency, higher efficiency of converter can
be achieved. The difference in efficiency goes up to 6% in
favor of the proposed algorithm.
V.
Diff = TSW-TPULSE
Diff > Up
.
RECTIFIER
In order to reduce the dimensions of reactive elements,
standard converter usually works in CCM at relatively high
switching frequency. Therefore, the switching frequency of
100 kHz was selected for a converter that works with fixed
switching frequency, and it was compared with the proposed
algorithm.
x
x
VIN ,iIN
CONCLUSION
Renewable energy sources often operate at power less than
rated. Applying the proposed algorithm higher efficiency can
be achieved in comparing to the case when converter works
with fixed switching frequency. This allows better utilization of
renewable energy and less heat of converter. The proposed
algorithm, for variable input power, maintains converter
operating point at the boundary between the DCM and the
CCM, and thus ensures maximum converter efficiency. This
algorithm was verified through simulations and the
experimental measurements on the boost converter prototype.
Experimental measurements have shown that it is possible to
achieve higher converter efficiency when technique of variable
switching frequency is applied. In the future work, we planned
to establish correlation between the parameter Diff and a
change of switching frequency ∆f, aiming at speeding up the
proposed algorithm.
Comparator
output
VDS voltage
Figure 11. Drain-source voltage and pulses at the comparator output in DCM
of the converter.
Figure 13. Converter efficiency as a function of input power for fixed and
variable switching frequency obtained by experimental measurements.
REFERENCES
[1]
Comparator
output
Control pulses
Diff > up
a)
Comparator
output
Control pulses
Diff < down
b)
Figure 12. Control pulses and pulses at the comparator output for: a) DCM of
the converter; b) CCM of the converter.
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