Implementation of Softswitched Frequency Unfolder
Technique for Single Phase Inverter
S.Urgun, Student Member, IEEE T. Erfidan, B. Cakir
Abstract: In this paper, a novel DC link softswitched
inverter is presented. The new inverter uses softswitched
frequency unfolder technique to provide soft-switching
feature while keeping pulse width modulation (PWM)
capability without high voltage stress The main obstacle
for inverters to operate at high frequencies is the
increasing switching losses with the increasing switching
frequencies. At high switching frequencies, the EMI noises
in the circuitry inrease and power density decreases. To
overcome these problems, switching losses must be
reduced. In this paper, with the help of proposed
softswitching cell, the power loss at the switching instants
are terminated and the operation of inverter at high
frequencies ensured. As a result, with the use of the
unfolder teacnique and softswitching cell, inverter
efficiency increased.
Soft-switching inverters use auxiliary resonant networks in
order to decrease or eliminate the overlap between voltage and
current at switching transitions. According to the placement of
auxiliary circuitry, soft-switching inverters can be classified as
DC-side and AC-side topologies. An attractive topology
should use a simple auxiliary circuit, in particular with a
minimum number of auxiliary switches [2-8].
Index Terms: PWM, driver, soft switching, frequency
unfolder, single-phase inverter, and microcontroller.
In general, the structure of the resonant dc link inverter is
simpler than that of resonant pole inverters. However, the
resonant dc link inverter restraints that all devices be switched
on and off simultaneously. The resonant pole inverter, on the
other hand, allows each phase to be independently modulated,
thus achieving higher output resolution. In other words, to
obtain the same level of output resolution, the resonant dc link
inverter needs to be operated at a higher switching frequency.
The purpose of this paper is to design a single phase soft
switched resonant dc link inverter operating at high frequency
with high efficiency.
I. INTRODUCTION
II. PWM TECHNIQUES
Since DC/AC inverters are able to supply alternate voltages
with adequate magnitude and frequency, they are widely used
in many applications such as motor drives, active- filters, and
uninterrupted power supplies (UPS)[1].
Fig.1 shows the proposed resonant dc link inverter structure
which comprises an auxiliary switch and switching poles S1/S4
and S2/S3. Although these switching poles can be controlled by
several ways, in this paper, frequency unfolder technique is
proposed. This technique, instead of generating PWM signals
on all switches of the inverter, generates PWM signals only on
the auxiliary main switch at the resonant side of the inverter.
S1/S4 and S2/S3 switch pairs changes their states only at half of
the inverter period (10ms in this work), therefore there is no
switching loss observed on these pairs. Thus switching loses
will be observed on a single switch, which will be terminated
with the proposed ZVT-ZCT soft switching circuitry. [9]
Nowadays there are several PWM techniques to the inverter
circuits, which make possible to obtain good output voltage by
low order harmonic elimination. The PWM techniques, to
reduce sonorous pollution and size of the transformer and
output fitler elements, need high switching frequency.
In high switching frequency, the switching losses are high;
resulting low efficiency, low power density, high EMI and
environment noise. Soft switching techniques are used to
overcome these drawbacks inverters [2-8].
S.Urgun is with the Civil Aviation College, Kocaeli University, Kocaeli,
41650 Turkey (e-mail: urgun@kou.edu.tr).
T. Erfidan is with the Department of Electrical Engineering, Kocaeli
University, Kocaeli, Veziroglu Campus Turkey (e-mail: tarik@kou.edu.tr).
B.Cakir is with the Department of Electrical Engineering, Kocaeli University,
Kocaeli, Veziroglu Campus Turkey (e-mail: bcakir@kou.edu.tr).
978-1-4244-1633-2/08/.00 ©2008 IEEE
Fig. 1. Resonant dc link inverter.
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Fig. 3. Softswitched inverter.
dsPIC
Microcontroller
Circuit
Fig. 2. (a) PWM signals generating, (b) SM switching signals,
(c) load current, (d) load voltage, (e) S1/S4 switching signals
In Fig. 2(a) PWM signals generating for SM is shown. As seen,
a reference sine wave Vref, is compared to a carrier triangular
wave Vcar, to obtain switching instants. After the comparison,
as seen in Fig. (2b), if Vref is greater than Vcar then SM is turned
on, otherwise it is turned off. From Fig. 2(e) S1/S4 and S2/S3
switches are turned on and off in every 10ms. This technique
is called frequency unfolding to obtain the line frequency of
50 Hz. In Fig. 2(c) and Fig. 2(d) load current and voltage are
shown respectively.
Gate
Pulse
PWM
Driver
+
VDC
-
Power
Circuit
1-phase
A.C.
RL
Load
Fig. 4. The block diagram of the implemented system.
As seen from Fig. 4, the necessary switching signals of the
implemented system are generated by a dsPIC and the level of
these signals are amplified by a driver circuit.
III. IMPLEMENTATION
In Fig. 3 the implemented inverter sytem with softswitcing
cell is shown. Here the cell circuitry is connected between the
supply andinverter. Softswitching circuit is creating the
softswitching conditions for SM switch before the DC PWM
pulses change.
For the implementation of fore-mentioned techniques
Microchip’s dsPIC30f4011 microcontroller is used, which has
six motor control PWM channels and can run up to 120MHz.
The block diagram and picture of the implemented system are
shown in Figures 4 and 5, respectively.
Fig. 5. The picture of the implemented system.
dsPIC microcontroller is programmed by ICD2 for sinusoidal
PWM technique. To obtain sine values a look-up table with
64word size is placed in program memory of the
microcontroller. SM switch and inverter switches are turned on
and off by the PWM module and timer via PORT of the
microcontroller, respectively [10].
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IV. RESULTS
In Fig. 6, switching signals of SM and softswitching cell for a
single PWM pulse is shown, respectively. As seen from here,
the softswitching cell is switched for creating the
softswitching conditons before SM is turned on and off.
Fig. 8. Current and voltage wave forms of SM for a single
PWM pulse in hardswitching operation, (b) power dissipation
of SM. (V/div=50V, A/div=2A, time/div=20µs and
power/div=100W).
Fig. 6. Switching signals for SM and softswitching cell
(V/div=5V and time/div=20µs).
Fig. 8(a) shows the current and voltage wave forms of SM for a
single PWM pulse in hardswitching operation. As seen, at the
turn on and off switching instants, current and voltage wave
forms occur simultaneosly, thus creating power loss. The
power dissipation at these instants are shown in Fig. 8(b).
Fig. 7(a) shows the current and voltage wave forms of SM for a
single PWM pulse in softswitching operation. As seen, ZVT
conditons are created before the switch is turned on, thus there
is no switching loss. During turn off switching, ZCT
conditions are created and there is no switching loss. Fig. 7 (b)
shows the power dissipation during switching instants and as
seen, it is almost zero.
Fig. 9. Load current and voltage wave forms(V/div=50V,
A/div=100mA and time/div=5ms).
Fig. 9 shows the load current and volatge wave forms. As
seen, current and voltage wave forms with 50 Hz are created
on the load.
V. CONCLUSION
Fig. 7 (a) Current and voltage wave forms of SM for a single
PWM pulse in softswitching operation, (b) power dissipation
of SM. (V/div=50V, A/div=2A, time/div=20µs and
power/div=25W ).
The main obstacle for inverters to operate at high frequencies
is the increasing switching losses with the increasing
switching frequencies. At high switching frequencies, the EMI
noises in the circuitry inrease and power density decreases.
To overcome these problems, switching losses must be
reduced. In this paper, with the
help of proposed
softswitching cell, the power loss at the switching instants are
terminated and the operation of inverter at high frequencies
ensured. As a result, with the use of the unfolder teacnique and
softswitching cell, inverter efficiency increased.
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VI. REFERENCES
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technique”, Power Electronic Drives and Energy Systems for
Industrial Growth, 1998. Proceedings., V. 2, p.p. 723 – 728,
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Bekir Çakır was born in Kocaeli, TURKEY, on 1962. He graduated from
Berlin Technical University and Kocaeli University. He is currently a
professor at Kocaeli Univerity Electrical Engineering department.
Satilmıis Urgun was born in Tokat, TURKEY on April 1, 1977. He
graduated from Kocaeli University. He is currently a instructor at Kocaeli
Univerity Civil Aviation College and studying for PhD degree in Electrical
Engineering department.
Tarik Erfidan was born in Stuttgart, GERMANY on April 19, 1972. He is
currently a assistant professor at Kocaeli Univerity Electrical Engineering
department.
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