AU J.T. 8(4): 191-195 (Apr. 2005)
Implementation of a Single-phase Unipolar Inverter
Using DSP TMS320F241
Narong Aphiratsakun, Sanjiva Rao Bhaganagarapu and
Kittiphan Techakittiroj
Faculty of Engineering, Assumption University
Bangkok, Thailand
Abstract
This paper presents the design and implementation of a single-phase inverter that
produces a symmetric ac output voltage of desired magnitude and frequency. A diode
bridge rectifier is used to rectify the ac line voltage. Unipolar PWM technique is
employed to control the output voltage magnitude and frequency. The digital signal
processor (DSP) of Texas Instruments TMS320F241 is used for the implementation of
the inverter
Keywords: Single-phase inverters, digital signal processor (DSP), unipolarswitching scheme, output voltage control of single-phase inverters, Matlab simulation.
1. Introduction
Single-phase inverters are widely used in
industrial applications such as induction
heating, standby power supplies and
uninterruptible supplies. A block diagram
representation of a single-phase inverter is
given in Fig.1-1. The inverter consists of four
switching devices (represented as ideal
switches) connected in the form of a bridge.
The control scheme is implemented using
TMS320F241 DSP controller (Techakittiroj et
al. 2003)
GS3
GS1
Vdc
C
A
Load
B
V
GS4
Vo = VA - VB
GS2
Fig. 1-1. Single-phase inverter
In the unipolar switching scheme (Ned et
al. 1995), the output voltage changes between
positive and zero, or between zero and negative
voltage levels. To produce a sinusoidal output
voltage waveform of variable frequency and
amplitude, a sinusoidal reference signal (Vref)
is compared with the triangular waveform
(Vtri). The amplitude modulation index (ma),
which controls the rms value of the output
voltage, is defined as
^
ma =
Vref
(1.1)
^
V tri
^
^
The V ref and V tri in equation (1.1) refer
to the peak amplitudes of the signals.
Leg A and B of the full-bridge inverter
are controlled separately by comparing Vtri
with Vref and Vtri with -Vref. The resulting
waveforms are used to control the switches as
follows:
In leg A:
(1.2a)
Vref > Vtri : GS1 on and
Vref <Vtri : GS4on
and
In leg B:
(1.2b)
-Vref > Vtri : GS3 on and
-Vref< Vtri : GS2 on
The PWM signals obtained are shown in
Figs. 1-2a and 1-2b. Note that GS4 and GS2
will be automatically created as the inversion
of GS1 and GS3, respectively.
PWM generated from Vref and Vcarrier
The dead band (Tdead) calculated
Aphiratsakun (2004) and TI (2000):
1.5
Magnitude
1
0.5
by
0
Tdead = Period x Dead Band Prescaler
xCPU clock
-0.5
-1
-1.5
0
0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
Time Second
0.02
(1.4)
1.5
Magnitude
1
0.5
0
PWM_PERIOD
-0.5
-1
-1.5
0
0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
Time Second
Count up
0.02
(a)
Count down
PWM1a
PWM generated from -Vref and Vcarrier
1.5
Magnitude
1
Dead Band
0.5
0
-0.5
-1
-1.5
PWM1b
0
0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
Time Second
0.02
1.5
Fig. 1-3. Symmetric PWM waveform
generation
Magnitude
1
0.5
0
-0.5
-1
-1.5
0
0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
Time Second
2. Circuit Description
0.02
(b)
The schematic diagram of the inverter
circuit implemented is given in Fig. 2-1. It has
two parts, the control circuit and the power
circuit. The shaded part is the control circuit
containing the DSP controller TMS320F241
that generates the PWM signals and also
provides soft start function.
Set point for the modulation index and
frequency are set by a computer through serial
interface. The low pass filter was designed in
such way that the output voltage waveform of
the inverter is sinusoidal.
To start with, the single-phase inverter
with the unipolar switching scheme is
simulated using simulink in Matlab and its
performance is studied. Later on, the singlephase inverter was implemented using DSP
TMS320F241 and its performance was studied.
A comparison is made of the results obtained
through simulation and experimental work
under the same operating conditions.
Fig. 1-2 PWM signals (a) For leg A
(b) For leg B
The comparison of the reference
sinusoidal signal with the triangular waveform
is done in the PWM generator of the DSP to
generate the control signals for the switching
devices along with the inverted signals with the
required dead band. A 16-bit counter register is
used to measure the frequency of the triangular
wave. A centered symmetric PWM signal is
used which has maximum count up of 216 and
count down 216. The PWM signals and the
control signals generated are given in Fig. 1-3.
Count up, count down, switching time and
dead band are calculated as shown by
Aphiratsakun (2004) and TI (2000):
Count up = Count down = PWMA_PERIOD
Tsw = 2xPWMA_PERIODxCPU clk
= 2xPWMA_PERIODx50 ns
(1.3)
192
NO Relay contact
220 V
50 Hz Supply
LC filter
single-phase
inverter
Single-phase
Diode bridge
R Softstart
+
C
450V
R
Load
Variable ac voltage
variable frequency
-
Modulation
set point
PWM
generation
Sine wave
calculation
Unit
Interface
Softstart
time
Dead time
selection
Relay
Coil: 6V DC
Frequency
Set Point
Switching
Frequency
TMS320F241F
DSP Controller
Serial
Fig. 2-1: Overall schematic diagram of single-phase inverter
Simulation waveforms
The tested conditions for the simulation
and experimental work are:
Prated
VLN
Cdc
L
C
ma
Vref
Vtri
Tdead
PWM output (V)
400
: 1.5 kW
: 220 V
: 1000 µF
: 2 mH
: 1.5 µF
: 0.8
: 50 Hz
: 7.5 kHz
: 5.6 µs
B
-
Diode
Bridge
-
v
Vdc
R
C
Filtered output (V)
Output current(A)
2 mH
-
L
iL
+
B
-
+
v
Cfilter
V_pulse
1.5 uF
Load
-
v
Vfilter
Vfilter
To Workspace
Vout
PWM
Vab
time
PWM
0.045
0.05
0.025
0.03
0.035
0.04
0.045
0.05
0.025
0.03
0.035
Time second
0.04
0.045
0.05
0.2
0
-0.2
4. Experimental
To Workspace3
i
+
A
pulses
0.04
Fig. 3-2. PWM waveforms of unipolar inverter
(simulation)
IL
220
0.035
0
-0.4
0.02
PWM
Inverter
+
0.03
-200
0.02
Vdc_scope
+
0.025
200
0.4
The schematic for the simulation of
unipolar single-phase inverter is given in Fig.
3-1. The waveforms of PWM output, filtered
output voltage and filtered output current,
obtained through simulation are shown in Fig.
3-2.
+
0
-200
-400
0.02
3. Simulation Study
A
200
To Workspace1
Clock
To Workspace2
0.8 Modulation index
Fig. 3-1. MATLAB simulation of single-phase
inverter
193
The schematic diagrams for the power
circuit and control circuit are shown in Figs.4-1
and 4-2, respectively. TMS320F241 DSP
controller (Techakittiroj et al. 2003) with
PWM and output ports is used for the
implementation. The four PWM signals have
been fed to the optocoupler (6N137) for the
isolation of gate drivers. Four discrete MOSFETs
(IRFP450) are used as switching devices.
IR2110, IC gate driver is used to drive the
MOSFETs in the upper and lower legs of the
inverter. In the power circuit of Fig. 4-1 the
single-phase diode bridge rectifier (6A6 GW)
with a 1000µF DC link capacitor (Cdc) is
connected to the single-phase ac power supply,
220V, 50 Hz to provide
R soft-start
IRFP450
220 ohm/ 20 W
6A4 GW
6A4 GW
Cdc
VL-N 220V
Csnubber
Vdc
50 Hz
Filter + Load
0.1 uF
1000 uF/350V
6A4 GW
S3
S1
R
IRFP450
G3
G1
6A4 GW
IRFP450
IRFP450
G4
G2
S4
S2
Fig. 4-1. Power circuit
current. The rms output voltage is measured by
rms meter and is found to be 220 V.
6N137
Optocouplers
+5 V
+15 V
Gate driver
+15 V
PWM
gate
pulses
+5 V
GS1
IR2110
+5 V
+15 V
GS4
G1
G4
DSP
Controller
+5 V
+15 V
G3
I/O
+15 V
G2
GS3
+15 V
IR2110
GS2
NO Relay contact
(a)
Relay: OMRON
G2R-1-E
Coil: 5V DC
+5 V
Fig. 4-2. Control circuit
a constant dc source to the inverter. The Cdc
must have high voltage rating (greater than 350
V). A snubber capacitor of 0.1µF is connected
across the inverter to protect it from high surge
voltages. A soft start resistor is used to reduce
the peak inrush currents, thereby reducing the
stress on rectifier diodes and Cdc.
Photographs of the unipolar single-phase
bridge inverter constructed in the laboratory are
shown in Figs. 4-3 (a) and (b). Fig. 4-4 shows
the recorded waveforms of unfiltered
sinusoidal PWM, filtered output voltage and
(b)
Fig. 4-3. Laboratory unipolar inverter board (a)
control circuit (b) power circuit
194
parameters are adjusted for giving fundamental
frequency rms output voltage of 220V at 50
Hz. With this unipolar inverter unit, further
research on single-phase inverters can be
carried out such as soft switching inverters,
single-phase UPS and single-phase induction
motor drives.
Waveforms
PWM output (V)
400
200
0
-200
Filtered output (V)
-400
0.02
0.03
0.035
0.04
0.045
0.05
0.025
0.03
0.035
0.04
0.045
0.05
0
-200
0.02
Output current(A)
0.025
200
References
0.2
0
Aphiratsakun, N. 2004. Effect of Sampling
Time, Switching Frequency and Output
Quantization Step on Vector Controlled
Cage Motor, Master Thesis, Assumption
University, Bangkok, Thailand.
Ned, M.; Undeland, T.M.; and Robbins, W.P.
1995. Power Electronics Converters,
Applications, and Design, 2nd ed., John
Wiley & Sons, New York, NY, USA.
Techakittiroj, K.; Aphiratsakun, N., Threevithayanon, W.; and Nyun, S. 2003.
TMS320F241 DSP Boards for Powerelectronics Applications. AU J.T. 6:168-73.
TI. 2000. TMS320F243/F241/C242 DSP
Controllers Reference Guide, system and
Peripherals, Literature Number: SPRU276C.
Texas Instruments, Texas, USA.
-0.2
0.02
0.025
0.03
0.035
Time second
0.04
0.045
0.05
Fig. 4-4. PWM waveforms of unipolar inverter
obtained through experimental work
Conclusion
A laboratory model of a unipolar singlephase inverter was successfully implemented
using DSP TMS320F241 and tested. The
inverter unit consists of four, discrete
MOSFETs connected as a bridge and drive
with a low cost driver. The experimental result
matched with simulation results. Although any
195