A low-cost dual programmable power source based on a three-leg inverter

Abstract

761 Setúbal (Portugal) Tel.: +351 265790000, fax: +351 265721869, e-mail: dfoito@est.ips.pt, mgaspar@est.ips.pt Abstract A low-cost dual power source solution suitable for several applications is presented. The power source is based on the common 2-level three-leg inverter which is one of the most popular topologies and is commercially available in low-cost power package modules. Although sharing the same topology, the two power sources are completely independent and programmable in term of desired waveform and control. A software tool was also developed to facilitate the dual power source configuration. The control techniques and power drives were designed to satisfy static and dynamic performance of several equipments such as electric machinery or demanding laboratory experiments which make it a very attractive solution to educational and investigation projects, among many others. Each power source are internally self-protected against short-circuits. Although not fully tested in all aspects, the implementation of this dual power source solution prototype allowed experimental confirmation of the expected performance.

Full text

Page 1

A low-cost dual programmable power source based on a three-leg inverter

D. Foito1, M. Guerreiro1, A. Cordeiro2

1 Escola Superior Tecnologia de Setúbal,
Instituto Politécnico de Setúbal
Campus do IPS – Estefanilha, 2910-761 Setúbal (Portugal)
Tel.: +351 265790000, fax: +351 265721869, e-mail: dfoito@est.ips.pt, mgaspar@est.ips.pt

2 Instituto Superior de Engenharia de Lisboa,
Instituto Politécnico de Lisboa
Rua Conselheiro Emídio Navarro 1, 1959-007 Lisboa (Portugal)
Tel.:+351 218317000, fax: +351 218317001, e-mail: acordeiro@deea.isel.ipl.pt




Abstract

A low-cost dual power source solution suitable for several
applications is presented. The power source is based on the
common 2-level three-leg inverter which is one of the most
popular topologies and is commercially available in low-cost
power package modules. Although sharing the same topology,
the two power sources are completely independent and
programmable in term of desired waveform and control. A
software tool was also developed to facilitate the dual power
source configuration. The control techniques and power drives
were designed to satisfy static and dynamic performance of
several equipments such as electric machinery or demanding
laboratory experiments which make it a very attractive solution
to educational and investigation projects, among many others.
Each power source are internally self-protected against short-
circuits. Although not fully tested in all aspects, the
implementation of this dual power source solution prototype
allowed experimental confirmation
performance.

Key words: power source, inverter, PWM, voltage
control, current control.


1. Introduction

The electrical energy allowed, in the last century, a
marked development of countries in various areas.
However, progress has led to the diversity of available
receptors fed with different voltage levels and/or
different frequencies. Due to advancements in Power
electronics many works were developed and focused on
the AC voltage and frequency regulation.
Application of microcontrollers and digital signal
processors (DSPs) in high-performance power electronic
systems has been a long term pursuing goal in
development of control solutions for power converting
systems. Today such digital devices can be effectively
used to realize advanced control schemes. Most DSP
instructions can be accomplished within one instruction
cycle and complicated control algorithms can be
executed with fast speed.
of the expected
This paper proposes a DSP-controlled power source
solution combined with a microcontroller device to
provide a PC USB connection, allowing an easy
configuration. The implementation
programmable power source, with independent reference
voltages or reference currents, allows an easy connection
between the available power and the receptor even
though their different characteristics. Besides the obvious
usefulness to do laboratory tests, the dual source allows
feed a device with two inputs or two receptors in
accordance with some criterion for exploration. This
flexibility shall be provided by a cheap power source.

of a dual
Duty-cycle
50 %
Square wave
Voltage or
current control.
PWM , SPWM
Ref 2
Ref 1
COM
Ph 1
Voltage or
current control.
PWM , SPWM
Ph 2
Time
delay
Time
delay
Time
delay
I2
U2
I1
U1

Fig. 1. Power source diagram using three-leg inverter.


2. Power Source

It is shown in this section, the implementation, control
and possible applications of a dual power source capable
of following, independently, two voltage references, or
one voltage reference and another reference current or
two current references.

Page 2

A. Implementation

The power source is based on the common 2-level
three-leg inverter (Fig. 1) that is fed with direct voltage
or a rectified alternating voltage.
The switches of one leg are operated with constant
switching frequency of 10 kHz, with a duty-cycle of
50%. The middle point of this leg behaves as a zero
reference voltage [1, 2]. The dsPIC outputs are the PWM
command signals to the semiconductors of the inverter
legs.
The output PWM signals to other inverter legs have a
maximum switching frequency of 5 kHz. Isolated gate
drives are also included in the power converter board.
This board is internally
short-circuits.
Figure 2 presents the developed dual power source
prototype during experimental performance tests.

self-protected against

Fig. 2. Dual source prototype during experimental performance
tests.

B. Control

The dual power source is DSP-controlled by a single
device (dsPIC30F4011) which receives analog values
from current and voltage sensors and generates internally
the sigma-delta PWM output command signals [3, 4] to
control independently each power source. The current
control scheme is based on a simple hysteretic
comparator (Fig. 3). The voltage control scheme is based
on a PI voltage controller which is not so simple to
control due to output filter delays (Fig. 4). In this dual
power source prototype several 2nd order output filters
were developed according with the desired output
frequency. Improvements must be done to design a
unique output filter with ability to adapt to different
frequencies.

dsPIC30F4011
DC Power
Supply
Power
Converter
Gate
Drives
Output PWM
signals
Reference
Load
i
+
-
Gain

Fig. 3. Hysteretic current controller diagram.
dsPIC30F4011
DC Power
Supply
PI
controller
Output PWM
signals
Reference
Load
u
+
-
Gain
Output
filter
Power
Converter
Gate
Drives

Fig. 4. PI voltage controller diagram.

The control board is also composed by an 8-bit
microcontroller (PIC18F4550)
master-slave architecture with the dsPIC device. The data
communication between them is accomplished by an SPI
(Serial Peripheral Interface) bus [5-9] available in both
devices. The micro master (PIC18F4550) device is
responsible for digital data communication between the
control board and a computer through a plug and play
USB interface. This device is also responsible to record
the configuration parameters of the dual power source.
The configuration parameters will be record in the
internal EEPROM of the master device and sent to the
slave device also through the SPI bus after the USB
detachment. The control board is also composed by an
analog and digital signal conditioning circuit to adapt the
current and voltage sensors feedback from the circuit
load. The dual power source control can be performed in
open or closed loop but connection of current sensors is
essential to provide additional current load protection. An
LCD display is also available to show configuration
parameters and error messages.
A general overview of the control board diagram is
presented in Fig. 5.

connected in a
USB-A
Connector
dsPIC30F4011
SPI Interface
Shift
SqrExtTriSine
Selection Buttons
LCD Display
USB
Interface
Personal Computer
USB-A
Connector
7
3
4
Pic
PIC18F4550
Analog and
digital
Signal
Conditioning
6
8
6
6
8
Output
PWM
control
signals
Current/
Voltage
feedback
sensors

Fig. 5. Control board diagram.

Page 3

To facilitate the dual power source configuration, specific
software was developed. Microsoft Visual Basic (VB) 6.0
was the object-oriented language used to create the
software application. Visual Basic was considered as a
powerful and flexible development platform, which
includes many useful tools for graphical interface and
data communications [10, 11].
Figure 6 presents a sample screen of the software tool
developed to program the dual power source. Using this
software becomes simple and flexible to choose between
internal (generated internally in the dsPIC device) and
external references, voltage or current mode and
respective peak to peak values, DC or AC waveforms,
frequencies, closed or open loop control options.


Fig. 6. Sample screen of the software tool developed for fast
configuration of the dual power source.


C. Applications

The concept of dual power source has been used with two
windings induction machines [1-3] and, seldom, with
direct current machines [2], however, its study must be
thorough and its application should be expanded to the
receptors whose feeding needs some voltage or current
control because it has a cheap and easy implementation.
These characteristics make it very attractive to
educational and investigation projects.
It is presented a not exhaustive list of applications:

1) Single-phase induction motor. The main and the
auxiliary windings work always together,
without centrifugal switch or any capacitor;
2) DC motor. The field excitation and armature
voltage or current are independently controlled.
3) Two permanent magnet DC machines (PMDC)
can be controlled with different references and
can be included in wheelchairs or carriers of
luggage or others objects;
4) Thermo-fans. A variable voltage can be applied
on an only resistance heating whilst the fan
motor can be have a regulated speed;
5) Dual battery chargers. There are various
charging algorithms to obtain three or four
stages (bulk, absorption, equalization and float).
Some stages can be implemented with constant
current and the others with constant voltage;
6) Telecommunication equipments. There are
several equipments in this application field that
demand different DC voltages, such as 12, 24
and 48V;
7) Laboratory experiments. Current rms value
(comparing thermal effects of various shapes of
the current), electrical machines under unusual
voltages, transformer behaviour fed with several
frequencies, electrochemical effects;
8) Avionics application field. This dual power
source can meet the evolving needs and
capabilities of a broad range of aircraft
equipments, ground support and test equipment
considering the 400Hz standard alternating
current.

There are also some limitations using this solution:

1) Due to inverter topology and control scheme,
only half of the DC bus voltage is available to
apply on the loads;
2) Nominal rms current supported by the switches
of the leg that operates at high frequency square
wave, with a duty-cycle of 50 %, must be equal
to the rms/average current sum of both power
sources;
3) Designed to maximum waveform frequency of
500Hz;
4) Care should be taken to avoid current or voltage
dependence between loads in open loop control
mode. This can lead to significant disturbances.

This power source prototype is not optimized up till now
to provide energy with quality requirements that some
equipments and standards claim but there are
improvements in progress to solve this.


3. Simulation and Experimental Results

The simulation results presented were implemented in
Matlab/Simulink software using the Power System
Blockset. Fig. 7 shows the simulation and experimental
results of an open loop current control using a sinusoidal
reference voltage (55 Hz) applied to a single RL load.


00,0020,0040,0060,0080.010.0120.0140.0160.0180.02
-1
0
1
-100
0
100
Time [s]
Current [A]
Voltage [V]

a)

Page 4

b)
Fig. 7. Open loop current control. Sinusoidal voltage reference
of 55Hz, single RL load, one power source: a) Simulation;
b) Experimental.


Figure 8 presents the simulation and experimental results
of an open loop current control using two sinusoidal
reference voltage ( 50 Hz and 400 Hz) in two identical
RL loads (dual power sources).

0 0.0050.010.0150.020.025
Time [s]
0.030.0350.04 0.045 0.05
0
0
Current1 [A] Current2 [A]

a)

b)
Fig. 8. Open loop current control. Sinusoidal voltage reference
of 400Hz and 50Hz, identical RL loads, dual power sources: a)
Simulation; b) Experimental.


Figure 9 shows the harmonic spectrum of a current
obtained with a single power source. A strong
fundamental frequency and low harmonic content can be
seen.


Fig.9 . Harmonic spectrum of a typical current waveform
obtained in open loop control.


Figure 10 shows the simulation and experimental results
of an open loop current control using a DC reference
voltage applied to a single RL load.

0 0,002 0,0040,0060,008 0.010.0120.014 0.0160.0180.02
-1
0
1
-100
0
100
Time [s]
Current [A]
Voltage [V]

a)

b)
Fig. 10. Open loop current control. DC voltage reference
applied to a single RL load, one power source: a) Simulation;
b) Experimental.


Figure 11 shows the simulation and experimental results
of an open loop current control using a square wave (65
Hz) and a sinusoidal wave (55 Hz) using two identical
RL loads (dual power sources).

Page 5

00.0050.010.0150.020.025
Time [s]
0.030.0350.040.0450.05
0
0
Current1 [A] Current2 [A]

a)

b)
Fig. 11. Open loop current control. Square wave voltage
reference of 65Hz and sinusoidal reference of 50Hz, identical
RL loads, dual power sources: a) Simulation; b) Experimental.


Figure 12 shows the simulation and experimental results
of a hysteretic closed loop current control using a
sinusoidal and square wave reference current with nearly
50Hz, applied to identical RL loads. In this result, the
low switching frequency is due to high hysteretic
bandwidth. This was purposely done to show details
about current waveforms. Hysteretic bandwidth is
typically very narrow.

00.0050.010.0150.020.025
Time [s]
0.03 0.0350.040.0450.05
0
0
0
0
Current1 [A] Current2 [A]

a)

b)
Fig. 12. Hysteretic closed loop current control. Sinusoidal
reference and square wave voltage reference of nearly 50Hz,
identical RL loads, and dual power sources: a) Simulation;
b) Experimental.


4. Conclusion

The flexibility of the proposed dual power source
prototype makes it suitable for many applications.
Considering the harmonic content, and comparing
reference signals and the respective obtained waveforms
it can be concluded that the performance of the dual
power source is good. The developed software is a useful
tool to fast configure the dual power source.
This paper also demonstrates that is possible to take
advantage of three-leg bridge inverters usually optimized
for three-phase applications to create solutions with
potential for other applications. In this case, a three-leg
bridge inverter has become a combination of two
independent programmable power sources. It is advisable
to use a low cost three-leg inverter to reduce the final
cost.
The main drawback of this solution is that only half of
the inverter voltage is available to apply on the loads, and
the rms/average current sum of both power sources
cannot exceed the nominal rms current supported by the
switches of the leg that operates at high frequency square
wave, with a duty-cycle of 50 %. Improvements must be
done in the prototype to design a unique output filter with
ability to adapt to different frequencies.


References
[1] D.G. Holmes, A. Kotsopoulos “Variable Speed Control
of Single and Two Phase Induction Motors Using a
Three Phase Voltage Source Inverter” IEEE Industry
Applications Society Conference, vol 1, pp: 613-620,
Toronto, Canada, 1993.
S.S.Wekhande, B.N.Chaudhari,
Sharma”A Low Cost Inverter Drive For 2-Phase
Induction Motor” IEEE 1999 International Conference
on Power Electronics and Drive Systems, PEDS’99, July
1999, Hong Kong.





[2] S.V.Dhopte, R.K.