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Abstract
This paper discusses the design and implementation of single phase PWM inverter using 8051 microcontroller.
The main features of 8051 based PWM inverter are simpler design, low cost, maximum range of voltage
control and compact in size. The designed PWM inverter is tested on various AC loads like AC motor and
intensity control of incandescent lamp in a closed loop environment.
Introduction
The pulse width inverters can be broadly classified as
Analog bridge PWM inverter [1]
Digital bridge PWM inverters [2]
The advantage of Analog based PWM inverter controller
is that, the level of inverter output voltage can be
adjusted in a continuous range and the throughput delay
is negligible. The disadvantages of Analog based PWM
inverters are:
Analog component output characteristics changes with
the temperature and time. They are prone to external
disturbances. Analog controller circuitry is complex and
bulky. They are non-programmable, hence not flexible.
On the other hand Microcontroller based PWM inverter
controller (Digital bridge PWM inverter) makes
the controller free from disturbances and drift, but the
performance is not very much high due to its speed limitation.
However to minimize throughput delay, some
microcontroller based PWM inverters, retrieves switching
patterns directly from memory so that calculation can
be minimized, but this technique demands more memory.
This drawback can be eliminated if switching patterns
are generated by executing simple control algorithms
[3]. Even after using simple control algorithms,
sometimes throughput delay may be substantial.
With the availability of advanced microcontrollers and
DSP [Digital signal processor] controllers [4], with many advanced features like inbuilt PWM generator, event
managers, time capture unit, dead time delay generators,
watch dog timers along with high clock frequency, the
limitation of speed, associated with microcontroller
based PWM inverters [5] can be neglected to some extent.
This paper presents a simple and cost effective technique
of implementing single-phase AC [alternating current]
voltage controller, used to control AC loads ,which
doesn’t demand very high precisions, using 8051 microcontroller.
The paper is organized as follows.
Review of PWM inverters.
Block diagram of controller.
Controller implementation (software and hardware).
Results and Conclusion.
2. PWM Bridge Inverter Review
Inverters can be classified as single-phase and three
phase inverters [6] and they are further classified as
Voltage fed inverter [VSI.], current fed inverter [CFI],
and variable DC [direct current] linked inverter. In Voltage
fed inverter, input voltage remains constant, in current
fed inverter [CFI], input current remains constant
and in variable DC [direct current] linked inverter, input
voltage is controllable.
4. Controller Design
Controller is designed by using simpler low cost components
like 8051 microcontroller, 8 or 12 bit Analog to
Digital Converter (ADC), 4×4 keypad, 4 chopper MOSFET
switches (IRFZ48) and speed/Intensity sensor.
The controller design can be explained under 4 sections
as:
Keypad interface with 8051 μc.
ADC interface with μc.
Generating PWM signals and gate signals using
8051 microcontroller.
Gate driver circuit implementation.
4.1. Keypad Interface
A 4×4 keypad is interface with 8051 microcontroller as
shown in Figure 5, through which four keys are accepted.
After accepting the four keys they are combined to represent
four digit required RPM, which actually represents
the external memory address, in which digital equivalent
of speed is stored.
For example if the keys entered are 1 (01), 2 (02), 3
(03), 4 (04), then they are combined as 1234 (RPM),
which represents External memory address, in which 8
bit digital equivalent of that speed is stored. Higher byte
of the memory address is stored in DPH [data pointer
high byte]. Lower byte of the memory address is stored
in DPL [data pointer low byte]. This method saves time
since it doesn’t require any program execution to convert
the entered speed in RPM into its digital equivalent. The
other method is to enter equivalent digital data of RPM
directly, provided a conversion chart is available [external
look-up table]. This technique will save some memory
access time, since communication with memory is
avoided.
4.2. ADC Interfacing
Whenever speed varies from zero to maximum, the speed
sensor O/P varies from zero to five volts respectively. An
8-bit ADC with resolution 1/28
is used to convert the
analog voltage to digital data. Minimum of 19.5 mv
change in voltage (corresponding change in RPM) is
required to change the digital state of ADC. This limits
the accuracy of the application. The logic of interfacing
ADC is as explained in the flowchart given in the Figure 7.
4.3. PWM Generation
8051 microcontroller do not have on-chip PWM generator.
It is implemented using ‘A’ register and any other register (R0-R7) as shown in Figure 8.
A count (ON period time) is loaded onto one of the
GPR (General purpose register), which can be called as
Duty cycle register and accumulator (‘A’) is loaded with
zero. Register ‘A’ is incremented in steps of one and
continuously compared with duty cycle register.
Gate Signal Generation
The generated controlled PWM signal itself will be one
set of gate signal (g1, g2) and other set of gate signals (g3,
g4) is generated using interrupt technique. The controlled
PWM signal generated is given to the external interrupts,
which is initialized as falling edge sensitive interrupt
type. When controlled PWM signal’s falling edge occurs,
an interrupt service routine meant for that particular external
interrupt is executed.
In the interrupt service routine, a delay is created equal
to the time, 7FH minus duty cycle register content, after
which, the port line is made high and is retained high for
the time duration decided by the contents of duty cycle
register (Figure 9).
The gate signal (vg1 vg2, vg3, vg4) are boosted to a
sufficient voltage level by Gate drive circuitry as shown
in Figure 10, so that they are capable of driving MOSFET’S
to the ON state, when the gate signals are high.
A transistor switch (with inverted gate signals as input)
is made used to boost the gate signal. The same DC
supply, which is used for inverter is also used to drive
the transistor by reducing the DC level using voltage
dividers. The other technique is to use opto-isolators.
Both of these techniques use the same inverter DC
source to boost up the gate signals, thus avoiding more
usage of DC sources.
5. Results and Conclusions
The designed application is tested by designing 60V MOSFET bridge inverter.
Harmonics are removed by using simple capacitor filter
and the AC voltage is stepped up to 220 V using
step-up transformer. The performance of application is
tested on various A.C loads and the plots of the same are
as shown in Figure 10. The design exhibits good results
for the load values of 50 ohm and 100 mH/ 10mH. A
simple PWM technique with 100% duty cycle variation,
which reduces hardware and software complexity, is
used rather than using the most often used complex sinusoidal
PWM technique (For Single-phase inverters).
Required dead time is generated through interrupt, which
avoids the usage of dead time delay generators. With
minor modifications the same work can be used to control
light intensity, temperature etc., The accuracy can be
further improved by using high resolution ADC’s and the
delay involved in the software can be overcome using
higher versions of controllers.