MICROCONTROLLER BASED THREE PHASE INVERTER

Project index: PRJ 012

BySANG GIDEON KIPCHIRCHIR

F17/2161/2004Supervisor: Dr.-Ing. W. Mwema

Examiner: Mr. Ogaba

INTRODUCTION

OBJECTIVE

This project presents a design that will attempt toconvert 12 V DC power from a solar panel to a threephase 120 V AC power at 50 Hz. The design is based onCMOS logic inverters made up of power MOSFETs and amicrocontroller.

DEFINITION

An inverter circuit is used to convert DC power to ACpower. The AC power produced can run regular ACappliances.

THE NEED FOR AN INVERTER• Conversion of DC power from solar panels to

drive AC loads.

• Uninterruptible power supplies.

• High voltage DC power transmission.

• Electric vehicle drives.

INVERTER DESIGN• A CMOS logic inverter was implemented using

power MOSFETs. Two sets of CMOS logic inverter circuits are used. There gates are driven by the anti-phase signals generated a microcontroller.

• With the use of microcontroller, the output frequency could be altered with easy since its software manipulation.

• To produce an ac signal, current is made to flow in one direction for half a period then reversed in the next half period. The duration of the period determines the output frequency.

Q1

Q3

Q2

Q4

T1

+12 V

L

H

N-MOS OFF

P-MOS OFF

A

B

• when the gate inputs of transistors Q1 and Q3 are L level (0 volts), andthe inputs of transistors Q2 and Q4 are H level (5 volts), transistors Q1and Q4 are turned ON while transistors Q2 and Q3 are OFF. Therefore,the electric current flows through the direction of A to B on theprimary coil of the transformer .

• Considering when the gate inputs of transistor Q5 and Q6 are Hlevel and the inputs of transistors Q7 and Q8 are L level. TransistorsQ6 and Q7 are ON while transistors Q5 and Q8 are OFF. Therefore,the electric current flows through the direction of B to A on theprimary coil of the transformer.

Q5

Q6

Q7

Q8

T2

+12 V

H

L

P-MOS OFF

N-MOS OFF

A

B

Gate drive signals

The gate drive signals were generated by the AVR microcontroller. The ATtiny26L AVR microcontroller was chosen as the most appropriate source of gating signal because it has the following characteristics:

• It has an internal oscillator with frequencies ranging from 1 MHz to 8 MHz

• Most of its instructions are single clock cycle execution therefore executes faster.

• It is programmed by connecting some of its pins directly to some pins of the computer parallel port.

The desired output frequency is 50Hz hence a period of 0.02 seconds. To obtainthe three phase square wave AC signal, the three phases must be 120˚ out ofphase.

For the three phase waveforms, at every one sixth of the period, one of thethree waveforms will either be changing from high to low or from low to high.

This is achieved using a progressive delay of one sixth of the period

R1, Y1 and B1 and corresponding anti-phase signals R2, Y2 and B2 are takenfrom the pins of the microcontroller.

0 3 6 9 12 15 18 21 24

R1

R2

Y1

Y2

B1

B2

0

0

0

• The microcontroller frequency used is1MHz, implying that one machine cycle takes 1microsecond.

• A delay of 3333 microseconds, was to becreated but instead, the two 8-bit registers wereloaded with a value 3580 and decrementing thevalue while monitoring the content of theregister. The value was attained after severaltrials and took care of the time taken to executethe various instructions.

• When the value is zero, then the microcontrollerclears one pin and sets another pin and thevalue loaded to the registers and decrementedagain.

Switching circuit• The switching circuit for each phase consists of two CMOS

logic inverters with their gates driven by two anti-phase signals from the microcontroller.

• The design is based on the saturated switch approach where high efficiency is achieved because transistors dissipate very little power.

Q1

Q3

Q2

Q4

Q5

Q6

Q7

Q8

Q9

Q10

Q11

Q12

T1

IRON_CORE_XFORMER

T2

IRON_CORE_XFORMER

T3

IRON_CORE_XFORMER

4

2

5

6

7

3

0

1

0 0

19

0

18

0

20

0

8

12 V 9

0

10

R1

R2

Y1

Y2

B1

B2

IMPLEMENTATION• IRF9540 PMOS and IRF830 NMOS power

MOSFETs were used in the actualimplementation of the CMOS logic invertermainly because they have a freewheelingdiode internally connected between theirdrain and source

• An LM7805 voltage regulator was used topower the microcontroller. Its input voltagewas 12V from the laboratory power supplyand the output was a stable 5.1V.

Gate drive circuitThe output of the microcontroller was a square wave ofamplitude voltage 2.2V. This voltage could not drive thegates for the CMOS logic inverter because the thresholdvoltage for the MOSFETs is 4.5V. For efficient switching ofthe MOSFETs, the gate drive voltage need to be in therange of 10-20 V. A BJT circuit was used for the gate drive.

R11kΩ

R2

10kΩ

Q1

BC107BP

connetion to the MOSFET gateconnection from the

microcontroller output pins

Circuit implementation

Microcontroller

Gate drive circuits

CMOS logic inverters

Transformers

• The anti-phase square waveforms generated by themicrocontroller pins were connected to the gates of theCMOS logic inverter. Both waveforms have the samefrequency and duty cycle of 50%.

RESULTS OBTAINED

• Waveforms obtained had 120 ˚ phase difference between red and yellow phase.

• The two waveforms were fed to the transformer primary windings and had same frequency and duty cycle of 50%.

• The waveforms of the red and blue phases were similarly obtained, and had relative phase of 120˚.

• The two waveforms were fed to the transformer primary windings and had same frequency and duty cycle of 50%.

• The waveforms of the yellow and blue phases weresimilarly obtained, and had relative phase of 120˚.

• The two waveforms were fed to the transformer primarywindings and had same frequency and duty cycle of50%.

CONCLUSION• The output voltages obtained at the secondary coil of

each transformer was 118 V AC at a frequency of 50 Hz.

• The three phases of the inverter implemented gave samevalues in terms of voltage and frequency. The currentthat the inverter can draw from the source will dependon the load to be driven.

RECOMMENDATION FOR FUTURE WORK

The following recommendations are suggested for betterperformance,

• To obtain a proper sinusoidal ac power output,harmonic reduction methods should be employed.

• To ensure high switching speed of order of 100nanoseconds, a proper charging and dischargingcircuit should be provided to every CMOS logicinverter gate.

THANK YOU!