ELG4139: DC to AC Converters

Converts DC to AC power by switching the DC input voltage (or

current) in a pre-determined sequence so as to generate AC

voltage (or current) output.

IDC

Iac

+

VDC V

ac

+

Harmonics Filtering

vO 1

+

L

Cv

O 2

(LOW PASS) FILTER

+

vO 1

vO 2

BEFORE FILTERING AFTER FILTERING

INVERTER LOADDC SUPPLY

Output of the inverter is “chopped AC voltage with zero DC component”. It contain harmonics. An LC section low-pass filter is normally fitted at the

inverter output to reduce the high frequency harmonics. In some applications such as UPS, “high purity” sine wave output is required. Good filtering is a must.

In some applications such as AC motor drive, filtering may not required.

Single Phase Half-Bridge Inverter

Vo

RL

+

VC1

VC2

+

-

+

-

S1

S2

Vdc

2

Vdc

2

Vdc

S1 ON

S2 OFF

S1 OFF

S2 ON

t0G

Also known as the Inverter Leg! Both capacitors have the same value. Thus the DC link is equally

spilt into two. The top and bottom switch has to be complementary. Meaning, If the top switch is closed (ON), the bottom must be OFF,

and vice-versa.

Q1 on, Q2 off, vo = Vs/2

Peak Reverse Voltage of Q2 = Vs

Q1 off, Q2 on, vo = -Vs/2

Single Phase Full Bridge

S1

S4

S3

S2

+

-

G

+

2

dcV

2

dcV

-

2

dcV

2

dcV

dcV

2

dcV

2

dcV

dcV

p

p

p

p2

p2

p2

tw

tw

tw

RGV

GRV '

oV

GRo VVVRG '=

groumd" virtual" is G

LEG R LEG R'

R R'- oV+

dcV

+

-

Single phase full bridge is built from two half-bridge leg. The switching in the second leg is delayed by 180 degrees from the first leg.

Q1-Q2 on, Q3-Q4 off, vo = Vs

+ Vs -

Q3-Q4 on, Q1-Q2 off, vo = -Vs

- Vs +

Performance Parameters

• Harmonic factor of the nth harmonic (HFn)

1

onn

o

VHF

V= for n>1

Von = rms value of the nth harmonic component

V01 = rms value of the fundamental component

• Total Harmonic Distortion (THD): Measures the “closeness” in

shape between a waveform and its fundamental component

1

2 2

2,3,...1

1( )on

no

THD VV

=

=

Design Constraints of a Pure Sine wave Inverter

Quantity Details

Voltage Convert 12VDC to 120 VAC

Power Provide 300 W continuous

Efficiency > 90% efficiency

Waveform Pure 60 Hz sinusoidal

Total Harmonic

Distortion

< 5% THD

Physical Dimensions 8” x 4.75” x 2.5”

Cost $175.00

Required Components for Design

Full-bridge

Inverter

Sinusoidal PWM

Controller

Low-pass

Filter

PWM Control

Circuit

Half-bridge

Converter Transformer

12 V DC Input

from vehicle battery)

120 VAC, 60 Hz, 300 W

Output

PWM Controller

• Produces two complementary pulses to control half-bridge transistors.

• Problem:

– Voltage may drop when the input voltage is decreased.

• Solution:

– A feedback network may be added for voltage regulation.

Pulse Width Modulation

Modulating Waveform Carrier waveform

1M

1+

1

0

2

dcV

2

dcV

00t 1t 2t 3t 4t 5t

Triangulation method (Natural sampling). Amplitudes of the triangular wave

(carrier) and sine wave (modulating) are compared to obtain PWM

waveform. Analogue comparator may be used. Basically an analogue

method. Its digital version, known as REGULAR sampling is widely used in

industry.

h x( ) if k x( ) c x( ) 1 if k x( ) c x( ) 1 0( )( )=1

Regular sampling PWM

Sinusoidal modulating

waveform, vm(t)

Carrier, vc(t)t

1 t2

t'1

t'2

t

t

p

p2

)(tvs

pwmv

Regular sampling waveform,

Software Flow Diagram (Dr. Yaroslav Koshka)

Initialize all variables

Count0 = 300 (300 duty cycles)

Has duty cycle been

reached?

Output 1 = high, Output 2 = low

duty cycle table (increment pointer) Output 1 = low, Output 2 = high

300 duty cycle

values?

Decrement Count0 by 1 Duty cycle and sampling period timer

One Sampling

Period? no yes

yes

no

yes no

Low-pass Filter

• 2nd order L-C filter

– Filters to retain a 60

Hz fundamental

frequency

– Few components

– Handle current

– Wind inductor (fine

tune)

PCB Layout

Case Study: Solar System Using Inverters Stand Alone; Simple Grid Tied; Grid Tie with Battery

Solar Schoolhouse and San Mateo College

Simple Grid-Connected System

Solar Array

Inverter Distribution Panel

Utility

Subpanel

Solar AC in from Inverter

Lightning surge arrestor

Stand Alone Residential System

Solar Array

Charge Controller

Battery

DC to AC Inverter

Distribution Panel AC

Distribution Panel DC

Small Stand Alone

System

(to power an office)

Solar Array

Charge Control

Storage: Battery

“Fuel Gauge”

Inverter DC to AC