The document discusses DC to AC converters known as inverters, which convert DC voltage from a source like solar panels or batteries into AC voltage that can power loads by switching the DC voltage in a predetermined sequence. It covers various inverter circuit designs like single phase half-bridge and full-bridge inverters, and considerations for inverter design like reducing harmonic distortion and outputting a pure sine wave. The document also provides examples of how inverters are used in solar energy systems to either connect to the electric grid or operate independently.

1. 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 Vac
+


2. Harmonics Filtering
vO 1
+

L
C
vO 2
(LOW PASS) FILTER
+

vO 1
vO 2
BEFORE FILTERING AFTER FILTERING
INVERTER LOAD
DC 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.

3. Single Phase Half-Bridge Inverter
Vo
RL
+

VC1
VC2
+
-
+
-
S1
S2
Vdc
2
Vdc
2
Vdc

S1 ON
S2 OFF
S1 OFF
S2 ON
t
0
G
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.

4. Q1 on, Q2 off, vo = Vs/2
Peak Reverse Voltage of Q2 = Vs

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

6. Single Phase Full Bridge
S1
S4
S3
S2
+
-
G
+
2
dc
V
2
dc
V
-
2
dc
V
2
dc
V
dc
V
2
dc
V

2
dc
V

dc
V

p
p
p
p
2
p
2
p
2
t
w
t
w
t
w
RG
V
G
R
V '
o
V
G
R
o V
V
V RG '

=
groumd"
virtual
"
is
G
LEG R LEG R'
R R'
-
o
V
+
dc
V
+
-
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.

7. Q1-Q2 on, Q3-Q4 off, vo = Vs
+ Vs -

8. Q3-Q4 on, Q1-Q2 off, vo = -Vs
- Vs +

9. Performance Parameters
• Harmonic factor of the nth harmonic (HFn)
1
on
n
o
V
HF
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
n
o
THD V
V

=
= 

10. 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

11. 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

12. 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.

13. Pulse Width Modulation
Modulating Waveform Carrier waveform
1
M
1
+
1

0
2
dc
V
2
dc
V

0
0
t 1
t 2
t 3
t 4
t 5
t
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)
t1 t2
t'1
t'2
t
t
p
p
2
)
(t
vs
pwm
v
Regular sampling waveform,

14. 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
ye
s
no
yes
no

15. Low-pass Filter
• 2nd order L-C filter
– Filters to retain a 60
Hz fundamental
frequency
– Few components
– Handle current
– Wind inductor (fine
tune)

16. PCB Layout

17. Case Study: Solar System Using Inverters
Stand Alone; Simple Grid Tied; Grid Tie with Battery
Solar Schoolhouse and San Mateo College

20. Simple Grid-Connected System
Solar Array
Inverter Distribution
Panel
Utility

21. Subpanel
Solar AC in from
Inverter
Lightning surge
arrestor

23. Stand Alone Residential System
Solar Array
Charge
Controller
Battery
DC to AC Inverter
Distribution Panel
AC
Distribution Panel
DC

24. Small Stand
Alone
System
(to power an
office)
Solar Array
Charge
Control
Storage:
Battery
“Fuel Gauge”
Inverter DC to
AC