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Chapter 4
DC to AC Converters
( Inverters )
Applications of Inverters
Conversion of electric power from DC type energy
sources to AC type load
– Battery
– Photovoltaic cell (Solar cell)
Power
– Fuel cell
As a part of composite converter
– AC-DC-AC frequency converter (for AC motor drive)
– AC-DC-AC constant-voltage constant-frequency converter (for
uninterruptable power supplies)
– AC-DC-AC Converters for induction heating
– AC-DC-AC-DC switching power supplies
2
Outline
4.1 Commutation
4.2 Voltage source inverters
Power
4.3 Current source inverters
4.4 Multiple-inverter connections and multi-level inverters
3
4.1 Commutation types
Basic operation principle of inverters
uo
S1
Ud
Power
S2
io
Load
uo
S3
S4
io
t1 t2
t
A classification of inverters
– Square-wave inverters (are discussed in this chapter)
– PWM inverters ( will be discussed in Chapter 6)
The concept of commutation
4
4 types of commutation
Device commutation:
Fully-controlled devices: GTO, IGBT, MOSFET
Power
Line commutation
Phase-controlled rectifier
Phase-controlled AC controller
Thyristor cycloconverter
Load commutation
Forced commutation
5
Power
Load commutation
Condition: Load current is leading load voltage
Application: capacitive load, synchronous motor
6
Power
Forced commutation
(capacitance commutation)
Direct-Coupled
With Coupling-Inductor
7
Another classification of commutations
4 types of Commutations
Device commutation
For fully-controlled
devices
Self-commutation
Power
Forced commutation
Line commutation
External
commutation
For thyristors
Load commutation
8
2 classes of inverters
Current Source Inverter
(CSI)
Power
Voltage Source Inverter
(VSI)
9
4.2 Voltage source inverter (VSI)
+
V3
VD1
C
Ud
V1
R io
VD3
L
uo
V2
VD2
VD4
V4
Power
-
Features
DC side is constant voltage, low impedance
(voltage source, or bulk cap)
AC side voltage is square wave or quasi-square
wave.
AC side current is determined by the load.
Anti-parallel diodes are necessary to provide
energy feedback path.
(freewheeling diodes , feedback diodes)
10
Single-phase half bridge VSI
Ud
2
VD
io R
Ud
Ud
2
1
U G2
L
uo
VD
V2
Power
U G1
V1
uo
Um
2
The current conducting path is
determined by the polarity of load
voltage and load current. (This is true
for analysis of many power electronics
circuits.)
io
t3
t4
t1 t2
t5 t6
V1
V2
V1
V2
VD1 VD2 VD1 VD2
The magnitude of output square-wave voltage is Ud/2.
11
Single-phase full bridge VSI
U G1,4
Operation principle
+
V3
VD3
VD1
C
V1
R io
Ud
VD2
Power
uo
Um
L
uo
V2
-
U G2,3
VD
V4
4
io
t3
t4
t1 t 2
t5 t6
V1
V2
V1
V2
V4
V3
V4
V3
VD 1 VD 2 VD 1 VD 2
VD 4 VD 3 VD 4 VD 3
The magnitude of output square-wave voltage is Ud.
The effective value of output voltage (or fundamental
output voltage) can be changed by changing Ud.
12
Single-phase full bridge VSI
Quantitative analysis
Fourier series extension of output voltage
uo 
4U d 
1
1

sin

t

sin
3

t

sin
5

t




 
3
5

(4-1)
Power
Magnitude of output voltage fundamental component
U o1m 
4U d

 1.27U d
(4-2)
Effective value of output voltage fundamental
component
U o1 
2 2U d

 0.9U d
(4-3)
13
Single-phase full bridge VSI
Output voltage control by phase-shift
uG1
+
O
V3
VD1
C
V1
R io
U
Power
d
L
uo
V2
VD2
-
VD 3
VD 4
V4
t
uG2
O
uG3
O
uG4
t

t
O
uo
io
O
t
io
t1 t2
uo
t3
t
14
Inverter with center-tapped transformer
—push-pull inverter
Load
Power
io
uo
+
Ud
-
V1
V2
VD1
VD2
15
Power
Three-phase VSI
180o conduction
Dead time (blanking time) to
avoid “shoot through”
16
Three-phase VSI
Basic equations to obtain voltage waveforms
For phase voltage of the load
Power
For line voltage
UUN  UVN  UW N  0
17
Three-phase VSI
Quantitative analysis
Fourier series extension of output line-to-line voltage
u UV 
Power

2 3U d 
1
1
1
1

sin 13t  
 sin t  sin 5t  sin 7t  sin 11t 

5
7
11
13



2 3U d 
1
k
sin

t

(

1
)
sin
n

t




n n


(4-8)
Magnitude of output voltage (line-to-line) fundamental component
U UV1m 
2 3U d

 1.1U d
(4-10)
Effective value of output voltage (line-to-line) fundamental
component
U UV1 
U UV1m
2

6

U d  0.78U d
(4-11)
18
4.3 Current source inverter (CSI)
Features
Power
DC side is constant
current, high impedance
(current source, or large
inductor)
AC side current is quasisquare wave. AC side
voltage is determined by the load.
No anti-parallel diodes are needed. sometimes series
diodes are needed to block reverse voltage for other
power semiconductor devices.
19
Single-phase bridge CSI
Parallel Resonant Inverter
Ld
Id
A
VT1
VT3
C
LT1
io
Power
LT2
R
VT2
LT3
uo
LT4
L
VT4
Switching frequency is a little higher
than the resonant frequency so that the
load becomes capacitive and load
current is leading voltage to realize
load commutation.
20
Power
Three-phase self-commutated CSI
120o conduction
21
Power
Three-phase force-commutated CSI
22
Power
Three-phase load-commutated CSI
23
4.4 Multiple-inverter connections
and multi-level inverters
Power
Series connection of 2 single-phase VSIs
24
Power
Series connection of 2 3-phase VSIs
25
Multi-level Inverters
Ways to deal with higher voltage and achieve better
waveform
– Series connection of multiple converters
– Series connection of multiple switch devices
Power
Major type of multi-level inverters
– Neutral point clamped multi-level inverter
– Flying-capacitor multi-level inverter
– Cascade H-bridge( series connected H-bridges)
In broad sense, previously discussed series
connection of multiple inverters is also called multilevel inverter.
In narrow sense, only NPC and FC structures are
called multi-level inverters.
26
Power
Neutral-Point-Clamped 3-level inverter
27
Power
Neutral-Point-Clamped 5-level inverter
Ud
'
'
28
Power
Flying-Capacitor 3-level inverter
V11
VD11
V12
VD12
Ud
U
V
V41
VD41
V42
VD42
W
29
Series connection of 3 H-bridges
U
V
Power
W
Ud
Ud
Ud
Ud
Ud
Ud
Ud
Ud
Ud
30