Advanced Power Electronics

1. Chapter 5 –DC/AC Conversion
1
Advanced Power Electronics (EE4007A/B/D)
24-10-2019

2. Outline
2
Chapter 5 –DC/AC Conversion
 DC-AC Converter (Inverter)
• Single-phase half-bridge inverters
• Single-phase full-bridge inverters
• Three-phase full-bridge inverters

3. 3
 Simply called inverters
 Converting from DC power to AC power
 Single or multi-phase
Chapter 5 –DC/AC Conversion

4. 4
 Switching devices of Inverters
 MOSFET
• For low power
• Very high frequency
• Easily controlled
 IGBT
• For low to high power
• High switching frequency
• Easily controlled
 Thyristor
• For very high power
• For very high voltage

5. 5
 DC-DC
Vo waveform
 AC-DC
Vd waveform
 DC-AC
VAo waveform

6. 6
 Voltage Source Inverters (VSIs)
 Adjustable voltage output
 For low to high power
applications
Chapter 5 –DC/AC Conversion
 Fed with constant voltage
 Commonly use insulated gate
bipolar transistors (IGBTs)

7. 7
 Current Source Inverters (CSIs)
 Adjustable current output
 Typically use gate turn-off
thyristors (GTOs)
Chapter 5 –DC/AC Conversion
 Fed with constant current
 Very high power and very high
voltage drives

8. 8
 Circuits of VSIs and CSIs:
 Same power circuit topology
 VSI with voltage control loop
 CSI with current control loop
Chapter 5 –DC/AC Conversion

9. 9
 AC power supplies
 Motor drives
 Variable speed drives (VSDs) for
induction machines
 Electronic drives for brushless DC
(BLDC) machines
Chapter 5 –DC/AC Conversion
Stator
Rotor
Three-phase voltage source inverter

10. 10
 Uninterruptible power supplies (UPSs)
Chapter 5 –DC/AC Conversion

11. 11
 Electronic ballasts (high frequency inverters)
 Florescent lamps
 High intensive discharge (HID) lamps
To produce high voltage to strike the
fluorescence to generate the light
Chapter 5 –DC/AC Conversion

12. 12
 Induction heaters
 Induction stoves
 Water heaters
 Industrial induction heaters
Chapter 5 –DC/AC Conversion

13. 13
 Very high power and high voltage AC motor drives
 Motor drives for motion control
 Robots
 Torque control
 Wireless charging
https://ricardo.com

14. 14
 Single-phase half-bridge inverters
 Single-phase full-bridge inverters
 Three-phase full-bridge inverters
Chapter 5 –DC/AC Conversion

15. 15
 Single-phase output
 For low power applications
Half-bridge single-phase inverter
One leg
 These two IGBTs and two diodes build one leg
(half bridge)
 Two legs form a full bridge
 D1 and D2 are called anti-parallel diodes
 Features
 Topology
 The node between these two capacitors is the neutral
point. Load is connected between A and N.
 The input capacitors, C1 and C2, share the input equally
Chapter 5 –DC/AC Conversion

16. 16
 Purely resistive load (0.5 duty ratio)
Gate signal of T1
Gate signal of T2
ON OFF
OFF
ON
T1 and T2 are complementary
Chapter 5 –DC/AC Conversion

17. 17
 Resistive and inductive load

18. 18
 Single-phase output
 For higher power applications than half-bridge inverters
1st leg 2nd leg
One full bridge
 Features
 T1 and T2, are on/off at the same time, and
T3 and T4 are on/off at the same time.
 Operation
 Also, T1 and T4 are switched on and off
alternatively, and T2 and T3 are
switched on and off alternatively
(The gate drive signals of the upper transistor
and bottom transistor for each leg are
complementary)

19. 19
 Purely resistive load (0.5 duty ratio)
ON OFF
OFF
ON

20. 20
 Resistive and inductive load

21. 21
 Constructed by 3 legs in parallel
1st leg 2nd leg 3th leg
 Topology
 3 phase outputs
 one phase is connected to the middle point of one leg
3 phase outputs
(A, B, C)

22. 22
Chapter 4 – AC/DC and DC/AC Conversion
 Delta-connected load (0.5 duty ratio)
 Operation
• The on-state sequence is T1-T2- T3-T4- T5 -T6 -T1
• The transistor is turned on with 60 degrees
difference, i.e., each leg is operating with
120 degrees phase difference.
360̊
180̊
120̊
60̊ 60̊ 60̊ 60̊ 60̊ 60̊
0 0
0 0
0 0
 
 
 
AB A B
BC B C
CA C A
v v v
v v v
v v v

23. 23
 Delta-connected load Vs Wye-connected load
A
A
B
C
B
C
Inverter Inverter
Under balanced load condition, RA=RB=RC
Delta (Δ) connection:
Line-to-line voltage = phase voltage
(vAB) (vA)
Wye (Y) connection:
Line current ≠ phase current
Line-to-line voltage ≠ phase voltage
Line current = phase current

24. 24
 Wye-connected load

25. 25
 Purely resistive load
(0.5 duty ratio) Gate signal of T1
Gate signal of T2
ON OFF
OFF
ON
Problem: the output voltage is square waveform. But for AC load, sinusoidal
waveform is required.
Solution: To obtain a waveform similar to sinusoidal, duty ratio shoud be changed.

26. 26
Solution: Sinusoidal Pulse-width Modulation (SPWM)
Problem now become: how to control the inverter with changing duty
ratio in order to obtain a sinusoidal output voltage
 VM is compared with VC
 If VM is greater than VC, turn on T1
 If VM is smaller than VC, turn on T2
Based on this switching scheme, what do the gate driving signals of T1 and T2 look like?
Modulation Index

27. 27
 VM is compared with VC
 If VM is greater than VC, turn on T1
 If VM is smaller than VC, turn on T2
If T1 and T2 are controlled using such driving signals (vgs1 and vgs2), what do the output
voltage vo look like?
2
in
V
2
 in
V
Complementary
signals
Vgs1 and vgs2 are complementary

28. 28
This output voltage vo now become such a waveform that the duration of
positive/negative voltage is different in every period.
2
in
V
2
 in
V
Tc Tc

29. 29
2
in
V
2
 in
V
Tc Tc
Tc

30. 30
Therefore, by controlling the single-phase half-bridge inverter using
SPWM scheme, we can generate a similar sinusoidal waveform.
2
in
V
2
 in
V
+ Higher Order Harmonics
Take the Fourier analysis of the above waveform, we can obtain
,1
ˆ
2
 in
o
V
V M

31. 31
 Simulated by PSIM
Vin=200V, M=0.8, fM=50Hz, fC=5kHz

32. 32
 Generation of gate drive signals in practical
Voltage
Comparator Inverted

33. 33
 SPWM
• Single modulation signal
• Two modulation signal with 180 degrees phase difference
T1 and T2 switch as a pair; T3 and T4 switch as another pair.

34. 34
 Single modulation signal

35. 35
+ Higher Order Harmonics
,1
ˆ 
o in
V MV

36. 36
 Simulated by PSIM
Vin=200V, M=0.8, fM=50Hz, fC=5kHz

37. 37
 Two modulation signals

38. 38
+ Higher Order Harmonics
,1
ˆ 
o in
V MV

39. 39
 Simulated by PSIM
Vin=200V, M=0.8, fM=50Hz, fC=5kHz

40. 40
 SPWM with one modulation signals  SPWM with two modulation signals
 Output voltage of the inverter is unipolar
 Output voltage of the inverter is bipolar


,1
ˆ 
o in
V MV
,1
ˆ 
o in
V MV
 The harmonics are lower.

41. 41
 SPWM with one modulation signals  SPWM with two modulation signals

42. 42
 3 single-phase half-bridge inverters in parallel

43. 43

44. 44
+ Higher Order Harmonics
+ Higher Order Harmonics
+ Higher Order Harmonics
Phase voltage

45. 45
+ Higher Order Harmonics
+ Higher Order Harmonics
+ Higher Order Harmonics
Line-to-line voltage

46. 46
 Simulated by PSIM
Vin=200V, M=0.8, fM=50Hz, fC=5kHz
Line-to-line voltage:

47. 47
 Simulated by PSIM
Vin=200V, M=0.8, fM=50Hz, fC=5kHz
Phase voltage:

48. 48
 Example 1 (Motor drive)
The three-phase full-bridge inverter can be used to drive the synchronous motor. The inverter
input is supplied by dc voltage source. The inverter three-phase output is connected to the
stator. The rotor could be permanent magnet or windings excited by dc current to produce
the magnetic flux. If the inverter is controlled using SPWM to generate three-phase ac
voltages, three-phase ac currents will be induced at the stator. Then, there will be
electromagnetic torque generated because of the stator magnetic flux and rotor magnetic
flux. As a result, the rotor will be “Pulled” by the stator to rotate.
Stator
Rotor
Frequency of the modulation signal – frequency of the inverter output voltage – frequency of
the stator current – rotating speed of the stator flux – rotor speed.

49. 49
 Example 2 (AC power supply)
The output of a solar PV panel is 120 V DC. Design an electric circuit that can
supply 80 V AC power to a three-phase load. This load can only be operated
in 50 Hz AC voltage.
Control system design
 Control objectives
 Control aims
 Control methods
DC/AC converter
80 V, 50 Hz AC voltage

50. 50
 Example 2 (AC power supply)
Related Youtube video: Solar Photovoltaic Generation Part 1: Pulse Width
Modulation (PWM) DC/AC Inverter
 Example 1 (Motor drive)
Related Youtube video: animation: How a VFD or variable frequency drive works
https://www.youtube.com/watch?v=DiKcKYbJ1A4
https://www.youtube.com/watch?v=OztKg7EV-Dk