1. Chapter 4 DC to AC Converters ( Inverters )
Power Electronics
Chapter DC to AC Converters ( Inverters )

2. Applications of Inverters
Conversion of electric power from DC type energy sources to AC type load
Battery
Photovoltaic cell (Solar cell)
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

3. Outline 4.1 Commutation 4.2 Voltage source inverters
4.3 Current source inverters
4.4 Multiple-inverter connections and multi-level inverters

4. 4.1 Commutation types Basic operation principle of inverters
Load S 1 2 3 4 i o u U d
A classification of inverters
Square-wave inverters (are discussed in this chapter)
PWM inverters ( will be discussed in Chapter 6)
The concept of commutation

5. 4 types of commutation Device commutation:
Fully-controlled devices: GTO, IGBT, MOSFET
Line commutation Phase-controlled rectifier
Phase-controlled AC controller
Thyristor cycloconverter
Load commutation
Forced commutation

6. Load commutation Condition: Load current is leading load voltage
Application: capacitive load, synchronous motor

7. Forced commutation (capacitance commutation)
Direct-Coupled
With Coupling-Inductor

8. Another classification of commutations
4 types of Commutations
For fully-controlled devices
Device commutation
Self-commutation
Forced commutation
Line commutation
For thyristors
External
commutation
Load commutation

9. 2 classes of inverters Voltage Source Inverter (VSI)
Current Source Inverter
(CSI)

10. 4.2 Voltage source inverter (VSI)+ - C R L U d V 1 2 3 4 VD u o i
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)

11. Single-phase half bridge VSIU d i o u V 1 2 VD u o U m i t 1 2 3 4 5 6 V VD G1 G2
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.)
The magnitude of output square-wave voltage is Ud/2.

12. Single-phase full bridge VSIo U m i t 1 2 3 4 5 6 V VD
G1,4
G2,3
Operation principle + V 3 VD 3 VD 1 C V 1 R i L U o d u V 2 o VD VD 4 2 V 4 -
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.

13. Single-phase full bridge VSI
Quantitative analysis
Fourier series extension of output voltage
Magnitude of output voltage fundamental component
Effective value of output voltage fundamental component
(4-1)
(4-2)
(4-3)

14. Single-phase full bridge VSI
Output voltage control by phase-shift + - C R L U d V 1 2 3 4 VD u o i

15. Inverter with center-tapped transformer —push-pull inverter

16. Dead time (blanking time) to avoid “shoot through”
Three-phase VSI
180o conduction
Dead time (blanking time) to avoid “shoot through”

17. Basic equations to obtain voltage waveforms
Three-phase VSI
Basic equations to obtain voltage waveforms
For line voltage
For phase voltage of the load

18. Three-phase VSI Quantitative analysis
Fourier series extension of output line-to-line voltage
Magnitude of output voltage (line-to-line) fundamental component
Effective value of output voltage (line-to-line) fundamental component
(4-8)
(4-10)
(4-11)

19. 4.3 Current source inverter (CSI)
Features
DC side is constant current, high impedance (current source, or large inductor)
AC side current is quasi square 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.

20. Single-phase bridge CSI
Parallel Resonant Inverter A C R L d I VT 1 2 3 4 T T3 T4 u o i
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.

21. Three-phase self-commutated CSI
120o conduction

22. Three-phase force-commutated CSI

23. Three-phase load-commutated CSI

24. 4.4 Multiple-inverter connections and multi-level inverters
Series connection of 2 single-phase VSIs

25. Series connection of 2 3-phase VSIs

26. Multi-level Inverters
Ways to deal with higher voltage and achieve better waveform
Series connection of multiple converters
Series connection of multiple switch devices
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 multi-level inverter.
In narrow sense, only NPC and FC structures are called multi-level inverters.

27. Neutral-Point-Clamped 3-level inverter

28. Neutral-Point-Clamped 5-level inverter

29. Flying-Capacitor 3-level inverter

30. Series connection of 3 H-bridges