8-1

Chapter 8

Switch-Mode DC-AC Inverters

• converters for ac motor drives and uninterruptible power supplies

8-2

Switch-Mode DC-AC Inverter

• Block diagram of a motor drive where the power flow is unidirectional

Power flow

8-3

Switch-Mode DC-AC Inverter

• Block diagram of a motor drive where the power flow can be bi-directional

Power flow

8-4

Switch-Mode DC-AC Inverter

• Four quadrants of operation

8-5

Full-Bridge DC-DC Converter

8-6

Half-bridge DC-AC Inverter

• Half-bridge inverter

(One leg of full bridge converter)

8-7

Square-Wave Mode of Operation

• Harmonics are of the fundamental frequency

h

VV

VV

AohAo

dAo

1

1

ˆˆ

2

4ˆ

8-8

Effect of Blanking Time

• Results in nonlinearity

8-9

Effect of Blanking Time

• Effect on the output voltage

8-10

Synthesis of a Sinusoidal Output by PWM

2

sinˆ

ˆ

sinˆ

2ˆ

11

1

1

d

tri

controlAo

controlcontrol

d

tri

controlAo

Vt

V

VV

tVv

V

V

vV

8-11

Synthesis of a Sinusoidal Output by PWM (2)

1

8-12

Half-Bridge DC-AC Inverter

• Capacitors provide the mid-point

8-13

Full-Bridge DC-AC Inverter

8-14

Bipolar Voltage switching

Full-Bridge DC-AC Inverter

8-15

Unipolar Voltage SwitchingFull-Bridge DC-AC Inverter

8-16

• Harmonic components around the switching frequency are absent

Full-Bridge DC-AC InverterUnipolar Voltage Switching

1

8-17

Sinusoidal Synthesis by Voltage Shift

8-18

Programmed Harmonic Elimination

• Angles based on the desired output

• IC circuit or microcontroller required

8-19

Summary of DC-AC Inverters

PWM

square-wave