Switch Mode Inverter

LECTURE #18

Switched Mode Inverter

1Switched Mode Inverter

118.1 Single-phase half-bridge inverter with resistive load.

118.1.1 Square-wave output.

318.1.2 Quasi square waveform

418.1.3 SPWM switching scheme.

618.2 Single-phase full bridge inverter with resistive load.

718.2.1 Unipolar PWM

818.2.2 Bipolar PWM

918.3 Single-phase full-bridge inverter with resistive and inductive load.

918.3.1 Square-wave output

1118.3.2 Quasi square wave output

1218.3.3 Pulse Width Modulation

18.1 Single-phase half-bridge inverter with resistive load.The half-bridge single-phase inverter as shown below could generate three types of output waveform; square-wave, quasi-square and PWM.

18.1.1 Square-wave output.

The load voltage VLoad is a square wave and having significant low-order of harmonic components as shown below. It is not favourable because the lower harmonics components required large LC to filter out the components.

18.1.2 Quasi square waveform

Also known as PHASE OPERATION. The magnitude of the fundamental is reduced and the quasi-square wave has no triplen harmonics component compare to square-wave as shown above. Therefore, the output waveform is better than the square-wave.

18.1.3 SPWM switching scheme.

The pulse-width-modulation signal is generated by comparing modulating signal (sine wave) with the carrier signal (triangular wave).

Vin = 100 VMa = 1.0

fs = 1 kHz

The quality of waveform generated using SPWM is much better than quasi-square output where no low-order of harmonics current present in the waveform as shown below. It is much easier to remove the high frequency components using small size of LC filter.

18.2 Single-phase full bridge inverter with resistive load.

All three types of switching as applied to half bridge inverter are also applicable to full bridge inverter. However PWM switching is commonly used in this circuit topology.

PWM switching

Definition of terms in PWM:

Two types of PWM: Unipolar and Bipolar.

18.2.1 Unipolar PWM

Vin = 100 VMa = 0.8

fs = 1 kHz

18.2.2 Bipolar PWM

Vin = 100 VMa = 0.8

fs = 1 kHz

The frequency spectrum of bipolar PWM is quite similar to unipolar PWM. However the magnitude of the harmonic component is larger than the fundamental. This technique has disadvantages:-

1. The output voltage jump form Vd to +Vd

2. Impose stress on the power switching devices because always perform switching for the whole cycle.

18.3 Single-phase full-bridge inverter with resistive and inductive load.

18.3.1 Square-wave output

When IGBT1 and IGBT2 turn on (assume negligible voltage drop across the IGBTs):

The above equation has a solution in terms of:

where time constant

At the start, the load current is zero.

1st half cycle (IGBT1 and IGBT2 turn on):

Start:

End:

2nd half cycle (IGBT3 and IGBT4 turn on):

Start:

End:

3nd half cycle(IGBT1 and IGBT2 turn on):

Start:

End:

Compute i(t..) until reach steady state condition.

18.3.2 Quasi square wave output

During on period:

1st on period (IGBT1 and IGBT2 turn on):

start: i(0)=0

End:

1st zero period:

start: i(t1)End:

2nd on period (IGBT3 and IGBT4 turn on):

start: i(t2)End:

2nd zero period:

start: i(t3)End:

Compute i(t..) until reach steady state condition.

18.3.3 Pulse Width Modulation

(i) Unipolar PWM

(ii) Bipolar PWM

EMBED Equation.DSMT4

EMBED Equation.3

EMBED Equation.DSMT4

EMBED Equation.3

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