Tools

Performance measurement and analysis of 1-phase IGBT inverter with sinusoidal PWM control

Theory

The circuit configuration of single-phase bridge inverter is shown in Fig. 1.

Fig. 1. Circuit diagram of single -phase bridge inverter.


Based on the operation of switches (S1, S2, S3, S4: ON/OFF-state) the operating principle of the inverter is explained below briefly. All possible switching states are given Table.1.

Switch (S1, S2): ON-state

Switch (S3, S4): OFF-state

Switch (S1, S2): OFF-state

Switch (S3, S4): ON-state


Fig. 2(a). Equivalent circuit when S1 and S2 are ON



Fig. 2(b). when S3 and S4 are ON



Table-1. Switch states for single-phase bridge inverter (Unipolar switching)

Switching States S1 S2 S3 S4 Vo
1 ✔ ✔ ✘ ✘ Vin
2 ✘ ✘ ✔ ✔ -Vin
3 ✔ ✘ ✔ ✘ 0
4 ✘ ✔ ✘ ✔ 0


SINGLE-PULSE-WIDTH MODULATION

Fig. 3. Typical waveforms for a single pulse PWM Technique.


  • From Fig. 3 it is seen that only one pulse per half-cycle and the width of the pulse is varied to control the inverter output load voltage magnitude.
  • The gating signals are generated by comparing a rectangular reference signal of amplitude ‘Ar’ with a triangular carrier wave of amplitude ‘Ac’.
  • The frequency of the reference signal determines the frequency of output voltage.
  • The ratio of ‘Ar’ to ‘Ac’ is the control variable and defined as the amplitude modulation index (M) defined as:

..(1)


The RMS value of output voltage is

..(2)

By varying ‘Ar’ from ‘0’ to ‘Ac’, the pulse width ‘δ’ can be modified from ‘0’ to ‘180o’ and the RMS output load voltage ‘Vo’, from ‘0’ to ‘Vs’. The Fourier series of output voltage expression is:

..(3)

Even harmonics are absent due to the symmetry of the output voltage along the x-axis. The pulse-width (δ) is

..(4)


where Ts=T/2.

SINUSOIDAL-PULSE WIDTH MODULATION (SPWM)

  • For realizing SPWM, a high-frequency triangular carrier wave is compared with a sinusoidal reference of the desired output frequency.
  • The intersection of carrier and reference waveforms determines the switching instants of the devices.
  • The PWM scheme is illustrated in Fig. 4 in which ‘vc’ is the peak value of triangular carrier wave and ‘vr’ that of the reference signal.
  • The switches of the inverter are controlled as per the PWM pattern generated.

Fourier series expression for the output load voltage waveform is:

..(5)
 
..(6)

 
..(7)

 

Fig. 4. Ideal waveforms for the unipolar SPWM generation.