Bipolar/unipolar of single phase inverter based SPWM

Two different switching strategies are used in Sinusoidal Pulse Width Modulation (SPWM) for controlling a single-phase inverter.

Bipolar SPWM:

• A single sine reference signal is compared with a high-frequency triangular carrier.
• If sine > triangle → output = +Vdc
• If sine < triangle → output = -Vdc

Result:

• Output voltage swings between +Vdc and -Vdc.
• Only two levels, so more harmonics.

🔸 Unipolar SPWM:

• Two sine reference signals (180° out of phase) are compared with the carrier.
• Based on comparisons:
• Output can be +Vdc, 0, or -Vdc.
• Switching between levels is more gradual.

Result:

• Lower switching stress
• Reduced harmonic content
• More efficient for most applications