An inverter is a power electronic device that performs the conversion of DC power into AC power at a desired output voltage and frequency. Inverters are the cornerstone of applications such as uninterruptible power supplies (UPS), variable frequency motor drives, and renewable energy grid integration.

5.1 Inverter Categories: VSI vs. CSI

Inverters are broadly classified into two main categories based on their input source characteristics:

• A Voltage Source Inverter (VSI) is fed by a stiff DC voltage source, meaning the input has limited or zero impedance. The output voltage waveform is controlled independently of the load current.
• A Current Source Inverter (CSI) is supplied by a variable current from a high-impedance DC source. In a CSI, the output current waves are controlled and are not influenced by the load.

5.2 Single-Phase and Three-Phase Inverters

• Half-Bridge Inverter: This is the most basic building block of an inverter, consisting of two switches connected across a split DC source.
• Full-Bridge Inverter: This common topology uses four switches to convert a single DC source into an AC output. Its primary advantage is the ability to apply the full DC source voltage across the load, producing an output voltage with twice the amplitude of a half-bridge inverter for the same DC input.
• Three-Phase Inverter: Conceptually, a three-phase inverter can be viewed as three single-phase inverters connected to the same DC source. The control signals for the arms of the inverter are phase-shifted by 120° relative to each other to generate a balanced three-phase AC supply.

5.3 Modes of Conduction

For three-phase inverters, there are two primary modes of conduction that determine how the switches are operated:

• 180° Mode: In this mode, each switching device conducts for 180° of the fundamental cycle. At any given instant, three devices are in the ON state.
• 120° Mode: In this mode, each device conducts for only 120° of the cycle. This means that at any instant, only two devices are conducting simultaneously. This mode prevents the possibility of the upper and lower switches in the same leg momentarily shorting the DC source during commutation, but it generally results in higher harmonic content in the output waveform compared to the 180° mode.

5.4 Pulse Width Modulation (PWM) for Harmonic Reduction

A simple inverter that switches once per half-cycle produces a square wave output, which is rich in undesirable harmonics. To improve the output quality, Pulse Width Modulation (PWM) is used. The goal of PWM is to reduce the total harmonic distortion (THD) in the load current. A Sinusoidal PWM waveform is generated by comparing the desired sinusoidal modulated waveform with a high-frequency triangular carrier wave. The resulting output is a series of pulses whose width is modulated to approximate a sine wave, significantly improving the harmonic profile of the output.

Beyond the primary DC-AC, AC-DC, and DC-DC conversions, specialized topologies also exist for direct AC-AC power conversion.