What are the basic multilevel inverter topologies?

A multilevel inverter (MLI) is a power electronic device designed to generate a stepped ac voltage level at its output by combining multiple lower-level dc voltages as inputs. This FAQ will cover the three basic MLI topologies: diode-clamped MLI, capacitor-clamped MLI, and cascaded H-Bridge MLI.

Conventional two-level inverter and MLI

A conventional two-level inverter (Figure 1 (a)) is a power electronic device that converts dc into ac with only two voltage levels: +V and −V, where V is the dc input voltage and a zero voltage level. An MLI (Figure 1(b)), on the other hand, generates more than three levels, and they are usually an odd number.

Figure 1. A simple circuit diagram illustrating how three and five-level MLI output can be generated. (Image: IEEE Access)

The circuits are the most simplistic representations for showing how multiple dc levels can form a stepped waveform at the output to resemble an ac output. Understanding these circuits helps us understand how the three basic MLI topologies operate.

The three basic MLI topologies

The three basic MLI topologies and their operating principles and clamping devices’ roles are discussed below.

Diode-Clamped MLI

A diode-clamped MLI uses diodes as clamping devices to limit the voltage stress on the power devices. This topology is commonly known as the neutral point clamped (NPC) MLI, especially in its three-level configuration, as shown in Figure 2.

The clamping action occurs when the diodes clamp the voltage across each switch to a fraction of the total dc bus voltage. For a three-level inverter, the voltage across each switch is limited to half of the dc bus voltage (Vdc/2). When more than three levels are desired at the output, the dc bus is divided into multiple voltage levels using capacitors in series. For an n-level MLI, n−1 capacitors are required.

Capacitor-Clamped MLI

A capacitor-clamped MLI, also known as a flying capacitor (FC) MLI, uses capacitors as clamping devices instead of diodes (Figure 3). This topology is designed to achieve multiple voltage levels by connecting capacitors in series within each phase leg.

Capacitors clamp the voltage across each switch, synthesizing multiple voltage levels. Unlike diode-clamped MLIs, these capacitors must be charged and discharged appropriately to maintain the desired voltage levels.

The capacitors are connected in series within each phase leg, and their voltages are regulated by switching actions. This configuration allows for more flexibility in voltage synthesis compared to diode-clamped MLIs.

H-Bridge inverter

Before understanding what a cascaded H-Bridge MLI does, it is good to understand the H-Bridge. An H-Bridge inverter is the most basic building block of a cascaded H-Bridge MLI to generate higher MLI levels. Figure 4 shows an H-Bridge inverter that converts dc power into ac power by reversing the polarity of the voltage applied to the load.

Figure 4. Circuit diagram of a three-level H-bridge MLI. (Image: Rakesh Kumar, Ph.D.)

The H-Bridge consists of four switches, MOSFETs or IGBTs, arranged in an “H” shape. The load, such as a motor or transformer, is connected across the middle of the “H”.

By opening and closing the switches in pairs, the H-Bridge can apply a positive or negative voltage across the load, converting dc to ac. For example, in Figure 4, switches S1 and S2 are turned on to apply a positive voltage, while switches S3 and S4 are turned on to apply a negative voltage.

Cascaded H-bridge MLI

A cascaded H-Bridge MLI uses multiple H-Bridge cells connected in series to produce a multilevel ac output from separate dc sources. Figure 5 shows a five-level cascaded H-bridge MLI containing two H-Bridges.

Each H-Bridge cell is connected to its dc source, which can be derived from renewable energy sources like solar panels or wind turbines. The switches in each H-Bridge operate at the fundamental frequency, reducing switching losses and allowing for high-power applications.

Figure 6 shows a five-level MLI’s hardware output using a resistive and an inductive load. A sine PWM scheme operates the switch so that even the PWM variations resemble a sine nature apart from the stepped output.

Figure 6. Hardware output of the five-level MLI using (a) resistive load and (b) inductive load (Image: Springer Nature)

Summary

Understanding the basic MLI topologies gives us a fundamental understanding of how MLIs work. Many new MLI topologies have been developed recently. Still, when given a closer look at these new topologies, most are either an extended version of the basic MLI topologies or a combination.