🔌 Mastering the Five-Level Multilevel Inverter: PWM vs Nearest Level Modulation Techniques

Multilevel inverters are gaining significant attention in modern power electronics for their ability to produce high-quality waveforms with reduced harmonic distortion. In this blog, we explore the Five-Level Multilevel Inverter (MLI) implemented using Pulse Width Modulation (PWM) and Nearest Level Modulation (NLM) techniques, along with its simulation in MATLAB/Simulink.

🧩 Understanding the Five-Level MLI Topology

The five-level multilevel inverter configuration presented includes:

• A single H-bridge.

• A bidirectional switch.

• Two DC voltage sources.

This topology enables the inverter to generate five discrete output voltage levels:+Vdc, +Vdc/2, 0, -Vdc/2, -Vdc.

A switching table defines the required combinations of switches (S1–S5, SA) to achieve these output levels. For example:

• +Vdc → Turn ON S1 and S2.

• +Vdc/2 → Turn ON SA and S2.

• 0V → Turn ON S2 and S4 or S1 and S3.

• -Vdc/2 → Turn ON S3 and S5.

• -Vdc → Turn ON S3 and S4.

⚙️ MATLAB/Simulink Implementation

The simulation setup in MATLAB includes:

• Two voltage sources (V1 and V2).

• The bidirectional and unidirectional switches.

• A resistive load.

The diagram visually demonstrates how each switch operates during mode transitions, showcasing real-time voltage waveform responses.

📈 Nearest Level Modulation (NLM) Technique

Nearest Level Modulation is a simplified digital modulation technique that operates as follows:

• A sinusoidal reference waveform is compared with predefined voltage levels (e.g., ±0.25, ±0.75).

• Based on the instantaneous value of the reference waveform, the closest discrete level is selected.

• This selection turns ON the respective switches to output the desired voltage level.

Example behavior:

• Sinusoid > 0.75 → Output = +Vdc

• 0.25 < Sinusoid ≤ 0.75 → Output = +Vdc/2

• -0.25 ≤ Sinusoid ≤ 0.25 → Output = 0

• -0.75 ≤ Sinusoid < -0.25 → Output = -Vdc/2

• Sinusoid < -0.75 → Output = -Vdc

This method offers fast and efficient switching with reduced complexity.

🌀 Pulse Width Modulation (PWM) Technique

PWM operates by comparing the modulating sinusoidal signal with a triangular carrier waveform:

• When the sinusoidal signal exceeds the triangular wave, a pulse is generated.

• These pulses control the switches to achieve various voltage levels.

PWM offers better harmonic performance but requires higher switching frequency and computational control.

🔄 Modulation Index and Output Behavior

A critical factor in both techniques is the modulation index (Ma):

• High Modulation Index (e.g., 0.9 – 1) → More voltage levels are produced, enhancing waveform quality.

• Low Modulation Index (e.g., 0.5 or lower) → Fewer voltage levels and reduced output voltage.

In the simulation:

• Changing Ma from 1 to 0.5 shows a drop in voltage levels and changes in inverter current.

• The output waveform and current are significantly affected by the modulation index.

🔁 Comparing NLM and PWM in Practice

Switching between NLM and PWM in MATLAB/Simulink highlights:

• NLM: Fast response, easy control, but slight performance degradation under rapid changes.

• PWM: Smoother waveforms at the cost of switching losses and complexity.

Users can adjust the modulation index in both methods to observe changes in output voltage and current, enhancing their understanding of inverter performance.

✅ Conclusion

The five-level MLI with NLM and PWM offers an excellent balance between performance, simplicity, and harmonic quality. Through MATLAB/Simulink simulation, users can visually grasp the impacts of switching logic and modulation strategies.