Page 56 – PWM Techniques for Multilevel Inverters
Subject: Power Electronics
Level: GATE / PSU / Interview Advanced
🔹 1. Why PWM for Multilevel Inverters?
PWM is used to:
- Control output voltage magnitude
- Reduce THD
- Improve waveform quality
- Balance capacitor voltages
🔹 2. Types of PWM Techniques
1️⃣ Level-Shifted PWM (LS-PWM)
Used mainly in NPC and FC inverters. Multiple carrier signals are vertically shifted.
Types:- PD (Phase Disposition)
- POD (Phase Opposition Disposition)
- APOD (Alternate Phase Opposition Disposition)
2️⃣ Phase-Shifted PWM (PS-PWM)
Used mainly in Cascaded H-Bridge (CHB). Carrier signals are phase shifted by:
Phase shift = 360° / number of carriers
Example: For 5-level → 4 carriers Phase shift = 360° / 4 = 90°3️⃣ Space Vector PWM (SVPWM)
Extension of conventional SVPWM to multilevel inverters. Provides better DC bus utilization and lower THD.
🔹 3. Comparison of PWM Methods
| PWM Method | Used For | Complexity | THD |
|---|---|---|---|
| Level-Shifted PWM | NPC, FC | Medium | Low |
| Phase-Shifted PWM | CHB | Simple | Low |
| SVPWM | All Types | High | Very Low |
🔹 4. Important GATE Concepts
✔ Phase shift in PS-PWM = 360° / carriers ✔ LS-PWM uses vertically shifted carriers ✔ SVPWM gives better DC bus utilization ✔ More levels → easier harmonic reduction ✔ Capacitor balancing important in FC
PWM Techniques Series – Shaktimatha Learning
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