403 - Neutral-Point-Clamped (NPC) 3-Level Inverter¶

Overview¶

The Neutral-Point-Clamped (NPC) inverter is a multilevel topology that produces three voltage levels per phase, reducing harmonic content and voltage stress on switching devices compared to conventional two-level inverters.

Difficulty: 3/3 (Advanced)

Estimated Time: 45-60 minutes

Learning Objectives¶

After completing this tutorial, you will be able to: - Understand the NPC 3-level inverter topology and switching states - Analyze neutral-point voltage balancing challenges - Design PWM strategies for multilevel converters - Compare performance with two-level inverters

Prerequisites¶

• Complete Tutorial 402: Three-Phase Inverter
• Understanding of PWM modulation techniques
• Familiarity with power semiconductor devices (IGBTs, diodes)

Circuit Description¶

NPC Topology¶

The NPC inverter uses four switches and two clamping diodes per phase leg to generate three voltage levels: +Vdc/2, 0, and -Vdc/2.

                    +Vdc/2
                       │
                   ┌───┴───┐
                   │  S1   │ IGBT with antiparallel diode
                   └───┬───┘
                       │
                   ┌───┴───┐
                   │  S2   │
                   └───┬───┘
                       ├──────── Phase Output (A, B, C)
               Dc1 ────┤
                       │
           0 (NP) ─────┼───────────────── Neutral Point (Capacitor Midpoint)
                       │
               Dc2 ────┤
                       ├────────
                   ┌───┴───┐
                   │  S3   │
                   └───┬───┘
                       │
                   ┌───┴───┐
                   │  S4   │
                   └───┬───┘
                       │
                    -Vdc/2

Switching States¶

Key Parameters¶

Theory¶

Advantages of NPC over Two-Level¶

1. Lower dv/dt: Output voltage steps are Vdc/2 instead of Vdc
2. Reduced THD: Better approximation of sinusoidal waveform
3. Lower switching losses: Devices switch at half the DC bus voltage
4. Common-mode voltage reduction: Smaller CM voltage steps

Challenges¶

1. Neutral-point voltage balancing: Unequal charge/discharge of DC capacitors
2. Increased component count: 4 switches + 2 diodes per phase (vs 2 switches)
3. Complex modulation: Requires space vector or carrier-based PWM

Neutral-Point Current¶

The neutral-point current determines capacitor voltage balance:

i_NP = i_a * f_a(d) + i_b * f_b(d) + i_c * f_c(d)

Where f(d) is a function of the switching state (0 for P/N states, ±1 for O state).

PWM Strategies¶

1. Carrier-Based PWM (PD-PWM):
2. Two triangular carriers, phase-disposed
3. Upper carrier: 0 to 1

4. Lower carrier: -1 to 0

5. Space Vector PWM (SVPWM):

6. 27 switching states (3^3)
7. Redundant states for NP balancing

Building the Circuit in GeckoCIRCUITS¶

Step 1: Create DC Link¶

1. Add two DC voltage sources in series (or capacitors charged by a rectifier)
2. Connect their junction to create the neutral point (NP)
3. Each source/capacitor = Vdc/2 (e.g., 400V each for 800V bus)

Step 2: Build One Phase Leg¶

1. Add 4 IGBTs (or ideal switches with antiparallel diodes) in series
2. Add 2 clamping diodes:
3. Dc1: Anode to midpoint between S1-S2, Cathode to NP
4. Dc2: Anode to NP, Cathode to midpoint between S3-S4
5. Phase output is the junction between S2 and S3

Step 3: Replicate for Three Phases¶

1. Copy the phase leg structure for phases B and C
2. Connect all to the same DC bus and neutral point
3. Connect phase outputs to a balanced 3-phase load (R-L or motor)

Step 4: Add PWM Modulation¶

1. Create three sinusoidal reference signals (120° phase shift)
2. Add two triangular carrier signals (PD-PWM):
3. Carrier 1: offset +0.5, amplitude 0.5, frequency = fsw
4. Carrier 2: offset -0.5, amplitude 0.5, frequency = fsw
5. Use comparators:
6. S1: ref > carrier1
7. S2: ref > carrier2
8. S3: ref < carrier2 (inverted logic)
9. S4: ref < carrier1 (inverted logic)

Step 5: Configure Simulation¶

• Simulation time: 60-100 ms (3-5 fundamental cycles)
• Time step: 0.1-1 us (depends on fsw)
• Solver: Trapezoidal

Expected Results¶

Waveforms to Observe¶

1. Phase-to-Neutral Voltage: Three distinct levels (+Vdc/2, 0, -Vdc/2)
2. Line-to-Line Voltage: Five levels (±Vdc, ±Vdc/2, 0)
3. Neutral Point Voltage: Should remain at Vdc/2 (balanced)
4. Output Current: Near-sinusoidal with load inductance

Performance Metrics¶

Exercises¶

Exercise 1: Basic Operation¶

1. Build a single-phase NPC leg with resistive load
2. Apply a 50% duty cycle to observe the three voltage levels
3. Measure the output voltage and verify the switching states

Exercise 2: PWM Modulation¶

1. Implement PD-PWM with m = 0.8
2. Compare output THD at fsw = 5 kHz vs 10 kHz
3. Question: What is the dominant harmonic frequency?

Exercise 3: Neutral Point Balancing¶

1. Connect unbalanced loads to phases A and B
2. Observe neutral point voltage drift
3. Challenge: Implement a simple balancing controller using redundant switching states

Exercise 4: Comparison Study¶

1. Build an equivalent two-level inverter (same power rating)
2. Compare:
3. Output voltage THD
4. Semiconductor losses
5. Common-mode voltage
6. Document your findings

Common Issues and Troubleshooting¶

Related Examples¶

• Two-Level Three-Phase Inverter - Compare with simpler topology
• Thermal Analysis - Add loss models
• Matrix Converters - Alternative AC-AC topology

References¶

1. Rodriguez, J., Lai, J.S., Peng, F.Z. "Multilevel Inverters: A Survey of Topologies, Controls, and Applications." IEEE Trans. Ind. Electronics, 2002.
2. Nabae, A., Takahashi, I., Akagi, H. "A New Neutral-Point-Clamped PWM Inverter." IEEE Trans. Ind. Appl., 1981.
3. Holmes, D.G., Lipo, T.A. "Pulse Width Modulation for Power Converters." Wiley-IEEE Press, 2003.

Circuit Files¶

Note: This tutorial folder will contain example circuits once created in GeckoCIRCUITS. To contribute a circuit, follow the documentation and save as: - npc_single_phase.ipes - Single-phase NPC leg - npc_three_phase.ipes - Complete three-phase NPC inverter - npc_three_phase_balanced.ipes - With NP voltage balancing control

Tutorial Version: 1.0 Last updated: 2026-02 Compatible with GeckoCIRCUITS v1.0+