Design of Power Electronic Converters: Bipolar PWM

Introduction

• Topic: Bipolar PWM in H-Bridge Converters
• Previous Lecture: Edge bridge converters and pulse width modulations (PWM)

Overview of Bipolar PWM

• Uses two switching combinations: TA+ & TB- and TA- & TB+, operated in pairs.
• Switching Scenarios:
  • VO = VDC: TA+ & TB- ON
  • VO = -VDC: TA- & TB+ ON
• DC Bus Current, IDC:
  • +IO when TA+ & TB- ON
  • -IO when TA- & TB+ ON

Waveforms in Bipolar PWM

• Carrier Waveform: Triangular with switching frequency Fs
• Reference Waveform (Voref): Used for comparison
• Logic:
  • Voref > Vcarrier: TA+ & TB- ON
  • Vcarrier > Voref: TA- & TB+ ON
• Output Voltage (VO): Switches between +VDC and -VDC continuously
• Leg Voltages (VAN & VBN):
  • VAN: +VDC (upper switch ON), 0 (lower switch ON)
  • VBN: Similar pattern for leg B
• Load Current: Assumed positive; depends on load and output voltage

DC to AC Conversion with Sinusoidal Reference

• Voref: Sinusoidal
• Carrier Waveform: Range 0 to 1
• Modulating Strategy:
  • Output voltage switches between +VDC and -VDC
  • VO average resembles Voref sinusoid

Modulating Waveform Equation

• For one switching period TS:
  • VO is averaged over TS
  • Equation: VO_bar = VDC * (2D - 1) where D is duty ratio
  • Modulating Waveform: Voref = (VO_hat / 2VDC) + 0.5*

Modulation Index (MA)

• Defined as VO_hat / VDC
• Can use peak or RMS value but peak preferred in this context
• Maximum Modulation Index for Bipolar PWM: 1
• Impact: Determines minimum required VDC for desired output

Capacitor Current Analysis

• DC Bus Current (IDC):
  • Average: IDC_bar = VDC * (2D - 1)
  • Ignoring switching frequency components
• Equations:
  • VDC * IDC_bar = VO_bar * IO_bar
  • Results in a second harmonic component in IDC
• Capacitance Requirement: Larger due to second harmonic (e.g., for 50Hz output, 100Hz component in capacitor current)*

Current Waveforms

• Instantaneous Output Voltage: VO from +VDC to -VDC
• Load Current: Increases when VO is positive, decreases when negative
• DC Bus Current (IDC): Includes DC, second harmonic, and switching frequency components
• Visual Representation: Through oscilloscopic waveforms

Key Points

• Choosing modulation strategy involves analyzing different waveforms and modulating waveforms
• Determine maximum modulation index for minimum VDC requirements
• Observe and analyze capacitor current ripples to determine capacitance size

Thank you!