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Lec 25: Optocouplers based Gate Drivers - II
NPTEL IIT Guwahati
Overview
This video compares two PWM modulation strategies for H-bridge converters: bipolar and unipolar. It details their differences in output voltage waveforms, switching transitions, insulation requirements, electromagnetic interference (EMI), effective switching frequency, and current ripple. The video then demonstrates simulations of both strategies using LTspice, illustrating the theoretical concepts with practical waveform examples and emphasizing the importance of choosing the right modulation strategy to influence converter component selection and overall design.
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Chapters
- Bipolar PWM switches output voltage between +Vdc and -Vdc, resulting in transitions of 2Vdc.
- Unipolar PWM switches output voltage between 0 and +Vdc, or 0 and -Vdc, with transitions of Vdc.
- Unipolar PWM offers advantages in insulation requirements and potentially lower electromagnetic interference (EMI) due to smaller voltage transitions.
- Unipolar PWM effectively doubles the switching frequency component in the output voltage waveform compared to bipolar PWM, even though the actual device switching frequency remains the same.
Understanding these fundamental differences helps in selecting the appropriate modulation strategy based on application requirements like insulation stress and EMI concerns.
The output voltage for bipolar PWM swings directly from +Vdc to -Vdc, while unipolar PWM swings from 0 to +Vdc and then 0 to -Vdc.
- Unipolar PWM results in lower current ripple compared to bipolar PWM for the same inductive load.
- Lower current ripple in unipolar PWM reduces the need for larger or more complex filtering components (inductors and capacitors).
- The switching frequency component in the DC current (Idc) is higher for bipolar PWM (Fs) than for unipolar PWM (2Fs), potentially influencing capacitor selection.
The choice of modulation strategy directly impacts the filtering requirements, affecting the size, cost, and efficiency of the power converter.
The current waveform in bipolar PWM shows a greater amount of switching frequency ripple than the current waveform in unipolar PWM.
- A simulation using LTspice demonstrates an H-bridge converter with MOSFETs and an inductive load.
- The simulation shows the triangular carrier waveform, the reference (modulating) waveform, and the resulting leg voltages (Van, Vbn).
- The output voltage waveform (VAB) for bipolar PWM is shown to switch between +100V and -100V.
- The load current waveform follows the sinusoidal reference but contains switching frequency components.
Visualizing the simulation helps confirm theoretical predictions and understand how modulation strategies translate into actual electrical waveforms.
The simulated VAB output voltage waveform directly switches between +100V and -100V, illustrating the bipolar switching behavior.
- Unipolar PWM simulation uses separate files for generating gate pulses based on comparisons between reference and carrier signals for each leg.
- The simulation shows leg voltages (Van, Vbn) and the resulting output voltage (VAB).
- For positive reference voltages, VAB switches between 0 and +Vdc (e.g., +40V).
- For negative reference voltages, VAB switches between 0 and -Vdc (e.g., -40V).
- The load current waveform in unipolar PWM also exhibits ripple, but less than in the bipolar PWM simulation.
This simulation visually confirms the unipolar switching characteristic (0 to Vdc or 0 to -Vdc) and its effect on the output voltage and current.
The simulated output voltage for unipolar PWM is observed to switch between 0 and +40V when the reference is positive, and between 0 and -40V when the reference is negative.
- Different modulation strategies produce different output waveforms.
- These waveform differences necessitate changes in the selection of converter components like inductors, capacitors, and switches.
- It is crucial to select and fix the modulation strategy before proceeding with component selection and detailed converter design.
- Simulation is a valuable tool for engineers to test and validate converter designs before hardware implementation.
Choosing the modulation strategy early in the design process ensures that the subsequent component selection and overall converter architecture are optimized for the intended application.
The choice between bipolar and unipolar PWM affects the required filter inductor size due to differences in current ripple.
Key takeaways
- Unipolar PWM offers advantages over bipolar PWM in terms of reduced voltage switching stress (Vdc vs 2Vdc), potentially lower EMI, and reduced current ripple, leading to smaller filter requirements.
- The effective switching frequency seen in the output voltage waveform is doubled in unipolar PWM compared to bipolar PWM, even with the same device switching frequency.
- Simulation tools like LTspice are essential for power electronic engineers to visualize and verify the behavior of different modulation strategies and their impact on converter performance.
- The selection of a modulation strategy is a foundational design decision that dictates the requirements for other converter components, such as filters and switches.
- Engineers must compare modulation strategies based on application-specific needs before finalizing the converter design and selecting components.
Key terms
H-bridge converterPWM (Pulse Width Modulation)Bipolar PWMUnipolar PWMVdcSwitching transitionInsulation requirementElectromagnetic Interference (EMI)Switching frequencyCurrent rippleLTspiceCarrier waveformReference waveformLeg voltageOutput voltage waveformLoad current waveform
Test your understanding
- What is the primary difference in output voltage transitions between bipolar and unipolar PWM, and why does this matter for insulation?
- How does unipolar PWM affect the effective switching frequency observed in the output voltage waveform compared to bipolar PWM?
- Explain why unipolar PWM generally requires less filtering than bipolar PWM.
- What role does simulation play in the design process of power electronic converters using different modulation strategies?
- Why is it important to choose a modulation strategy before selecting specific converter components?