Why Does the Inverter Waveform Not Align with Theoretical Predictions?

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Discussion Overview

The discussion centers around the discrepancies between the observed inverter waveform and theoretical predictions, particularly in the context of load characteristics and current behavior. Participants explore the implications of inductance in the load, the behavior of diodes during switching, and the nature of sinusoidal current in relation to voltage changes.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant concludes that the load has some inductance and discusses the conduction through anti-parallel body diodes when switches are off.
  • Another participant suggests that the sinusoidal nature of the load current causes polarity changes at specific points, leading to voltage changes despite the current being zero at those instances.
  • A third participant agrees with the sinusoidal current reasoning and posits that the voltage across the capacitor at certain points is responsible for diode switching, indicating a simpler understanding of the problem than initially thought.
  • A later reply introduces the concept of discontinuous inductor current in SMPS designs, noting that this can lead to floating switch voltages, although there is a correction regarding the nature of inductor current.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of the inverter waveform, particularly regarding the implications of load characteristics and current behavior. There is no consensus on the correctness of the initial conclusions drawn about the waveform behavior.

Contextual Notes

Participants acknowledge the complexity of the problem, including the assumptions about load types (RLC) and the behavior of current and voltage at specific instances. Some statements reflect uncertainty about the precise nature of the current and its effects on voltage changes.

Who May Find This Useful

Individuals interested in inverter design, SMPS systems, and the behavior of electrical components in relation to sinusoidal currents may find this discussion relevant.

cnh1995
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Homework Statement
Find the pole voltage waveform Vao.
Relevant Equations
Waveform plotting based on KCL and KVL.
20201108_141805.png

The answer given is d).

Seeing the problem statement, it can be concluded that the load has some inductance, and when the switches S1, S2, S3 and S4 are off, conduction takes place through the anti-parallel body diodes (freewheeling action).

Seeing the triggering pulse waveforms, S1-S4 ON, Vao= +Vdc/2
S1-S4 OFF, D2-D3 ON, Vao = -Vdc/2.
This is only seen in d). So I can eliminate a), b) and c).

But I find d) incorrect as well.
The load current at "wt=pi" is 0 as per the problem statement. This means neither the switches nor the diodes are conducting at wt=pi. So how can Vao jump from -Vdc/2 to +Vdc/2 at wt= pi?
Shouldn't it be floating at some unknown value?

What am I missing here?
Any help is appreciated.
 
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Since the load current is sinusoidal it is changing polarity when it crosses zero at pi. This will cause the commutating diode pairs to switch, that is why the voltage changes. I suppose at the precise instant the current changes you could argue it's floating, but that's virtually zero duration.
 
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DaveE said:
Since the load current is sinusoidal it is changing polarity when it crosses zero at pi.
Yes, I agree with your reasoning.
Looks like the load is RLC type, and the voltage across C at wt=pi is responsible for diode pair switching.
I believe I was overthinking; it wasn't necessary to know the type of load and its components . Only 'the current is sinuoidal' part is enough to answer this question.

Thanks a lot! I have to stop overthinking.o0)
 
In SMPS designs where the inductor current is discontinuous (i.e. the energy stored is all used in each cycle), then you do see the switch voltages floating at times. In practice this almost always looks like part of a sinusoidal resonance of parasitic L & C. So your intuition wasn't too far off.

edit: Oops, inductor current is never discontinuous. I should have said load, or input current.
 
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