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Bilevel and trilevel PWM

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Almas Sahar

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Pulse width modulation is varying the on and off time of Duty Cycle so that we can achieve our required average voltage at output,it is a simple understanding about this, can someone please elaborate what is Bilevel and Trilevel PWM?
 

I wonder if this is similar to 3-level, 5-level, etc.? It creates a PWM sinewave, by switching rapidly between, say,
the 0 & 33% level for a while,
then between 33 & 67% level,
and finally between 67 & 100% level.

Then back down in reverse order.
It creates a smoother sine shape than plain switching between On and Off.
 

right, but what is the technique of achieving Tri-Level or 5 level PWM?
 

I can't see any advantage in it.

Brad, if that was done, the technique woould be just as complicated as generating a continuous sine wave so I can't see any advantage in doing it.

There are three and five level 'modified sine' circuits that are much better than simple square wave switchers but they don't use PWM.

Brian.
 

Hi,

First, I really don't know the answer.

I guess it is like a fullbridge.
In bilevel mode it's output can be positive or negative only. PWM it and you can create (filtered) AC sine output voltage...

In trilevel i guess you can have three states: negative, zero and positive.
This may ease filter design and/or improve overall efficiency.

Klaus
 

what is Bilevel and Trilevel PWM?
Click to expand...
Bilevel and trilevel PWM are different modulation schemes that can be generated with a regular full bridge.

Bilevel (2-level also called bipolar PWM) switches either +VDC or -VDC to the output.
Trilevel (3-level, also called unipolar PWM) uses a third state with zero output.

Example:

+0.5 VDC is outputted in 2-level PWM by setting the duty cycle to 0.75.
In 3-level PWM, +0.5 VDC is generated by switching between +VDC and 0 with 0.5 duty cycle.

The advantage of 3-level PWM is lower ripple voltage (respectively lower ripple current with same output inductor). Also doubled ripple frequency with same number of switching events.
 


I imagine it involves several mosfets in parallel, switched so that some are turned on at low amplitude of the sine shape, others turned on at greater amplitude.
Click to expand...

As far as we are discussing voltage driven inverters (= inverters utilizing a DC bus in contrast to less frequently implemented current driven inverters) multi-level involves a series connection of multiple output stages with separate DC bus. The 3-level inverter gets along with a single DC bus because it can sum up the output voltage of left and right half bridge to generate the total output. Starting with 5-level, multiple stacked or isolated DC supplies are needed.
 

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