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Figure 1 is where a 10v square wave (red) is driving a filter and the output of the
filter is shown as the blue wave.
Figure 2 is what happens when we increase the duty cycle of the wave from 50
percent to 70 percent. The sawtooth ramps up.
Figure 3 is what happens some longer time after we change the duty cycle to
70 percent and keep in that way. The sawtooth eventually ramps up and
stays at that average level.
If we change the PWM duty cycle again, the sawtooth will ramp either up or
down depending on the new duty cycle.
The square wave has a duty cycle of 50 percent, meaning half the time it is
high and the other half of the time it is low.
The 70 percent duty cycle wave is high for 70 percent of the time and low
for 30 percent of the time.
Note the time scale for Figure 2 is longer than for the other two figures in order
to show the way the sawtooth ramps up and then stabilizes.
Also note that if we decreased the PWM duty cycle back to 50 percent the output
(blue) sawtooth would decrease back to around the 5v level.
The way we control the average output voltage is by changing the driving waves
duty cycle, which is called pulse width modulation. There are other ways to do
this same thing other than PWM, but this is called pulse WIDTH modulation because
we vary the *WIDTH* of the high pulse while keeping the total period time constant.
Take the time to examine the relationship of the pulse width to the output voltage.
We vary the pulse width in order to be able to change the output voltage. This
allows us to use a switching waveform to control an analog voltage and is often
much more efficient in power converters that way.