Re: PWM in a switching power supply

BradBrigade wrote: 
John Popelish wrote:


If the transformer is a voltage output (produces some ratio of the
primary voltage when the switches are on) and zero the rest of the
time), then, yes, the peak output voltage is essentially independent
of the duty cycle.  but those kind of transformers also require an
additional LC filter that outputs a voltage about equal to the average
input voltage, not the peak.  Holding the peak voltage for a smaller
part of the cycle lowers the average voltage.



OK, I got it, the output is filtered to create a stable voltage that is
the average of the duty cycle.  But now I'm wondering, what is the
purpose of the transformer?  If you want to convert 100V to 10V, why
not filter the output straight from a PWM with a 10% duty cycle?
What's the difference?

Energy is passing from transformer to filter only during the "on" part of the duty cycle, The larger the % on time, the lower peak energy that must pass during the peak to satisfy the average energy flow.

You could also move an automobile by setting sticks of dynamite off behind it a small percentage of the time, but the peak forces would have to be pretty high to make up for the average force needed to keep the car moving. The windings in the transformer and the switch on the primary side are also affected by the peak verses average energy flow. For this reason, one normally wants to have the duty cycle just hit 100% at full output load and minimum input voltage, to keep the minimum duty cycle under other conditions as high as possible.

Good question.
.

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