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# Power stage transfer function for flyback

#### cupoftea

Hi,
Why is it that the page 235 (point 1) of Basso, as attached, says that for the BuckBoost equation 2A-22 (pg 232) to be used for a flyback, the c(out) should be referred to the primary along with the load.....But why does it make no mention that vout will be changed?
(ie changed to what the equivalent buckboost would give)

The equations given in the book are for a Buckboost

As you know, to do feedback loop on flyback, the flyback must first be converted to the "equivalent" buckboost converter.

#### Attachments

• 235.pdf
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• 232.pdf
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Hi, on a similar note...
The Power stage transfer function for a buckboost in CCM is given on page 233 as equn 2A-24.
This contains a zero wz3, for which calculation requires knowing what is the value of "Cs", which , as page 233 says, is derived from the PWM Switch model.

Do you know the whereabouts of this "Cs" calculation?

If this cannot be calculated, then equn 2A-22 on page 232 will need to be used instead.

#### Attachments

• 233.pdf
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The flyback and buck-boost resemble each other because their core generates all power sent to the output stage. Specifically, the collapsing flux field provides current. This current goes through the diode to the reservoir capacitor. The capacitor charges to a greater volt level which is hard to predict. Then the load drains it to a level which is hard to predict.

I suppose that could be why the author claims similar equations for both flyback and buck-boost?

It's common for a flyback to need a snubber on the primary. But the buck-boost is not known as needing a snubber.

I don't know how to answer about a difference between DCM and CCM.

...thanks, they do say on the page that Cs is in an equation which is ">Fsw/2"....which in fact gives you a range in which that Cs exists......and if you adjust the Cs value within that range, then the overall bode plot doesnt really change that much, givien conservative compensation.
But would be good to know the equation for "Cs", for the sake of completeness.

I suppose that could be why the author claims similar equations for both flyback and buck-boost?
Yes indeed you have hit the nail on the head......in fact, to do a feedback loop equation on a flyback, the flyback must first be converted into its "equivalent buckboost" converter. This equivalent will have the same duty cycle, the same vin, the saem peak current in the fet, and the same max power, but its vout will obviously be different......so too will its Cout, and obviously ESR.
For some reason, no feedback loop text anywhere actually makes mention of this need for conversion.