Expression for small signal controller transfer function for "type 3" compensator.

Expression for small signal controller transfer function for "type 3" compensator.

Please help in converting the given expression for a "type 2" compensator to a "type 3" compensator in a flyback SMPS. This concerns the application note AN57 by Power Integrations. (linked below)


Referring to AN57… equation 25, page 14 shows the expression for the small signal transfer function of the controller in a topswitch based flyback SMPS. This expression is a “type 2” compensator and corresponds to the schematic of fig 23, page 14.

If this compensator is converted to a “type 3” compensator, as in fig 26, page 15, then how should equation 25 be adjusted to produce the correct small signal transfer function? (note that converting to a “type 3” compensator just involves adding RF4 and CF2 across RF3).

AN57:
https://ac-dc.power.com/sites/default/files/product-docs/an57.pdf

Re: Expression for small signal controller transfer function for "type 3" compensator

Strictly speaking, that isn't really a type III compensator, in my opinion. It takes feedback from two different "output" nodes, and therefore can be described with a different transfer function for each. Putting the feedback network in parallel with RF2 should yield a type III compensator.

But this is a somewhat standard compensation technique, I will check my reference books to see if I can find an analytical solution for it.

Re: Expression for small signal controller transfer function for "type 3" compensator

Equation (3-66), page 292 of the basso book gives the type 3 expression which comes from adding the RF4 resistor and CF2 capacitor as in the power integrations app note (AN57).
Basso actually states that putting an RC network across the upper divider resistor only works if the optoLED resistor (RF3) is fed from a fixed bias voltage and not the vout of the SMPS.
Interestingly, on page 297, Basso does indeed say that the TL431/opto configuration does not lend itself well to a type 3 compensator. This , he says, is because the optoLED resistor is involved in both the gain setting as well as forming a pole zero pair. Basso states that sometimes an RF3 resistor (optoLED resistor) which gives the correct gain at the crossover frequency, and sufficient optoled current at light load, sometimes results in an impossible solution.
It is in these situations that Basso says that the classic type 3 opamp solution (inferred by Mtwieg) should then be used instead.

Re: Expression for small signal controller transfer function for "type 3" compensator

So, your question really is..?

Re: Expression for small signal controller transfer function for "type 3" compensator

the question is still, ..what is the expression for a type 3 compensator for the topswitch as shown in AN57 ? (link to AN57 is in top post).

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Basso's book pages shown here don't apply to the topswitch