Type 2 error amplifier for flyback cannot be done without knowledge of RHPZ?

[treez]

Hello,
On pages 212 to 217 of “Power supply cookbook” by Brown, it goes through a type 2 error amplifier feedback compensation scheme for a CCM flyback in current mode. It discusses the placement of the error amplifier's poles and zero, but does not do this with respect to the power stage RHPZ. Why is no mention made of the RHPZ? Surely an error amplifier pole and zero cannot be placed without first knowing where the RHPZ is? (what frequency its at). I mean, the high frequency pole of the type 2 error amp must be below the RHPZ frequency, surely? So how can the book be properly compensating this flyback without assessing the location of the RHPZ?

 

[mtwieg]

In general it's good to know the RHPZ frequency before designing a control loop, but if you follow conservative cookbook design procedures you will end up with the RHPZ at least 5-10 times the resonant frequency f0 of the power stage. And since typically you set the crossover frequency fc to be at least 2-3 times the resonant frequency, you should end up with the RHPZ safely beyond fc, and it shouldn't be a problem.

Even so a good engineer should check a design by plotting the loop response over all line/load conditions and ensuring the phase margin and crossover frequency is always acceptable.