Hi,

I did the attached TOPswitch-HX Flyback design in PI Expert.

I then used the equations given by Power Integrations in AN-57 to calculate the crossover frequency, gain and phase margins.

However, the equations given in AN-57 give nonsense Bode Plots (as attached in Excel). The AN-57 feedback equations give a crossover frequency above 100kHz, (the switching frequency is only 66khz)

AN-57:

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjfwabJmJGAAxUhV0EAHWp1BLkQFnoECBsQAQ&url=https%3A%2F%2Fwww.power.co…

Do you know how to calculate the feedback loop for this TOPswitch-HX design?

It is Offline Universal flyback 24vout and 800mA out.

What value would you take for the TL431 open loop gain?