Voltage mode not with type 2 compensator

[cupoftea]

Hi,
Do you agree that a voltage mode converter should only be compensated with a type 2 compensator if the crossover frequency is less than the power stage LC resonance frequency? Else you just dont have enough zeros to use to get enough phase lift, and your phase margin will be inadequate.

 

[mtwieg]

Hi,
Do you agree that a voltage mode converter should only be compensated with a type 2 compensator if the crossover frequency is less than the power stage LC resonance frequency? Else you just dont have enough zeros to use to get enough phase lift, and your phase margin will be inadequate.

For voltage mode control this is generally a good rule of thumb. Unless the converter is always operating in DCM, in which case you can have a higher fc.

 

[cupoftea]

Thanks, the problem comes when you have an offline flyback in ccm at 90-120vac mains....and it uses an older Power integrations control chip, such as TOPswitch HX, and because your transformer has too low NP/NS, the Power integrations software wont do it for it....so you then have no means of knowing what is the error amplifier and modulator transfer function, and you cant calc the xover frequency.

--- Updated Aug 9, 2023 ---

The attached shows the problem with voltage mode in ccm, when you dont get given the modulator transfer function from the chipmaker.......you can go unstable.....and whereas with current mode you can get to stability , usually by just reducing the loop gain...this is not necessarily so in voltage mode/ccm......with voltage mode/ccm, you have to have calculated your crossover frequency....otherwise, goodness help us.!

--- Updated Aug 9, 2023 ---

..We mess with the double poles of voltage mode/ccm, at our extreme peril!

--- Updated Aug 9, 2023 ---

..after all that, forgot the attachment...here it is...