Flyback in voltage mode or current mode, and CCM to DCM

[treez]

If one is designing a Current mode CCM flyback, then when the load gets lighter, such that it goes into DCM, then this DCM is always stable in current mode. (as long as it had been stable in CCM)
In a CCM Voltage Mode Flyback, lightening the load such that it goes into DCM can actually cause instability, even if it had been stable in CCM….is this true?

 

[Easy peasy]

Depends on how crappy your control electronics is, for CCM you usually need a very slow damped control loop to counter the RHP zero, this should usually be fine as the unit enters DCM as the power stage then has less phase lag...

 

[mtwieg]

I'm having trouble thinking of a way for the voltage mode one to become unstable when shifting to DCM. The RHP zero disappears, and your conjugate pole pair becomes a single real pole, so your phase should get a significant boost. The gain will also increase, but I'd be surprised if it would be enough to decrease your stability margins. But I can't say it's impossible either. Might be a publishable study if done properly.

 

[treez]

I know what you mean when you say gain increases, because as you know, DCM comes with lightening load, and gain increases with lighter load...however, as you know, generally speaking, DCM has less gain than CCM (eg a flyback in dcm has generally less gain than one done for the same load in ccm)

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App Note AN57 on fig. 13, page 9 shows that a voltage mode flyback that is stable in CCM, can go unstable in DCM.
(the plot shows vout when load is stepped from high load in CCM, to lighter load where its in DCM)..

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

So the golden question is……Is it possible for a Current Mode Flyback that was stable in CCM , to become unstable when it gets lighter loaded and goes into DCM?….i say no, because a DCM current mode Flyback SMPS is inherently stable…do you agree?

This clearly means that Voltage mode is disadvantageous for Flybacks, when compared to current mode..you agree?

 

[treez]

Pg 10, (halfway down on Right hand side) confirms that for a current mode flyback...if you get it stable in max. load, min vin, then it will be stable at all other conditions of line and load, including when it goes into DCM...
https://www.fairchildsemi.com/application-notes/AN/AN-4137.pdf

 

[Easy peasy]

Not much help for a volt mode system though... at light load you have an un-damped output cap fed from an inductive source, this is always a problem for any volt mode system, so unless you add appropriate feed-forward at the volt sensing resistors, compensate the opto coupler, and other things related to the particular control IC, and use the minimum output C you can get away with, the response of your volt loop will have to be slow and damped if you want stable operation under DCM at light loads and high mains...

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I note in app note 4137 above - they don't show any transient performance of the power supply, I can guess why...

 

[treez]

I note in app note 4137 above - they don't show any transient performance of the power supply, I can guess why...

thanks, I will have to guess at what you are referring though, as I am not sure. AN4137 concerns a current mode flyback chip.

 

[Easy peasy]

your whole thread is about current mode flyback... vs voltage mode flyback...

 

[treez]

yes, and I always took it that faster transient response is easier to achieve in current mode as opposed to voltage mode.