Fyback-Continuous mode Right Half Splane Zero-Meaning??

[themaccabee]

Dear all,

I ve read that a right half S plane zero gets introduced in the system while the flyback converter operates in CCM(Continuous conduction mode).How a zero gets introduced in this configuration.IS there any hardware change in CCM.? Can some body help me in understanding this???


Regards
Stephen.

 

[Jay Carp]

The zero gets introduced into the system when computing the control-to-output transfer function in the small signal model:

Gvd(s) = v(s)/d(s)

See pages 294-297 in Fundamentals of Power Electronics by Robert Erickson and Maksimovic for the details. They derive it for a buck-boost, which is basically a flyback with a transformer.

As far as hardware changes go, I am not 100% sure, but your controller will most likely have more complicated compensation than if you were to operate your flyback in DCM. The only problem with that is that DCM flybacks aren't really good for output powers over 60W or so.

 

[permute]

to see the zero, as well as another effect, it is useful to plot the ideal current flowing in the inductor, as well as the output current.

this will be a triangle for the current flow in the inductor, and the output current will be the current flow during the discharge phase.

To increase output, the output current must increase. to increase output current, the peak current in the inductor must increase. to do this, either the input voltage must change, or the charging time must change.

If the charging time increases, the peak current will increase. but now there is a shorter discharge time. The output current for this cycle is lower. in future cycles, the output current will be higher.

The key point is that your controller cannot respond to this initial decline in output current.