Formula for Right Half Plane Zero in a Boost Converter 4

[DHambley]

Lloyd Dixon gave us this equation long ago for the corner frequency of the RHP zero for a Boost converter:

F = (1/2pi)* R/L * (1-D)^2/D
= (1/2pi)* R/L * (Vi/Vo)^2/D

Other app notes from TI give this equation:

F = (1/2pi)* R/L * (1-D)^2
= (1/2pi)* R/L * (Vi/Vo)^2
There's no D (duty cyle) in this denominator!

National Semi gives this formula:

F = (1/2pi)* 1/L * (Vin*D)/Iin

Well, these three just don't match! Any thoughts on who's right? I've always used the first formula above.

DH

 

[DHambley]

Well, looks like this thread took some tangents to different subjects. I see I confused some people initially by stating "transfer function" when I was actually analyzing the closed loop equation with V-in fixed at a constant value. To bad, this seems to have induced rude remarks. The original problem wasn't the transfer function from Vin to Vout. (Of COURSE these two vary by '1/D'. That's obvious.) Sorry for the confusion. It was the closed loop response w.r.t. pulse width which was my issue. (When we're talking about Vin being a contant value, we aren't talking about the small-signal transfer function from input to output.) For this circuit, V-in is a slowly changing value, the frequency of which is much lower than the RHPZ, so again, it's not input-to-output transfer function which was the original issue.
By the way, the circuit has been in production for 2 months now. It works fine from 5V low-line to the 60V transient. I had found and verified the correct equations shortly after the first post. The closed loop tests show the RHPZ where the calculations say it should be.

 

[cabraham]

No problem. I'm glad we agree and that your closed loop tests affirm the correct RHPZ frequency value of (R*(1-D)^2)/L, for the boost topology. We did go off on a tangent but covered some interesting stuff.

This stuff is good to go over and review. None of us really know as much as we think we do. Twice a week in grad school class I am reminded of my limitations. Peer review and re-examining our own positions can only help us. If I'm wrong, I benefit by being corrected. Even when I'm right, reviewing my steps and thought process is a good sanity check. BR.

Claude