What's wrong with coupled inductor SEPIC converters?

Orson Cart said:

Unfortunately - In the real world - when you build and run up a Sepic or a Cuk, you find that the coupled version is harder to stabilise than the un-coupled, if you write the complete transfer function for the power stage (now with mutual inductances, and magnetising inductances in the transformer/choke), and not a simplified version, you find there are more terms in the transfer function and the net effect is more phase delay making stable control harder to get - esp at light loads/no load where the RHP's are lowest in freq.

Click to expand...

Okay I'm going to have to ask you to back this up with some math or citation. Here's my state space model for a coupled SEPIC in CCM. Its transfer function is a mess, which requires MATLAB to compute, but I can verify it still gives four poles and three RHP zeros, independent of K.
\[
x=\begin{bmatrix}
i_{L1} \\
i_{L2} \\
v_{C1} \\
v_{C2} \\
\end{bmatrix}
\\

u=\begin{bmatrix}
Vg \\
d \\
\end{bmatrix}
\\



A=\begin{bmatrix}
0 & 0 & \frac{-Dk-\bar{D}}{(1-k^2)L_1} & \frac{-\bar{D}(1-k)}{(1-k^2)L_1} \\
0 & 0 & \frac{D+\bar{D}k}{(1-k^2)L_2} & \frac{-\bar{D}(1-k)}{(1-k^2)L_2} \\
\bar{D}/C_1 & -D/C_1 & 0 & 0 \\
\bar{D}/C_2 & \bar{D}/C_2 & 0 & \frac{-1}{R_L C_2} \\
\end{bmatrix}
\\
B=\begin{bmatrix}
\frac{1}{(1-k^2)L_1} & \frac{(V_{C1}+V_{C2})(1-k)}{(1-k^2)L_1} \\
\frac{-k}{(1-k^2)L_2} & \frac{(V_{C1}+V_{C2})(1-k)}{(1-k^2)L_2} \\
0 & \frac{-I_{L1}-I_{L2}}{C_1} \\
0 & \frac{-I_{L1}-I_{L2}}{C_2} \\
\end{bmatrix}
\\
C=\begin{bmatrix}
0 & 0 & 0 & 1 \\

\end{bmatrix}
\\
D=\begin{bmatrix}
0 & 0 \\

\end{bmatrix}
\]

I'm glad you found 3 RHP zeroes, most people can only find two, even in good quality papers such as:

http://www.ing.unibs.it/~richelli/matindustriale/sepic7.pdf

Regards, Orson.