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# A transfer function block that has only zeros

## A transfer function block that has only zeros

(OP)
I am trying to model a current mode dc/dc converter as shown by Dr Ray Ridley, which requires a block that has a double zero and no poles. (http://www.ridleyengineering.com/downloads/curr.pdf page 9)

The block is H=1+s/(wn*Q)+s^2/(wn^2) where wn and Q are constants and s is the laplace variable. How can I make this block in simulink? The built in functions won't allow more zeros than poles.

### RE: A transfer function block that has only zeros

Your "H" is a second order system. Why not convert directly to state space form, and then model it with an SS block in simulink.
just a thought,

Fe

### RE: A transfer function block that has only zeros

You have H = [1+s/(wn*Q)+s^2/(wn^2)]

You could model the inverse function
Hinv=1/[1+s/(wn*Q)+s^2/(wn^2)]

Then plug Hinv into where it says H in the negative feedback loop here:
http://en.wikipedia.org/wiki/Closed-loop_transfer_function

Now you have transfer function:
Y/X = G / [1+G*Hinv]

Make G a really big scalar gain constant so that the 1<< G*Hinv and the denominaotr becomes approximately G*Hinv and
Y/X ~ G / [G*Hinv] = H

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