Question

We are trying to model a DC/DC buck converter using matlab’s ode23 solver. When we try to run our code the following errors come up:

??? Error using ==> odearguments at 91
The last entry in tspan must be different
from the first entry.

Error in ==> ode23 at 171
[neq, tspan, ntspan, next, t0, tfinal,
tdir, y0, f0, odeArgs, odeFcn, ...

Error in ==> buck2 at 13
      [t,x] = ode23(@event1,[t0 tf],x0);

When we take out the code that modifies the initial condition array the code runs without errors but does not produce the expected result.

This is our code:

function buck2
close all
clear all
clc

t0 = 0; 
tf = 1;
x0 = [0 0];  % Initial conditions

for i = 1:10
    if (1 <= i <= 4),
      [t,x] = ode23(@event1,[t0 tf],x0);
      nt = length(t);          
    else
      [t,x] = ode23(@event2,[t0 tf],x0);
      nt = length(t);   
    end   
    t0 = t(nt);
    x0 = x(nt);
end

plot(t,x)

function xdot = event1(t,x)
L = (12.12e-6);
C = (19.5e-6);
RC = (2.5*19.5e-6);
xdot = [(24/L) - (x(2)/L); (x(1)/C) - (x(2)/RC)];

function xdot = event2(t,x)
L = (12.12e-6);
C = (19.5e-6);
RC = (2.5*19.5e-6);
xdot = [-(x(2)/L); (x(1)/C) - (x(2)/RC)];

Answer 1

here’s the problem:

in the loop, the first iteration:

you call the following

[t,x] = ode23(@event1,[t0 tf],x0);

t0 = 0; and tf = 1;

then further down the loop:

t0 = t(nt); %so t0 = 1;

next for loop iteration:

[t,x] = ode23(@event1,[t0 tf],x0); 

in other words:

[t,x] = ode23(@event1,[1 1],x0);

solution:

either modify t0 = t(nt); or update tf with tf = t0 +1;

update:

also, you should correct the following x0 = x(nt,:);