Hello there oodes,
Well, theoretically with a perfect 0.75 duty cycle and components that are perfect with no losses and a little output averaging filter, you'd see an output voltage that is exactly 9.000000 volts. That's not 9.000001 volts nor is it 8.999999 volts, but exactly 9 volts plus or minus nothing.
Now if you figure in the error in floating point math and the errors dont average out, you might see a very small difference of 1uV or something like that, probably more typically 1e-12 volts plus or minus.
With imperfect components though, you'll see lots of different kinds of variations. The list is quite extensive...
1. The diode forward voltage will pull the output down slightly, not up. That's because it represents a power loss.
2. The diode reverse leakage, which will affect it less in most cases.
3. The inductor series resistance will decrease the output also.
4. The switch series resistance will decrease the output.
5. The capacitor ESR will decrease the output, but it plays in combination with the switch resistance too.
6. The capacitor value itself. An increase in value can cause more peak drop across the switch resistance and input source resistance.
7. The pulse timing accuracy which comes in at least two forms: a. The pulse setting itself, and b. the rise and fall times.
8. The output resistance, which causes a drop in the inductor ESR and the switch ESR so decreases the output.
9. The input voltage source series resistance, which of course causes a loss.
So you see every single component causes a drop in the output voltage from the theoretical with perfect components.
If you improve the diode choice you'll get closer results, or better yet get rid of it entirely by going to a synchronous buck (another switch).
Probably a big factor is the rise and fall times of the pulse source. You can't set them to zero you have to choose some
reasonable value like 1ns. If you choose 0 then LT spice will choose for you which will cause problems.
I'd also set the values for the ESR for all the components too rather than leave them blank or set them to zero. Choose some small value like 0.001 or even lower.
To average the output without appreciable loss, connect a 10k resistor in series with a 0.1uf cap and take the output voltage reading from across the cap and increase the simulation time somewhat until the voltage levels off. Once the voltage levels off the loss is extremely small.
It is entirely possible to set the values of the components to see very near theoretical results in the simulation.