Input impedance of error amplifier in SMPS?

[cupoftea]

Hi
Do you agree, that in the attached (LTspice and PNG) boost converter,
the input impedance of the error amplifier is 47k + 17500R_in_parallel_with_2500R?

 

[stenzer]

Hi,

this should be correct. You can also check this in your simulation by plotting the voltage at the cathode of D1 divided by the current towards the FB pin e.g. I(R4). Here you will see most likely some impact of the compensation network at higher frequencies. I haven't run your simulation.

BR

 

[D.A.(Tony)Stewart]

What's your point?

--- Updated Jun 2, 2024 ---

Did you see how I eliminated your overshoot?

 

[cupoftea]

Thanks, the point is that we need to know the zin of the error amp so we can calc its transfer function so as to do the bode plots.

Thanks, I think you reduced r11 to reduce overshoot....

 

[D.A.(Tony)Stewart]

Yes and that forced some static Isense feedback, not a fix, but it blocked some instability.

You have two compensated loops; Isense and FB are two nodes, which combined need correction.