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# Power Tip #3: Damping the input filter --- Part 1

9/5/2008 00:00 AM EDT
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8/6/2014 4:05:10 AM
Dear Robert Kollman,

I have read your article and found it really helpfull for myself. However, I would like to know one thing here.

While calculating negative input impedance of the converter you wrote:

Negative input impedance

This voltage-current line has a slope that essentially defines the dynamic impedance of the power supply. The slope of this line is the negative of the input voltage divided by the input current. That is, with Pin = V • I, we have V = Pin/I; and so dV/dI = –Pin/I2 or dV/dI ˜ –V/I.

This approximation is a bit of an over-simplification because the control loop impacts the frequency response of the input impedance. But many times, this simplification is good enough when current-mode control is involved.

Could you please explain that in the presence of Current-mode control, how this simple assumption is good enough?

Thank you very much.

Regards.

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