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8th September 2019, 19:47 #1
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SMPS feedback compensation..origin pole is "pole at origin"?
Hi
http://www.ti.com/lit/an/slva662/slva662.pdf
..pg 4 , equation 8 of the above doesnt make sense....because "fp0" is zero Hertz...it has to be zero Hertz, because its a pole at the origin?
C1 would work out as infinite.

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9th September 2019, 00:58 #2
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Re: SMPS feedback compensation..origin pole is "pole at origin"?
At the top of page 4 of the apnote you specified:
"Offering an origin pole, one zero, and one highfrequency pole, ..."
in the gain plot of figure 4 on page 5, it appears they put the pole "at the origin" at a
convenient place, about 3500 Hz
see
http://web.mit.edu/2.14/www/Handouts/PoleZero.pdf
bottom of page 12:
"Any poles or zeros at the origin cannot be plotted on the Bode plot, because they are effectively
to the left of all finite break frequencies. However, they define the initial slope. If an arbitrary
starting frequency and an assumed gain (for example 0dB) at that frequency are chosen, ..."
so you can choose where to put that pole
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9th September 2019, 17:54 #3
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Re: SMPS feedback compensation..origin pole is "pole at origin"?
For the type 2 compensator, it is usually sufficient to implement an inverted zero at fz1 without bothering about fp0. An inverted zero at fz1 determines the slope between fp0 and fp1.

Akanimo.
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9th September 2019, 18:51 #4
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Re: SMPS feedback compensation..origin pole is "pole at origin"?
I guess I'm not clear on the definitions here either.
When I design Type II compensators think in terms of the frequency at which the leftmost pole slope would cross 3db if there was nothing else going on and call that the first pole frequency. That's not zero, its probably 10010k for most designs.
Note that the topology is inherently an integrator and any value of caps will give you the infinite DC gain you expect.
Bottom line make sure to simulate. LTSpice AC sim does this stuff easily.
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