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For a voltage-mode buck-derived converter the open-loop frequency response of the converter is dominated by the 2-pole LC output filter. This gives a flat passband at low frequency, a relatively sharp resonant peak at the cutoff frequency, followed by a steep -2 slope (-12dB/oct or -40dB/dec) above the cutoff frequency. At some higher frequency beyond the cutoff frequency in the stopband the ESR of the output filter capacitor begins to dominate and the -2 slope changes to a more shallow -1 slope (-6dB/oct or -20/dc) Above this frequency the output filter is essentially an LR filter instead of an LC filter.
When closing the feedback loop around a switching converter the design goals are:
1. To get as high a DC loop-gain as possible to minimise steady state error. (In other words to get the output voltage to equal precisely what you set it to be, even if it takes a while to get there.)
2. To get the overall loop-gain to cross 0dB (unity gain) at as high a frequency as possible this gives a wide bandwidth to the control loop. What this does is ensures that the control loop responds quickly to changes in the line voltage or output load, and acts to bring the output voltage back to the setpoint as quickly as possible.
3. Ensure that the phase shift around the loop is substantially less than 180 degrees when the loop gain crosses 0dB. This is called phase margin. If there is 180 degrees phase shift at the point where the loop gain crosses unity the control loop will oscillate for ever because there is too much phase lag. Decreasing the phase lag a little bit gives a control loop that rings in a damped oscillation in response to step changes. Decreasing it further increases the damping until you have a nicely damped response to those line and load step tests.
4. Ensure that the gain is well below unity at the point where the loop phase shift eventually crosses 180 degrees. For the same reasons explained above, you want healthy gain and phase margins or else the transient response will overshoot and ring!
The main problem for a voltage-mode buck converter is that the phase shift slews very quickly through 180 degrees around the resonant frequency of the output filter and it doesn't head back towards 90 degrees until the ESR of the output capacitor takes effect. This 180 degrees is made even worse if you try to include any integral action in the control loop to minimise stead-state error as an integrator adds another 90 degrees of phase lag!
The popular solution for the buck converter is a type-3 compensation network that has integral action at low-frequencies only to minimise the steady-state error and achieve excellent DC regulation. A zero and pole pair are then placed either side of the resonant frequency of the output filter to cause a phase-lead bump in the compensator at exactly the region where the output filters double-pole causes a dramatic phase lag. The further apart the zero and pole are the more phase lead the compensator kicks in at the troublesome frequency. (Output filters with high ESR electrolytics give lower phase lag around the cutoff frequency, and require less of a phase-lead bump to compensate them with sufficient phase margin.)
Finally another pole in the compensator causes a falling low frequency characteristic to filter out high frequency switching noise. The cutoff frequency for this pole is typically above the crossover frequency for the control loop anyway, so it doesn't contribute noticeably to the phase shift or degrade the phase margin at the crossover frequency.
That is just a very very quick summary of how and why a voltage-mode buck converter is compensated.
The thing about adding a portion of the sensed inductor current into the feedback amplifier actually moves the controller away from voltage-mode control and more towards current-mode control. With current-mode control you are effectively controlling the inductor current directly in the output filter, so you can model the output capacitor as an integrator fed by a variable current source. This removes one of the poles from the transfer function and makes closing the feedback loop easier. The downside is that current-mode control typically has lower loop gain and higher sensitivity to switching noise for light loads. The advantage is that loop compensation is easier and you get s/c protection for free.
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interleaved boost converter, circuit ok?