Taming the buck with a Type III compensator

Tuning feedback loops is a topic I've found to be fiendishly good fun, perhaps because of how obtuse the final result is. There is no at-a-glance way to see the connection between your high-level goals (closed-loop bandwidth and phase margin) and the implementation (the R and C values). Consequently, I make it a point to document the hell out of these circuits with parameterized simulations so that my future self has some hope of understanding and adjusting them later on.

A lot of applications are usually tolerant of suboptimal compensator design. I've participated in a few designs where these circuits were plucked from the datasheet reference design and never touched. There's a tradeoff between having a little bit of ringing vs. having an engineer model, tune, and test, and also adding to the BOM complexity with a bunch of different passive part values.

It's been a while since my EE degree so I had to refresh my memory [2]. This is related to PID controllers (https://en.wikipedia.org/wiki/Proportional%E2%80%93integral%...).

Type I is proportional only.

Type II compensation adds a zero (to increase phase margin [1]) and a pole (to filter high-frequency noise) to the compensation network, resulting in proportional-integral (PI) control.

Type III compensation introduces two zeros and two poles into the control loop, making it a proportional-integral-derivative (PID) control scheme.

... and if you want to go one step further, MPC [3] also exists:

https://iaeme.com/MasterAdmin/Journal_uploads/IJARET/VOLUME_...

[1] https://en.wikipedia.org/wiki/Phase_margin

[2] https://eng.libretexts.org/Bookshelves/Industrial_and_System...

[3] https://en.wikipedia.org/wiki/Model_predictive_control

I am impressed by the xschem developer ! from the article: https://github.com/StefanSchippers/xschem/issues/238

...looking at something like this built with surface-mount parts in the modern day reads like someone trying to do high-speed rail with steam locomotives. I mean, you can do it (see https://en.wikipedia.org/wiki/LNER_Class_A4_4468_Mallard#193... !), but... please just learn to solder SMT parts. It really is easier, I promise you.

To go SMT you'll need to add fine-gauge solder wire (0.015"/0.4mm), more flux than you think you need, and either a second iron or soldering tweezers (amazing but expensive). You should add get some T4 paste, a dental pick to spread it, and a "reflow plate" (that means old electric skillet from the secondhand store). And you'll want an inspection microscope but those are really useful in general. That's all you need!

Stop building with dinosaur parts and you can use shiny new things like the LM70660 which can do that entire design in about 1/10th the space and double the reliability: https://www.ti.com/lit/ds/symlink/lm70660.pdf

Are there pictures of the Type III Compensator physical reality?