I built a voltage regulator for an old truck I own. The circuit uses a voltage controlled oscillator with a square wave output to drive the gate of a mosfet that controls the alternator's field. The voltage regulation feedback loop uses a voltage divider from the alternator output/vehicle battery, connected to the reference pin of a TL431. The TL431 controls the vco.
Now, for a bit of a digression. Years ago I built a boost converter with a similar feedback loop, the major difference being that it used a bjt instead of a TL431. But it worked on the same principle, with a voltage divider from the output to the base of the bjt, which controlled a blocking oscillator. The output voltage wandered up and down and was very unstable, so I put a capacitor from the collector to the base of the bjt. That fixed the problem and made the output voltage stable.
So, having had this experience, I put a cap from the cathode to the reference of the TL431 in my truck regulator. When I installed it in the truck, the alternator output regulation was unstable; the voltage jumped up and down like crazy. After much head scratching, on a hunch I took wire clippers and removed that cap from the circuit board. Then the regulator worked GREAT. System voltage perfectly stable, good load regulation.
Perhaps I could be forgiven for thinking the boost converter and the vehicle voltage regulator would operate on the same principle and that a cap would have the same effect, in view of the fact that in both circuits the cap provides negative feedback in the controlling element of the feedback loop. But the two circuits have opposite behavior, and adding a cap to the controlling element in the feedback loop has exactly the opposite effect in each circuit. The boost regulator was unstable without a capacitor, and stable with one. Contrarily, the alternator regulator was stable without a capacitor, and unstable with one.
At best I have some intuitive sense that the presence of a big mechanical device that drives the load and resides within the loop changes things dramatically. But I'm lacking in knowledge of theory and could use elucidation. Any pointers?