Using 100 amp mopar alternator to charge batteries out of a vehicle
I think the newer electronic Vreg's continuously change the alternator field current to adjust the voltage output, whereas the older relay type just turned the field on and off.
Observations with a voltmeter may lead you to this conclusion, but a scope will show that solid-state regulators also control the field by "on"-"off" but at a faster rate. Electronic regulators use "on"-"off" control to reduce transistor losses, improve efficiency, reliability and transient load response. Even the first (1971) Mopar electronic regulators work that way. The mechanical regulators work in a similar fashion, but the switching speed is much slower due to the mechanical inertial of relay, and delay related to collapsing field in relay coil. A comparator that drives a transistor, is orders of magnitude faster. The faster switching speed improves regulation and greatly reduces undesirable noise transients.
While the transistor turns "on"-"off", on a repeating basis, the coil current is fairly smooth, holding the value necessary to regulate the alternator output. The field is an electro-magnet, with significant inductance, so field current is stored, and freewheels in the "off", then "on" results is a nearly linear charge ramp, until the desired alternator output voltage is reached. The switching interval matches with the need for varying load, losses and alternator RPM. View with a scope, will reveal a very small saw-tooth riding onto of the field signal. Observation of the field driver transistor base or gate will reveal the digital control signal.
The term SMPS, is "switched mode power supply", it is the most common voltage and current, means of regulation today.