When talking about lab power supplies I am aware of two typologies, linear and switcher. The linear supply rectifies the AC into DC and then uses a series pass element to burn off the excess power to give you a constant voltage or constant current depending on the control mode.
The switcher rectifies the AC into DC and then uses a series pass element to switch the output on and off. The switching is such that when averaged it give either a constant voltage or constant current depending on the control mode.
The last topology uses a triac to chop up the AC before the transformer so that after the output of the transformer and the rectification the final output is either constant voltage or constant current depending on the control mode.
Would this topology be called a switcher or does it have a name i'm not familiar with?
simulate this circuit – Schematic created using CircuitLab
EDIT: Per @FakeMoustache I added and inductor to the switching circuit.
Best Answer
I have seen the triac PSU circuit before. I have heard it called "Primary side phase control". It is old school and has a bad peaky input current waveform and lots of output ripple. The power factor and output ripple on the primary side phase control PSU is worse than an uncontrolled diode rectifier.
Where I have encountered it is on battery chargers manufactured in a time where output ripple was not a concern and nobody worried about power factor. In most applications the desired output voltage is much lower than the incoming mains. This means that a cheap TO220 triac can be used for a say 1KW or more charger. The more orthodox secondary side phase control would need 2 SCRs rated at the load current with a center tapped secondary. In the 1970s when power semiconductors were very expensive or unobtanium the triac circuit was tempting. Transformer magnetising inductance and leakage inductance are important transformer parameters that you have to get right if you expect reliability.