Oh Gawd!
SMPSUs! In the mid 1990s, I was unemployed and did 6 month's unpaid work experience for a firm specialising in bespoke PSU manufacturing. My job was to test them before they left the factory and fault find any customer returns.
The firm ( since gone under and the factory site now a legal eagle's nest ) had contracts to supply cable TV companies with PSUs for distribution boxes ( at the time all the UK's streets were being trenched for cable TV! ) and high current PSUs for overhead projector ( OHP ) lamps among other things.
Even though I had circuit diagrams it was a mystery to me how they worked but I was usually able to diagnose some faults and fix them. One design was absolute shite, the PWM timing capacitors were electrolytic!!! This model was overwhelming sent back to us for repair and I had to drill out the rivets to open the case, replace ALL the electrolytics and cross fingers, test them uncased then once working use the riveting machine - it got to the point I knew the part numbers by heart!
The OHP PSUs were interesting, they had no output smoothing capacitors or rectification at all - I think they only had inductors on the output and the waveform was a kind of amplitude modulated rectangular waveform. I think they were 12V 10A output. The size of the units were maybe 100mm by 160mm in area. A separate board with fuse and rectifier was connected to provide DC to the cooling fan and boy, the number of times the fuses were blown thanks to clumsy assembly at the OHP's factory! It was a boring job checking those crappy little boards.
Having said all that, the experience I gained in that factory was useful even if it didn't lead to a paid job there.