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One of the basic circuit to step down input voltage is to use a buck converter topology. Some power supplies reduce input voltage using non isolated type while others use isolated type (example use toroidal, EI transformer).
If I am designing a bench supply, which should I prefer?
Mains (low) frequency transformer first is the old school way. Depending on the application, the whole supply can be extremely simple to design as the transformer already provides isolation. It results in a heavier unit because of the mains frequency transformer and unit cost is higher.
A switching PFC is the modern day way. It involves significantly more design (usually 2 switching stages), but it's better in terms of the aspects discussed above. Also the PFC stage shapes the current drawn from the mains plug, making it that of a resistive load. I'm not familiar with the regulations with regards to this, but you may not be allowed to go above a certain power level without doing something for the current to look good (PFC).
I guess I'd choose based on:
Power level (nominal and peak). If it's high, the transformer can be massive and also you don't want to draw current that's not PF correct.
How many units will be made, as to me there's a trade-off between development cost and unit cost.
I understand I'm not providing a definitive answer here, but hopefully I'm helping you get a view of some factors involved.
While the existing answer by Alex Lopez is very good, I want to point out there's one more and very popular design of bench supplies.
It consists of:
So e.g. if you want 5V output, the selected tap will be around 10-11V (of course poorly regulated), and the final linear stage would drop off the excess voltage as heat. At 30V output the highest tap will be used (e.g. 34-36V) so even in the worst possible case the efficiency won't be too bad.
I've seen this design in a bunch of entry-level bench PSUs (the typical 30V/3A units or sometimes 30V/5A), but I was amazed to see it recently in a hefty 600W PSU (60V 10A).
This PSU was incredibly heavy! It was if the entire PSU was a solid block of metal! I can compare it with our lab's 315W unit, which is about four times lighter (but it is a switcher + linear postregulation design).
In conclusion, the weight considerations should not be overlooked, especially at higher power levels.
