Transformer vs. switched-mode power supply

Question

I'm working on making my own bench power supply, I want up to 7-8 amps. The switched mode power supplies are much cheaper than the transformers. Why would one choose a transformer instead of switched mode, if they're more expensive and heavier? Is longevity really the only reason? Is a 15v, 8 amp transformer better at pushing its 8 amps than a 15v, 8 amp switched-mode power supply?

Answer 1

In engineering, it's usually fairly straightforward to get the wanted, specified, behaviour. The spec says 15v 8A, you can usually get 15v 8A.

The more difficult bit is to tradeoff the different costs that come with any given approach.

One cost you mention in your question, longevity, or fear of a short life, is quality. While high price and a brand name aren't necessarily guarrantees of good quality, a price too low to be true and absence of a data sheet are often guarantees of bad quality. A SMPS can be built for a few dollars by almost anyone, big iron OTOH is unlikely to get produced like that. Are you going to spend the time on research and money on quality when you could buy a cheap knockoff?

There are other costs to the different approaches. Each sends back rubbish into the mains, just different rubbish. The transformer, if feeding a standard diode/capacitor rectifier takes current in big, 3rd harmonic producing pulses, a problem for the supply if on a large scale, but generally no problem at all at this level. The SMPS sends back swathes of RF, usually only suppressed enough to meet EMI regulations, if you're lucky. If you want to do sensitive measurements in those bands, you may have to search to find better gear.

The SMPS is much lighter. Maybe not much of a difference today, but when your shelves are groaning, and when you're old and wizzened and you're groaning (how would I know about this?) then lighter instruments can be beneficial.

Draw up a list of all the costs and benefits of each approach, and see if there's a killer reason for one or the other, for you. If not, flip a coin.

As you're making a bench power supply, that's presumably going to be adjustable. One way to adjust the output is with a linear regulator, the other is with SMPS. The linear will be quieter, but much less efficient than the SMPS. One common way to get the best of both worlds is to use a linear regulator as the adjustable output stage, preceding that with an SMPS regulated to keep the minimum practical drop across the linear stage.

The adjustable SMPS will be preceded by some form of transformer stage, both for isolation from the mains, and to step the voltage down to something appropriate for the output. That can be either big iron + recitifiers + capacitor, or can be SMPS to fixed output, or if it has enough control range, to variable output to drive the last linear cleanup stage. It does actually make sense to use a big iron + rectifier + caps that's then followed by a non-isolated buck converter, as you can use much smaller caps in the rectifier than would normally be the case, making best use of the varying capacitor voltage.