Hello,

Did you already have a look at the wiki:
http://en.wikipedia.org/wiki/Switched-mode_power_supply

Bertus

 

Thanks guys, lots of interesting info there.

Bertus - yes I did look at quite a few links and there are lots of how-they-work type descriptions but not why they became ubiquitous over the last decade or so. This thread has done a great job of answering that.

 

I'm with bountyhunter. Fast, cheap mosfets and IC's to drive them became available. Try making a switching supply with only bipolar transistors, no IC's, and you will see the problem.

In the (almost) early days, the most common type was based on the blocking oscillator, a second transistor which is controlled by the regulation opto shunts its base.

This circuit occasionally turns up with a MOSFET in it, but it takes a fair bit of extra circuitry to control the gate correctly.

The last one I saw was in a Sky box I scrounged on Freecycle to pull apart to see what's in it.

Having traced out the circuit of the MOSFET version - I haven't bothered using the design because of its complexity.

 

I don't think it's been mentioned but I think another reason for going to SMP in wall-warts is that the idling power loss can be made less than with a standard linear supply and there are government efficiency standards requiring that power to be below a specific level, generally well below a watt. Reduces the "vampire" power loss due to wall-warts.

 

They used an oscillating solenoid core within some cylindrical coils, and a high quality adjustable contact set at the end to make it oscillate and probably to do rectification too, I can't remember the specifics. They were military spec equipment labelled as DC-DC converters.

So you don't even need semiconductors or vacuum tubes to make a SMPS!

There were two types of the electro-mechanical vibrator type, synchronous and non synchronous, the non sync. type provided a AC output for the input to a low voltage to high voltage transformer where the secondary was then rectified.
The synchronous type switched the primary and the secondary simultaneously to provide a DC out eliminating the need for a rectifier.
Max.