Switch-Mode Power Supplies.

[Yorkie]

As to 'transformers' AFAIK, switched mode power supplies don't use a 'mains transformer' in the conventional sense to reduce the AC voltage from mains potentiial to that which is required. Basically, I think they usually consist of a bridge to rectify the incoming AC to DC, still at HV, then a smoothing cap, with the HV smoothed DC passing to a DC-DC converter, which switches at a frequency of 100 kHz to 1 MHz. A Pulse Width Modulator circuit then regulates the DC output voltage by adjusting the ON/OFF durations of switching transistor, so you get a short period of full on DC, then a period of full off, and the process repeats itself at the desired operating frequency to give the desired output voltage. A feedback circuit monitors the output voltage and adjusts the frequency of the DC-DC converter to stabilise the voltage -the power supply's pulse-width modulating (PWM) circuit comparing the supplied 5VDC output to an accurate Voltage reference so that an error-correcting feedback signal develops. This signal adjusts the relative ON and OFF durations of the DC-to-DC converter, thus holding the output at the required voltage. I believe that SMPSs are very fussy about having electrolytics with low ESRs, and the proliferation of SMPSUs in computers and as mains adaptors, which perhaps explains why ESR meters have come into vogue in recent years.

There's a simplfied explanantion of how SMPSs operate, which is pitched at about my level, here:

http://ecmweb.com/power_quality/electric...r_quality/

They tend to use toroids rather than conventional mains transfomers, which is a big advantage in terms of keeping down the weight, size and cost of the PSU. For such things as plugtop adaptors for say laptops, SMPSUs aren't not much bigger than the plugtop itself. I use one for an Apple Macbook Pro - I've never looked at the output on a scope but I can imagine it's not a pretty sight! Maybe I'll take a squint at it, if only out of curiosity:omg: