We all know that switching power supplies are wonderful things. They eliminate the bulky transformers used in linear supplies and provide efficient power conversion. They can step up or step down. Some are even smart enough to handle both functions if the input supply drops too low. And they've gotten easy to use: Pick the chip that has the right specs for input voltage, output voltage, and current; add a cheap inductor and maybe a discrete FET or two; lay it down on a board -- done.
But have you ever paid attention to the noise that these things generate and radiate? Go get a portable radio with an AM detector and hold it near any plug-in wall charger or the last switcher you designed. Tune around the AM broadcast band, or if you can find a radio with some shortwave bands, listen there.
Older, cheap switchers running at 20kHz tend to produce raspy-sounding noise throughout the MF and HF spectrum, with peaks spaced 20kHz apart. And since the 20kHz switching clock is not all that stable, it drifts around with temperature, load, and pretty much any other stimulus. Higher-frequency switchers do much the same, with big signals at multiples of the switching frequency. And they also put out a lot of wideband noise between the harmonics of the switching frequency.
This noise radiates, using internal wiring of the power supply as the antenna, and it also gets conducted as a common-mode signal through the input and output leads which serve as nice antennas and radiate the noise energy everywhere. In some cases, when the CM noise gets to the wall socket, there's an imbalance somewhere nearby, and the common-mode signal becomes differential-mode, and is carried far away by the power-wiring/transmission line.
Good old dumb linear ac-dc supplies didn't do this.
I've noticed some power supplies, notably in PCs, are missing the filters on the line voltage input. I have heard that some manufacturers install the filters on the sample used to pass FCC and CE certification, then omit them in the production model. By the time someone complains and enforcement is initiated, the model is long out of production.
The noise that escapes the enclosure can often be knocked down by the application of appropriate ferrite material. That's the little doughnut-shaped thing on the cord of your laptop's power supply.
I also heard about a trick to pass the FCC specs that involved deliberately modulating the switching frequency such that the noise was smeared out over a wider band so that the noise power in any narrow measurement band was lower. I call that meeting the letter of the law, but not the spirit.
So why is this a problem? It is pollution on a grand scale. Most urban and suburban environments now exhibit noise in the MF and HF spectrum that interferes with various services, and as the problem gets worse, more services will be impacted. If we can start to take steps to reduce this pollution source, like a river with a polluting factory shut down, things will be clean again.
But whose responsibility is it? Is it the chip makers? Can the problem be fixed with either better designs or better app notes on how to eliminate radiated and conducted noise? How about the equipment manufacturers taking some responsibility for better use of the chips, and honesty in leaving the filters in the BOM for mass production? Is it more enforcement (and if so, who will fund it)?
What has been your experience in dealing with switcher noise? How did you fix the problem if the design wasn't your own? Did you get satisfaction from the responsible party?