Electronics > Beginners
When are SMPS not used these days?
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lordvader88:
I see they are used in oscilloscopes, even my 1980s analog scope. How is it actually cleaned up enough, to not interfere with something as sensitive as a scope ? Or do they use circuits that can cancel it out or ignore it ?

And digital stuff like computers use them all the time. What types of stuff, besides maybe tube amp's, still use main's transformers ?
wraper:
High resolution bench DMM.
mariush:
My nightstand lamp uses a 20VA transformer (has one 12v halogen bulb).
The transformer actually has just the right amount of weight to prevent the lamp from falling on its sides or shaking.

You may use them in devices where noise can affect it (radios, transmitters etc) , or maybe in devices that use negative and positive voltages (small audio amplifiers, small boomboxes etc)
Siwastaja:

--- Quote from: lordvader88 on June 23, 2019, 05:27:23 pm ---I see they are used in oscilloscopes, even my 1980s analog scope. How is it actually cleaned up enough, to not interfere with something as sensitive as a scope ? Or do they use circuits that can cancel it out or ignore it ?

--- End quote ---

You have two slightly wrong assumptions here:
1) Oscillosscopes being extra sensitive. Most are not; think about the fact that most digital scopes digitize at only 8 bits! The timebase accuracy is more important, most scopes more or less suck at measuring low-level noise, or getting very accurate voltage readings. There are exceptions on the high-end market, of course.

2) Switch-mode power supplies being noisy. They can be designed to be fairly clean. A properly designed one can be surprisingly clean even without filtering, e.g., by using as high frequency as possible, reducing edge rate a bit at the cost of efficiency, using RC snubbers, keeping layout good and switching loop area minimal, with ground plane shielding all around it, and good low-ESR, low-ESL capacitors at the input and output (so that the ripple currents actually can go through these local capacitors and won't take a longer loop over the "load" side capacitors!)

Adding LC filtering to kill the remaining ripple current (fairly low frequency differential mode noise), or ferrites for really high frequency common mode noise, can reduce the noise further.

If you see a linear supply in a modern device, it's most likely for legacy reasons, such as a design reuse. It's also possible that they really need some very low-noise operation and designing such low noise SMPS with enough post-filtration would end up being too complex or expensive.

They may also have design culture around simplicity and robustness, and while an SMPS can be made very robust, long-life and low noise, it requires more understanding to do so, and more trust on your SMPS engineers that they are capable of pulling that off.
Siwastaja:
At some point they realized that for high precision work, you need to have local regulation anyway, just where the load is. These typically do not handle large currents, because each site uses their own regulators, the burden is shared. They want to use local regulation for DC accuracy, as well.

At the same time, something else happened as well: now there are linear regulator ICs with really excellent Power Supply Rejection Ratios, even at high frequencies, and they aren't even expensive. Compare to older regulators, if you try to post-filter your noisy SMPS with a LM317 or 7805, it won't do shit after a few dozen kHz, just passing it through. But some modern regulators regulate at MHz or tens of MHz ranges just fine, which is the territory where you'd otherwise need some real inductors and capacitors, which then again could have their own resonances where they are ineffective or even make the situation worse, unless you dampen them which again makes them less effective over a wider range.

But now if the linear regulator is capable of taking care up to some MHz, you are only left with really high-frequency noise which is dealt with ferrites, very small SMD caps, proper layout, shielding etc. Suddenly, an SMPS isn't so bad after all.

They need to deal with noise, anyway: it reflects back from all the other SMPS's, and this may include some very crappy ones, for example, old ones that passed less stringent regulations back then, or some illegally performing imports. If you just use a mains transformer (which has some coupling capacitance from input to output) and a 78xx style linear stage, assume it's guaranteed to be very low noise, and call it a day, you may fail due to external noise getting in through power line.

So, say, your 10x better than average SMPS design inside your low-noise product may not produce more noise than what's already getting in because of a 10x worse than average PSU in the neighbor's power outlet! Which brings us to filtration, and local regulation and local shielding.
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