Electronics > Beginners
When are SMPS not used these days?
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rstofer:

--- Quote from: Dave on June 24, 2019, 06:15:37 pm ---
--- Quote from: mariush on June 23, 2019, 05:44:07 pm ---maybe in devices that use negative and positive voltages (small audio amplifiers, small boomboxes etc)

--- End quote ---
Bipolar supplies are trivial to produce with isolated switching topologies.

--- End quote ---

They're pretty easy with 78xx and 79xx devices also.
David Hess:

--- Quote from: Siwastaja on June 23, 2019, 08:23:53 pm ---
--- 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 ?
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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.
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Oscilloscopes and some other test instruments are sensitive and take special precautions when switching power supplies are used.  Careful attention is paid to magnetically shielding the CRT of an analog oscilloscope and to preventing ground loops.  In addition, unconventional switching topologies were often used in the past.

Tektronix had a steady progression of series resonate and push-pull soft switching and current driven topologies from the 70s through the 90s.  The sensitive parts of the differential signal paths included plenty of common mode isolation.  Remove the shield from around the CRT and the display will be rendered completely useless; Tektronix did not use mu-metal shields without reason.

An unusual example of the attention Tektronix paid to these issues is shown below.  What is the extra transformer winding and C908 doing?  Apparently, they introduce a counter current between two points in the chassis to cancel EMI.  Also notice that there is no secondary side regulation but there are secondary side LC filters.  Feedback for regulation only comes from the primary side but later they switched to feedback from the secondary side and added linear post regulation.


Modern analog instruments like digital voltmeters and function generators often use linear power supplies.  I suspect this is because of excessive common mode noise and poor common mode isolation in commonly available regulators.
Siwastaja:

--- Quote from: bdunham7 on June 24, 2019, 04:22:08 am ---A lot of consumer audio equipment still uses mains transformers and linear power supplies.

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Yes, because audio amps don't need voltage regulation. Regulated linear supply is highly inefficient, heatsinking cost alone is considerable. In an audio amp, you don't need that, it's just the transformer, rectifier and filter caps. Simple, highly efficient. More cost in copper and iron, but in the big picture, not too much difference.

Another reason is that audio equipment has mostly stayed the same or very similar, and made by the same companies, using their legacy designs, because why not, they work and have a good market case. (Of course there are totally new audio product segments, like wireless bluetooth speakers, but they do not indeed use linear power supplies.)
Zero999:

--- Quote from: Siwastaja on June 25, 2019, 06:59:29 am ---
--- Quote from: bdunham7 on June 24, 2019, 04:22:08 am ---A lot of consumer audio equipment still uses mains transformers and linear power supplies.

--- End quote ---

Yes, because audio amps don't need voltage regulation. Regulated linear supply is highly inefficient, heatsinking cost alone is considerable. In an audio amp, you don't need that, it's just the transformer, rectifier and filter caps. Simple, highly efficient. More cost in copper and iron, but in the big picture, not too much difference.

Another reason is that audio equipment has mostly stayed the same or very similar, and made by the same companies, using their legacy designs, because why not, they work and have a good market case. (Of course there are totally new audio product segments, like wireless bluetooth speakers, but they do not indeed use linear power supplies.)

--- End quote ---
Strictly speaking, a transformer with a rectifier and capacitor is not a linear power supply. A linear power supply has a pass device acting a resistor to dissipate the difference between the input and output voltage across it.
tszaboo:
I had sensitive analog circuits (few ppm accuracy, few KHz bandwidth)  running from switching power supply. Once you know your supply, you can filter out the noise and that's it. You can always exchange a little bit of efficiency for precision analog, so so there is no real reason to use it. In fact filtering 100KHz and harmonics is a lot simpler than 50Hz.
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