Low Profile Desktop Power Supplies

[Tom D]

I'm curious about the design and components used for these kind of power supplies. Reason being: I needed one of these to replace a defective one I had.

I ordered a CUI model from Digikey.com with the same specs as my original one. It's one of those desktop supplies(regulated output) with a detatchable 2-prong power cord and DC power jack.

What surprised me was how small this power supply was compared to my original one; and it works just as good or better than the original which was 3 times the size.

Now, I'm assuming they use surface-mount technology with some of these desktop supplies? Also, the transformer must be very small as well?

Is anyone familiar with these designs?

[mrfeedback]

These nowadays contain a switchmode supply as opposed to a conventional transformer and linear regulator stage.
long term experience of switchmodes is that they are capacitor dependent, so for longevity, turn off the input power when not in use.

Eric.

[Tom D]

Originally posted by mrfeedback
These nowadays contain a switchmode supply as opposed to a conventional transformer and linear regulator stage.
long term experience of switchmodes is that they are capacitor dependent, so for longevity, turn off the input power when not in use.

Eric.

Thanks. So, are you saying that these supplies do not use transformers? and they are not as reliable or long lasting?

[Lligior]

Small transformers just cost too much...
Apparently it is cheaper to use a complicated control circuit with voltage and current sensing. If it makes you feel better they do usually an inductor...

[mrfeedback]

Tom D, in a typical SM power supply the incoming 240V is rectified and this gives around 330v, smoothed by 400V rated smoothing/resevoir capacitor.
This 330V DC is chopped at a fast rate (20-40 kHz) into the primary winding of a small ferrite cored transformer, and the secondary (secondaries) are rectified and smoothed and filtered to produce the output DC voltage.
SMPS's rely on a controller circuit that monitors the DC output voltage and vary the primary current chopping rate or duty cycle in order to keep the output voltage constant regardless of secondary load (within rated limits of course).
When operating correctly SMPS give close to perfect output voltage regulation, and with relatively low power loss, unlike a conventional linear regulator stage.
The downside of SMPS's is that the controller circuit relies on capacitors to smooth the controller stage supply, and to smooth feedback voltages, so as capacitors age the performance of the SMPS will degrade, and may even cause the output voltage to increase as particular capacitors dry out, and cause consequent downstream failures.
This failure mode is common in TV's and VCR's, and is dependant on time of the supply is connected to the mains, and not just when loaded.
When a TV is switched to standby mode, the SMPS is still operating, so for this reason I always turn off at the mains/master switch.

Eric.

[Tom D]

Hey Everyone,

I know this is an old thread I started a while back; but I was searching the forums and could not find a specific answer to my new question.

Concerning Switchmode Power Supplies: Now that just about every DVD/DVD-Audio/SACD Player has a Switchmode power supply; Have there been improvements or advances in their designs? Or; is it just a more "cost effective" method these days?

Is the "Linear" power supply still superior for Audio? Thanks.

Tom

[dhaen]

Hi Tom,

Almost all consumer electronic equipment (and most other) use switched-mode power supplies. It is down to cost and size: in the continuing fall of prices, nothing else could be used.
Probably the only exception is power amplifiers, though even they are now moving towards switched-mode.

Which is best for performance?
Well convential linear supplies are still quietest, but in the future that may change.
As for reliability; I'd go for a cool running linear any day, but most of the time we don't have the choice

Cheers,

[Tom D]

Thanks. Would you say that a switchmode supply is really not a "bad" thing with DVD players opposed to Amplifiers?

Many people seem to say that switchmode supplies in DVD/CD players make for poor or inferior sound because of the Video circuitry.

So; which one is the real culprit for degrading sound; the switchmode power supply or the video circuits? Or both?

[dhaen]

Hi,

Would you say that a switchmode supply is really not a "bad" thing with DVD players opposed to Amplifiers?

Good and bad design elements can occur in both applications.

Many people seem to say that switchmode supplies in DVD/CD players make for poor or inferior sound because of the Video circuitry.

So; which one is the real culprit for degrading sound; the switchmode power supply or the video circuits? Or both?

I suppose either could affect sound quality.

The picture is much bigger than just the power supply. Just spend a few hours browsing the Digital forum. The issues raised about CD players are often valid for DVD players too.

At this point I'll hand over to the many in the Digital forum who are more experienced than me

Cheers,

[annex666]

As to the question of whether a swith-mode supply is "better" than a linear equivalent (VA not performance) it is a matter of what the use is.

Due to the design of switch-mode supplies they inherantly produce a lot more high frequency output than a linear supply (due to them being based on a switching circuit that "pulses" stored energy from an LC circuit at a high frequency), but are much smaller and more efficient - they are also much cheaper to produce (as previously mentioned) so therefore find themselves in much of the "consumer grade" equiptment.

Their use in digital circuitry should show them in best light, as a digital signal (once polluted with noise) is much less of a problem than an analogue signal with the same noise.

One thing that should be realised though is the D/A sections (and indeed all circuitry after the DAC) of DVD-A/V, CD and SACD should idealy be powered by a linear supply, due to it's lower high frequency harmonic components.

Unfortunately to save money manufacturers tend to use the same supply system for both digital and analogue sections of the player - but this is DIYaudio so that can surely be fixed...