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Guest Zeppelin

What is holding switch mode PSUs back?

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Guest Zeppelin

Something I've been wondering for some time:
Despite the obvious advantages of switch mode power supplies, they are still not used (not usually) in either lab supplies or audio equipment supplies, I don't see why? ???
Any ideas?

Thank you

P.S. I'm of course aware of their oscillation but this can be easily filtered out, can't it?

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I think that the reason is that the main advantages of SMPS are underutilized in such applications. That means that we don't need a high cost and difficult to design PSU for a home audio amplifier that powers from mains voltage and has a noticeable volume, nor on a lab power supply. Having efficient of 70-80% and up doesn't offer much in these applications (instead offer much in battery powered applications). Among all these parameters, we add the non existed problem (on linear psu) of EMI/RFI emmitions that can be attenuated using filters and shields but can't be eliminated.

In many cases the switching frequency is in audio spectrum or a little above and that makes the problem more complicated.

So, we gain more than we loose not using SMPS in such applications. That's the reason!

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Hi Guys,

Today SMPSU’s are found frequently in home electronics equipment. Almost every VCR, DVD players, tuners, TV-sets and similar has got them and so do every computer laptop or other computer. The smaller and lighter SMPSU’s are cheap to make and they save weight and space in the containers shipped form Asia to US and Europe. The cabinets can be made smaller since the temperature rise and the need for cooling is less, and no bulky transformer. However I haven’t seen any in audio power amplifiers for home use yet. Audio power amplifiers for vehicles always got switchers for the step-up voltage converter. Most SMPSU’s run in the 30 – 200 kHz range and should not cause any hum in audio equipment, perhaps the high current demand in a power amp is the reason? But then again today’s computers carry PSU’s up to 700W and more at very high current levels so I’m not sure about that.
BTW, my lab-PSU (0-40V 0-5A) is a switcher too! ;D

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Hi Zepplein,

Here is some info about my bench PSU, I’ve got the one with 0 – 40V & 0 – 5A, it’s also available as 0 – 20V & 0 – 10A.
http://www.kjell.com/filarkiv/SUPPORTPDF/41-50/44/44311/44311.pdf

Here is another one similar with a different name on it: http://www.elfa.se/elfa-bin/setpage.pl?http://www.elfa.se/elfa-bin/dyndok.pl?lang=en&dok=5645.htm

Works really well too! ;)


post-929-1427914308875_thumb.jpg

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Hi Zeppelin,

The 100% load swing means that a current swing (in this case) 0 to 5A creates less than 10mV change in the output voltage and less than 5mA in the output current!
Ripple at nominal load (in this case 5A) is +- 15mV RMS.
Not bad, I’d say! ;)

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Those who work in the industry have probably heard of a company called Vicor. It was founded by a guy named Dr. Patrizio Vinciarelli... When I was working for LTX,  I was fortunate enough to have had a working relationship with Patrizio and Vicor when in it was in it's infancy (just a bunch of guy's in a office condo

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Where I live I can buy a computer PSU (450W) for less than $20US. As everyone knows they are multiple voltage PSUs very compact and light. It shouldn’t be difficult to produce say a 2 X 40V 2 X 8A for the same amount and with the same physical size. 

BTW. A 40V 40A unit is not a bench PSU it’s a welder! ;D

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First, sorry for the typo... my XHR 40-25 is 0-40V, 0-25A not 40A, but hey... 1KW is 1KW.

The SMPS in a computer is a discontinious mode flyback. The switching edges all ring like hell and the ripple is a bitch to get down. The reason "they" use this topology is that it's "CHEAP" to build (you get what you pay for). You don't need a output inductor (actually the transformer isn't a transformer at all, it's a coupled inductor, whose secondary "kinda" acts like a ouput inductor) and you don't need a flywheel diode. This saves $$$. The down side is that this is the worst possible topology to use when you want "clean" DC. A forward converter will have ripple levels an order of magnitude lower just for starters and can easily be made even lower with an additional small LC.

A computer PSU isn't very compact when you consider DC-DC converters like Vicor's first generation units, back in the mid 80's (VI-200: 4.6" x 2.4" x 0.5" (116,9 x 61,0 x 12,7 mm) and 6.0oz./170g) were up to 50w/cubic inch and some models up to 90% efficient. The computer PSU isn't even close to that power density. In fact, the entire amp, with power supplies could probably be put into the volume of a computer PSU. Like I said, if money isn't an issue, sure you can do it, but an off-line flyback isn't the way.

Here's the primary FET and output ripple for a 24V 4.5A DC-DC

post-14697-14279143101953_thumb.jpg

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A lot of them use POL (point of load) converters to further filter/regulate the voltage or even use an intermediate bus architecture. Nowadays, even digital control is used to set the output voltages of POL's in order to set CPU speed. It's come a long way since "the old days"!! You also have to consider, how "crappy" is crappy!!! In an audio application, is 20mV or 200mV too much ripple... I don't know because I've never tried it!!

Oh... BTW, the reason the ripple frequency in that scope shot isn't the same as the drain waveform frequency is because it a 2 phase converter (interleaved)

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I don’t really see a computer PSU as a DC-DC converter, more like an AC- DC converter or primary switched DC-supply. I believed computers where more sensitive than they obviously are since they seem to accept dirty (cheap) DC. I have seen many HiFi stereo amplifiers with only a pair of rectifiers and some electrolytic caps barley filtered at all and still no hum. In one resent case (a receiver 2X100W) which only had 2X2200uF on each side of zero, 55 Volts at standby and about 45 V - 4A at 80W output. I mean if they can get away with this it cannot be very critical!

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