smps not used in lab power supply - Page 1
Forum section: electronics / projects

author: electros6
author_rank: Regular Contributor
timestamp: May 16, 2014, 03:42:33 am
content: hi guys ,I got small question. why smps is not used to convert ac to dc in lab power supply instead of transformer. using smps reduce the size isn't it?

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 16, 2014, 04:35:52 am
content: in a lab supply you care about noise more then heaviness and efficiency, but they exist. their niche items so their priced up due to lack of customers, so its often easier to use a big heavy transformer rather then to design filters and deal with switchmode regulator problems. trying to get uniform specifications along variable output voltages and currents is alot harder to do with a switchmode supply then a linear supply.

author: macboy
author_rank: Super Contributor
timestamp: May 16, 2014, 05:50:18 pm
content: I have two different supplies that use SMPS. They are an SMPS followed by a linear regulator. The SMPS provides the input side voltage for the linear regulator, about 5 V or so above the output voltage. This reduces the power dissipation in the pass transistors. The linear regulator ensures a cleaner output and faster response to transients than a pure SMPS design could. Their outputs are definitely noisier than a "pure" linear design, but due to their efficiency (and high output power) they do see their share of use.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 16, 2014, 06:55:31 pm
content: smps is suitable for high power dont care noise design. i suspect most of consumer products are dont care noise design. things will be 180 degree different if you are T&M nuts.and btw, both smps and linear psu are using transformer. thats what make it heavier or lighter from the other.

author: diyaudio
author_rank: Frequent Contributor
timestamp: May 16, 2014, 07:45:59 pm
content: Quote from: electros6 on May 16, 2014, 03:42:33 amA Linear design is simple and less expensive that's all, they more bulky though, cheaper to maintain and design. I don't buy into the SMPS is noisy excuses. SMPS are efficient, noisy bad designs yes...good ones (filters and pre-regulators iron out noise) they lightweight too and efficient to a theoretical efficiency of +/-90%, they much more complex to design though..Most manufacturers actually get another company to design a reliable SMPS. Sensitive laboratory equipment uses SMPS majority of the time.

author: pipe
author_rank: Contributor
timestamp: May 17, 2014, 08:54:23 am
content: Quote from: diyaudio on May 16, 2014, 07:45:59 pmBut laboratory equipment has a fixed output voltage and a reasonably steady load current, leading to very predictable switching noise that is "easy" to filter out. When you have to design your switch mode circuit for an output between 2 and 30 volts and a load current of 0 to 5 A, it get much more complicated.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 11:54:41 am
content: Nonsense about noise. Probably the real reason that SMPS are not used in many power supplies offered is that they are so simple to build and work so well and a very simple circuit can deliver up to 5-amps without any noise problems.All this means is that you could so easily build a SMPS yourself and not bother to waste a lot of money on complicated and obsolete analog power supplies.Any switching noise can be virtually completely eliminated by a tiny inductor-capacitor filter at a SMPS output.If you paid big bucks for a SMPS based bench power supply and looked inside the case and found so few components, you would think, Why in the #z!Q# did I pay so much for so little circuitry I could build myself. I am just paying for a nice cabinet!On the other hand when you look inside your linear bench power supply and see so many components you think, "Wow, this is really complicated..I couldn't have built this myself so easily, I am glad I paid so much for so much complicated circuitry!"The circuits below only require a power switch, power transformer, bridge rectifier and a electrolytic filter capacitor to have the raw DC power to feed the input of these switching circuits so to then have an efficient, simple bench power supply solution.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 17, 2014, 12:14:31 pm
content: I Have an Voltcraft VSP 2405 HE power supply which is switchmode.It is based on an TL494 (http://www.ti.com/lit/ds/symlink/tl494.pdf).Schematic (http://www.produktinfo.conrad.com/datenblaetter/500000-524999/511142-sp-01-en-Labornetzgeraet_VSP2405HE.pdf)I cannot say anything about noise and ripple, but what is do know is that it makes an awful whining sound when I'm using a low voltage 3-5Volt with less then 500 mA. And I do that a lot. It gets even worse if the load isn't linear but pulses it's current consumption.That might be a good reason NOT to get a cheap switchmode supply.It's actually designed by a company named Sonodyne in mumbai, Voltcraft just put their case around it.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 12:31:47 pm
content: Jeroen3, The circuit link you've posted does not use switching technology nor does the circuit use a TL494. The TL494 is a 35+ year old chip, a very much older technology SMPS chip with a low switching frequency that creates the problems you notice. The new designs I show here do not. Doesn't sound like you've bought a SMPS power supply with a good design! The newer chips do not create any audible sounds.It is a myth that someone has stated here that SMPS power supplies are complicated and "difficult to design" as you can see from the simple circuits I have posted.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2014, 01:09:04 pm
content: Quote from: Paul Price on May 17, 2014, 11:54:41 amwill that cover the 1-30V, 0-30A, and charted performance for the requirement of some lab grade psu?

author: diyaudio
author_rank: Frequent Contributor
timestamp: May 17, 2014, 01:09:48 pm
content: Quote from: Paul Price on May 17, 2014, 12:31:47 pmWhat a funny comment.The TL-494 old yes..., useless far from that word, the chip is exceptionally good, it has two error amplifiers that can operate in both voltage and current mode, it has an oscillator capable of up to 400kHZ (yes this is fast even for today's standards) it also has an adjustable dead-time section capable of manipulating and imposing duty cycle.Majority moderate power to high power SMPS electronics uses frequencies just under 100Khz, that is in push pull mode +/-50Kz per phase or single ended mode this is directly as a result of the magnetics, PCB layout and switching losses.The only advantage using any SMPS controller with speeds in the excess of 300Khz+ is aiming for a confined layout which results in smaller magnetics, double layer PCB (ground plane design) and tons of analysis in the RF and component parasitics that needs to be accounted for.

author: wraper
author_rank: Unknown
timestamp: May 17, 2014, 01:18:56 pm
content: Quote from: Paul Price on May 17, 2014, 12:31:47 pmWhat so special about those step down converter schematics you posted? There is no mains isolation, therefore you need another SMPS anyway. Nor it's low noise by no means.Can you compare that with HP E3611A linear lab PSU: Ripple & noise (v/i): <200 µVrms, <2 mVpp / <200 µArms, 1 mApp?P.S. "Audible sounds"?This is not about sound properties.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 01:26:25 pm
content: Mechatrommer, A 0-30V 0-30Amp power supply is very expensive, powerful, massive, "laboratory" power supply, and should not to be confused nor compared with something like the cheap 1.2-15V, 0-500mA or 0-1.5A "bench" power supplies most of the hobby-experimenters here are using to power up their projects.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 01:30:02 pm
content: Wraper: There is no need to use a SMPS power supply to be the power input,why not keep things simpleand use the same power transformer-rectifier-filter cap approach and still enjoy the simplicity, easy to build, low-cost, low-noise output design of a SMPS bench supply.Nonsense: "tons of analysis in the RF and component parasitics that needs to be accounted for"Enclose a power supply in a metal project case and it is shielded. Few hobby or experimenter types will find a need to concern themselves about these supposedly unavoidable "RF" effects, and after shielding in a metal box, I doubt if they can be detected at all without a NASA grade lab equipment setup, so how is this going to create a problem?I constructed high-frequency SMPS switches on simple perfboard with resulting very good performance. There is really no need for expensive double-sided PCB's. Instead, I get excellent performance with point to point wiring with attention to use ample size wires to create good power and good grounds connections.

author: wraper
author_rank: Unknown
timestamp: May 17, 2014, 01:38:55 pm
content: Quote from: Paul Price on May 17, 2014, 01:30:02 pmWith such approach you will get bad things from both designs: big transformer and huge ripple. Moreover step down converter cannot be used without a load and have very bad load regulation compared to linear PSU. There is no current limiting with such simple schematic too. BTW, low noise? There will be tens of millivolts ripple, maybe even hundreds in some conditions. And you will get huge common mode noise too.

author: TMM
author_rank: Frequent Contributor
timestamp: May 17, 2014, 01:58:56 pm
content: For low power circuits the traditional linear supply is a no-brainer. When you start drawing significant power, SMPS starts to have advantages.One significant advantage of a SMPS is that it can have active power factor correction giving it a PF approaching 1, where as the PF of a typical transformer, bridge rectifier linear supply is pretty awful. The traditional linear supply isn't immune to noise either because the transformer spews out RF interference at a 100Hz repetition rate due to the smoothing caps drawing large currents at the peaks of the mains voltage.

author: diyaudio
author_rank: Frequent Contributor
timestamp: May 17, 2014, 02:00:47 pm
content: Quote from: Paul Price on May 17, 2014, 01:30:02 pmQuoteThen its good for you.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 02:01:55 pm
content: Wraper: You do not get any ripple, these switchers are highly regulated and power line ripple is not present at the output without an overload condition, same as a linear circuit.It is as easy to add current limiting to a switching power supply as to a linear one.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 02:18:40 pm
content: For those of you who doubt SMPS power supplies and simple perfboard are unlikely bed fellows, consider that you can create large area, low-resistance, lo-inductance high current wiring on an ordinary perfboard by making cut strips of copper sheet (available from your local hobby or hardware store, sometimes sold as "Shim Stock"). These moderately thick copper sheets (.1-1.5mm) can be easily cut with ordinary scissors to create the few "double-sided PCB equivalent) high power handling "traces" that you want for high power SMPS circuits.In the few instances i could not avail myself to copper sheeting, I have cut small strips of single-sided blank PCB stock to create the few "traces" for ground and power wiring upon a perfboard.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 02:37:46 pm
content: TMM is it really a no brainer?These drop-in compatible 7805 (same size) switching regulators can do a fine job.

author: wraper
author_rank: Unknown
timestamp: May 17, 2014, 02:48:27 pm
content: Quote from: Paul Price on May 17, 2014, 02:01:55 pm

author: wraper
author_rank: Unknown
timestamp: May 17, 2014, 03:20:26 pm
content: Quote from: Paul Price on May 17, 2014, 02:01:55 pmNo ripple you say, search word ripple in the datasheet.

author: wraper
author_rank: Unknown
timestamp: May 17, 2014, 03:27:45 pm
content: This is very precise current regulation indeed. 3.5-7A +35/-25% regulation is just awesome .

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2014, 06:30:24 pm
content: The LM22679 eval board output 3.3V at 5amps and shows 20mV P-P ripple (the ripple is a 500Khz sawtooth ramp waveform). This is without a simple additional small inductor/filter small val. ceramic capacitor, and when this filter is added would reduce this ripple easily >10 fold.

author: qno
author_rank: Frequent Contributor
timestamp: May 17, 2014, 06:46:40 pm
content: Modern lab power supplies do have a switching powers supply.They also have Power Correction Factor.In the near future power supplies with large transformer and a diode bridge will not be allowed on the market as a new design.This is due to the bad current load on the net.The current drawn by a transformer with a diode bridge and capacitor is not a sinus and has a high content of 3, 5 and 7t harmonic.This is why they are banned for CE marking for power > 75W.

author: c4757p
author_rank: Super Contributor
timestamp: May 17, 2014, 07:17:52 pm
content: I've learned to consider "banned in Europe" with a grain of salt. "Banning things" seems to be something of a sport over there

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2014, 09:51:18 pm
content: the more they know, the more they ban, neverending story...

author: madires
author_rank: Super Contributor
timestamp: May 17, 2014, 10:22:25 pm
content: Quote from: qno on May 17, 2014, 06:46:40 pmHaven't heard anything about that yet. Any references?

author: Jeroen3
author_rank: Super Contributor
timestamp: May 17, 2014, 10:55:08 pm
content: The germans have some regulation regarding solar panel inverters since they made the grid unstable on a typical <50% clouded day.Germans overall seem to really like all sorts of certifications, it's a pain selling something there. You actually need to have it emc-tested for a lot of money before you market it, instead of the other way around the people usually do.But prohibiting big transformer devices from CE marking is new for me.

author: madires
author_rank: Super Contributor
timestamp: May 18, 2014, 12:45:25 pm
content: Quote from: Jeroen3 on May 17, 2014, 10:55:08 pmReally? I tought that the CE conformance is the same for all EU members.

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2014, 03:04:57 pm
content: Quote from: electros6 on May 16, 2014, 03:42:33 amThey have been around since the 1980's. Maybe even sooner. I have several from HP. Usually they are a bit more expensive and some (the early ones from Delta) don't have a very good load regulation. The major advantage is low weight and low heat. Most circuits I work on don't care about noise and most linear regulated power supplies aren't exactly low noise either. IIRC my HP supplies emit about 40mVpp noise (with a load attached).

author: poorchava
author_rank: Super Contributor
timestamp: May 18, 2014, 03:16:13 pm
content: They are used in some high power supplies like those made by Delta. For example at work we have a couple of 60V/30A supplies. Those would have been huge if they were linear and would quite likely require liquid cooling as well.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 18, 2014, 04:06:48 pm
content: Silicon Chip Magazine has come up with a project as mentioned by the OP. Have a look. Its over 3 months. Next month I think will the final part I am hoping the kit suppliers pick this one up. 0V to 40V @ 100Wtotal. Not too bad I think,

author: qno
author_rank: Frequent Contributor
timestamp: May 18, 2014, 08:54:31 pm
content: FYIEN 61000-3-2 is a proposed guideline.I remember someting special for lighting appliancesGoogle "power factor correction regulations"

author: Jeroen3
author_rank: Super Contributor
timestamp: May 19, 2014, 05:38:47 am
content: Quote from: madires on May 18, 2014, 12:45:25 pmIt is. You just don't have to get something tested and certified to put the sticker on.

author: TMM
author_rank: Frequent Contributor
timestamp: May 19, 2014, 10:18:34 am
content: Quote from: Paul Price on May 17, 2014, 02:37:46 pmI meant as in the whole architecture of a bench power supply - transformer, rectifier, caps, linear regulator. Reliable, serviceable and cheap.

author: Rerouter
author_rank: Super Contributor
timestamp: May 19, 2014, 11:26:25 am
content: equally easy to troubleshoot at a glance, the higher the frequency the switch-mode the more black magic you need to pry apart to find out a bootstrap cap has lost capacitance, or a self resonance point has shifted to a point that it makes the output glitch under high loads, etcthough i will say i have seen some very novel 2 transistor and single op amp constant current / constant voltage supplies (plus 7 resistors) from around the late 80'sSwitchers could be on a similar par if you didn't have to pick for a voltage or current range, if both of them are wide it sometimes is easier to make it multiphase and power up more regulators as needed, which is added complexity,For very high power switchers become almost the only option, as heat losses become a large consideration,