Choosing transformer for a switching-mode PSU - Page 1
Forum section: electronics / projects

author: ttodorov
author_rank: Regular Contributor
timestamp: January 01, 2015, 03:42:31 am
content: Hello,I am designing my own PSU based on the Linear device LT8612, which is a 6A-capable step-down switching regulator. My design needs to provide only 4A max DC current at 30-32V, which is within the capabilities of the chip. What I have a problem with is sourcing a suitable power transformer at reasonable prices. I can find a 160VA tororidal tramsformer with 18-0-18 secondaries for about US $80, but is this the transformer I need and can I find anything cheaper?To my understanding, the 18-0-18 secondaries in series will give 36V AC, or 36 * 1.414 = 51V DC after rectification. But this will be without load. The LT8612 has a recommended max input of 40V DC, so first question is whether the 51V will drop low enough when the switcher starts pulling current as to not damage the chip? Or do I have to look for a 28V AC transformer (28 * 1.414 = 39.5V DC after rectification)?Second question: If I need to provide 4A @30V to the PSU load, do I need to provide the same current to the switcher? If I remember my basics correctly, converting from a higher voltage to a lower voltage given a set output current requirement, requires less input current - do I have this right or is this total bull? Btw, the LT8612 works with better than 90% efficiency, if that matters... Unfortunately, I have't seen a datasheet for a switching regulator that would also explain how to choose a transformer given the design requirements. Anyway, I also searched for "transformer" or "switching supply" here on the forum and I did not find anything to answer my questions sufficiently, so I created this topic.Hope the questions were straight and understandable enough to not confuse potential "helpers"... ;-) Answers highly appreciated!Best regards,todor

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 01, 2015, 04:23:44 am
content: Why not an off-the-shelf power supply?There are few reasons to use iron core transformers anymore. Mostly they're used for applications where the simplicity provides higher reliability and maintainability (usually for industrial purposes), or the overload capacity is important (short-circuit current adequate to start motors, clear fuses). There may be some relatively small applications where they're still cheaper (wall warts?), but even those are being encroached by ever cheaper and more compact switchers.Tim

author: MrAl
author_rank: Super Contributor
timestamp: January 01, 2015, 04:28:03 am
content: Quote from: ttodorov on January 01, 2015, 03:42:31 amHi,You can not allow a chip that takes say 40v to get 50v for any amount of time. If this happens then you must take steps to correct this. If 40v is the absolute max, then you should not even run it at 40v but some lower value like 35 volts.For the converter i assume you are using a Buck circuit. If the input is 30v and the output 15v at say 2 amps, then the input current is roughly 1 amp. That's because it acts like a transformer with a turns ratio of 2 to 1.Of course there is the efficiency which means it will draw about 10 percent more current than that, say 1.1 amps.If you have 32v in and 30v out however there is almost no scaling so with 4 amps out it will require slightly more than 4 amps in. With 35v in and 30v out the scaling is minimal, so the input current will only be about 5 percent less than the output current.

author: Alex30
author_rank: Regular Contributor
timestamp: January 01, 2015, 04:48:59 am
content: Buck regulators do tend to require less input current at higher input voltages, however I cannot provide much more information on this because I don't know. But with this in mind, you sometimes have to be careful about initial start up, as when the device is powered on the input voltage tends to take a few ms to rise to a higher voltage as the caps are charged etc. This is an issue because the buck may pull more amps than what you would expect until the input voltage has risen high enough. Think delayed startup.Also in regards to transformers etc. As I am a hobbyist I am not that enthusiastic about setting up my own transformers, as setting up an enclosure where I could mount a transformer safely and take all the design precautions I would want to take is mostly beyond my capability at home. In addition to this, transformers areveryexpensive for the hobbyist, as we do not benefit from bulk ordering. I would recommend just getting a wall adapter if possible as it will be much cheaper than buying the transformer alone. Sometimes due to your power requirement it will be hard to find an adapter that suits and you have to buy your own transformer etc but most of the time you don't have to. Also I would never buy a wall adapter or transformer off ebay. Full of crappy power adapters that are not capable of delivering the rated power and are downright dangerous, not only for being a fire risk but also electrocution. My favorite way around this is to buy used laptop adapters (don't buy the new ones on ebay as they are generally knock offs). They tend to be at nice DC voltages and capable of several amps. Ripple is also quite minimal with these adapters too so that's nice.Only downfall is some of them use strange plugs and wires. I bought a 20V 7A laptop charger that had a weird plug on there. I thought no worries, chop that off and solder on a DC plug, only to find the cable inside was made up of an outer sheath wire that was the ground and within that was a ridiculously large gauge wire. It was impossible to wire this to a DC plug without replacing the cable entirely. Since those little boxes are not made to be opened up, I would have to dremmel it open and mess around, so it's just sitting in my parts bin for now.So yeah, check out used laptop adapters and make sure they have a standard DC plug to save yourself the trouble.

author: theoldwizard1
author_rank: Regular Contributor
timestamp: January 01, 2015, 01:37:28 pm
content: Wind your own !The secrets are in this application (post #47)ExcellentIT(7300)Designing one is an iterative process. Select different core style (EE and ETD are the most common) and sizes as well as wire size.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 01, 2015, 01:48:27 pm
content: It is not clear why you want to make a SMPS but use a conventional "heavy iron" step-down transformer? Is there some special requirement you didn't mention that suggests this solution? Otherwise, it seems like you are almost choosing the "worst of both worlds" and making things much harder on yourself. I agree wtih T3sl4co1l, that it would seem much more logical to simply buy a commercial SMPS module. Unless your objective is to learn arcane power supply design or something?

author: prasimix
author_rank: Unknown
timestamp: January 01, 2015, 02:14:12 pm
content: Quote from: MrAl on January 01, 2015, 04:28:03 amI didn't have yet experience with LTC controllers but this is the fact at least with various TI controllers. You can burn it in a matter of second if absolute ratings are exceeded. So as MrAl suggested it's better to go with 35VDC (24VAC). If 40V is a limit and you need 30-32V on output you are already in region of buck-boost controller.Quote from: theoldwizard1 on January 01, 2015, 01:37:28 pmHm, ttodorov asked for low frequency EI or toroidal transformer that will be connected to AC main before rectifier bridge. Mentioned application is intended to make calculation for high frequency transformers for application such as flyback, SEPIC etc. BTW, you link does not work here is the correctone.

author: ttodorov
author_rank: Regular Contributor
timestamp: January 01, 2015, 02:42:37 pm
content: Thank you all for the answers!Here is the full story. In the July 2014 edition of LT Journal there is a nice circuit for what the author claims to be a very stable, minimal ripple bench-grade PSU with 0-24V and 0-3A capabilities. It is based on the LT3082 linear regulator, which can do 1.5A but can be paralleled for multiples of the output current. The LT8612 switching converter acts as a pre-regulator, providing the output voltage + 2V to the linear regulators, in order to keep everything cool and reduce conversion loss due to heat. I am changing the circuit to add digital control and voltage/current display for set and actual values, using a Cypress ARM Cortex-M0 mcu with ADC and DAC. I was able to get all the chips from Linear as samples, and I bought a lot of the MCUs when Cypress were offering them for US $1 a pop.So yes, I could pay $450 for a new Rigol PSU, but then, when do I get to learn something?As to the transformer specifics... The circuit in the magazine was designed to provide 24V max and with absolute max of 42V input to the LT8612 pre-regulator, this is enough headroom for it, so a lower power transformer can be used. The decision to increase the max voltage to the load to 30V is mine. So if I use a 28VAC transformer, the rectified voltage is 39.5VDC. I am hoping that with 42V absolute maximum input, the LT8612 will be ok with 39.5, especially as it is soft-start capable and I can control the inrush current that way. I am thinking of adding a relay in front of the PSU output so I can disconnect the load at will, so the ramping up of the voltage/current to the pre-regulator will not be that bad. The LT3082 have short-circuit protection, so the pre-regulator should not see any bad spikes beyond what it can handle during operation. So the correct transformer is the last missing piece in my puzzle.If after all is said I have this correctly, then with a 28VAC transformer I would need it to be able to provide at least 3.2A. Taking in mind the conversion losses and the 90% efficiency of the switching pre-regulator, that goes up to around 3.7A. So I should be looking for either a transformer able to provide 28VAC at 4A or a DC converter to provide 40V at 4A.Which is to say, after looking at transformer prices at mouser, newark and allied electronics, I see why lab psu units cost so much :-(Wish everyone a happy new year!

author: ttodorov
author_rank: Regular Contributor
timestamp: January 01, 2015, 02:50:18 pm
content: Here is the original circuit, if anyone is interested:http://www.linear.com/docs/45095

author: prasimix
author_rank: Unknown
timestamp: January 01, 2015, 03:12:35 pm
content: Yes, nice circuit. You can always try but I'm afraid that your margin is unacceptably small. LTC does not have such a great offer when you goes above 40V like TI for instance. I'm currently working (and learning) on my programmable PSU and currently testingLM5118as pre-regulator for main andLM5574for bias supply. Both of them goes easily beyond 50V.It seems that working beyond 50V can be challenging even for some manufacturers of pasive component. Look what is happened to some (not all)AVX capacitorsthat is rated 100VDC when connected to the 67VDC.

author: wraper
author_rank: Unknown
timestamp: January 01, 2015, 03:30:39 pm
content: Quote from: ttodorov on January 01, 2015, 02:42:37 pmMaximum input voltage is not related with inrush current. You can easily destroy the IC even with output completely disconnected. Did you read the datasheet?ABSOLUTE MAXIMUM RATINGSNote 1: Stresses beyond those listed under Absolute Maximum Ratingsmay cause permanent damage to the device. Exposure to any AbsoluteMaximum Rating condition for extended periods may affect devicereliability and lifetime

author: wraper
author_rank: Unknown
timestamp: January 01, 2015, 03:35:42 pm
content: Do not forget about transformer tolerance and load regulation (rated voltage is under rated load, it noticeably increases without load), mains voltage tolerance.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 01, 2015, 04:55:25 pm
content: Things are clearer when you disclose the whole picture. Note that there is a very big difference between a fixed-voltage PSU and a variable BENCH SUPPLY. If you had identified your project as a bench supply, and as a modification of an existing design, you might have got different response from the start. It is rarely to your disadvantage to leave out details (unless the post starts running to thousands of words).

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2015, 05:30:42 pm
content: Why not just buy a SMPS to use for the front-end?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 01, 2015, 06:24:09 pm
content: Quote from: Hero999 on January 01, 2015, 05:30:42 pmTtodorov is trying to design a VARIABLE voltage-output "pre-regulator" for a bench supply. That would appear to preclude any sort of fixed-voltage linear or SMPS solution.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2015, 06:54:39 pm
content: Quote from: Richard Crowley on January 01, 2015, 06:24:09 pmNo it wouldn't. What I meant was as he's thinking of powering the SMPS with a mains transformer and rectifier, he might as well power it with a fixed voltage SMPs.

author: ttodorov
author_rank: Regular Contributor
timestamp: January 01, 2015, 11:42:16 pm
content: If I want to use the suggested schematic (unfortunately I am not as advanced to be able to design my own bench PSU from scratch) and go with the LT8612 as a pre-regulator, then I cannot use something that supplies lower voltage to it - at least not without designing a boosting pre-pre-regulator circuit as the LT8612 is not a SEPIC converter. According to the original article the pre-regulator is supposed to deliver the desired output voltage of the PSU + at least 1.7V. So if I want to do 0-30V (or 32V for some headroom), then the LT8612 must be able to supply 1.7-34V. Both the article and the datasheet recommend to supply the LT8612 with 5V more than the max design output, so that makes the requirement for the AC-DC stage at least 39V DC. I found some schottky diodes (SBR10U40CTFP) for the rectifier bridge, that can take 10A forward current (150A surge) and have only 0.25V forward voltage drop, so my thought of using a 28VAC transformer is right at the edge, but should be still doable.As to the absolute maximum of 42V supply for the LT8612, would a shunt circuit with a mosfet and a voltage reference between the rectifiers and the pre-regulator guard it from spikes higher than 42V?Thanks again for all your help!

author: Alex30
author_rank: Regular Contributor
timestamp: January 02, 2015, 01:42:38 am
content: Quote from: ttodorov on January 01, 2015, 02:42:37 pmQuote from: ttodorov on January 01, 2015, 11:42:16 pmI think you mean LT3081, which has a max dropout voltage of 1.5V. Not sure why they say to supply +5V to it? Maybe so that when you turn the voltage up the tracking pre-reg can keep up? I honestly would think that you could supply less than 5V without a problem. If it was me I would just go with 2V dropout. Maybe someone will tell me i'm wrong though.

author: theoldwizard1
author_rank: Regular Contributor
timestamp: January 02, 2015, 01:48:04 am
content: Anyone trying to build their own power supply is obviously on a budget !24V or 24.6V are pretty common (inexpensive). 28V are not common.This one on Jameco is only 2A.A pair of these12.6V 4A transformerswould probably be closer to your goal

author: ttodorov
author_rank: Regular Contributor
timestamp: January 02, 2015, 02:09:10 am
content: Quote from: Alex30 on January 02, 2015, 01:42:38 amYes, that is correct - I meant LT3081. Anyway, the (max VoutPSU+5V) is for supplying the LT8612, not the linear regulator.Quote from: theoldwizard1 on January 02, 2015, 01:48:04 amWhile that is most probable, it is not necessarily always the main factor. I am trying to develop my own digital control and display for the PSU using an ARM micro with its inbuilt ADC and DAC and that is my main interest. I just thought that a PSU with such characteristics has a lot of added value, even though I already have a both a linear and a switching bench units. I am well aware that building my own with enclosure and everything will top the combined price of the 2 inexpensive ones I have - but it will have better specs and more features. And it is something that I can get some use out of after it is done.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2015, 02:11:38 am
content: We mean, just a commercial SMPS at the input, instead of the iron core transformer + rectifier.The post regulator stuff is all unchanged.28VAC is far too much. You should have a 50% margin on maximum ratings. Reliable design goes something like...24VAC off the transformer: which will actually be more like +/-10% for extreme  line variation.Ok, make it 26.4VAC.Which makes up to 37.3VDC (no load).We want to clamp that with a TVS, to protect against surge and overload. The TVS needs to handle 37.3VDC for eternity.The TVS' actual breakdown will be more like 44V, and will actually clamp more like 55V under surge conditions.(This is most relevant if you've got, like, a hotplugging situation, or dirty automotive power (which however comes with the added burden of load dump) -- whereas, for an iron-cored transformer and filter circuit, you get some surge protection from the transformer, due to saturation, and from the capacitor, due to its sheer size. It's also a lot of potential current capacity for a mere TVS to withstand -- suppose the transformer were accidentally supplied from 240V instead of 120V: for a fraction of a cycle, it would dump 56VAC into the circuit with tons of amperes, toasting the TVS, maybe even blowing it open and damaging the rest of the circuit, before the AC line fuse finally opens.)For the subsequent circuitry to survive while the TVS is doing its clamping, it needs to be rated over 55V, maybe 60 or 80V. (50% above 37V is 55V, so I wasn't lying on margin, earlier.)From a 24VAC transformer, a 40V chip is already very marginal. Yeah, it'll work, and probably work for a long time. But one day you come over, turn it on, and *pffst...bang*, out comes the magic smoke.So, if you can find an 80V buck chip (usually advertised for automotive, industrial and telecoms applications), very much equivalent to the one you're eyeing right now, you can use the 28VAC transformer just fine.Tim

author: DanielS
author_rank: Frequent Contributor
timestamp: January 02, 2015, 02:30:46 am
content: Quote from: Alex30 on January 01, 2015, 04:48:59 amNot quite: if your load requires 50A, your buck switch, buck inductor and the decoupling caps upstream from it still need to be able to pass 50A. The power supply upstream from the buck converter's decoupling caps on the other hand only see the averaged current assuming adequate buck converter input decoupling is present. If your buck regulator is operating at 10% duty-cycle while providing 50A to its load, it will draw 50A from its input decoupling caps only 10% of the time and the upstream supply will see 5A average.If you want to trade increased duty cycle for lower switch current to feed a higher-current lower-voltage loads, you would need a transformer-based converter topology.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2015, 02:51:23 am
content: Quote from: DanielS on January 02, 2015, 02:30:46 amPeak*Peak is important for some things (like if there's series inductance to the switch, you need to know that the energy stored there is handled appropriately!), but RMS is important for others (like cap ESR and wiring). RMS = sqrt(D) * Pk for a "pulsed DC" waveform, so if it's a large ratio converter (like, 50V down to 5V, so D ~= 0.1), you stand to save quite a bit (~0.32 x) relative to dimensioning everything for sheer 50A capacity.Of course, you need the inductor rated for minimum inductance at maximumpeakcurrent, so that's another potential gotcha to look out for. Mainly with ferrite cores, where saturation is fairly sharp. Powdered iron and composite types saturate slowly, so this isn't as big a deal.Don't forget saturation of thecapacitors, either, if you're using a lot of ceramics. Not for voltage ratings reasons, but for saturation: you generally want to usetriplethe voltage rating if you're using type II dielectrics (X7R -- please avoid anything worse than X5R, like Z5U, etc., honestly), just so the capacitance value doesn't drop off so much. This isn't a concern with conventional (electrolytic, aluminum polymer, tantalum, etc.) or type I (C0G, SL) types.Tim

author: Alex30
author_rank: Regular Contributor
timestamp: January 02, 2015, 02:56:03 am
content: Quote from: DanielS on January 02, 2015, 02:30:46 amBut I was talking about the actual power supply or transformer feeding the buck switch. I thought the capacitor at the input of the buck smooths out the switching load such that the transformer will only see the maximum current output of the buck or less.

author: DanielS
author_rank: Frequent Contributor
timestamp: January 02, 2015, 03:22:29 am
content: Quote from: Alex30 on January 02, 2015, 02:56:03 amYou are correct - if you do your buck converter decoupling right, your AC transformer, rectifiers and 100/120Hz input filter only need to worry about providing the buck converter's averaged (filtered/decoupled) worst-case current. (On top of all the usual AC-related rectifier and capacitor sizing stuff, of course.)

author: Alex30
author_rank: Regular Contributor
timestamp: January 02, 2015, 03:46:46 am
content: Quote from: DanielS on January 02, 2015, 03:22:29 amWhich is why I was saying that for example if you have a buck converter that has Vin=20V Vout=10V which at say 90% efficiency means that if you want 4A out you only need 2.2A in. But if Vin takes a while to rise to 20V, and the buck is trying to supply 4A, then the supply current to Vin will start at 4A and fall to 2.2A as Vin rises to 20V. This is why you have to consider slow turn on if your supply can't deliver 4A.

author: ttodorov
author_rank: Regular Contributor
timestamp: January 02, 2015, 09:07:09 pm
content: Wow, this turned out to be a very interesting discussion! Thank you all for contributing.So I looked into using a ready-made SMPS of the laptop-like kind, just because it will allow way more flexibility in selecting the enclosure for my project, since there is not going to be any bulky 4x4x4in transformer. Well, guess what? There are none to be found, which deliver 36V @5+A. What I could find was 48V @2A. The other option is to use an industrial power supply from Hengfu, which delivers 36V @4.2A, but that seems borderline for my requirements as well. Plus, the supply frame is about 8x4x2in, so this is in no way going to be more compact than using a transformer. I might have to reconsider what was suggested in a previous message and switch to a higher rated TI switching pre-regulator and go with a 48V supply, which will need less current for the input because of the more headroom down to the 30V-32V PSU max output requirement.On the other hand, I foundthis transformerat MPJA.com, which delivers 24VAC @10A for US $30. I might just order one and see what is coming out of it after rectification with and without load.I have another question with regard to protecting the pre-regulator from voltage spikes when the conditions are already marginal (say, when the input to it is already close to its absolute maximum rating). Would putting a resetable fuse (e.g. 4A hold current, 6-8A trip current) in series with a crowbar circuit (e.g. LM431 and a triac) in between the rectifier bridge and the input of the pre-regulator work reliably to prevent the LT8612 from blowing out?Best,Todor

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 02, 2015, 09:38:10 pm
content: Yes, finding a "line lump" power supply at a higher-than-average voltage that will produce 150W is unlikely.However MPJA also sells this unit which should be more than adequate...36 Volt Power Supply, 8.5A, Switching, Hengfu US$65.00http://www.mpja.com/36-Volt-Power-Supply-85A-Switching-Hengfu/productinfo/16490%20PS/I would doubt that a polyfuse would be suitable (fast enough) to protect against spikes in your scenario.One of the corollaries of Murphy's Law is that semiconductors will fail in order to protect fuses.

author: Marco
author_rank: Super Contributor
timestamp: January 02, 2015, 10:29:04 pm
content: Quote from: Richard Crowley on January 01, 2015, 06:24:09 pmSomething with a simple non resonant flyback controller shouldn't be hard to mod ... it doesn't really care if the output voltage is lower than designed (other than ripple increasing).

author: ttodorov
author_rank: Regular Contributor
timestamp: January 02, 2015, 11:45:42 pm
content: Quote from: Richard Crowley on January 02, 2015, 09:38:10 pmActually, my thinking is that the crowbar should be there to protect the pre-regulator from spikes. If the spike is short, then the voltage reference will turn off the triac and the operation of the PSU will resume normally. Otherwise the triac from the crowbar will continue conducting and start heating up. The polyfuse is there to prevent the triac from blowing up eventually. For that it shouldn't matter that the polyfuse is slow as long as I choose a beefy enough triac that could handle a lot of watts. Right?

author: Alex30
author_rank: Regular Contributor
timestamp: January 03, 2015, 03:06:09 am
content: If you can't work out a decent solution for an adapter and are still not keen on using your own transformer then you could try and get an adapter that can deliver lots of amps (like 8A @ 19V) and then buck boost it to your needs. Kind of a bit hacky using a buck boost -> buck step down but relatively simple.

author: ttodorov
author_rank: Regular Contributor
timestamp: January 03, 2015, 05:26:15 am
content: I just ordered some TPS54560 step-down converters from TI (up to 60V input) to use instead of the LM8612. It will give me more margin for the input and more options to choose for the power supply. At this point I have narrowed down the options to these:1) Go with a EI bobbin transformer. Price range around US $30. But no PFC, so it will get really hot when the actual load is disconnected from the my PSU.2) Go with a toroidal transformer. Price range around US $50-60. More efficient than EI, less space in the enclosure, less heat in idle mode (when the load is disconnected), a lot less EMC(??), and maybe(??) the efficiency will offset the lack of PFC in my circuit.3) Go with a industrial power supply or a power adapter. Industrial power supply can be found cheapest of all, but the size is 8x4.5x2 inches, which really sucks. Besides, the cheapest units that can be had for less than US $20 are off ebay, so how much can I trust them? On the other hand, Mean Well have nice laptop-like 160W adapters (found it onMouserfor US $56), which also provides PFC. As it will be outside of the enclosure, it will also let me choose or design a compact enclosure for my project, which would be really neat. But that's a hell of a lot more components, that can fail than the first 2 options...Thanks,TodorThere is something else that I've heard, so maybe somebody more knowledgeable can confirm or set me straight. When rectified, EI transformers will produce Vdc = 1.414 * Vac, but that is without load. When loaded, they will drop down closer to the their Vac rating (how close?)... On the other hand, toridal transformers will not drop down, when loaded. Is this correct?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2015, 06:34:15 am
content: Quote from: ttodorov on January 03, 2015, 05:26:15 amEhh...what?PFC is an under-load thing (power factor correction, meaning, the phase shift or harmonic content of the AC load current drawn by the equipment).Magnetization current and core losses control how warm it gets at idle, which is certainly not a problem for any standard transformer which is capable of operating continuously at rated current (it's always hotter under load than when unloaded).QuoteThe power factor will actually be slightly worse, because the supply impedance will be lower, and therefore, larger current gulps will be delivered to the filter capacitor.Power factor has little effect on the performance of the circuit. All that entails is using a transformer rated for amps AC RMS = 2 * DC load amps, give or take. (In other words, a power factor of about 0.5 is typical for capacitor-input filters.)QuoteQuality SMPS aren't terribly more vulnerable than iron cored transformers. How much reliability are you expecting? Do you want to fly this in space? Is it medical where peoples' lives depend upon it? If the answers are no, then you probably have a typical lifetime in mind (or some idea about it). 2000 hours would be pretty awful for benchtop equipment (you accidentally leave it on overnight for a few months, and poof... there go the capacitors), but 10khr and up is pretty reasonable (several years continuous operation, or decades under infrequent use).A monolithic power supply is also vastly easier to replace if down time is a problem. Plug in a replacement and you're out about ten seconds (plus the cost of a new power supply, true). The transformer method is easy to work on, but takes longer to repair.PFC isn't a big deal. Maybe if you're going to build ten of these, and run them all at once on a test bench, then you'll have some worries about wasting line capacity -- a 120V 15A circuit offers 1800VA capacity, but at PF = 0.5, you can only use 900W of that. The rest is NOT "going up as heat", it's simply that watts and VA are two different quantities, and it's the nature of the current waveform that's causing the extra heating (and therefore elevated RMS current, for a given power delivery) in your line wiring.Outside of those kinds of situations, PFC is more of a feel-good thing. It's also being pushed more and more for consumer applications (because reducing AC line burden is a good thing), but probably doesn't factor into the requirements of lab equipment.QuoteUnder load, the drop depends on supply impedance. An EI transformer with side-by-side (bank or bobbin wound) construction typically has quite a high impedance, resulting in higher power factor (maybe 0.7ish?) and lower full-load voltage. VDC = 1.25 x VAC(rms) isn't unreasonable. Less would be surprising, and I'd wonder just how hot the transformer is getting at that point.Toroidial transformers have lower impedance, so the drop is less, but still present.Note the drop also depends on filter capacitance. A very large capacitor, supplied from a very low impedance source, will drop almost no voltage between charging peaks. A very small capacitor, even if supplied by a low impedance, will discharge so quickly that the output voltage tracks the supply voltage: VDC output will never be less than 0.9*VAC (the average level) for this reason.You also have to consider a possible 10% fluctuation in line voltage, so some days it might be floating high (or jump relatively quickly as nearby loads switch on and off), say to 132V (from 120V nominal), or pull down to 108V (or even less under brownout conditions). Together with the load variation (above), your circuit should be able to deal with a good +/-20% variation. Less dropout, the minimum of this range defines the maximum regulated output voltage from a linear or buck converter. Likewise, the maximum of this range should be used to define power dissipation (linear regulator -- worst case voltage drop), and the point where protection (if included) should begin to pull in (TVS, crowbar, etc.). Finally, some margin further above this, sets the voltage rating of the devices.Tim

author: prasimix
author_rank: Unknown
timestamp: January 03, 2015, 10:35:21 am
content: Quote from: ttodorov on January 03, 2015, 05:26:15 amI hope you'll inform us how it works especially if you are going to use the same tracking control built around Q3. I'd like to suggest to test it first as stand alone without connection to the rest of the cirucit.

author: tcsaba101
author_rank: Contributor
timestamp: January 03, 2015, 06:38:40 pm
content: Nice discussion!My experience is that smps makes a lot of noise on the workbench.I try to keep off all the smps units very far from my bench when I am using my scope (Rigol DS2072).Sometimes I see more effects of the switching PSU-s, than my circuits signal.At the beginning it drove me crazy: what creates this waweforms in my circuit?Then I found out to change some power units to better quality and put in distance.The E2050 GPIB-LAN converter case was the worst. I used a 5V/1A simple wallplug adapter (all coming from china) what was close to the scope.It has caused a horrible noise on the signal. Finally I have changed the supply to an old smps what was supplied by an external CD reader I don't use for a while.Still I have some noise on the signal, but at least I see what I want to measure.So I am thinkong to change all the PSU-s around the workbench to toroidal transformer based ones.It is a bit of out of the original goal of this thread, but do you have some ideas how to handle this issue?Csaba

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 03, 2015, 06:51:25 pm
content: Quote from: tcsaba101 on January 03, 2015, 06:38:40 pmAnd yet, your scope has a substantial SMPS right there behind the display and circuit board!

author: tcsaba101
author_rank: Contributor
timestamp: January 03, 2015, 08:23:38 pm
content: Quote from: AcHmed99 on January 03, 2015, 07:07:17 pmCan you suggest models from low consumption 10VA till some hundred VAs?

author: tcsaba101
author_rank: Contributor
timestamp: January 03, 2015, 08:26:43 pm
content: Quote from: Richard Crowley on January 03, 2015, 06:51:25 pmBased on the weight I suspected there is an smps in the scope, but it is still under warranty, I haven't looked into.So there is solution at least...

author: prasimix
author_rank: Unknown
timestamp: January 03, 2015, 09:08:48 pm
content: According to LTC it's possible to build SMPS which is possibly better then many linear PS. Check i.e.DN316 - Ultralow Noise Switching Power Supplies Simplify EMI ComplianceandAN70 - A Monolithic Switching Regulator with 100µV Output Noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2015, 12:19:12 am
content: Quote from: prasimix on January 03, 2015, 09:08:48 pmYes, the problem is not topology, the problem is responsibility (and cost). You get a $3 converter on eBay, you have no room to complain, you bought exactly what you should've expected! Irresponsible designs like those are not only noisy and illegal radiators, but dangerous to public safety (as has been mentioned before -- like the teardowns of those counterfeit "Apple" plugs).Even a very noisy device can be filtered 'til the cows come home. The power supply in my TDS460 probably isn't very remarkable itself, but it is absolutely chock full of inductors, evidently the amount of filtering they required to meet spec (some analog, and including lots of high speed low jitter ECL).Tim

author: ttodorov
author_rank: Regular Contributor
timestamp: January 04, 2015, 08:05:59 am
content: Quote from: prasimix on January 03, 2015, 10:35:21 amGot a tracking number for my TI buck converters, so we'll see how that goes. Unfortunately, both kinds I ordered (54560/54561) are thermal pad packages, so I have to order a board for them through a pcb fab. Since I am not willing to pay premium shipping from China, that'll take about a month. And if life happensTM, that could take consequently longer, so it might be a while until I report my experience for those interested.I don't see why the same tracking circuit wouldn't work for the TI chip though... The TPS has about same internal reference for the feedback voltage, about 100mA less, so maybe just tweak the resistor to ground a bit... And I am thinking, with a higher input voltage pre-regulator, I should replace Q3 with another PNP transistor that has similar characteristics to the 3906, but can handle at least 60V collector-base voltage, something like the BC856. Otherwise it should just work the same.Anyway, at this point I decided to go with a 48V/3.34A Mean Well desktop power adapter. It costs $25 more than going the transformer route, but then the price difference between a nice flat enclosure that will only fit my power converters+control MCU+LCD and a big one that will also have enough space for transformer and fans is more that $30...Thanks for all the advice, it is really helpful!Regards,Todor

author: prasimix
author_rank: Unknown
timestamp: January 04, 2015, 10:31:39 pm
content: I didn't see either but just asking. Anyway I think that playing with feedback loop should not be taken lighthearted. This part of the system is very sensible and stability easily comes into the question (or be improved as in the case of adding properfeedforward capacitor). So my suggestion is to try chosen controller first with simple resistor network then measure if transistor based circuit can really provide proper Vref required for error amp and after that try to replace resistor network with it.

author: ttodorov
author_rank: Regular Contributor
timestamp: January 06, 2015, 06:37:05 am
content: Quote from: prasimix on January 04, 2015, 10:31:39 pmI will certainly pay attention to that 1nf cap from the original circuit. I am designing my circuit with a lot of test points, so if I see some unexpected behavior, I will be sure to at least empirically narrow down the correct feedforward capacitor ;-)Ans as to your post from several days ago, where you shared the image with the busted ceramic caps... I do not thinks that is a voltage rating issue, or at least not solely. I have been reading the TI datasheets on the TPS54560 (to be used as a pre-regulator) and the TPS54160 (going to use this one as either 1.5A 48-to-12V or dual rail +/- 15V to drive the MCU, LCD and some 16bit DACs and a 16bit ADC, with additional linear regulators of course). So in these documents there are some nice explanations about how important the capacitor rms current rating is. I've been looking at mouser.com for the parts I need and so far I haven't seen this rating on any of the offered ceramic SMD caps, so you might have overlooked this calculation as well. The docs describe how capacitors tend to emit the magic smoke, when this value is overlooked, in conjunction with not enough capacitance, when there is a sudden demand on the current - say from no load to full load - and the switcher is working at lower frequency for better efficiency.... Apparently the error amplifier in the regulators need 2 clock cycles or more to catch up to the changed conditions and the capacitors must take up the slack in the mean time. Who woulda thought!?

author: prasimix
author_rank: Unknown
timestamp: January 06, 2015, 09:23:09 am
content: Yes that's possible. The strange thing is that destroyed capacitors were not there alone. There was 2x2200uF elco in parallel with them and that's happened without any load connected. Simply increasing input voltage by switching from 36VAC to 48VAC was enough for instant damage.

author: ttodorov
author_rank: Regular Contributor
timestamp: January 06, 2015, 04:05:34 pm
content: Interesting. Do you know, if the electrolytic ones were destroyed first or after? Is it possible that destroying one type generated a spike high enough and long enough to destroy the other type?