Power Supply Design : Questions about Dual Supply and Transformer - Page 1
Forum section: electronics / projects

author: LittleRain
author_rank: Contributor
timestamp: August 17, 2020, 09:22:16 pm
content: I just designed a switching PSU that converts 120V AC to 42V DC, and is capable of 20amps.Just realized I need to create a dual supply for an audio amp I am creating, so I want to convert my single supply design into a dual supply.Every dual supply schematic I look at, the common ground is connected to the center tap of the transformer.But my single supply PSU does not have a transformer on the high voltage side, so I'm just wondering,where would the common ground point even be, and is it even possible to create a dual supply without a transformer?Do I need to add a transformer, or a second bridge rectifier, for a dual supply?I was trying to draw something up with a second bridge, but I just don't see how I would do it.Or could I just invert the voltage after the buck converter/capacitance multiplier?I would add a transformer, if I could find one that can do 20 amps on the secondary, but they are all like $100, unless its only 12V.On aliexpress they sell a +/-35V dual PSU for around $50 CAD, so I know its possibly to make it fairly cheap, but I'm just not seeing how right now.So if anyone has a transformer recommendation it would be very much appreciated. It needs to be 120V(or 120/240) on the primary, and be at least 40/42v 20a on the secondary(or should it be 90, with a center tap?).If anyone can push me in the right direction, that would also be greatly appreciated.I just feel like I'm missing something, and I really don't want to have to buy the one on ali just to figure it out...Oh and as for adding an inverted buck converter,how do I make an inverted capacitance multiplier with a mosfet, would I use a P-channel?My single supply uses a darlington for its capacitance multiplier, but finding a BJT capable of 480 watts is tricky, so for the dual supply design, I'm going to change it to a mosfet.Thanks for any help you can provide.

author: MosherIV
author_rank: Super Contributor
timestamp: August 17, 2020, 09:38:33 pm
content: HiCan you tell us what specification you want this psu to be?Your post was not clear what voltage and current capability the 2nd voltage rail needs to be.From your post, it sounds like you have only found designs for dual rail linear psu.That is why they all use transformers.A mains based switch mode power supply will still use a transformer for isolation.The transformer will be for high freq (probably 35KHz or higher) and probably custom made.My understanding for multiple output supplies, is that one rail (primary) will be used to control the freq or pulse width for the transformer. Other rails will just be buck converters running off the primary rail.

author: LittleRain
author_rank: Contributor
timestamp: August 17, 2020, 10:02:16 pm
content: Hi Mosher,Thanks for the reply.So my dual supply needs to 480watts, +/-42V DC, 20amps from +42 to 0.Not entirely sure how you would rate amperage for a dual supply.The dual PSU I'm looking at is this one.It says its a switching supply, and it has 2 transformers on it. Small one of course would be for the PWM generation, and then not entirely sure how the large one is used.I was thinking of purchasing a core and wiring the transformer myself, but not confident that I would be able to do that correctly.Thanks again.Edit: Actually I probably only need something like 10A per rail, the audio amplifier I'm designing this for can use 12 amps per channel, but that's before I realized it wasn't 0 to +42, and is actually -42 to +42.

author: MosherIV
author_rank: Super Contributor
timestamp: August 17, 2020, 10:50:35 pm
content: HiSo you want+42V 20A-42V 20AI do not think there are 2 transformers on that smps, one of them looks like a choke/inductor to surpress emc.Since you want symetrical rails and since it is audio, it is much easier. The load on positive and negative should be symetrical (unless you have some wired waveform that has more positive or negative half).If the transformer is wound symetrically, you pretty much only have to regulate on rail and the other rail will track but in other polarity.Yes, you will have to join the 2 transformer outputs in series to get the opposite polarity voltage.

author: Benta
author_rank: Super Contributor
timestamp: August 17, 2020, 10:58:44 pm
content: Quote from: LittleRain on August 17, 2020, 10:02:16 pmThat's the most horrendous POS I've ever seen, and I've seen quite a bit. But OK, it's your money and your (and probably your family's) life.Can it get cheap enough?

author: langwadt
author_rank: Super Contributor
timestamp: August 17, 2020, 11:00:00 pm
content: Something missing here. What does your supply look like? if it is isolated, which it absolutely should be, it has a transformer.Why do you think you need a 480W transistor for a capacitance multiplier?

author: LittleRain
author_rank: Contributor
timestamp: August 18, 2020, 01:36:52 am
content: Quote from: MosherIV on August 17, 2020, 10:50:35 pmI initially thought it was a choke, but then thought it must be a transformer to run the mosfet gate at a lower voltage, but I am very new to PSU design, I mostly build USB devices, so yeah you are probably right.Ah ok, guess I will have to find a transformer that doesn't cost $100.Quote from: Benta on August 17, 2020, 10:58:44 pmHaha I didn't plan on actually using it, I was just going to use it for a basic reference. I only have a small idea of how dual supplies work.But how can you tell its a horrible POS just by looking at it?And how much should a reasonable power supply rated at 500 watts cost?Quote from: langwadt on August 17, 2020, 11:00:00 pmIt is not isolated, I wanted to add a transformer, but I could not find one that can handle 20 amps for under $100, and I only really wanted to spend $20 CAD max on one.I uploaded a 3d rendering, I don't actually have the boards yet.As for capacitance multiplier, the only place I see current flowing is through the FET/BJT.I used this for reference.Edit: As for an affordable transformer, is this really how much I should be expecting to pay for a transformer capable of 500watts and 20 amps?Mind you, in this image the secondary winding is not even close to the voltage I need. I want to build a buck converter, not a boost converter lol.Any transformer above 40, 50 volts, are hundreds of dollars.How the hell do people make and sell non-logic-level electronics for a price people can afford?Am I searching wrong or something?

author: TimNJ
author_rank: Super Contributor
timestamp: August 18, 2020, 01:58:47 am
content: Quote from: Benta on August 17, 2020, 10:58:44 pmIt doesn’t look that bad, apart from maybe questionable capacitors. What’s your rationale here?

author: TimNJ
author_rank: Super Contributor
timestamp: August 18, 2020, 05:25:06 am
content: By the way, can you please explain what you're trying to do here? Please share a schematic. From your 3D render, it doesn't seem like it's isolated? (I see transformer T1, but that does not appear to be in the main power path. Maybe stepping down for your low voltage circuitry.) Unless you have a system which is extremely well insulated, (AKA "double" insulated), you need a proper transformer.Youcouldput a big 50/60Hz transformer before the switch-mode supply...but this basically defeats the purpose of using an SMPS in the first place.Regarding dual supplies. If you have two isolated power sources (that is, transformer isolated), you can wire them in series and get call the center point "0V". The only requirement is that each individual output is not earthed. Sometimes an SMPS's output negative is tied back to earth. If you try to wire two earthed-output power supplies in series, you wind up shorting one of the power supplies out. So, they need to be truly floating. (You can still tie the center-point to chassis earth, if you'd like.)

author: bdunham7
author_rank: Super Contributor
timestamp: August 18, 2020, 05:32:28 am
content: Quote from: LittleRain on August 17, 2020, 09:22:16 pmAre you saying that your PSU is mains powered but not isolated?In a nutshell, that's probably not suitable for an audio application even if you did find a way to do it.

author: MosherIV
author_rank: Super Contributor
timestamp: August 18, 2020, 07:30:59 am
content: QuoteMost people, professional or hobbiest, do not make their own mains Switch Mode Power Pupply.Part of the reason is that the high freq transformers are not available off the shelf, they are made to order due to the characteristics being very specific to each psu.Most audio hobbiests either build simple unregulated linear psu orBuy a SMPS rated for their project.It is not impossible to make your own SMPS, getting the parts and getting the design to work right is difficult.The reason why these off the shelf SMPS are so cheap is that they are made in huge volumes, and with the cheapest parts possible (see others comments about the capacitors). The large volume also means that the custom transformer can be made to order at a cheap price.No you are not going to find a transfors in that power range cheap!As others have said, if your current psu design does not have a transformer to isolate from mains - abandon the design - it is highly dangerous (leathal) !

author: LittleRain
author_rank: Contributor
timestamp: August 18, 2020, 08:18:47 pm
content: Quote from: TimNJ on August 18, 2020, 05:25:06 amI'm just trying to build a +/-42V AC to DC PSU for an audio amp, here is schematic for my single supply version, since I can't use it for an audio amp, I'm going to turn it into a lab bench PSU(after I figure out how to isolate it for a price that doesn't break the bank).No it is not isolated, you are correct T1 is for low voltage circuitry.Hmm, yeah that just leads me back to transformer issue, I can't find one that isn't a couple hundred dollars with the VA rating I need, and then I would need 2 of them.Thanks though that is good to know.Quote from: bdunham7 on August 18, 2020, 05:32:28 amNo its not lol.Hmm didn't think about that, I did add a ton of capacitance for noise, but I am not very knowledgeable with analog circuitry, I have more of a programming background and mostly work with logic level digital circuits.Quote from: MosherIV on August 18, 2020, 07:30:59 amMan how do they afford to make a linear PSU, guess they aren't worrying about cost...Yeah that makes sense, I just don't understand how they can sell it for that much and still make a profit. You'd think you'd be able to build your own for around the same price as their marked-up cost.I think I might try to make my own isolation transformer, unless I can find a similar one they used in that link from china...This is starting to be a pretty good learning experience, possibly a little more than I currently have time for.Edit: Do you by chance know if stainless steel can be used for a core? I have tons of stainless steel sheet metal, and can get as much as I need just down the road for free.And I can print the plastic bits, not sure what its called, the former?Is it really that bad to not have it isolated? I see tons of documentation online from TI, and other big IC makers who design buck converters specifically for non-isolated purposes.Yup they are very expensive haha!I already ordered them though so I really want to see this project through, I will surely isolate it though.Luckily I found some high power transistors for a decent price, all be it from China.All in all I've spent around $100 on the project, but I'll have enough components to make 5 of them(at least when I get more LV transformers, only getting 1 or 2 to start), and of course that doesn't include my isolation transformers.And thanks to everyone for their input.

author: Dulus
author_rank: Contributor
timestamp: August 18, 2020, 09:07:39 pm
content: Holy s*** this guy's making a buck converterLittlerain you're either trolling us or you seriously dont know what you're doing.Let me point out some highlights:transistor Q4 blows, and you're shaking hands with rectified mains.MCU doesent wake up properly, Q4 latches up, aaaand you're shaking hands with rectified mainsyou cant drive a POWER FET gate with a 100ohm and 1k pull-up to 10V..... i cant even find out where to startwhat will the FET transistors do at the first few cycles of mains. Clearly there will not be any low voltage supply ready yet, however you'll have rectified mains at the FET inputs...I dont mean to discourage you or anything but, this is serious business and you're going in a very wrong direction.If you're determined to carry on, please share picturesWe may even start betting on which components would blow first

author: Benta
author_rank: Super Contributor
timestamp: August 18, 2020, 09:27:51 pm
content: I'm submitting this for the Darwin Awards.Anon-isolatedbuck converter for converting 120 VAC to 42 VDC for a power amplifier? And extending it to +/- 42 V?Man, you can't touchanythingon that amp without getting killed... input jacks, loudspeaker leads, you name it.

author: TimNJ
author_rank: Super Contributor
timestamp: August 18, 2020, 10:18:51 pm
content: Quote from: LittleRain on August 18, 2020, 08:18:47 pmAaah...I see your confusion. Yes, it is very, very bad. Thereareappliances and devices out there with non-isolated power, but those devices make itessentially impossiblefor the user to come in contact with anything dangerous. As I mentioned before, this is called "double insulated". You may have seen the term on an AC power tool or kitchen appliance. A power amp is surely not double insulated, unless you take some very particular precaution. Any exposed jacks, metal housing, and that's instantly defeated.The TI documentation you mention is referring to stepping up/down a voltage from an already isolated source. For example, in a computer, the point-of-load DC/DC converters near the CPU are non-isolated buck converters...but the input voltage is derived from the isolated ATX power supply. Another example is a battery source that needs to be stepped up or down.

author: TimNJ
author_rank: Super Contributor
timestamp: August 18, 2020, 10:31:39 pm
content: And you will never beat the cost of commercial audio products. When I was about 16, I thought I could make a 3 x 50W amp that was cheaper and better than anything you could buy in a store. Let's just say I stopped keeping track of the cost after my 5th or 6th Mouser order, each of which was about $100.Oh well. But, hey it's a lot of fun!

author: LittleRain
author_rank: Contributor
timestamp: August 18, 2020, 11:35:22 pm
content: Quote from: Dulus on August 18, 2020, 09:07:39 pmHey man, its my first time working with AC, I kind of thought it was obvious I don't know what I'm doing, but you don't need to take the piss out of me, what was your first time working with AC like, I'm generally curious?And this is exactly why I'm here, I need help with designing it, this is a forum exactly for learning electronics, so come on man, there are better ways to get your point across.Also my initial plan, before I even made the post, was to add a transformer, but again, they are $300 friggen dollars for anything close to what I need, which is 1KVA, its why I asked if anyone could recommend a transformer in my initial post.And this guy here says adding an isolation transformer will stop GFCI protection?I'm getting conflicting information everywhere, so its been difficult to learn what is right and what is wrong in the matter of a few days.But even if I add a transformer, it will still be 120V DC after rectification, so if Q4 blows then whats the difference, other then the fact now I won't have GFCI protection, or is that post wrong(Honestly curious about this also)?Is it floating power rails that make it less dangerous?So then if I get a transformer that can convert 120 to 42(which I cant, there are none on the shelf, again anything close to spec is $300), what is the point of a buck converter for cases like this, efficiency?Maybe you can let me know if I'm wrong about this, I added Q5 there for when the low voltage side/MCU is warming up, believe it or not, but I did think about that, I'm just not sure if I went about it the right way.If I were to take a guess at why this wont work, it would basically be a floating gate, is that correct or no?So how would I go about fixing this?Whats wrong with +10V? The VGSS max for that PFET is +15V, and VGSM is +25V?Here is the datasheet, I would appreciate if you could tell me what I'm doing wrong with the 10V.Edit: Oh I just noticed the 100r resistor R9, I just forgot to remove it so calm down lol. The circuit was different before I added the transistors, have you honestly never left a component in by accident after changing it?Like I said in other replies, I come from a programming background, and only have been working with logic level DC for years, this is literally my first AC/(HV DC) (side)project.I'll admit I'm rushing this, maybe a little too much, I have boards I want to order for my main projects, and was going to add this to the order.I actually do appreciate the pointers you gave, but you gave them in the wrong manner, and you could of provided a way to fix this issue as well, instead of making fun. Remember, this is a place for learning.I will post pictures though, and video if you want.If you have eagle, and of course if you want to, I'll post the schematic and you can show me what changes you would make.Quote from: Benta on August 18, 2020, 09:27:51 pmOh god not me haha.No the 120 to 42V was before I realized I needed a dual supply, I was going to make a new design using the same components, and yes with a transformer because I couldn't see any other way to get a common ground, and because everyone else told me it would be too dangerous.I don't even think you could create a dual supply with that design unless you stepped it up to 84V, and created a virtual ground, but don't worry that is not going to happen, the components are not rated for that, and I'm not up to finding/buying new ones.Just to be clear though, I did know it was dangerous, but I saw it as my only option other than buying a $300 transformer.But yeah, I've decided on making my own transformer, I have tons of steel sheet metal, and silicone/epoxy, all I need is 12-14 gauge wire and I should be set.This is actually whats on the back of the boardEdit: why aren't the BBCODE attachments working properly?Anyways the image is on the bottom of the post.Quote from: TimNJ on August 18, 2020, 10:18:51 pmOhh ok, yeah that makes much more sense. I do really need to stop rushing this, maybe order these with my next batch of boards instead.Quote from: TimNJ on August 18, 2020, 10:31:39 pmYeah I hear ya, I actually have about 6 1000 watt rack mounted amplifiers already, I really don't need another amp, but yeah I was mainly doing this for the learning experience, and as you said, just for fun.

author: TimNJ
author_rank: Super Contributor
timestamp: August 19, 2020, 12:05:08 am
content: I feel like I've seen (and have been part of) very similar discussions on this forum about working safely with mains, but don't know the names of those topics off the top of my head. Maybe others can point you towards them.In general, two issues, not to be taken lightly:1. Working on/debugging your mains powered project2. Using your mains powered projectWhen working on a mains powered project, you'll want to probe the non-isolated side of your power supply with your scope. To do this, your device under test needs to be powered through a lab isolation transformer. This lets you put the scope'searthedground clip virtually anywhere you want on the board. Without an isolation transformer, you are limited to putting the ground clip on earth only, which is not useful. So, if you want to design a 1KW power supply, you need a 1KVA isolation transformer, just for bench testing.Assuming you built an isolated supply, once all your debugging is done, and the design is buttoned-up, you can remove the 50/60Hz isolation transformer, because you no longer need to poke around with the probe.The above isolation transformer is a big chunky iron-core low frequency transformer. Remember, its main purpose is to help you safely debug.But, you still need to provide galvanic isolation in the finished product. That's accomplished by a (relatively) small ferrite-core switching transformer. (This is one of the reasons we use switch-mode power supplies in the first place.) It doesn't really make sense to use a big 50/60Hz low frequency transformer in conjunction with a switching supply. The beauty of SMPS is down-sized magnetics.As noted a few times above, one of the issues with mains is that it is earth-referenced. We humans are also earth referenced, as long as we are standing on the earth, or a floor, etc. So, if some fault condition happens, there's a direct loop between the low-impedance main supply through your body. A transformer breaks that loop, since there is no galvanic connection, thus limiting the current through your body to the small parasitic leakage current through the transformer.

author: TimNJ
author_rank: Super Contributor
timestamp: August 19, 2020, 12:18:49 am
content: I encourage you to try building a mains powered supply...just there are many pre-requisites to do it safely. 500W-1KW is not a trivial task. I also don't think you've considered things like power-factor correction and EMI. No one is going to hunt you down with your hobby project, but in general, the commercial supplies meet these international requirements. The ones on AliExpress might at least try to.If you want to try building something, I'd recommend <100W to start.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 19, 2020, 12:49:55 am
content: Quote from: TimNJ on August 18, 2020, 10:31:39 pmAmplifier boards and surplus telecom PSUs are really cheap nowadays, definitely well under the cost of a preassembled amplifier of equal specs. I guess it depends on if you call the board a "commercial audio product".

author: TimNJ
author_rank: Super Contributor
timestamp: August 19, 2020, 12:57:50 am
content: Quote from: NiHaoMike on August 19, 2020, 12:49:55 amMaybe? I don't know...in my experience, everything added up pretty quickly...that is, taking the "Hi-Fi" route, circa 2010. (Class A/B, big-ass toroid, big heatsinks, etc.) I too recall the eBay PCBs being quite inexpensive. But, after all the mounting hardware, connectors, enclosure, etc...it wasn't cheap anymore.I also should note that I have *ahem*tastefor expensive parts.

author: MosherIV
author_rank: Super Contributor
timestamp: August 19, 2020, 08:27:06 am
content: HiUsing a buck converter to reduce mains voltage is unusual to say the least and highly dangerous as others have pointed out.Commercial SMPS rectify mains to high voltage DC, this stage is highly dangerous because1. It is HV2. It is DC3. Mains means that there is a LOT of powerThis HV DC is then pulsed at high freq (40KHz and higher) into a high freq transformer with a high power switching FET. Either the freq or the pulse width is varied. The output side of the transformer is smoothed with output cap and monitored for output voltage. The monitor is typically done with an opto isolator. The transformer and opto isolator provide the galvanic isolation.The regulation and pulse control is done by dedicated controller chip.The high freq makes the transformer physically smaller than mains freq transformer. The smaller transformer also makes them far cheaper than the mains freq equivilant. These high freq transformers are custom made for each power supply design.This type of SMPS is most common for mains psu. They are tricky to design and get right, mainly due to the transformer. No you cannot get then off the shelf, they are designed and made to order.What I am try to tell you is that these small high freq transformers are not available by looking at electronics suppliers. I got the feeling that you were looking at main freq transformers that step down.An alternative to your approach is to use a lower output voltage transformer eg 12V and use a boost converter instead. However, to get 20A you will need 12V 80ish A transformer! Common voltage (12V is a good one) are more common than the 42V ones you are looking atAnother way to do it is to parallel up transformers, just make sure the transformers are wired in phase!Eg you can use 2 45V 10A transformers. The disadvantage of using parallel transformer is that it is much heavier and a little less efficient.

author: Dulus
author_rank: Contributor
timestamp: August 19, 2020, 11:53:39 am
content: QuoteI'm from 220Vac land, so it was terrifying. And i got zapped a couple of times from the main filter cap that's charged to 300VDC and pulsing like a motherf***....Dont make a big fuss about it, we should be able to tease people a little bit.I really dont want to stretch the discussion, but other people may read these posts in the future and get carried on the way you are.So i reiterate:Mains AC is dangerous.People can die. It may not be you.You clearly dont know even the essentials about mains power electronics, yet you go the distance, even finishing the layout before clearing your schematic with experienced people.Dont do it. You'll hurt someone.You cant build a high power supply on the first run. Learning doesent begin this way.No architects first project is a skyscraper.Start small, start simple. As you can see, people are here to help. And have a bit of fun on the sideAnd again i'm saying it, if you are determined still, even after this many warnings....Take pictures

author: Pawelr98
author_rank: Regular Contributor
timestamp: August 20, 2020, 01:49:58 pm
content: Quote from: MosherIV on August 18, 2020, 07:30:59 amMaking a switching power supply is fairly easy.No off the shelf transformers ?Winding a switching transformer is easy.Just take a transformer from an ATX power supply and rewind it.Primary on such transformer is usually 20T+20T or so.Unwind the first part of the primary and then remove the windings you don't need.Wind the new ones, put a good layer of isolation and wind the primary again.Design problems ? Feedback-less IR2153 is not very safe (no overcurrent protection) but it's pretty much the simplest supply one can make.IR2161 does have overcurrent protection, still no regulation.Those chips however may run with a primitive feedback when in resonant mode.If you want proper regulation then jump at 494 or any more recent IC.Reusing an ATX power supply may be a better option, if you know how to modify it to output +-48V.At such power level (500W) half-bridge is a reasonable topology.Then I was also making Flyback supplies for low power levels using UC3845 just fine.Off-the-shelf transformers but with secondary rewired in series instead of parallel for higher output voltages or symmetrical ones.