Designing an audio amplifier switchmode power supply (500W, flyback) - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 02:35:13 pm
content: Hello!just evaluating an idea of building a customized high powered class D audio amplifier with a switchmode power supply. The amplifier should be capable of outputting 500W into 4ohms (1kHz, sine). However I am pretty unsure for what power and how should I design the power supply.I am sorry beforehand, this might be a longer reading, but any of your help or insight is welcome! Just also want to state first, I am no expert in SMPS design, but not a beginner either. I understand some of the math involved, so please be patient with me and my ideas.As we all know, audio is a signal with high crest factor (common value about 10dB with a reasonable signal), however this amp shall be used for bass primarily, so the crest factor expected might be way lower. Does anybody have a good guesstimate of what the typical crest factor for an audio bass signal in a typical PA setup might be?I would like to try something new for me, which is trying to design a double-switch flyback power supply. It seems to be a reasonable choice for this application: No output chokes are required (less space taken), easy and simple currentmode control (which in a DCM mode makes a nice constant power limit set by the peak current if frequency is fixed). Zero current switch on (but rather high current switch off), simple transformer design.A design goal is to place the amplifier within an 1U 19" enclosure, meaning I have a 40mm height inside, allowing for about 33mm of component height at best, including capacitors (which seems to be of no issue) and the transformer (which is a sort of issue).Within those 33mm I am only able to squeeze an ETD34 core in there. ETD39 would be more nice, however with the component selection available, I am able only to do the ETD34, but I think this might be doable, under a few conditions:I already know for sure, that 200W can be pushed using an ETD29 core using the same double-switch topology. (My colleagues at work have already tried that successfully, but that was a completely different project). Therefore I think it might be possible to do 300W continuous power with ETD34 and the same topology. What I think it could be done here, is to design the supply it is able to deliver 500W short term, but only 300W continuous (more copper cannot be squeezed on to the ETD34 bobbin, without going to silly A/mm2levels).300W continuous supply power would allow for a 2.2dB crest factor if I calculate that right.10 log (500W short term / 300W cont)I think this might be a reasonable for an bass amp. This however requires the PSU to have a rather large capacitor bank on its output, that can be used as the storage for peak power delivery. (My educated guesstimate is most if not all amps today are made this way).To help the transformer not to overheat during longterm high power load, a thermistor in the winding might help a lot!Operating frequency of the supply is expected to be 100kHz on an ETD34, CF297 material. The supply should be operating in DCM mode only (seems it gives smallest possible transformer) with a deltaB of 0.3T. That might seem a lot at 100kHz, hence the CF297. The core loss seem to be still within reasonable limits.I am not sure how to make the guesstimate of the output capacitor bank. The supply shall produce 2x70V. Would 4700uF per rail be enough to do the job? (remember the supply will be capable of 500W short term. (The peak power output of an amp is double at the max output amplitude, so the caps should hold enough charge for say 20Hz sine peak). How would you calculate this?What do you think about my ideas? Are they completely off or reasonable? Do you see any possible problems there?Thank you for your help,Y.

author: Benta
author_rank: Super Contributor
timestamp: July 01, 2017, 03:12:38 pm
content: At those power levels, flyback is not the optimal choice in my opinion. The magnetics get large, because the core has to store the full energy.In your shoes, I'd start looking at forward topologies, either single switch, but more likely half- or full bridge. The transformer for a single switch converter is a bit more complicated, as you need a deflux winding, for bridges it's a simple primary/secondary type.Additionally, forward converters are very good a delivering instant power when needed.

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 03:36:18 pm
content: You maybe missed the point, that a forward type of convertor has a choke on the secondadry side, that stores the energy? So now we have two magnetic parts, instead of only one, that you consider a bit larger. (still nowhere near that large as the two would require in a forward topology).If you try to use a forward converter without any current regulating inductor in the path, you will at least nicely blow the transistors up.I am aware (and have actually seen) that some amplifier designs use a forward converters without any current regulating elements (secondary chokes), but I consider these to be stupid cheap designs. These do not provide any voltage regulation on the output and current limiting them in an overload condition is a rather issue.Design of the following converter is even more simple than the flyback one. It even gets more interesting if the Nt:Np ratio is 1:1 and the collector gets capacitively coupled to the anode of D1 to nicely limit the voltage spiking at the switch device due to leakage inductance.But what are you gonna to do about L1? (No matter if single switch, half- or full- bridge forward converter). Or what exact topology are you speaking about?EDIT: Even thinking about the instantaneous energy delivery, the flyback is better then the above one, that needs at least few periods to increase the current through L1 (assuming L1 works in CCM as usual) to react for a stepped load increase. The flyback does not do any of that, when working in DCM.

author: Benta
author_rank: Super Contributor
timestamp: July 01, 2017, 03:48:58 pm
content: Well, if you've already made up your mind, there's no reason to ask for feedback.I'm aware of the need for a choke, but with the semiconductors available today, increasing the switching frequency can decrease its size.Another input to you, as your constraint seems to be height: try looking at using planar cores. The PCB contains the windings, and the two core halves are placed on each side of the PCB, with the core legs going through holes in the PCB.

author: stj
author_rank: Super Contributor
timestamp: July 01, 2017, 03:55:53 pm
content: what's your mains voltage?

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 04:01:48 pm
content: Benta: Not trying to offend anyone, but I was asking about the the power levels and crest factors, not which topology I have to choose. As was stated in the first post, I want to try using double switch flyback, as this seems to be nice application for it.What kind of switching frequency with modern parts are we talking about, if you think you will make it as small as the flyback one, or at least come close enough to the flyback size? As I think that is nowhere possible, unless using cosmic technologies from NASA, which is my exact description of planar transformers. They are not viable for single quantity designs.Using planar transformers and 200kHz+ switching speeds is possible. It might be size effective, but certainly not cost effective. This is not a competition to make a smallest PSU with GaN transistors at 1MHz.Using flyback topology is both cost effective and size effective. Does not require as high switching speeds.I have not made my mind, I have used (or tried to use) arguments, why I have decided as such. If you think competitive forward converter could be made, please be exact, how it can be done.stj: Thank you for asking, I have forgot to mention I live in the 230V 50Hz side of world

author: Benta
author_rank: Super Contributor
timestamp: July 01, 2017, 04:24:37 pm
content: OK, your design. I don't understand this, though:QuoteWhy not? Planar cores can be bought from Farnell, Mouser etc. (TDK, Ferroxcube...). Nothing cosmic about them, it was just a suggestion based on your mention of height restriction.

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 04:33:30 pm
content: And who will pay for the multi-layer and/or high-oz PCB that goes inside the planar transformer? Maybe you can get manufactured these cheap, certainly not me. Hence why I never thought about using planar transformer. It would cost half the rest of the design. The planar cores can be of course sourced for cheap, even from a local distributor we have here.

author: nctnico
author_rank: Super Contributor
timestamp: July 01, 2017, 04:38:59 pm
content: Quote from: Yansi on July 01, 2017, 04:01:48 pmI'd consider using a transformer with a double primary winding so your gate drive circuitry can be simpler. Also don't forget you'll need a PFC stage at these power levels. You should also consider that the output inductor in a forward converter works similar to one that is in a flyback converter. The output voltage and current will ramp up slowly over a few cycles (depending on the primary cycle-by-cycle current limiter).

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2017, 04:51:00 pm
content: For an audio amplifier there usually is no need for strict voltage regulation. In addition at 500 W one might have to consider PFC on the input side anyway and PFC often provides some voltage regulation. So with PFC there should be no problem in not having extra voltage regulation in a forward converter.Even without PFC (one might get a around it, as the typical use is low power) chances are that an audio amplifier can tolerate the typical variations in mains voltage.The choke at the secondary side of a forward converter only stores a small fraction of the energy - just the fraction needed for regulation. So maybe 10% of that in a flyback.A flyback converter is possible, but it needs larger magnetics and larger caps. Not sure suitable cores are made in that size. A flyback is cost effective at low power, like less than 20 W. Essentially all ATX supplies use a kind of forward converter, a flyback is for USB chargers and the like.

author: Benta
author_rank: Super Contributor
timestamp: July 01, 2017, 04:52:49 pm
content: QuoteYou didn't mention that it will be a Dalo-pen PCB.I'll shut up now.

author: Pitrsek
author_rank: Regular Contributor
timestamp: July 01, 2017, 04:55:12 pm
content: Why not LLC?

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 05:08:24 pm
content: I'd plan to use a bootstrapped gate driver. If this will result in any problems, will replace it with a simple 1:1 pulse transformer. Driving the double switch flyback requires the duty cycle always less than 50%, so the gate transformer drive circuitry can be very simple.You're right that the PFC is currently required for any devices over about 100W in our country, but keep in mind this is an experimental device that might never be used more than once in a lab and much nastier devices without PFC do exist in the kW power levels, including those audio power amplifiers.Do you have any specific ideas, how to make any forward topology as small as the flyback one, without using excessively expensive solutions? (Excluding the crude and cruel cheap unregulated nonsenses and LLC resonant, that I currently do not want to mess around with by any means)Pitrsek: I am not as familiar with LLC designs. It might get complicated with the transformer design, compared to the flyback. The LLC might however get a % or two more of efficiency, but I think I can leave it there, cooling should not be of an issue.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2017, 05:26:20 pm
content: Quote from: Yansi on July 01, 2017, 02:35:13 pmRegarding the PSU, this doesn't save you much.The switch, inductor and anything handling parasitics (snubbers, peak voltage and current ratings, etc.) all have to be rated for maximum, period.Your savings are limited to efficiency and heat dissipation. You can get away with slightly smaller transistors and diodes, placing the efficiency peak around the average operating level rather than the maximum level. You will have less steady-state power dissipated by the semiconductors, transformer and filter capacitors, so the heatsink can be somewhat smaller, and the capacitors somewhat cheaper.Mind that these limitations are also associated with lifetime and reliability. If you want to make a high efficiency, high reliability amp, you don't really have any room to save.QuoteLet's check those assumptions first:1. Flyback only uses one magnetic component, the transformer (really a coupled inductor). True. Compare LLC (two magnetic components, though they're usually integrated, the first 'L' as leakage in the transformer) and forward (transformer and choke).2. Flyback draws "100%" ripple, on both primary and secondary sides. [Full wave] forward converters draw, at most, half the input ripple of the flyback converter, and arbitrarily small output ripple.3. Flyback is half-wave. [Full wave] forward converters use the full B-H curve of the transformer. This halves the size of the transformer. (Half wave forward converters still have some advantages, like the low output ripple, but generally have similar limitations as flyback converters.)4. Because flyback has "100%" ripple on both primary and secondary sides, you needdoublethe filter capacitors. That is, for filtering, you are normally limited by ripple current ratings, not by capacitance value. Now, it may not be exactly double, because the primary side capacitors serve double duty for mains filtering (assuming this is an offline application), and the values for mains filtering are determined by value rather than rating, so they have extra rating to spare. But that's not unique to the flyback converter. Thus, the main difference is the amount of secondary side filtering required.5. Accounting by whole components only, misses the fact that the filter inductor is usually not very large. It can also be very cheap: it can be made from powdered iron instead of ferrite, and can therefore be made much larger, giving lower secondary ripple current, needing even less final capacitance. And, because flybacks are just so noisy, you may need the additional LC filter anyway, just to keep the output quiet.6. Resonant designs (like LLC) can benefit from ZVS as well as generally lower harmonic energy in both primary and secondary circuits. This makes filtering (on both sides) much easier. EMI filters take up space and BOM cost, too!Above 100W, flyback performs increasingly worse. Forward is recommended, whether two-switch, half bridge or full bridge. Resonant is also very attractive, though it's more complicated to design from scratch (if you're taking that approach), so that I would again recommend forward. (If you're planning on using controller ICs anyway, it isn't much difference to implement either.)QuoteYou answered yourself: if thesupplydelivers the power, then you don't need a "bank" of capacitors to fill in. (You could make a power supply that hard limits at, say, 300W, but then you'dneedthat bank of capacitors, and have to pay for them -- all the while getting worse regulation under peak load conditions. What a waste!) Just use whatever capacitance is required for the supply design, and local bypass if/as needed.Tim

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2017, 05:26:42 pm
content: If it doesn't need to be regulated, then how about a Royer converter?There's an example of such a circuit on the data sheet for the TC33167. Yes, I know that IC is only rated to 5A, so wasn't suggesting it, just the isolated pre-converter shown on page 12. If it needs to be regulated, a suitable buck regulator can be added to the output.http://www.ic72.com/pdf_file/t/85248.pdfEDIT:Is the 500W rating peak power? If so the PSU only needs to be designed to give 100W continuously and a large filter capacitor can be used to supply the peak demand.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2017, 05:30:14 pm
content: Eww, no current limiting, no protection... and obviously, no regulation.Neat that they show saturable reactor commutation. That's quite common (if not necessarily intended), and gives good efficiency.Self oscillating circuits can be useful, but after you've patched all these problems, you might as well have started with a proper switching design.Tim

author: PChi
author_rank: Frequent Contributor
timestamp: July 01, 2017, 05:32:27 pm
content: Another disadvantage with the flyback is that it gives CIN and CO a hard time and they will need to be larger to survive the ripple current.

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 05:35:25 pm
content: Quote from: Kleinstein on July 01, 2017, 04:51:00 pmSure true, 10% of that energy, but that does not make the choke 10 times smaller, does it? Usually the output choke is executed as an iron powder core, which size is like half the transformer itself. Please tell me if I am wrong.I do not think it is true that flyback is only for USB chargers. I have seen too many applications with 100+ W flybacks that I could agree with you. It might have been true when there weren't cheap transistors that could be run efficiently at 100+ kHz and such peak current levels. Last time it was a Quint Power industrial PSU from Phoenix Contact that I have analyzed. Also a double-switch flyback. 250W output power on EE40 core if I remember that right. But a quite old design I think. The large peak currents were handled by a bunch of ceramic caps, then a small choke and two standard electrolytic ones.Yes the high powered flyback does produce high output peak currents, but that is of no issue for this kind of situation, when the output cap bank will be made of high number of paralleled smaller low ESR caps (you can not do any different, if height is an considerable restriction). To be exact, currently evaluating the possibility of having a group of 10 paralleled Nippon KY series 470uF/100V per rail. Or Nichicon PS series, have available handful of those already.Thank youT3sl4co1lfor your input in this situation, will answer after a while.Hero999: That is a complete nonsense. You need transistors rated for twice the input voltage. That would mean at least 800V transistors in my case, not mentioning the severity of problems with primary leakage inductance. Why mentioning such circuit? Are you a troll?

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 06:05:13 pm
content: T3sl4co1l: Regarding the current ripple both on the primary and secondary side: Due to height restrictions, both the input and output capacitors will be split into a multiple ones in parallel. On the secondary is no problem, the primary side is a bit worse, but still well within the specifications. (I know, I have not posted any exact values calculated, but I am fiddling with the design a lot now).Regarding the secondary ripple voltage and its filtering: It is more of an issue of ESR of the caps, than capacitance. There will be enough caps in parallel to have low enough ESR to filter that pretty out I think.With the capacitor bank on secondary comes this next on to mind, where unfortunately I can not agree with you:It is a nonsense to design the PSU capable of delivering the full instantaneous power. For a 500W amplifier (rated with a continuous sine power), the instant maximum power drawn at the amplitude is double, i.e. 1kW. I see no reason to design the PSU for 1kW of instantaneous power. This is where the crest factor comes into place.The peak output power can be therefore taken from the secondary capacitor bank, allowing for a supply with lower power ratings. What I suggested above, is to design the PSU capable of delivering 500W instantaneous power and 300W continuous.The question however was, if that is enough or not. These figures can be estimated based on the crest factor of the amplified signal. However now I have finally figured out how to simulate that, so I will give it a try.The underpowered flyback solution requires cap bank on the output, that is for sure, so your point in money being thrown at the caps instead of a beefier PSU is a good one.But that is really the way the very high powered class D amps are made today. (Correct me if I am wrong please) Or does anyone think the supply in them can deliver 5kW+ instantaneous power? Certainly no. It's all about the crest factor and burst signal ratings.Will do and try to make some simulation and calculation and will weigh in different PSU solutions for this.

author: diyaudio
author_rank: Frequent Contributor
timestamp: July 01, 2017, 06:47:27 pm
content: Quote from: Yansi on July 01, 2017, 02:35:13 pmThis is probably not the best forum for this. head over towww.diysmps.comnever worked on mains SMPS ?safety first my friend.I would recommend you look at a push pull converter, but see above most of the answers you searching for has been answered.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2017, 07:27:15 pm
content: Electrolytic buffer caps get more cost and size efficient at high voltage (e.g. 400 V). So there can be an advantage of having the buffer before the converter. However a +-70 V at the output is already relatively high and there thus might not be that much advantage of having the buffer on the primary side.With a flyback converter the magnetics need to be sized for the peak power (e.g. core saturation). In a forward configuration the transformer part is limited by thermals, not saturation. So the size of the transformer is more by average power. There is not that much extra cost for a higher peak power rating. It is only the transistors and rectifiers that need to be larger - though these parts too can have some reserve for peak power.A flyback has a kind of hard power limit, with essentially no reserve for peak power. A forward converter, like a conventional transformer might deliver something like 3 time is nominal power for a short time. So there is not just nominal rating, but also a higher peak rating.For a flyback design, it might be more convenient to have buffer caps at the output side and thus a converter designed for less than peak power. With a forward converter it might very well be better without much buffer on the secondary side but higher peak rating. The options to compare would be something like a 300 W flyback with quite a lot of secondary buffer and maybe a 200 W forward converter that allows for a 600-1000 W peak rating, but only a little secondary buffer (e.g. good for a few ms).The output inductor can get smaller, than the flyback transformer. With magnetics power rating often scales a little faster than weight (like a power of 1.2 to 1.5) - however iron powder cores possible for a filter inductor also have higher saturation levels. So like the transformer they can have extra reserve for peak power before going into saturation. Normal rating might be more limited by core loss and not saturation. So the inductor might be more than 1/10 the size, but still considerably smaller.It also depends on the frequency used. A forward converter might not need such a high frequency, but it could in principle. It is more like a possible lower frequency is an other degree for optimization. A lower frequency would somewhat increase the magnetics - though possibly less than with a flyback, but this would be more than offset with advantages at other parts.

author: Marco
author_rank: Super Contributor
timestamp: July 01, 2017, 08:50:09 pm
content: Isn't it possible to apply the push-pull class D signal directly to a high frequency transformer at mains voltage and avoid the separate switching power supply?

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 08:54:05 pm
content: Quote from: diyaudio on July 01, 2017, 06:47:27 pmI have a degree in power electrotechnics / energetics and drive/traction, I think I know how to handle mains voltages and currents, but thanks for your concerns! I always try to be safe. I also have all (or at least access to) the equipment required for mains operation and measurement.No, I probably won't participate at diysmps. I know this forum exists for some time, but never was interested enough. I think this forum is bestThey almost recommend you to float a scope in the "safety thread read that first". (found that thread few hours ago)http://www.diysmps.com/forums/showthread.php?72-Necessary-Equipments-needed-for-SMPS-development/page1Also, those suckers should really repair their registration "random question" that thinks that addition precedes multiplication.(got definitely pissed off at that, after being in war with it for quite some minutes)Kleinstein: Having a buffer cap bank on the primary side seems not the greatest idea, with regards to the power factor (I would leave this without a PFC).The flyback can be design for 500W power, however the transformer underrated for currents only equal to 300W continous load. Thats what I meant to try. At 500W, the peak primary peak current seems to be at about 10A, while the rms being about 3.8A, which is still reasonable, no unreal figures there.Yes, the 500W will be a hard power limit, defined by the primary peak current limit and frequency. (assuming the DCM operation, at CCM it won't limit precisely at constant power)I have cobbled together a simple simulation, to try to estimate the secondary cap bank value. The simulation is not accurate (not bothered with calculating the exact shape of current drawn by the class D stage), but it turns out this approach might completely be invalid.If the supply hard limits power (i.e. outputs almost constant current to the cap bank) and the amp goes loaded with a LF signal (like 20Hz), the cap bank discharges too quickly - it would require an enormous cap bank to store all the energy for the halfwave cycle of load. The supply would not be able to will the energy in, quickly. Doh!T3sl4co1l seems always to be correct and I stay being corrected. The supply cannot be design this way (flyback or not). It must be able to handle much higher peak power levels (which in the end is almost impossible to do with the flyback, at this power level).I will still make a prototype of that flyback just to measure at least it's efficiency at such power levels. I will rethink the topology completely and will probably make use of a forward type of converter and might try implement an active PFC corrector. That might be way betterMarco: It is possible, but the transformer would then see the audio spectrum frequencies in its flux --> leading to a very large heavy iron transformer that would then need to transfer the 20Hz signal(Similar situation as a PWM chopped big iron transformer in some UPS systems with sinewave outputs.)

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 10:00:56 pm
content: And what will you do with the reactive currents from the load? Remember that the speaker is not pure R load, but has quite some L in it. (not mentioning the impedances of passive crossovers in speaker cabinets). That would simply not work.

author: Marco
author_rank: Super Contributor
timestamp: July 01, 2017, 10:01:54 pm
content: Ahh tried to delete it, yeah never mind. You would need many more switches to make it all work. Not much point.

author: jbb
author_rank: Super Contributor
timestamp: July 01, 2017, 10:02:45 pm
content: Quote from: Yansi on July 01, 2017, 08:54:05 pmOh dear, not a good sign.Quote from: Yansi on July 01, 2017, 08:54:05 pmQuote from: Yansi on July 01, 2017, 08:54:05 pmIf you're concerned about spending inordinate amounts of time stuffing about with design issues, and can fork out a little bit more per unit, I suggest you have a look at the Power Integrations products. I've used their TOPSwitch line before and they generally worked well.TheHiperPFS-3 PFS7529His an integrated boost PFC (includes MOSFET, boost diode and controller) semiconductor device rated 405W continuous (or 900W if you have real electricity and don't need to operate below 180 V RMS)TheHiperTFS-2 TFS7708His an integrated 20W flyback + 340W two switch forward controller (includes all MOSFETs and drivers).Remember as well that efficiency is very important. In a 1U rack enclosure, there isn't a lot of room for fans and heatsinks. That means that if you have to get lots of heat out you will need to deploy little high speed fans which are very noisy. Which isn't a good feature in audio equipment...Quote from: Marco on July 01, 2017, 09:55:29 pmThat would be like using a high frequency transformer with an AC-AC output stage. It can be done (seecycloconverters, but could have terrible implications for sound quality. And would require quite a lot of design effort to make sure it didn't blow up or radiate copious amounts of EMI.

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2017, 10:20:24 pm
content: I see no point in using these overpriced integrated modules. I am not as new into power conversion design that I have to be so afraid of laying some stuff on a PCB myself. But interesting point, never seen any of these devices.Currently thinking about the PFC... If I should implement that a a transition mode one or CCM one. TM one requires less inductance, however large flux swing and peak currents. The CCM one has low peak current, but requires large inductance (at least twice that for the TM one) and also ultrafast rectifiers (which is not as bad issue).Not easy being cheesy, if one wants the inductor simple on a toroidal core... Powder cores do not like high flux swings (compared to ferrite), but have low permeability so one cannot implement any of the two so easily.EDIT: Regarding the cooling: I have already characterized suitable heatsink I have available, seems I could pull away 100W from the 1U box withotut much difficulty. The heatsink I have is segmented and one segment can take 30W with low airflow. (the segment has about 1.3K/W). Almost four segments could be fit inside.

author: jbb
author_rank: Super Contributor
timestamp: July 02, 2017, 12:09:53 am
content: Hi YansiI agree that those integrated modules do cost more - and nail down some design parameters! - than using separate parts. I guess the important question will be how many units you're making (i.e. how does the engineering cost per unit compare to the more expensive part). And whether they will achieve sufficient efficiency (you could get to 98% for the PFC and 99% for the DCDC if you really want to) to meet your cooling options.From Digikey, in $NZ:PFS7529H $7.29 ea or $3.65 @ 1000 pieces (Note: designed for CCM operation and include ultrafast diode)TFS7708H $10.05 ea or $5.43 @ 1000 piecesI expect that the integrated modules will save a bit of board area (may be relevant as a 1U rack can fill up quickly) and quite a chunk of design & testing time, so it comes down to you.On toroid inductors: toroids have many nice properties but are a pain to wind! That's why most production equipment will use some kind of split core + bobbin. Assorted options:Powered iron: pretty crap above 20 kHzFerrites with slit: allegedly available, I've never tried to source any. May radiate some EMI from the slit (probably OK if carefully positioned within metal rack case).Specialty powder materials e.g. Sendust, [urlhttps://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores]Kool Mu, MPP. More expensive than other options but can be veryI suspect for an audio application, a powder toroid is a good choice because it will have minimal stray field.On transformers: you'll probably wantreinforcedinsulation for the isolating transformer. If you go toroidal, that guarantees you'll need triple insulated windings for the secondary (for safety analysis, the core should be considered to be conductive and live to line voltage). If you go for a bobbin-wound, this opens the opportunity to use normal magnet wire with insulating tape layers (gives better fill factor.

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2017, 12:32:42 am
content: How many pieces was stated in the first post: Either one or noneI have access to a handful of power components including MOSFET, IGBT, SiC and diodes, or almost any kind of controller IC for no cost for any private prototype application. Therefore I try to avoid buying any modules, as I also do all of this as a hobby, to extend knowledge and have some fun.I don't agree with ur statement about iron powders, they pretty decent but are only good forspecificapplications.I know that toroidals with a slit exist - I have come across these some time ago in a Micrometals catalogue. Their availability here locally is like almost if not total zero.Kool Mu / Sendust is what I would like to go with. If I am not mistaken, these are also iron powders, but different type. These are not cheap, but still can be sourced for reasonable $. Quite a few local distributors have them.Like $1.5 for a 33mm OD Koolmu u60 at one off, which is nice. Two of these stacked on top of each other would probably make a nice CCM PFC at 100kHz. (It is up to the MOSFET + diode selection then)I would go with bobbin wound transformer with the standard yellow 3M-1350 isolation. Probably connecting the secondary ground right to PE, to increase safety. I can have the transformer tested at 4kV, but well... I think the code calls for 8mm creep distance between the windings, so 4mm spacers from the side should be used inside the transformer.Sourcing a TEX of larger diameter would be real pain in the ass. This stuff aint no cheap.

author: jbb
author_rank: Super Contributor
timestamp: July 02, 2017, 01:19:24 am
content: Quote from: Yansi on July 02, 2017, 12:32:42 amA more discrete design will defintiely be more of a learning experience. And more rewarding when it finally works :-). There's nothing like the first hard-power on test (i.e. when you think it works and have removed the variac and current-limiting light bulb).Quote from: Yansi on July 02, 2017, 12:32:42 amSounds like you're on top of it.Connecting the secondary ground to PE is a big help safety-wise, as faults will be noticed :-). When looking at the winding design, remember that you have to get the leads out of the winding pack too - I think people often use little silicone sleeves for this. IIRC, it needs to either be double-walled or >= 0.4mm wall thickness to count as 'reinforced.'Also remember to check the distances from the mounting pins to the primary and core.And of course, the end result is that the reference book saying 'use a window fill factorkof 0.7' is telling you big fat lies. I found that 0.3 - 0.4 is more reasonable...Quote from: Yansi on July 02, 2017, 12:32:42 amThe triple-insulated stuff is hard to source. I think traditionally hobbyists get it by calling a distributor and begging... There is even triple-insulated Litz with square cross section (to improve window fill factor) and split red & green strands (so that you can make centre-tapped secondaries) available, but I've never been brave enough to ask for a price check.I know that people love it for small quantity designs because you can just bring bare ends out of the transformer and solder them to the board the appropriate distance away - no need to worry about all that intricate creepage geometry stuff.Should you feel adventurous in the future, LLC convertersusethe leakage inductance for converter operation. This means you can use a sectioned bobbin to help achieve the clearance you need.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2017, 07:03:23 am
content: Quote from: Yansi on July 01, 2017, 05:35:25 pmIt's not nonsense. It's a Royer converter. A similar circuit is used in CFL drivers, in LCDs with cold cathode lamps and electronic halogen lamp transformers. Apart from being simple, it's very efficient, often 95% or more.The transistors only have to be rated to the peak input supply voltage.Quote from: T3sl4co1l on July 01, 2017, 05:30:14 pmIt's powering an audio amplifier, which doesn't need a tightly regulated supply. Protection can be added in the form of a fast blow fuse.Old style amplifiers typically uses a transformer+rectifier+smoothing capacitor with no regulation or protection, other than a fuse and they worked perfectly.

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2017, 07:37:07 am
content: I insist on that the circuit is nonsense and shall never be used (and hopefully never was used) on mains voltage. It might work with those funny american 110V, but will not work economicaly with our angrier volts. As I pointed out, you need stupidly overrated transistorw and will have extreme problems with leakage inducancte.Why not making a self oscillating half-bridge as todays fluoro bulb use? That would be much better idea. (same ideas were exploited in the PC-AT power supplies).No, CFL converters are not Royer. If you talk about those small lamp CFL drivers with low voltage input. They are CFPR (current fed parallel resonant) converter. That is not a Royer converter. Just no. People often confuse this, though it have some similarities. Even wikipedia warns about this. Royer converter has saturation driven commutation, whereas the CFL inverters use resonant commutation in collector tank.//EDIT: Typo.

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2017, 07:40:17 am
content: Quote from: Hero999 on July 02, 2017, 07:03:23 amDefinitely NOT enough!In an push-pull converter like this, the opposite transistor to the switched-on one sees double the dc link + voltage spiking from leakage inductance.In a 230V environment, you would need at least 800V and even that might not be enough.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 02, 2017, 10:29:14 am
content: Quote from: Yansi on July 02, 2017, 07:40:17 amWhile you are correct that a push-pull is not a very good choice for operation off of 230VAC mains, note that a forward converter with a reset winding experiences the same operating conditions, and this was a very popular choice for 250-350W PC power supplies for many years.That aside, the two-switch flyback could be a reasonable choice at 500W if supplied by the nominal 385-400VDC input from a PFC front end,ifthe output voltage is relatively high - say, greater than 100V - because the current waveforms in the flyback are very peaky, especially in DCM.And speaking of operating mode, at this power level CCM would normally be recommended, except it is very difficult to stabilize due to the infamous right half plane zero, and so it is a terrible choice if the load is highly variable and dynamic... like an audio power amplifier. Operating in DCM, however, increases the already high ripple current that both the input and output capacitors have to handle. In fact, when you factor in the volume/cost of all the components comparing a two-switch flyback to a two-switch forward of the same power level you will likely find little difference between the two. Yes the forward converter requires two magnetic components, but the ripple currents are relatively low so the capacitors are much smaller/cheaper while the transformer is also much smaller as it is not an energy storage element.Also note that flybacks absolutely cannot be operated open-loop as is often done with forward-mode converters in audio amps - you can't just set a duty cycle and forget it, in other words - so there is no avoiding the need for loop compensation. In CCM it is not unusual to see the crossover frequency for the voltage feedback loop set at less than 1kHz.I definitely recommend the use of triple or quad insulated wire for the primary. I have ordered small rolls (~1 lb) from MWS Industries in the US, but I'm sure there are European distributors of the same. The specific insulation type I mainly use is180C polyurethane-nylon, as it is both directly solderable and UL listed for use in transformers (et al.).

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2017, 11:10:45 am
content: QuoteAs I have mentioned above in this thread, when the reset winding has the same turns number as the primary, the reset winding can be AC coupled to the switch collector to form a very impressive voltage clamp for the voltage spiking from leakage. The voltage gets hard clamped at 2xVbus at the collector this way. The collector operating voltage is then well defined.You are not going to do the same with the push-pull, as there is no as easy way to clamp the collector voltages in a push-pull.Also, the reset winding may have less or more turns than the primary. If more, then allowing for lower reset voltage being added to the Vbus on the switch collector, but limiting the maximum available duty cycle. But as stated above, most effective solution is the collector coupling. (See below)(I don't think I need any tripple or even quadruple isolated wire. I am not designing a class II device. It is a class I and the output GND can be directly grounded to PE.)EDIT: With a push-pull you can make a sort of clamp like that on second image, but not as simple and great.

author: ocset
author_rank: Super Contributor
timestamp: July 02, 2017, 01:11:05 pm
content: This is a bass amp, so yes youll need 500W continuous rating for 1kw peak.But the peaks are long because its a bass amp, so flyback is not looking good.Youll need PFC, because the instantaneous power peaks are long because of bass amp.So do a BOOST PFC followed by a Full Bridge converter with split secondary......make sure to couple the output inductors of the dual outputs...wind them on the same core....(wind them the way that doesnt make the inductance of the coils cancel each other.)..so use the corkscrew rule etc.I will post some docs on amplifier power supplys....in a few mins

author: ocset
author_rank: Super Contributor
timestamp: July 02, 2017, 01:14:05 pm
content: OK here are the amplifier PSU attachmnets

author: ocset
author_rank: Super Contributor
timestamp: July 02, 2017, 01:15:21 pm
content: OK more attachements on Amp PSU....each post only allows one attachment

author: ocset
author_rank: Super Contributor
timestamp: July 02, 2017, 01:21:14 pm
content: it wont let me send you more attachements, the best ones are to come, let me try and find a way to send them to you.....do you want the one that tells about why your output inductors must be coupled in a current mode bridge power supply with split output?, its from ti.com

author: ocset
author_rank: Super Contributor
timestamp: July 02, 2017, 01:25:44 pm
content: By the way, if you can tell me how to attach a link to my google drive stored course on SMPS's, then i can send that to you...it is full of SMPS tricks which will be good in your endeavours.Is it possible for me to post a link to my google drive document here?If it is, then its yours(pg 3 of the attached tells of why you need coupled inductor outputs for dual output bridge in currnt mode.)I bet you could get away with a pfc followed by two 500W flybacks...(500w is peak). Have a flyback for each of the 70v outputs.Sorry but pushpull is awful for use on a pfc output, mainly in terms of the reflected voltage level being overly high.LLC is poor for split outputs because its more difficult to coulel each sec coil to the core in an LLC Trxformer...but ok if you have an LLC for each of the 70v outputs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2017, 02:01:09 pm
content: Quote from: Hero999 on July 02, 2017, 07:03:23 amConsider those applications: they are very consistent. (C)CFLs only have a brief overload period during which the filament heats up, or the discharge breaks over. The rest of the time, the load is constant. It's a very different environment from an audio amplifier supply.QuoteTwice peak, plus extra overhead because the BJTs exhibit dynamic recovery (see MJE13007 RBSOA for example).The circuit can be rearranged to a half bridge, but then you have the CFL circuit, which isn't quite the same.It could also be adjusted to use MOSFETs, but those are still going to be more expensive than a modest voltage half bridge (i.e., 900V+ devices, versus 500/600V devices in a half bridge).QuoteNo. Not even no!The fuse protects the wiring.NOTHING protects the transistors, in the "old style" amplifiers. It's no surprise that they regularly suffer from destroyed outputs -- not just one, but in pairs or more! Cascading failures!The better amplifiers at least had current limiting, foldback limiting even, but they also failed from time to time.It takes a more subtle monitoring and protection circuit to make a truly solid amplifier. So too, the power supply.A fuse will not save any transistor in any design. The circuit itself must be protected and safe. The fuse is still provided, in case all else fails and the thing begins to burst into flames -- but that's far outside the range of normal operation.The goal is to bring as much of the operating range as possible, into safe, stable, and preferably continuous, operating range. Maybe not 100%, but maybe 99.99997% can be achieved. That is, decades between failures, under occasionally adverse conditions. Extremes of temperature, or voltage and current (including transients and RF). When operation can't be continuous, it should degrade gracefully. A power supply should shut down on overheating, or overvoltage: not explode.Sure, a simple circuit can operate for hours, weeks, years without a failure. Maybe your power is amazingly clean; maybe you just got lucky and never had a surge. Maybe you never had a hot day, or you didn't shove it in a stuffy corner with a dozen other heaters. That doesn't mean anyone else will have that experience.Tim

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2017, 07:40:07 am
content: Quote from: Yansi on July 02, 2017, 07:40:17 amSorry you're right. The transistors need to be rated to double the peak voltage because when one transistor turns on, it delivers a pulse to the turned off transistor's collector, via the transformer which is in series with the DC supply, hence doubling the voltage. As mentioned above, a half bridge would not suffer from this issue.

author: jbb
author_rank: Super Contributor
timestamp: July 06, 2017, 10:47:11 am
content: Quote from: treez on July 02, 2017, 01:25:44 pmLLC cross regulation (I.e. Outputs being equal) can be improved by winding the secondaries very tightly together, e.g. using bifilar windings. But it's never perfect.Using a centre-tapped secondary and a full bridge rectifier to make the +Ve and - Ve supplies does help provide a nice solid voltage clamp for the rectifier diodes. Also, if one side of the supply is unloaded it can drift quite high... a bit of ballast resistor might be required.

author: Yansi
author_rank: Super Contributor
timestamp: July 06, 2017, 11:45:28 am
content: Well... Have put a bit more thought into it and came with this:If I use a PFC, it will provide enough pre-regulation for the amplifier, as there is only a little voltage ripple on the output of the PFC.I could probably use then a non-regulated forward converter to supply the low voltage for the amp. However I am not sure how should I implement one, so it can be made reliable during startup (inrush current charging the output cap bank) and during overload conditions.As far as I can tell, in the professionally made amplifiers, they usually use a halfbridge converter after the PFC, with no secondary side choke for voltage regulation. The primary current is sensed by a current transformer. Question is, is that enough to make the converter safe in an overload condition? Could it react fast enough? As the current rise slope at overload (or even output short circuit) will be limited only by the leakage inductance of the transformer (which might be enough, as the primary current transformer circuit can be designed to have almost an instantaneous reaction). It would have to do a cycle by cycle current limit, or could probably enter a hiccup mode if the overload persists for a longer period. What do you think about that?Startup of the supply then could be made by a rather slow softstart with the duty cycle rising slowly up to the max for about a second or so. (This is also what I have seen in some designs).

author: Yansi
author_rank: Super Contributor
timestamp: July 09, 2017, 02:52:08 pm
content: No response, huh...

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 09, 2017, 03:42:21 pm
content: Thats sad.....Many control chips integrate the soft start and it's essential for that kind of psu.As for current limiting the usual problem is trading off speed of reaction against not reacting to the inevitable leading edge spike, this is where having an output inductor helps as it slows down the worst case DIDT.Personally when not trying to save money I always use output inductors for a more reliable less stressed solution.

author: Ghydda
author_rank: Regular Contributor
timestamp: July 27, 2017, 10:14:22 pm
content: I can't help thinking what the switch losses will be in a DCM-driven PFC unit pulling in on average 5-6 joules every 10 milliseconds with a power crest factor of 2. Yikes.Also, the skin effect on the inductor copper will make the power losses in the copper soar to new heights if not dealt with.Interesting challenges.I shall monitor this thread with interest.Godspeed to the thread owner.Sendt fra min Nexus 6 med Tapatalk

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 28, 2017, 01:15:47 am
content: If you're just making one or a few, get a surplus telecom "rectifier". You should be able to find one for far less than the cost of trying to build one yourself, as in $20 for 2kW.

author: oldway
author_rank: Super Contributor
timestamp: July 28, 2017, 10:10:21 am
content: I worked in SMPS repair at a major power supply manufacturer and I had the opportunity to experience many different topologies.The one we preferred was that of the forward topology with 2 mosfets because it was rare that the mosfets explode in case of error or breakdown... It seems safer than other topologies.So I would chose this topology but with two interlieved 250W forward converters (fase difference of 180° between the two converters) with a common PFC ....NB: power audio amplifiers requires often symetrical power supplies, so the solution of two interlieved converters may be of great interest.....one converter for + , the other for - supply

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: July 29, 2017, 09:24:15 pm
content: I would have suggested using a TL494 for the control IC, and a push-pull live side, as 500w is getting towards the limits for sensible flyback designs.I assume you need dual symmetrical +- supplies. Although if it's a bridge amp you only need a single supply.A series inductor is needed for a forward converter, but flyback converters do not need one. Forward converters are essentially a classic voltage stepdown transformer, while flyback converters are essentially constant current sources.With a forward converter you don't need two synced sections; since the output is symmetrical you just need a center-tapped secondary and four diodes to give a pair of +- supplies. The series inductor in this case has dual windings on the same core. You can't do this with the flyback converter because the output isn't symmetrical.I can see no advantage in using two FETs in series, as in your diagram. This doesn't make it push-pull since both have to be ON for any current to flow.All in all, SMPS design is an advanced task though, so expect a few blowups and a lot of time and frustration on getting it right. I would go for a couple of stock modules, or maybe modify a stock module to give dual output voltages.Regardless, consider the safety aspects carefully.

author: oldway
author_rank: Super Contributor
timestamp: July 30, 2017, 11:50:01 am
content: QuoteThe worst choice ever...1) TL494 as control IC is a poor choice because it is not fast enough for limiting current in case of failure or short circuit...It does not protect the Mosfets efficiently....You should choose a current mode pwm controller IC like UC3842 (100% duty cycle) or UC3844 (50% duty cycle)...that's far more reliable.....2) push pull topology is the worst for fast responding SMPS like it is needed for an audio amplifier.When there is a peak current, fast responding control must vary very quickly the conducting time of the Mosfets.The conducting times from the 2 Mosfets are then differents, that applies a dc voltage to the primary of the transformer and the high dc current saturates the transformer.Inductance of the primary became very low, there is no more current limiting inductance for the mosfets.... Current became very hight, and....they explode ...!!!It is still worst with such voltage as high as PFC dc voltage (more or less 400V)Mosfets must be designed for 2 x 400V more a safety margin, then 1000 or 1200V Mosfets.QuoteThere are a lot of advantages: full recovery of magnetizing energy, Mosfet must be designed for only half the voltage of the one mosfet forward converter, no catastrophic failure even if one Mosfet goes short circuit.Using two interlieved forwards converters has also some advantages: ripple is reduced and of 2 times greater frequency (both primary and secondary)...it's sometimes easier and cheaper to make 2 converters of 250W than one of 500WNB: Be aware of "Bus pumping" phenomena with half bridge classe D audio amplifierhttps://www.infineon.com/dgdl/an-1071.pdf?fileId=5546d462533600a40153559538eb0ff1

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2017, 08:59:44 pm
content: Quote from: oldway on July 30, 2017, 11:50:01 amWell, sure, if you follow the ancient appnotes, and don't put current protection on it.You don't have to use it in voltage mode. It's quite possible to use it in average current mode. Indeed, it's quite preferable to do so, for precisely the reason you give!Downside: you need to add an external op-amp to achieve voltage regulation.A small price to pay for such a cheap and versatile modulator.QuoteWell, UCC3808 and related chips are neat. They don't suffer from subharmonics. (Curiously, before I purchased some UCC3808s to play with, I built the block diagram with discrete logic, which did suffer from subharmonics. This might've been due to unbalanced delays in the type-T flip/flop, or drivers.)You still have the same disadvantage of the peak current mode control, however.QuoteWell, you wouldn't use "push-pull" (two NMOS, source to ground, drains to CT'd primary) for mains operation."Push-pull" is often used ambiguously, referring to the bridge configuration as well. Bridge is perfectly serviceable for offline operation.Anyway, 1000V MOSFETs have performance comparable to any other type, these days, so it's not a big deal. (There are other reasons to avoid PP, though.)QuoteThese are spot on.Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2017, 11:50:25 pm
content: The amplifier does not require close regulation of its supply voltages so if you need to step up the voltage and as a bonus supply multiple output voltages, why not use a high frequency inverter? This is similar to Hero999's suggestion of a Royer converter but even simpler.Vicor actually makes these as "bus converter modules".

author: oldway
author_rank: Super Contributor
timestamp: July 31, 2017, 08:57:19 am
content: QuoteAverage current mode is slow acting protection and only works efficiently if the rate of increasing current (dI/dt) is limited by an inductance of sufficient value.That's the case when there is an output short circuit, but not if a rectifier diode goes to short circuit...inductance is then too low and average mode current limitation is not fast enough to protect the mosfets....a current mode pwm controller IC limit peak current every single pulse...that's far more reliable.Average mode current does also not protect Mosfets from overcurrent due to primary saturation....a current mode pwm controller IC does it very well.For me, push-pull is push pull, not bridge configuration.I don't care if"Push-pull" is often used ambiguously, referring to the bridge configuration as well.", that's wrong....let's name the configurations by their correct names !!!!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2017, 11:04:22 am
content: Quote from: oldway on July 31, 2017, 08:57:19 amAgain... without current protection.Note that, it's not that average current mode control is "too slow" -- for faults in those locations,that current isn't being sensed at all!That is, indeed, the most wonderful advantage of the peak current mode controller. It's just that it suffers from economy of scale.And if you want to chuck a current transformer on the primary side, you're welcome to do that; then all these concerns are handily met. And with the cost and EMI savings of CCM operation!If not the cost savings, what with all these transformers and extra control circuitry.Alternately, you could employ desat protected gate drivers. Because, really, if you're worried about burning up friggin'diodes, surely the transistors are far more vulnerable!But this is usually too expensive to justify, until you get into the several kW range. Engineering is holistic optimization, after all: from design reliability to final product cost. Optimization, not minimization or maximization! Putting 20 year parts into a 5-year lifetime product isn't optimal!Tim

author: P.Mouse
author_rank: Contributor
timestamp: July 31, 2017, 11:31:22 am
content: I see a lot of theoretical ideas, which is nice to brainstorm, but real world circuits work very different.That is especially true for SMPS and (Class-D) power amplifiers.I am thinking about enough snubbers and board layout to prevent ringing and noise.Also, a SMPS for an amplifier is a whole different thing. Regulation isn't necessary.In fact, sometimes even unwanted because you need to design a power supply that can provide good peak power, because of the dynamic loads.PFC is rarely been seen in audio power supplies (even today).

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2017, 02:24:26 pm
content: Quote from: T3sl4co1l on July 31, 2017, 11:04:22 amWhat do you mean by "suffers from economy of scale"? Do you mean that it is more complex and therefor less economic to design into a small number of units?I prefer peak current mode for the reasons you identify and more.

author: oldway
author_rank: Super Contributor
timestamp: July 31, 2017, 03:13:11 pm
content: QuoteCurrent sensing magetics:http://www.power.pulseelectronics.com/current-sensing

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2017, 03:19:33 pm
content: Quote from: David Hess on July 31, 2017, 02:24:26 pmThe high ripple current requires quite more electrolytics than otherwise; a forward converter needs less transformer, and though an inductor is needed, it can also be cheap (powdered iron).I have been seeing two-switch forward and flyback more often, at higher power levels (like computer PSUs). May suggest electrolytics are getting better, moving the economy crossover point to higher power levels.Tim

author: ocset
author_rank: Super Contributor
timestamp: September 02, 2017, 01:10:47 pm
content: Yansi you said you took apart amps and saw no output inductor......i did same and i found same, ..the topology they use is some kind of variation of an LLC...it looked pretty crude and possibly even just relying on vout/vin = ns/np for a full bridge llc which switches at the upper resonent frequency of the llc converter, in kind of an "on/off mode" to get the regulation.I dont think vout regulation always has to be really tight with a class d amp as the control circuit can sometimes reject power supply vout variations....as long as their not way excessive.