50v 20A linear power supply - Page 1
Forum section: electronics / projects

author: M0UKD
author_rank: Contributor
timestamp: October 17, 2015, 05:21:52 pm
content: Hi all.I am building a 50v 20A power supply. I have a 1000VA toroidal transformer which has a 230v primary and a 40v, 25A secondary.I was going to build it unregulated, as I can accept some voltage drop under load. I bought the 40v transformer in the hope that I would get around 56v once rectified, but as we have 240v mains and not 230v (infact its usually around 245v here), I am getting a peak rectified no-load voltage of around 63v, which is a bit high for the intended application.So now I am thinking of making it 48v regulated, but I have some questions.1) What would be a good way to go about this? I have used 2N3055 pass transistors in the past, but there is probably a better way?2) What would be a good drive regulator that works up to 50v? Perhaps I could make my own out of an op amp and have that sense directly off the output of the pass transistors?I'd appreciate any thoughts or suggestions.Thanks, John.

author: fcb
author_rank: Super Contributor
timestamp: October 17, 2015, 05:50:44 pm
content: Do you care about noise on the output?Your going to be overloading the transformer a little unless you have 100% efficient regulator, so I would consider building a large buck-converter (or several - polyphase) rather than a linear regulator.If you are going down the linear regulator route, please god anything but the 2N3055 (unless you have loads of them onhand) - the hFE is terrible.

author: mariush
author_rank: Super Contributor
timestamp: October 17, 2015, 05:59:11 pm
content: If you'll configure the power supply to output 1v at 25A , you'll basically dissipate (40v x 25a ) - (1v x 25a) = 975 watts . Good luck with that. Even with very good heatsinks and forced air cooling, you're looking at about 50-80 watts per transistor of dissipated power...This kind of power supply is done with multiple secondary windings, connecting them as needed using relays or other methods.For example, for a 24v DC power supply, you'd have 2 14-16v AC secondary windings. For voltages below about 12v, just the first winding is connected to the rectifier, so the actual losses in the transformer will be much smaller. As user goes above around 12v, the second winding is connected in series with the first.Another thing... transformers are labeled as AC, if you have a voltage transformer that says on the label 40v 25A , that's AC, RMS. When you rectify this to DC using a bridge rectifier, you'll have a DC voltage with a peak equal to Vac x 1.414 = 40 x 1.414 = 56.6v , minus the losses in the bridge rectifier that are about 1.5-3v depending on rectifier choice (and if you use 2 or more in parallel or not). The DC current will NOT be 25A, it will be about 0.62 x Iac = ~15-16A ... 0.62 is a constant chosen that works with most transformers but could be a bit on the low side for a 1000w transformer.But anyway, what you have to get out of this is that with a 40v AC 25A transformer, you can't get higher voltage AND same current, current will drop to 15-16A.And, that 56v is a PEAK DC voltage, the voltage will go between 0 and peak voltage 50-60Hz times 2 a second, you need capacitors to keep the minimum voltage as high as possible (or convenient for you). A basic formula to approximate capacitance you want is C = I / [ 2 x Frequency AC x (V dc peak - Vdc minimum) ]so for example if you're in EU with 50 Hz mains and keeping in mind you may at some times get only 220v at mains therefore the peak dc voltage from transformer may be only about 54v minus 2v in bridge rectifier, so about 52v DC. You also have to keep in mind that the pass transistors or linear regulators will drop about 1-3 volts on them... let's say 2v. If you put the numbers in the formula you will draw the conclusion that you'd need an almost infinite capacitance to get your 48v DC power supply with this transformer. With a more realistic minimum output voltage of about 45v (so you'd get 42v DC at the psu connectors), you'll be looking at :C = 17 A / [ 2 x 50 Hz x ( 50v - 45v) ] = 17 / 500 = 0.034 Farads or 34.000 uF of capacitance for 17A of current (because your transformer won't really do more current without overloading).PS. you'd also have to be careful with the fuses for such power supply, especially with toroids, as they draw much higher current when they're plugged in. You'd probably need to use 30-40A time delay fuses.Such large capacitance would also cause the transformer to push a lot of current into them, trying to charge up, so it wouldn't hurt to also have a PTC (a resistor in series with the fuse) to slow down the transformer. As current goes through the PTC, it will heat up so it will allow more current through it... initially little current goes through, so the rectifier and the fuses will be happier, as it heats up the whole 25A or more will be available. A relay can be configured to short out the PTC after a few seconds , after power supply is turned on (so you won't waste power on the PTC).Going to attach the schematic of a power supply or two, to see what I mean about using relays to connect several windings in series.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 17, 2015, 06:20:53 pm
content: You can often pick up surplus telecoms SMPSUs in this voltage current range - they are sometimes called "rectifiers". Super high quality and usually very quiet.

author: M0UKD
author_rank: Contributor
timestamp: October 17, 2015, 08:14:06 pm
content: Thanks for the replies.I am not building a variable supply, so no worries about 975 watts dissipation. Just a fixed 50v (ish) supply.15A is probably the maximum I would need from this supply, so that's fine. It's for powering an RF amplifier in SSB, so it wont be a constant 15A.I do have a lot of 2N3055H's, but would prefer something with more gain, so the driver circuit doesn't need to be hugeThanks Mike, I do have a Eltek SMPS 1000 48v switcher, but the reason I am building a linear supply is because it is a little noisy, when used with communications equipment. Plus its something to buildAny suggestions for a 50v regulator (like the LM317 but with higher voltage capabilities >65v) and better pass transistors than the 2N3055?Thanks!

author: dom0
author_rank: Super Contributor
timestamp: October 17, 2015, 11:24:51 pm
content: - 1000 VA transformer for 1 kW DC output is far too small- Use any regulator circuit you like, e.g. the classic HP regulator riding the positive output- Audio power transistors may be interesting for the pass elements, of which you probably need at least ~8 at this output power- Permitting a high voltage ripple on the filter caps reduces power loss in both the pass elements and the transformer. Make sure that you have enough headroom for the regulator and low-line conditions. Too small headroom can easily be seen when 100 Hz spectral lines appear on the output (e.g. spectrum analyzer, DSO FFT)- You also want to take a look at the ripple current of the filter caps. You'll want to use quite a few, as they are lossy, too, and have poor thermal conductivity.

author: akis
author_rank: Frequent Contributor
timestamp: October 18, 2015, 08:24:29 am
content: On smaller scales I have found that the combo of LM317 bypassed by a suitable transistor, in this case the MJL3281 and MJL1302, results in blown LM317s. No idea why, all protections were in place.Since you want a fixed voltage, go old style, use a zener/FET combo for the voltage reference at the base of the pass transistors.You can have a tiny resistance (say 0.05R or less) before the pass transistor to sense the current and use a switch and a resistor network straight on a transistor to ground the voltgae reference when too much current goes through. I say use a switch like "Low/High". So when you are testing and you have leads all over the bench, you keep the switch to "Low" at which point it triggers at a small current, say 1A.In addition there are simple slow start circuits you can use to power up th toroidal, I use them on anything at or over 300VA. The circuit is basically two relays the first relay closes as soon as you press "On" but feeds the toiroidal through a series of large 10R resistors, and after a set delay (250ms) a second relay opens which bypasses the resistors. By that time the toroidal is chanrged and you are OK. I can upload the schematic if I find it somewhere.

author: akis
author_rank: Frequent Contributor
timestamp: October 18, 2015, 09:09:16 am
content: This is a slow start for a 300VA toroidal I made a few years ago, works like a dream, can be improved slightly on the small transistor that opens the second relay (maybe a beefier one)

author: rikkitikkitavi
author_rank: Contributor
timestamp: October 18, 2015, 09:27:41 am
content: I add to dom0 , at a load of 1000W, with a a capacitive filter PF will be low and effective RMS current drawn from the transformer will be equal to a 1500-1600VA load, resistive.Hence you can only expect about 650W continously from a 1000VA transformer.It is possible to overload short durations of course.

author: dmills
author_rank: Super Contributor
timestamp: October 18, 2015, 06:12:26 pm
content: If it is for a SSB set, then short duration is the name of the game (About 10dB power backoff ratio even for highly compressed audio), but due the the increasing drain mismatch a 6dB reduction in RF only gets you a 3dB reduction in current demand (and a 3dB fall in amplifier efficiency), .Me I would start with that switcher and clean it up rather then starting with the enormous iron, those Eltek supplies are not that bad (Assuming you mean the double eurocard convection cooled affairs), the result will be a supply that ONE person can lift.Now if I was going for it linear with big iron, I might be very tempted to cheat and loose most of the excess voltage across a modest 'buck transformer' wired into the primary side of the big iron, with tap changed manually or with a handful of thyristors and some zero crossing optos driven by window comparators on the DC side.The other option (Which would be the big iron approach I would probably take) is a switcher on the secondary side, which is enabled only when you go key down, you have got ~10ms or so while the TR relays switch and the bias comes up to get the amplifier rail up and stable. If you used one of those 4 chip buck/boost controllers you could minimise the DC rail capacitance and actually allow the reg to drop into boost mode at the bottom of the cycle which would improve the power factor and reduce transformer heat at the expense of increased ripple current in the bus caps.If going fully linear, mosfets are your friend (No base current supply issues), hint put the mosfets in the negative return, and float the transformer secondary, that way you can easily drive the gates far enough above the drains.Regards, Dan.

author: Kalvin
author_rank: Super Contributor
timestamp: October 18, 2015, 06:22:02 pm
content: How about using 12V lead acid batteries or similar (you might even find a 48V battery which will make your design much simpler) to provide the peak current needed. Then, build a (simple) charger which will keep the batteries charged.

author: poorchava
author_rank: Super Contributor
timestamp: October 18, 2015, 06:37:00 pm
content: You can use TIP35Cs as pass transistors. They are cheaper, ale have parameters similar to those of 2N3055 and are in much more friendly package.Sent from my HTC One M8s using Tapatalk.

author: dom0
author_rank: Super Contributor
timestamp: October 18, 2015, 08:12:13 pm
content: QuoteMOSFETs are almost never your friend in power linear.Also, base drive is not a real issue in a linear supply, since the drive currents flow through the load as well, and can be sourced from the main transformer, too.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 19, 2015, 12:52:00 am
content: Maybe a hybrid approach would make more sense? As in a small linear supply and a large switcher, with the switcher only on during transmit.

author: akis
author_rank: Frequent Contributor
timestamp: October 19, 2015, 06:52:44 am
content: At half the max current draw, sat ay 7A, there will probably be 57.5V-58.2V output past the 20mF caps. The ripple will be around 4.5V p2p. The pass transistors will have to drop around 8V @ 7A this is already 56W of heat. At 1A the output will be around 59V and the pass transistors will need to drop 9V @ 1A = 9W. Maybe you will have more losses on the diodes and on the windings, which means the heat will be spent on the diodes and on transformer) but I think my estimates are correct.

author: Theboel
author_rank: Frequent Contributor
timestamp: October 19, 2015, 11:10:51 am
content: I believe You will use LDMOS devices, If I can give You advice use 48v is a little bit high, I suggest to use not more than 45vYou will lose some power output but Your devices is safer. my sugestion based from my experience use LDMOS in broadcast transmitter.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2015, 05:43:15 pm
content: At that power level I would consider using some passive power factor correction through an inductor. This can save on the needed capacitor and lowers peak currents and output voltage under high load. The inductor may be heavy and a little difficult to obtain, but is more reliable than capacitors under high ripple current. The lower voltage can also be used as an advantage here.I would also prefer BJT for the power stange - it's much easier to parallel BJT than MOSFETs. The current per transistor is likely limited by the SOA during startup - so one will likely need quite some transistors (e.g. 15 x 2N3055). At that current level Hfe is not that bad, even with the 2N3055. I would also prefer a package like the TIP35 for easier mountig - heat removel is likely not a critical point in this application. The base current could come from the main supply, possibly without PFC to have a little extra headroom here and thus less drop for the main current.

author: rikkitikkitavi
author_rank: Contributor
timestamp: October 20, 2015, 11:22:13 am
content: Then you need a transformer with higher secondary voltage to compensate for I*X voltage dop.Inductor will be largish.And it will be ridicously largish to get above a utilization of more than 75-80% of the transformers VA rating even if rms value of the charging current spikes reduces.Easy to simulate i LtSpice or similar.

author: dom0
author_rank: Super Contributor
timestamp: October 20, 2015, 12:27:09 pm
content: You'd put the inductor on the primary side. Saves a lot of copper, and power dissipation. Common practice with high-power primary side SCR-preregulated supplies for e.g. emergency battery chargers.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 20, 2015, 01:40:37 pm
content: For the mechanical size of the inductor it does not make any difference whether it's on the primary or secondary side of the transformer. It's the same amout of energy that has to be stored. It may be easier to wind, as the wire is thinner, but more windings are needed. The advandatege of having the inductor on the primary side is, that it can also help a little to reduce incrush current during turn on.In large supplies the inductor get more attractive, as magentic divices often get better when they get larger. while with large capacitors cooling gets more difficult. You will not get a good ( > 0.power factor easy, but the inductor can help to get away from the really poor powerfactor (possibly < 0.5) you can have with a large torroidal transformer and large caps. It may just be inpractical not to have an inductor to limit current spikes.

author: rikkitikkitavi
author_rank: Contributor
timestamp: October 21, 2015, 06:22:31 pm
content: True.One could get some of the inductance through a very leaky transformer though.For the hobbyist perhaps the finding a scrap Xformer , rewinding it and cutting a air gap is cheaper than obtaining huge caps for the rectifier.I have seen some largish power amplifiers with huge cap banks and toroids with inductors to limit charge current peaks. FEX H/K AVR7300/7000 (toroids > 1000VA)

author: dom0
author_rank: Super Contributor
timestamp: October 21, 2015, 07:22:11 pm
content: Making an air gap in an existing transformer could be interesting: You'd need to completely disassemble it, cut/stamp the sheets, recoat them, reassemble, reweld, rewind, repot it. If you just take an angle grinder and cut into the core you'll short the sheets out, which would make the core very lossy (eddy currents).Making a toroidal transformer (- for mains frequency, that is, a wound core) with an air gap is not possible.

author: rikkitikkitavi
author_rank: Contributor
timestamp: October 21, 2015, 08:32:38 pm
content: Depends on the transformer core.C core, air gap is built in per design.Toroid- well cutting it means you have loads of metal band all over, or cut it half way, attach a binder around the cut half and cut the other half... been there done that. Cutting ferrite toroids is a different matter though...EIs are the best and the worst. If it is a pure E /I which has not 180 twisted layers (to reduce magnet leaks) you can simply knock out the I.Cuttin all cores means you have to fray the cut ends of steel laminate sheets of course to prevent shorts.Like pointed out, a separate inductor will be huge. Probably same size of a EI transformer of equal rating as mains transformer.I attached a simple LTspcie that I used to size inductors. And necessary extra secondary turn to end up at the same rectified voltage. (about 30% more)You also needs to add a lot of secondary to increase the voltage to compensate for impendance voltage drop. This increases the VA rating, but you gain of course current rating lowering necessary VA due to better PF.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 22, 2015, 01:57:09 am
content: If space is of little concern, get a microwave oven transformer, cut off the existing windings, then add your own winding with the correct number of turns to get the required inductance.

author: pelud
author_rank: Regular Contributor
timestamp: October 22, 2015, 04:20:34 pm
content: If it doesn't have to be regulated, why not just put a suitably sized variac on the primary side of your transformer? That way you can also set the voltage to whatever you want.