0-70V, 0-5A Lab Power Supply Design - Page 1
Forum section: electronics / projects

author: H713
author_rank: Regular Contributor
timestamp: April 08, 2020, 05:24:17 am
content: For testing audio amplifier designs, it is useful to have a fairly beefy bench power supply. My current supply is capable of 10-34V at about 3 amps, and it has proven to be insufficient. I have decided that it is time to build a bigger and better linear bench power supply. Here are my minimum requirements. As a side note, this is by no means my first power supply design project.1) 0-70V2) 0-5A3) Extremely now noise/ripple. I'd like to see less than 2 mV peak-to-peak.4) Linear, which complicates #55) Dual power supply in a 2U rackmount enclosure.6) Fan-cooled is acceptable7) Adjustable crowbar circuit to avoid damage to low-voltage circuits (like mixing console channel strips) in the event of a catastrophic failure. This will be a manual control that will likely see seldom use, however, it is essential to have for this application.Obviously it is impractical to dissipate 700W in the series pass elements when operating at low voltages and high currents. As such, some form of "pre-regulator", which will probably come in the form of a Triac "dimmer" circuit on the primary of the power transformer, will be necessary.Even still, the pass elements present an issue because when the output is shorted while it is set to 70V, the main filter caps will take some time to bleed down after the pre-regulator adjusts for the reduced output voltage. Traditionally these would be a BJT in a TO-3 package. I have an alternative idea, however.An application note from International Rectifier suggests that for linear operation, older high-voltage MOSFETs with a high Rdsonwill be less likely to suffer from secondary breakdown. Testing of the FQA8N90C and the 2SK3675 verified this. A single 2SK3675 or FQA8N90C on a substantial heatsink was successfully able to handle 40V at 6A in its linear region. None of the BJTs I tested came close. I was not successful in killing either of the MOSFETs that were tested. I do intend to test with the full 70 volts, however, I need to dig a larger transformer out of storage in order to do so.I haven't seen any linear power supplies using MOSFETs in this way, so I'm curious about what I'm neglecting to consider here. I was planning to use six of them in parallel for each of the two power supplies, and they will not be seeing the full 70 volts dropped across them for more than a second.Here's the basic idea I've been simulating for the regulator circuit. Nothing complicated, and I'm using the IRFP240 and 2N3019 SPICE models because they're fairly generic. This is a generic schematic, obviously there will need to be changes made so that it is stable into capacitive loads, etc. R7/R11 and R15/R16 will be replaced with pots. Yes, there will be limits imposed to ensure you can't set the thing to current limit at 20 amps.

author: moffy
author_rank: Super Contributor
timestamp: April 08, 2020, 07:05:42 am
content: Best way to do the variable voltage while keeping noise down and power dissipation, is to have a multitap transformer, that changes taps in say 10v or 20v increments. You need a controller for the relays but it's fairly straight forward.

author: H713
author_rank: Regular Contributor
timestamp: April 08, 2020, 07:10:05 am
content: I thought about doing tap switching, but that would require a custom transformer. I agree that it would be the simple and robust way to do it and I may still see if it can be an option without having to wind a custom toroidal transformer.

author: moffy
author_rank: Super Contributor
timestamp: April 08, 2020, 07:22:22 am
content: Yeah, a bit of a pain unless you use multiple say 5A/20v (secondary) transformers in series. But then packaging is a pain.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2020, 08:26:09 am
content: It is very common to get transformers with dual output windings, like 2 x 24 V. So a single transformer would already give 2 voltages.With only 2 U hight one does not have the hight for a single large transformer anyway. Keep in mind that continuous 5 A DC need something like a 8 A AC rating, unless extra PFC is used.With 2 transformers for each half one would have 4 taps (possibly more with different voltages). Even just 2 taps nearly cuts the worst case loss to halve. In addition it reduces the voltage seen by the transistors which helps with the SOA.For the tap switching it is still the question if one wants relays on the AC side or an electronic version at the DC side (e.g. 2 transistors in series). Both have there pos and cons.Normally an audio amplifier has to work with a normal unregulated supply, so there is no real need to have super low noise for the high power part. One may not need / want normal lab supply grade current limiting. There can be extra peak current, and once the voltage drops too much chances are one would prefer a full off than holding the peak current. This can reduce the worst case power loss quite a bit. I would consider the regulation more for getting adjustable voltages and to have some protection.There are some HP supplies that use MOSFETs in linear supplies. It is not that unusual.

author: CHANDRASEKARAN.V.T
author_rank: Newbie
timestamp: April 08, 2020, 08:37:51 am
content: I am not sure if I can post my requirement which is as follows:I need to design an ISOLATED DC-DC CONVERETER WITH 1KV ISOLATION:INPUT:12VDC+/-1VOUTPUT 1:12VDC---30maOUTPUT 2:12VDC---30maEfficiency is not a serious consideration.Wish to use LM3524D as the control IC with feed back.But designing this transformer with a toroid is my problem and can I get some support.RegardsVTC9980522856BENGALURU,INDIA

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: April 08, 2020, 10:24:10 am
content: Quote from: CHANDRASEKARAN.V.T on April 08, 2020, 08:37:51 amInstead of hijacking this thread, start another one related to your project.

author: Zero999
author_rank: Super Contributor
timestamp: April 08, 2020, 11:19:57 am
content: 2mVp-p ripple on 70V is an insanely crazy low specification of under 0.006%. Is it even possible? Just use batteries.I don't see why the original poster needs all of the above. A decent audio amplifier should be fine with an unregulated power supply consisting of a large transformer and filter capacitor. A variac, with a suitable mains transformer, rectifier and smoothing capacitor should be sufficient for testing most audio amplifier designs.Any pre-regulator circuit will introduce a significant amount of noise and ripple on the secondary side. Worse still it will be high frequency ripple, which is less easily to get rid of, as the supply rejection of op-amps tends to decrease, with frequency.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 08, 2020, 02:24:28 pm
content: Quote from: H713 on April 08, 2020, 05:24:17 amI'd recommend against a triac or SCR based pre-regulator if you want to achieve low noise etc. Your main regulator should be able to cope with the ripple produced by the pre-regulator, as it would have to do so anyways. But a "dimmer-like" circuit can produce large stray magnetic fields at line frequency repetition rate having high frequency content (depending on your transformer / choke / bulk cap) that might couple into sensitive audio circuits. So you'd better go for a tapped transformer.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 08, 2020, 03:00:02 pm
content: Why dont you simply buy like : an sorensen xantrex kukusui or smps based psu ??/It will be simpler and some of them already have or are near the needed specs ??Sorensen DCS80 - 13EKikusui PAD70-5you have this one selling right now for an incredibly low price ?? untested an weight a ton lollSorensen DCR 80-20Bhttps://www.ebay.com/itm/Sorensen-DCR-80-20B-Variable-Regulated-DC-Power-Supply-0-80VDC-0-20A-1800W/113890939218?My kikusui pad70-5 give 1 -13 mv of ripple under a full load, sure the psu has been re-capped, on par with the manual, but i've paid 50$ usd to have it minus the shippingJust push a button and adjust the curent limitter, i love this.https://manual.kikusui.co.jp/P/PAD_L_TYPE1_2_E.pdfMy Kepco ATE 75-8 is slightly noisier, have to re-cap it. It has an scr crowbar protectionBut a 2mv rippleuse batteries as primary source. And sure many of theses psu are kinda smps or a mix of smps and phase controlled psu since you're going over 500w and up.An linear one wil give tons of heat and power losses ....

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 08, 2020, 03:21:10 pm
content: I have a lab supply what have classic output range 2x 36V 2A with specified 1mVpp ripple output and some fixed outputs +-15V with 2mV ripple at full loadIt uses thyristor preregulator with linear post regulatorBut it is huge beast and as new cost a fortune and there is not much what can be done smaller when want do it linear with mains transformer and reliable

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 08, 2020, 03:44:34 pm
content: You don't need a custom transformer to achieve several raw DC voltages, just a dual secondary transformer and a bunch of relays:Low range: secondaries in parallel, FW bridge rectifierMid range: secondaries in series, FW bridge rectifierHigh range: secondaries in series, voltage doubler rectifierI'll let you figure out the relay switching logic. Probably best achieved with a small microcontroller.

author: H713
author_rank: Regular Contributor
timestamp: April 08, 2020, 03:56:56 pm
content: Ripple isn't a huge deal up at 70V, and I should have mentioned that it is much more of a concern in the 24V range where it will likely be used to test class-A mixing console circuits. Given that the 0-500V linear supply I built had ripple below 10mV under load, I feel that I should be able to get this significantly lower.I should note that I have decided to build this rather than purchase a Sorensen or Xantrex PSU because going onto eBay and clicking "Buy" isn't much of a design exercise, is it?Regarding noise from the Triac/SRC "dimmer" pre-regulator: I was going to use an LC network to try and filter out the worst of the high-frequency noise before the primary of the main transformer. That said, I will be looking more seriously into tap switching since it is a much simpler solution.I can try a doubler circuit for the high range, but I suspect that the voltage sag under load will be problematic unless I useexcessively largefilter capacitors. I need to do a few calculations to see if it's going to be realistic. If it is, that solves the problem.

author: David Hess
author_rank: Super Contributor
timestamp: April 08, 2020, 04:59:11 pm
content: Since I try not to require regulated supplies for my audio power amplifiers, I use a conventional unregulated transformer/rectifier/capacitor power supply preceded by a variac for testing.

author: H713
author_rank: Regular Contributor
timestamp: April 08, 2020, 05:10:44 pm
content: I should note that while theprimaryuse for this will be audio power amplifiers, it will see use for testing other more sensitive circuits. As an example, class A mixing console circuits operating on +45V rails. It's still probably overkill, but I think it will be an interesting challenge to design.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 08, 2020, 06:08:34 pm
content: Filtering the extra frequencies from a scr regulator is not so easy, as the frequencies are relatively low. It is possible but still not so simple.Large filter caps can have the disadvantage of getting a even lower power factor, especially if there is no series inductor to improve the power factor.A switched mode converter has the advantage that passive filtering is easier with the higher frequency noise. So the choice is not that clear.For the current rating one could have peak current limits higher than the long time continuous rating. Especially transformers are relatively slow to react and can deliver more than nominal power for quite some time. This is especially sufficient for the audio output peak currents.

author: H713
author_rank: Regular Contributor
timestamp: April 08, 2020, 07:11:26 pm
content: Alright, here's where I'm currently at:I believe I have a way to do transformer tap switching without having to design and order a custom transformer. The Antek AN-3236 has a pair of windings on both the primary and the secondary. Assuming that I use one transformer for each power supply, that gives me three steps, which should be workable.Yes, this PS will probably be digitally controlled using a microcontroller. In addition to the tap switching, it will also manage the speed of the cooling fans. This brings me to a question regarding microcontrollers:If I want to digitally control both transformer tap switching and speed of the cooling fans (dependent upon heatsink temperature), would this require two separate microcontrollers or could one handle both processes?

author: Zero999
author_rank: Super Contributor
timestamp: April 08, 2020, 08:04:51 pm
content: My advice would be to not use a microcontroller to control the tap switching. Use comparator(s) on the output of the power supply to control the tap switching circuit. That way it will work irrespective of the microcontroller crashing and the same goes for the fans.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 08, 2020, 09:57:48 pm
content: I would refrain from digital control other than providing the set values via D/A converters.I think that those PS that use multiple MosFets in a linear mode must have had the benefit of a builder who could ask everything from a supplier, like very very tight selections. The only appliance which I have personal experience with was a custom unit which was used to weld hermetic cases for hybrids. If one of the Mosfets did break ranks even a bit, mass extinction ensued. While it is certainly feasible, you would have to do the selection yourself, which could be costly. If you want to try anyway, APT, IH (now Renesas) and IR had appnotes on the topic.Where I see problems is the 2HE requirement. Even assuming that you use fans with tunnel-type heatsinks, the heatsink has to have some capacity for heat which means mass, and therefore size.I would also recommend to give that brute a line switch with holding coil/undervoltage trigger and wiring the failure alarm contacts from the fans into this circuit alung with the thermal switches in the transformer and those at the heatsink.Concerning the preregulator, careful consideration should go into the control law. It is quite easy to produce instability here. Also, there are some alternatives to switching windings by relay or using phase control. Switching by triacs would be instantaneous and could be fast enough to make it work as a cycle-to-cycle switch. I have to concede that varying phase angle between the line voltage and current in the secondary could make this tricky.Also, there is the magnetic amplifier (Transduktor), which introduces some noise by saturation, but certainly less than a phase control.As you are doing audio power amplifiers, you might be already familiar with the McPherson circuit, where dissipation is limited by split rails. An example (using single transistors for a rather small and compact PS) was once published by Ian Hickman in the Electronis&Wireless World. My research led me to the original publication by McPherson, which was aimed at AF amplifiers. I unfortunately do not have it myself (IEEE transactions),

author: splin
author_rank: Frequent Contributor
timestamp: April 08, 2020, 10:18:35 pm
content: If it is one off (or only a few required) it should be fairly easy to add additional windings to a standard toroidal transformers (provided the centre isn't potted!).For example, I have a 2x50V Vigortronix 625VA, VTX-146-625-250, which requires only 1.68 turns/volt. So an extra 12V secondary would only need 20 turns. The centre has a diameter of around 38mm so plenty of room to accomodate extra windings. Starting with a 2 x 15V transformer you would get the advantage of lower copper losses in the secondary at lower voltages, whilst adding the extra windings with progressively finer wire to make it a bit easier to wind.Alternatively, you could carefully cut the existing secondary to add your own taps. In the case of my 625VA, the existing secondary is 2 x 84 turns of 1.25mm dia wire in one layer. This wouldn't be feasible for much smaller transformers of course with secondary windings having many more turns of finer wire.A further example; my 225VA 2 x 50V Vigortronics VTX-146-225-255 has 2 x 150 turns of .82mm so 3 turns/volt (with 30mm inner diameter).Or remove the existing secondaries and rewind. Some transformers have the primary winding on top of the secondary(s) but that isn't the case for my Vigortronix. Jerry Walker has some really well presented videos on transformers and has done lots of rewinds. This one is good where he designs a linear supply to power an RD6000 switching supply and rewinds a pair of toroids: He actually rewinds the transformers in part 4. Search for 'RD6006 Linear Part4' (without the quotes).

author: duak
author_rank: Super Contributor
timestamp: April 09, 2020, 12:03:47 am
content: The hp 6002A is a supply of similar rating that uses thyristors to switch the transformer secondary windings. The general design is written up in this hp Journal issue:https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdfThe authors go into the algorithm of how the windings are selected. Some older hp supplies have two voltage ranges that the user can manually select, kind of like a transmission in a vehicle. Automatic range switching is nice, but can sometimes be fooled close to the transition point(s).I've come to appreciate the design of hp and Lambda lab power supplies. Most use a floating regulator where everything is referenced to the positive output terminal. This makes the voltage and current feedback loops much easier to stabilize as the error amplifier does not need to swing over the full range of the raw DC input. There are a lot of bells and whistles in the 6002A that aren't needed here.Foldback Current limiting can protect the pass transistors by reducing the current limit as the voltage across them increases. Not much could be worse than to have a pass transistor in the supply fail shorted while powering a test circuit. You could easily have two corpses afterwards.

author: H713
author_rank: Regular Contributor
timestamp: April 09, 2020, 05:46:37 am
content: Quote from: Neomys Sapiens on April 08, 2020, 09:57:48 pmI'm hoping to mitigate the issues with the FETs in a few ways. For one, I'll be using 900V rated parts with a fairly high Rdson. Additionally, I will be using quite a few of them in parallel to have as much overhead as possible. The idea is that there will be enough beef in the series pass transistors that they will survive even if the tap-switching fails. They will also have some rather generous source resistors in order to help with the current sharing.Heatsinks are not a problem either- I currently have an Ashly FTX-1000 on my bench that is beyond repair (No seriously... it's cooked.https://www.diyaudio.com/forums/solid-state/352468-ashly-ftx-1000-failure-analysis.html#post6154245). The heatsinks and chassis may end up being used for this project, unless I come up with something better.Also, I have plans of running the heatsinks live and not trying to isolate the MOSFETs from the heatsink, so the thermal interface should be quite good.

author: magic
author_rank: Super Contributor
timestamp: April 09, 2020, 06:27:18 am
content: Maybe you really need a fancy high-spec 24V supply and another more relaxed one for 70V?As for MOSFETs, whole threads are written on DIYAudio on how to use them for amplifier output stages. It seems the most "interesting" problem is their very high fT and resulting proclivity to crazy fast oscillations.I have actually had a surprising encounter with this problem when a simple ON/OFF rail switching FET went into tens-of-MHz oscillation during transition and coupled noise into the control circuitry which promptly turned it off. The rail never came up even though everything seemed right

author: H713
author_rank: Regular Contributor
timestamp: April 09, 2020, 06:35:18 am
content: When I was testing the FETs for this project, I initially had about 1 meter of 20 AWG wire connecting it to the filter caps, leading it it to oscillate so violently that it was picked up on an oscilloscope almost 10 feet away. 1 single MOSFET, 1 meter of wire = violent oscillator. Silly me had thought "It's just one FET bolted to a heatsink. I don't need no stinkin' bypass cap!"Yes, a separate power supply for 24V is ideal (and I do have one), however, I would like to design this to perform well at 24V.

author: magic
author_rank: Super Contributor
timestamp: April 09, 2020, 06:54:51 am
content: One Bob Cordell built quite a few FET power amps and wrote a bunch about it over the years on DIYA and in his articles and books. He recommended tight layout, base stoppers and local Miller compensation of the outputs IIRC.Also on short circuits and overload protection, there is a big MOSFET vs BJT thread on DIYA where people tested some jellybean power FETs for pulsed SOA and found them to withstand quite a lot.The question of SOA of switching power MOSFETs recurs on this forum every now and then but the discussion usually revolves around quoting app notes because it seems few people here are crazy enough to actually have tried to push the limits of jellybean parts

author: H713
author_rank: Regular Contributor
timestamp: April 09, 2020, 07:30:24 am
content: I do not like to trust datasheets for this kind of thing- the intense competition in the power MOSFET market means that the numbers can be a somewhat idealistic.Testing is easy enough to do. A big transformer, bridge rectifier and some decently sized caps, plus a power supply with a 10 turn pot to control the gate. I tested the 2SK3675 and the FQA8N90C and found them to be quite robust in this application, pushing them past 150 watts of dissipation. They don't handle HV linear operation too well, but they have seemed fairly tough in this application.As an interesting aside, the little IRF640 actually held up pretty well- probably why BSS used them in the EPC780.The HV PS design thread I started a while back had much discussion about the issue of MOSFET DC SOA. Discussion remained quite civilized.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 09, 2020, 02:53:40 pm
content: Just a thought:because of the risks already mentioned with failing Mosfets, would a crowbar circuit be a good idea?Of course the crowbar's threshold could not be a fixed value, but would have to track the actual voltage output setting.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 09, 2020, 04:04:16 pm
content: Quote from: H713 on April 09, 2020, 05:46:37 amThe voltage/current/power headroom was not what I had in mind. In the mentioned application, the MosFets died during parameter changes due to differences in transconductance. So you'll have to match for this and have ample beef in the driver stage to keep them in line. The worst case is when one is sticking out during a change in drive which coincides with a change in the load circuit.Result:BANGzoingtinglecrashMoo: literally!, as I saw TO3 caps leaving the base violently, when we tried to rebuild the thing with new MosFets.Should not discourage you. The appliance I'm referring to was handling far higher currents and a load circuit which, depending of the tool condition and the cycle time, was varying apruptly.

author: David Hess
author_rank: Super Contributor
timestamp: April 10, 2020, 11:48:23 am
content: A pair of more common transformers could be used with a toggle switch to wire the secondaries for series and parallel operation. Unfortunately though for audio applications, higher current will be needed at higher voltage so this is of limited use. But the same limitation works well with fold-back current limiting; in an audio application, full current is not needed at low voltages so build that into the power supply to considerably relax the power dissipation requirements.Or use multiple small transformers with their secondaries in series to provide the taps. Two transformers with center tapped or dual secondaries yield 4 output levels.

author: H713
author_rank: Regular Contributor
timestamp: April 11, 2020, 03:29:45 am
content: I expect that the 3 levels that would be available with the Antek transformer (two primaries, two secondaries) would be sufficient. Maybe not the best, but sufficient. As was mentioned, it's pretty easy to add windings as well, but we'll see if that's necessary.Doing some back-of-the-envelope, worst-case-scenario napkin math, if the voltage rails were 90 volts (and never sagged under load), with 3 steps I the maximum power that would ever have to be dissipated would be 150 watts. That's a lot, but not unmanageable, and in practice it would never be that high because 1) the rails will be less than 90 volts, and 2) they will sag under load. Even if I only used 3 FETs per side, each FET would stay below 50 watts of dissipation. For the sake of having a safety factor, I'll probably use five or six.

author: graybeard
author_rank: Frequent Contributor
timestamp: April 15, 2020, 01:16:55 am
content: I see a couple of fundamental flaws in your schematic.The emitter of Q6 is directly driving of M1 through M4. There is no current path to keep that transistor turned on. You need to add some sort of current path to keep Q6 in it's active region. Perhaps a resistor from gate to source across the MOSFETs.You have the four MOSFETs in parallel. In SPICE where they match ideally this will work fine, but in real life the MOSFETs will not match. You will need to include a small source degeneration resistor series with the source of each MOSFET, and you will need to do your best match the devices on a curve tracer. You might have better luck replacing the MOS series pass devices with NPNs, each with a emitter degeneration resistor to help mismatches and prevent thermal runaway.There may be other flaws, but those two stuck out at me.

author: H713
author_rank: Regular Contributor
timestamp: April 15, 2020, 03:59:17 am
content: The simulation file is a rough topology. No doubt there will be numerous compensation capacitors that need to be added as well, and I suspect that I will need grid stoppers to stop this thing from going ballistic into capacitive loads. The need for a gate-source resistor is a good point- I had initially drawn the schematic using NPN transistors and neglected to add this when I decided to switch to FETs.I couldn't be bothered to add the source resistors to the LTSpice simulation since its main purpose was to verify the voltage and current controls. Source resistors will be employed in order to minimize issues with current sharing. I was initially planning on using rather generous source resistors of about 1 ohm, which would then be mounted to the heatsink.I also considered using BJTs for the series pass transistors, but the performance wasn't as good. They are still on the table, the MJL21194 being the most likely candidate. Suitable BJTs will be more costly than FETs, require significant current to drive the base (most BJTs of this power level have a relatively low hfe), and have a smaller SOA than the FETs that I have been looking at. As mentioned previously, in testing the FQA8N90C, I was unsuccessful in killing it, even with 6 amps at 45 volts (or something like that). Numerous times I dumped the entire 65 volt, 8800 uF capacitor bank into the FET under test, with a peak current of something like seven amps, and it survived. More testing still needs to be done on the FETs to ensure they can take what I want to throw at them, however, they performed better than the BJTs in the testing I have done thus far.I'll probably test a 2SC5200 and an MJ21194 just to see what they can realistically take, but as of right now the FETs look far more interesting. Many linear audio amplifiers using paralleled HEXFETs have been built, so I expect I should be able to work out the current sharing, possibly using fairly large source resistors mounted to the main heatsink.I'm quite busy at the moment, with numerous other projects that take priority, so testing for this power supply has progressed rather slowly.

author: H713
author_rank: Regular Contributor
timestamp: July 18, 2020, 08:09:38 am
content: Welp, I should have listened. Even with huge 1 ohm source resistors, the FQA8N90C-F109 FETs won't share current well at all, in some cases one FET will be handling 3A while another will take less than an amp. I'm hesitant to go above 1 ohm for the source resistors, since anything above that will have to be at least of the 10W variety, ideally 25W. This starts to get pretty big and inefficient.With some reasonable (if not particularly tight) matching I believe one could get a pair of these FETs to share current reasonably well, and it could make for anexcellentrugged and low-cost bench supply in the 2-3A range.I'm planning to use NJW21194G BJTs ($3.55 in 10 qty at DigiKey) and I'll just have to live with using BJTs in a darlington configuration. I'm going to try really, really hard to avoid having to use a triple. Triple EFs are a pain and they can be a b!tch to stabilize, so hopefully I can get away without the second driver transistor.I did some preliminary testing using a pair of 2SC3281s with an MJE15032 driver. My stock of 2SC3281s are pulls, and using a DMM on diode check to "kind of sort of" match the vbe, I picked the two worst. They still current share just fine with 0.22R emitter resistors. This isn't really surprising, since BJT power amps rarely bother to match the output devicestooclosely, and they usually us 0.22R emitter resistors.I still have to test my current limiting circuit, but the voltage regulation part works fine and seems to be reasonably easy to stabilize, mostly by borrowing techniques from power amp design including base stoppers, output inductor (which only helped a tiny bit) and an output zobel. This is in addition to the compensation cap, of course.I'm going to design this power supply such that the transistor SOA can take the full 80V across them at 6A. I don't want them to ever fail. The tap switching will only be to minimize power dissipation. My plan is to control tap switching, fan speed and crowbar range through a microcontroller. I am aware that this can raise issues if the microcontroller crashes, so extra care will be needed to make sure that it goes into a safe fault mode rather than blowing up. Since I'm "digitally challenged", I'll be using training wheels on the microcontroller- an Atmega328 programmed using the Arduino IDE.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2020, 09:19:50 am
content: Current sharing with fets is tricky. There is a way to use 1 extra bjt per fet for feedback to ensure current sharing. The bjts form a kind of differential amp from the current sharing resistors. This is used in some supplies. See here:https://www.eevblog.com/forum/projects/why-mosfet-is-used-as-a-power-transistor-on-the-aim-tti-ql355tp/msg3137964/#msg3137964The bjts may still be the easier choice.

author: magic
author_rank: Super Contributor
timestamp: July 18, 2020, 01:28:28 pm
content: I have heard of some crazy people using FETs to drive power BJTs to increase "beta" and "fT" of the driver...No clue how it turns out in practice. A readily apparent problem is the need for higher supplies.

author: H713
author_rank: Regular Contributor
timestamp: July 18, 2020, 07:53:28 pm
content: Quote from: magic on July 18, 2020, 01:28:28 pmKyocera did this in one of their receivers back in the day... sounded good, but it had an annoying tendency to blow up. I considered it, but I my experience with doing this in power amps has been that stability is even more tricky than with the EF triple.At this point, I think the BJTs will be an easier option. I suspect that I can get away without using a triple, if am willing to pass more current through Q8. With the MJE15032 I am using here, I anticipate this will be just fine.

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2020, 10:47:15 pm
content: Quote from: magic on July 18, 2020, 01:28:28 pmBipolar transistor beta and fT drop at high currents so that sort of makes sense although I cannot say that I have seen that configuration. A composite transistor with power MOSFET driven by a bipolar transistor is common though to boost the transconductance of the MOSFET.

author: Pawelr98
author_rank: Regular Contributor
timestamp: July 18, 2020, 11:39:50 pm
content: Quote from: magic on July 18, 2020, 01:28:28 pmNot really a problem.The pump charge can produce the required extra voltage without any extra winding, using existing transformer.No special elements required, just plain 50Hz charge pump.Zener may be added for limiting the max voltage though.With this extra voltage the Vdrop is at maximum the Vcesat of BJT which is ~0.6-1V or almost nothing for mosfet transistors.During normal operation the unregulated voltage can be barely higher than the output voltage, providing good efficiency.Tap switching is one thing to consider.With simple comparators and maybe some logic gates it shouldn't be a problem.SCR preregulator can be used but has to be filtered well.There's also main capacitor switching.Lower capacitance means lower mean voltage which in turn means lower average power dissapation.Ripple will go up and this has to be accounted for when testing and when setting rules for the switching.Series resistors somewhere within the design is not efficient but does a decent job at protecting the pass element.I like to use those when testing "rough" loads which can be unpredicible.But this I only used for HV 150-400V DC supply. 0-300ohm 25W wirewound potentiometer that goes in series with the transformer secondary.If the load is too great then the main capacitor voltage will sag, along with OCP limiting the current.By adjusting the resistance I can allow higher voltage at certain currents.If one searches through some military surplus parts then finding 1ohm adjustable high power resistors is possible.But the same can be implemented by a combination of relays/transistors and normal high power resistors.

author: H713
author_rank: Regular Contributor
timestamp: July 19, 2020, 05:12:19 am
content: Here's the test setup so far. Seems to work okay, there are a few changes that I need to make in order to optimize stability and ripple rejection. I should be able to get it quite a bit better than it is now, and I suspect I can lower the value of C4 considerably, which is always nice since the current limiting is a bit more effective.Stabilizing the current limiting circuit was not easy. I had initially thought that I could get it to behave with a small cap from the output of the op-amp to the inverting input, but all this did was lower the frequency of the saw-tooth oscillation. Ultimately the only thing that did anything useful was the series RC circuit consisting of C1 and R18. This of course made the voltage regulator part of the circuit a little squirrelly. A 470 pF from the output to the inverting input of the voltage regulator op-amp seemed to make things worse, so this may come down to careful tweaking of values. I've never had much success with simulating or calculating compensation capacitors, as it is difficult to account for parasitic properties of components. I'm sure some of the real analog geniuses (who are a whole lot smarter than me) are able to calculate / simulate values and have them work perfectly.A need for higher supplies would not have been an issue. An easier solution to the charge pump is to simply spec a slightly higher voltage power transformer. One of the goals here is to try and minimize how much noise and ripple the regulator circuit has to reject. I don't care how efficient this thing is as long as it can still happen in a 2U enclosure (probably from a Crest CA9). Switching converters are efficient, but the noise they introduce can be difficult to shut up. I thought really hard about using a buck converter as a preregualtor, but I decided that I'd rather deal with slightly reduced efficiency by using tap switching than have to worry about that noise making its way to the output.

author: magic
author_rank: Super Contributor
timestamp: July 19, 2020, 07:22:39 am
content: Quote from: H713 on July 18, 2020, 07:53:28 pmQuote from: David Hess on July 18, 2020, 10:47:15 pmAs usual, lots of creative analog technology is found on DIYAudio.For example, John Curl from Parasound expressed preference for that kind of topology in the first few pages ofthis thread; it looks like the motivation in his case is just a general preference for FETs combined with the difficulty of protecting FET outputs from shorts. So that's another commercial product which uses such output stage and they make claims of good reliability. What kind of sorcery goes into making it work, we don't know.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2020, 09:16:45 am
content: For the high power level one can hardly get around tap switching or some preregulator (e.g. SCR or switched mode).Tap switching can be electronic with the pass-elements in series. So the transistors would only see a reduced voltage. This can be easier on the soa.It depends on the topology if the extra drive voltage is problem. For the usual floating regulator it is not.

author: H713
author_rank: Regular Contributor
timestamp: July 19, 2020, 09:34:23 am
content: Agreed, DiyAudio is the playground for some very talented analog designers. For an audio amplifier, I wouldn't hesitate to play with such a topology. The big issue is probably less about oscillation and more an issue of thermal stability, but it's probably no worse than the EF triple in that regard.For any sort of lab power supply below 37 volts, it's really hard to justify doing this kind of thing these days when you could use a uA723, LM317 or other IC to control it. Sure there are some challenges to work around (like the high reference voltage in the 723), but these are easier problems to work around than the challenges that come from doing it with op-amps and transistors.I still don't love my current limiting circuit, I'm sure there's a more elegant way to do it. The traditional way (which is how the 723 does it) is great, except for that the vbe of the current limiting transistor changes with temperature. As a result, I find that it's both touchy and drifts like crazy. If anyone knows of a more elegant way to do what I'm trying to achieve here (and maybe one that's easier to stabilize), I'd love to hear about it.

author: magic
author_rank: Super Contributor
timestamp: July 19, 2020, 09:58:25 am
content: TLV431 is sort of transistor with more thermally stable (but also higher) Vbe.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2020, 10:03:50 am
content: The usual topology for a lab supply (especially with more than some 25 V) is the floating regulator. Here current regulation is naturally with an OP.The main positive aspect of the simple Vbe type current limit is the simplicity and speed - so it could still be used as an additional fast emergency limit.The lm317 is not really helping for lab supply and the lm723 is not much more than a possibly (some versions) good reference and a poor OP with access to the compensation. Using OPs and a separate reference is not more complicated. One has to think about the compensation anyway.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2020, 06:42:12 pm
content: Quote from: H713 on July 19, 2020, 05:12:19 amA base-emitter shunt resistor should be added across the 2N3055 output transistors. Otherwise there is no way to remove charge turning the transistor off which implicates the closed loop stability. Darlington transistors have this shunt resistor built in for exactly this reason. If emitter ballast resistors are used, which should be done also for current sharing, then the shunt resistor can encompass the emitter ballast resistors also.The alternative is to drive the base of the output transistor with a class-ab driver instead of class-a like you have.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: July 19, 2020, 10:05:17 pm
content: @H731: would you possibly post the LTSpice-file for the schematic above?

author: H713
author_rank: Regular Contributor
timestamp: July 19, 2020, 11:42:22 pm
content: Sorry, I meant to post this earlier.

author: b_force
author_rank: Super Contributor
timestamp: July 20, 2020, 12:39:29 am
content: For just audio purposes I would make a much simpler design.Which is a VARIAC followed by a isolation transformer with symmetrical outputs.Just make a simple bridge rectifier followed by a MOSFET regulator (capacitance multiplier) where the voltage is set by resistors.For repair purposes this is also a nice approach, because quite often you want to turn up the voltage very slowly.Having a VARIAC in general is nice to have anyway, so you kill two birds with one stone (poor birds)

author: Pawelr98
author_rank: Regular Contributor
timestamp: July 20, 2020, 12:48:49 am
content: Quote from: Kleinstein on July 19, 2020, 10:03:50 amTraditional Vbe current limiter was, in my experiments when designing a fairly fast digital I-V curve tracer, a much more stable approach compared to differential opamp circuit.If one can get a hand on a NPN Germanium transistor then this approach is very efficient and simple.Not very thermally stable (as Ge parts have bigger thermal drift) but still an interesting addon.I have used a GT404 russian NPN Ge transistor for testing my design.Required just 50-55mV Vbe drop to activate the current limiter.Collector was connected to COMP pin on the 723 chip.Compared to usual 650mV drop this is pretty much nothing.Had to limit the current to around 2A. Voltage wise (leakage currents, gain) it was boucing around in the +- 50mA region which is OK.When enclosed in a small space with hot elements it can and most likely will have a very noticeable thermal drift but I have not tested it in my design as it was not meant for long term operation, just few seconds at most.Quote from: H713 on July 19, 2020, 05:12:19 amAlso a thing to note.With bipolar transistors it makes more sense to sense the current at the emitter.Base currents with a few big power transistors is not really negligible. It could be as much as 100mA (beta of 50).Charge pump is a simple design.Two diodes, two capacitors.There is no switching intererence/ripple as it operates strictly on 50/60Hz voltage on the main rectifier bridge.Components cost pennies and make the control circuitry supply "independent" of the main supply.That is you can drop 1V at the pass transistors instead of say 3V because of control circuitry requiring higher voltage just to go through pass transistors Vbe drops.Of course one may simply use an extra winding (with toroid transformers just add turns of any wire around the core) or something like that to provide that extra voltage for the control circuitry.To give an example of why such design improves efficiency.Main supply provides 72V under load.If you use the main supply for both control circuitry and pass elements then the maximum output voltage will be 69V as there's 1V missing to overcome the pass element base-emitter junction voltage drop.To get around this, you bump the supply voltage to 73V. At 5A it means extra 5W which has to be dissapated.With a charge pump (or extra winding, doesn't matter) the main supply voltage is still 72V but the control circuitry has 80V available.Now the maximum output voltage can be 71V. At 70V you only drop 2V*5A=10W instead of 15W like in previous example.And when choosing a off-the-shelf transformer it's often not possible to say "oh, I would like an 0.5Vac extra on the main winding to get over the regulator Vdrop". So the 5W difference can evolve into 10W or more.Also if done right then the control circuitry will get a ripple-free voltage for controlling the pass elements.Another diode+capacitor connected to the main filter capacitor means almost no ripple as the current draw of the control circuitry is pretty much nothing compared to the main pass elements.

author: H713
author_rank: Regular Contributor
timestamp: July 20, 2020, 02:33:44 am
content: @b_force, the problem is a lack of adjustable current limiting. When doing initial tests on a circuit that I don't necessarily trust, either because it is new or intermittently failing. Once I am confident that it is behaving well, I may turn that maximum current up to something like 3A- just so I don't vaporize PCB traces if something stupid happens.Again, the charge pump is an interesting idea but I don't think it's necessary. The heatsinks being used here are out of a Crest CA9, and they will be used with forced air cooling. I'm aware that I can improve efficiency by running the control circuitry off a higher voltage, but in this case it is not necessarily a priority.I may play some more with the Vbe current limiter. The speed of this type of current limiting circuit meant that it worked very well in my HV power supply, which has suffered countless output shorts without issue. I may also be able to get around the thermal drift issues through physical design to make sure that the transistor used stays at a relatively constant temperature. If I can come up with a way to allow variation between about 100 mA and the full current (6A), without a stupidly twitchy control, then that will work. I may be able to get around the issue with a switch for high and low ranges.Also, what is not shown in the schematic yet is a diode-capacitor network to feed the control circuitry, which drastically improves the ripple rejection.

author: H713
author_rank: Regular Contributor
timestamp: July 20, 2020, 04:03:53 am
content: Here's a version with the Vbe current limit circuit. R1 and R12 represent a 5K pot. Yes, there would be limiting resistors so that the operator can't disable current limiting. Diode D2 serves two purposes. Firstly, it protects the series pass transistors from an external high voltage. You laugh, but I've blown up multiple power supplies this way. It is also there so that I can get the full adjustment range without the need for a 6 ohm 200W resistor. If desired, I could add a "low current mode" by switching an additional diode in series with D2. That said, my current bench supply has a minimum current limit of 250 mA and Irarelywish it had a lower current limit range.This won't be as accurate as the version with the op-amp, but it's quite rare that accurate current limiting is needed. Fast current limiting, on the other hand, is pretty handy.

author: H713
author_rank: Regular Contributor
timestamp: July 20, 2020, 08:05:51 am
content: Interestingly, the Vbe current limiter suffered from similar instability issues to the op-amp based one. This (combined with the fact that a series RC network to ground from the base of Q6 stabilized it) leads me to believe that it is a stability issue with the series pass transistors. I seem to have shut them up with a 470 pF cap from the base to collector of Q6.In testing, using a BD139 for Q7 showed relatively minimal thermal drift and generally performed well. Thermal drift over 2 minutes was good enough, less than 15 mA. The main issue that showed up is very poor ripple rejection while current limiting, even at lower frequencies (120 Hz). This may not be an issue, however, as I intend to use a capacitance multiplier in the filtering circuit anyways to achieve a relatively quiet DC bus without the need for 30,000 uF of filter capacitance, which takes up valuable chassis space (remember, this is a dual power supply in a 2u enclosure). Huge filter caps also have a big inrush current, which would require efforts to mitigate in order to avoid welding relay contacts in the tap switching circuit.I intend to do a bit more testing on the op-amp based current limiting circuit tomorrow, then make a final decision. Beyond that, things are looking good and I am planning to start my PCB layout in the next few days.

author: magic
author_rank: Super Contributor
timestamp: July 20, 2020, 08:17:15 am
content: You will probably get better PSRR by replacing the Darlington's base pullup resistor with an active source.Also with higher loop gain, did you add that resistor as David told you?

author: H713
author_rank: Regular Contributor
timestamp: July 20, 2020, 08:45:29 am
content: Yes, I added that quite a while ago, just forgot to include it in the Spice schematic.

author: b_force
author_rank: Super Contributor
timestamp: July 20, 2020, 03:25:26 pm
content: Quote from: H713 on July 20, 2020, 02:33:44 amJust add a current limiter?That's not so hard, quite a straight forward circuit actually.

author: Pawelr98
author_rank: Regular Contributor
timestamp: July 20, 2020, 10:20:04 pm
content: Quote from: H713 on July 20, 2020, 02:33:44 amSuch network (control circuitry capacitor) as shown on the drawn schematic ?Fairly simple way to cut off the ripple from the main tank capacitor.Using shottky diode may be better as long as there's a 30-40V zener diode in parallel to protect it from accidental reverse breakdown during shutdown or when taps get switched (main capacitor bank going much lower than the control circuitry capacitor). Most common diodes are rated for 40-45V reverse voltage.However there still will be some 60Hz ripple as the diodes in the main bridge are not equal as they have slightly different voltage drops.It's not going to be much but any RC filter for the control circuitry supply should be tuned for <60Hz rejection.Another small reason to go with boosted voltage as there's headroom for filters on the control circuitry side.If active current source is to be used the boosted voltage will be even more handy.As for current limiting using NPN transistor.Put the BE juction directly across the current shunt.Remember about BE junction beeing low voltage zener which can conduct in reverse.When applied across the diode it won't be protected in any way.In this case the current limit will be dependent on not only the temperature but also the current.Diode drop will change in roughly 0.5V to 1V range which is quite a lot.Extra source of error is the collector-base leakage current (disconecting the base pretty much made the transistor conduct all the time, shutting down the supply for good).For silicon this is fairly low but with germanium I had far much better results when adjusting the potential of the emitter instead of base.Far less dependent on the voltage put across the current limiting transistor.As for current limit adjustment.Either have the main shunt made out of few smaller shunts in series and switch the emitter (base will generally work too, but switch resistance should be low to minimize the influence of collector-base leakage current) between different points using CD4051 (digital control) or some mechanical switch.That or add schottky diodes in series with the emitter.Either one works.Resistive dividers over the main shunts are, even with silicone transistors, rather tricky.My bench power supply uses 100ohm potentiometer over the main output shunt and I can easily tell it's nonlinear.Classic 723 circuit with default internal current limiting transistor used.

author: duak
author_rank: Super Contributor
timestamp: July 20, 2020, 11:05:45 pm
content: The question was asked if there was a simple solution to the temperature dependancy of a VBE current limiter. Some older Lambda linear supplies use a custom Lambda designed chip with an on board heater to stabilize the zener reference (and the other bits too). I thought that an LM35 temperature sensor and heater transistor thermally connected to the current limit transistor could hold themselves at, say 75 C. If done properly, there should be at least a 10X improvmenet in the stability of the current limit. If the VBE sense transistor & heater were near the pass transistors and received some of the heat, and their case temeratures went above 75 C, it would naturally reduce the current limit.By the way, I built my first bench supply in 1974 and used a 723 regulator with a modified VBE limiter that lit an LED when in current limit. The stability wasn't half bad at low currents, since things didn't get very hot. At high temperatures, there was a natural reduction in the current limit that was a bit of protection.The next thought I had was to make the limiter less thermally sensitive by using another transistor to balance the VBE variation. The attached circuit shows the basic VBE limiter using a differential pair. Note, I run linux and don't have any spiffy ECAD packages so I used Oregano which is somewhat limited. Hopefully, the schematic is clear enough. I could provide the .oregano file and perhaps a SPICE file but I can't promise they'd be useable.Q8 is the basic VBE current limiter sense transisitor. Its base is driven by the current sense resistance voltage. Due to superposition the voltages on the ballast resistors to combine to give the total load current. It works for any number of sense resistors as long as they and the combining resistors are the same values. The base of Q3 is driven by a variable voltage corresponding to the current limit setting. The negative end of the current limit reference is connected to the bottom end of the current sense resistance which also happens to be the output voltage Vout+. When the voltage on the base of Q8 is more positive than that of Q3, ie. reached the current limit threshold, Q8 will conduct more and draw current from the base of the pass section driver Q5 and reduce the output voltage.Q4 sources current to pull up the input of the pass section and R4 must be adjusted to provide enough drive current to reach full output voltage at full load current.Q2 limits the voltage applied to Q3 to reduce its power dissipation so that its temperature (and thus VBE) and hFE are more closely matched to Q8. D6 is optional and increases Q3's VCE by one diode drop. Q3 and Q8 work with VCEs on the order of 1 V and hFE is somewhat important. I haven't simulated or tested this part of the circuit but I think it will work.The rest of the circuit deals with the currents to operating the diff pair and current limit reference. The diff amp emitter currents are taken to V-- with a current sink. This sink must be able to draw more than the current source pulling up the drive can provide. The current limit reference sink must likewise draw more current than the corresponding source can provide to prevent the output voltage from drifing upward when unloaded.It looks more complicated than it actually is as much of it is needed for any supply. The current source and sink for the current limit reference could be eliminated by using a floating supply like a DC-DC converter or just another transformer, rectifier and caps. Op-amps could also be used but this circuit offers fast response as it really doesn't clamp and wind up like an op-amp. BTW, D1 is probably not needed but in the worst case, it allows simple VBE limiting albeit at a higher current level.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2020, 11:43:40 pm
content: Quote from: duak on July 20, 2020, 11:05:45 pmUse a differential pair as shown in your example. In the past, Tektronix commonly did it that way in their linear regulators and implemented fold-back current limiting as well. Another advantage is that the sense voltage can be lower than Vbe.In cost sensitive designs, the differential pair might be a diode and transistor instead of two transistors but there is hardly a reason to do that today. There might be a suitable fast operational amplifier but the only advantage would be precision.

author: H713
author_rank: Regular Contributor
timestamp: July 21, 2020, 01:03:50 am
content: Quote from: David Hess on July 20, 2020, 11:43:40 pmThis is essentially (but not exactly) what I was trying to do with the op-amp based current limiter.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2020, 02:46:18 am
content: Quote from: H713 on July 21, 2020, 01:03:50 amThe difference is that the two transistor differential amplifier is so much faster but lacks the precision of an operational amplifier although performance is relative. Those old Tektronix regulators with two differential amplifiers, one for voltage control and one for current limiting, were plenty precise under load and line variation.In a power supply, I might implement fast current limiting with Vbe or differential pair as part of the pass element protection and a slower but very precise adjustable current limit using an operational amplifier. I like using integrated regulators as power pass elements which results in exactly that except the fast current limiting is built in although with thermal and SOA protection.Check out the schematics on old Tektronix oscilloscopes for examples.