Project: High power constant current programmable load - Page 1
Forum section: electronics / projects

author: Allen
author_rank: Contributor
timestamp: May 25, 2012, 07:07:16 am
content: For my senior capstone design project for electrical engineering college, I will be designing a building a high power, constant current, programmable load. Since so many people are posting or requesting dummy load DIY projects, I decided to start a thread documenting my project. This project will go on for several months and is planned to be finished by mid December. I also hope to pick up a few pointers from more experienced designers to help keep me from falling into "traps for young players".Here are the ultimate specifications that I hope to achieve with this project:Max power: 4.2 kWMax current: 1,000 amps (must be able to draw full current at 2 volts)Max voltage: Undetermined. Must be able to handle at least 4.2 volts, but will most likely be able to handle much more.Max error: 1%Required features:Microcontroller controlledLCD display (multiple if needed)Cut-off voltage (load cuts off after source voltage drops below a certain threshold)Powered from 120V outletOptional/desired features:Display drawn charge (Amp-hours)Display consumed power (Watts)External thermocouple that can be used to monitor temperature of battery being tested and cut-off load if max temp is exceeded.Ability to log voltage, current, drawn charge, and consumed power as well as some way to make this data available to a computer for analysis. Maybe log to a USB drive.I will update this thread as my project progresses.

author: EEVblog
author_rank: Unknown
timestamp: May 25, 2012, 07:15:07 am
content: Quote from: Allen on May 25, 2012, 07:07:16 amBut actual analog loop control?Dave.

author: Allen
author_rank: Contributor
timestamp: May 25, 2012, 07:38:19 am
content: Yes, analog loop control. I don't think the uC would be good for directly controlling something so non-linear.

author: Allen
author_rank: Contributor
timestamp: May 25, 2012, 07:48:24 am
content: For the main load section, I plan to use power MOSFETs with current sense resistors.For the current sense resistors, I think the following would be a good choice: 100W, 10 mOhm, 1%, four terminal resistor (TGHGCR0100FE)http://www.ohmite.com/cat/res_tghg.pdfI plan to use 12 of these resistors (one for each MOSFET) which will give up to 83.3 amps and 69.4 watts power dissipation each at full load.For the power MOSFETs, I think the following would be a good choice. ST Microelectronics, N-channel 100 V, 0.0045 ?, 220 A, ISOTOPSTripFET Power MOSFET (STE250NS10)http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00002654.pdfI plan to use 12 of these in parallel giving 83.3 amps at full load. I like this MOSFET because it has a low junction to case thermal resistance of 0.25 K/W as well as a large base area of a little over 1 square inch.I determined 12 of each would be needed for heat dissipation reasons. I will post my thermal calculations soon.

author: sonicj
author_rank: Frequent Contributor
timestamp: May 25, 2012, 08:33:11 am
content: interesting!why the individual current sense resistors vs a single high current shunt? the 12 resistors would run around $300usd while a single 1000A shunt is around $50usd.4.2V really limits the usefulness of the product outside of lithium testing. 6V, 8V & 12V lead acid cells will be around for a long, long time...my 2¢ fwiw-sj

author: amspire
author_rank: Super Contributor
timestamp: May 25, 2012, 08:59:59 am
content: I would probably look at current sense resistors at around 0.0005 ohms to 0.001 ohms instead or 0.01 ohm.The reason is that if the resistor can heat up 60degC at full power, you can only expect about 0.3% accuracy and that would be a pity if you look at the cost. Buying very low offset opamps costs very little by comparison, and you can end up with a much cheaper current sense resistor and a much higher accuracy.The transistor looks good, but devices with that kind of packaging are expensive compared to several devices in a TO247 package. It may be much easier to achieve your results with 42 100W modules, then your current 12 modules which will dissipate 280W on the mosfet and 70W on the current shunt. 280W means you have to keep those mosfets below 80 DegC and if the ambient can be up to 55 degC (as life gets hot near a 4200W heater), your heatsink has to be able to get rid of that 280 W per mosfet with an effective thermal resistance of less then 280/25 = 11 W/degC.That is very ambitious.At these sort of numbers, you can make your life much easier by just going for big numbers of much cheaper and more conservative lower power modules. A channel heatsink with a single fan and four mosfets can probably dissipate 200-300W. In a 19 inch rack width case, you make be able to fit 4 of these to get up to 1050W. Four of these cases can add up to the 4200W.That would be a total of 64 65W mosfet modules. If each used a 0.001 ohm shunt resistor, a SMD 2W 100ppm shunt resistor costs about 50c, a IRPF250 is under $2, the opamp about $1 so you are looking at under $10 including PCB per module. The SMD shunt resistor would hardly warm up, so you will get about 10 times the precision of your current design.Richard.

author: Allen
author_rank: Contributor
timestamp: May 25, 2012, 05:56:11 pm
content: Quote from: sonicj on May 25, 2012, 08:33:11 amThe reason I want to use a separate current shunt on each MOSFET is so that each one can be regulated individually. I don't know how how consistent the characteristics of each MOSFET is. I don't want a situation where one MOSFET is pulling 50 A and the one next to it is pulling 200A.As far as the voltages, I plan for it to be able to handle higher voltages, but not while pulling 1,000A. I will probably have something in the software that will limit current based on voltage.

author: Allen
author_rank: Contributor
timestamp: May 25, 2012, 07:08:54 pm
content: Quote from: amspire on May 25, 2012, 08:59:59 amI considered a larger quantity of TO247 type transistors, but then I would have to mount them to a PCB. Would PCB conductors be able to handle that much current?Or would I need to run wires from each module and consolidate them at the primary terminals?

author: Scarionn
author_rank: Contributor
timestamp: May 25, 2012, 10:21:36 pm
content: Just remember of the TC (temperature coefficient) of all your parts. Because 1% at 1000A can be very hard to get.

author: nctnico
author_rank: Super Contributor
timestamp: May 25, 2012, 10:28:57 pm
content: I'd go for regular transistors. These work like current sinks already so controlling them is much easier.

author: Scarionn
author_rank: Contributor
timestamp: May 25, 2012, 11:02:23 pm
content: There is other things to think about instead of the transistors , like the 1000A connectors,PCB, the cables, and how compensate their resistance.Because you will need a cable with very low resistance less then 0,00005 to get 1% of precision.

author: amspire
author_rank: Super Contributor
timestamp: May 25, 2012, 11:26:14 pm
content: Quote from: Allen on May 25, 2012, 07:08:54 pmNo problem. If you had 64 mosfets, then each one is handling 18A. a 1cm wide track on a 2oz copper PCB can handle that with about a 10 degC temp rise.Quote from: Scarionn on May 25, 2012, 11:02:23 pmNo - the cable has zero effect on the precision. The cabling resistance just has to be low enough to allow the sink to meet the 2V 1000A spec. Aiming at 0.0005 ohms would be a good target. If that was achieved with 4 rack width cases in parallel, each one would have to be less then 2 milliohms wiring resistance.Increasing the PCb track width to about 32mm on a 2oz copper PCB would reduce the temp rise of the tracks to about 1 deg, and would probably make the total PCB resistance about 1 milliohms.The IRFP250 I suggested may prove slightly marginal for on resistance - a 60A mosfet would make it easier to reach the targets.Richard.

author: Allen
author_rank: Contributor
timestamp: May 25, 2012, 11:47:27 pm
content: Quote from: amspire on May 25, 2012, 11:26:14 pmWhat about where all the modules are connected together in parallel? Would you do that with off board wiring, or could that somehow be done on the PCB?

author: EEVblog
author_rank: Unknown
timestamp: May 26, 2012, 12:18:43 am
content: Quote from: Allen on May 25, 2012, 11:47:27 pmOn first thought I'd do it all with off-board wiring.Build your XX number of individual constant current loads, each entirely separate, each with it's own heatsink and maybe fan and PCB etc.and then simply wire them across your input terminals, maybe via some form of big arse bus bar arrangement.So your bus bar is the only thing that has to carry all the current, and your input connectors screw directly into the bus bar.Your individual modules then draw 1/XX amount of that current, which is much easier to handle with wiring and PCB etc.Your digital control circuitry then knows what total current you need and simply divides that by the number of channels and then sends that command to each module.Probably not the cheapest way, but the most reliable and foolproof in design I think. It would tolerate failure of several modules. And for yur report you could go into math about likelihood of failure and redundancy maybe?Dave.

author: Scarionn
author_rank: Contributor
timestamp: May 26, 2012, 01:14:39 am
content: Quote from: amspire on May 25, 2012, 11:26:14 pm

author: IanB
author_rank: Super Contributor
timestamp: May 26, 2012, 01:15:48 am
content: I don't really get how people come up with these specs like 1000 A, 4.2 kW, 2 V?Is it serious, or do people get their brain removed and replaced with a wet sponge when they go to college?For instance at 1000 A in the system every milliohm of resistance is going to drop 1 V. Greater than 2 milliohms and your entire 2 V voltage drop is gone. (And 1000 A through 1 milliohm will dissipate 1 kW. Bye bye PCB tracks. They will vaporize.)To have any practical device working will presumably need thick copper bus bars as fat as fingers, welded connections, giant cooling fans and ridiculously expensive parts.Just to humour me, what is the industrial application of this device?

author: amspire
author_rank: Super Contributor
timestamp: May 26, 2012, 01:43:57 am
content: Quote from: IanB on May 26, 2012, 01:15:48 amIt can probably be built for under $2000 and I do not call that ridiculously expensive.The need? It is not that hard to imagine needs. Testing large stationary lead-acid cells. Testing 4KW DC welding supplies. Testing supplies that are going to power superconductor magnets. Who knows?If Allen wants to build a 1000A load, that is fine with me.Once you decide to design a monster like this, you may as well push the specs as far as you can. If Allen needs 1000A at 4.2V, it makes perfect sense to spec the design at 1000A at 2V, and if possible, 1000A at 1V would be even better.Richard.

author: Scarionn
author_rank: Contributor
timestamp: May 26, 2012, 01:57:07 am
content: Quote from: IanB on May 26, 2012, 01:15:48 amYou are right. At last post I said he will need a 0.0005 ohms wire to maintain it on spec, but he will need a 0.00005 at least to maintain it on spec. 50W of dissipation.

author: amspire
author_rank: Super Contributor
timestamp: May 26, 2012, 02:18:29 am
content: Quote from: Scarionn on May 26, 2012, 01:57:07 amI see what you mean. At 1000A current, a wattmeter measuring the internal terminal voltage is accurate for the load's dissipation but inaccurate for measuring the DUT's dissipation. Good point.To check the real total load dissipation, an external precision current shunt or DC clamp plus a voltmeter across the leads of the DUT are needed.Richard

author: Scarionn
author_rank: Contributor
timestamp: May 26, 2012, 02:26:26 am
content: DUT?

author: amspire
author_rank: Super Contributor
timestamp: May 26, 2012, 02:27:18 am
content: DUT = Device Under Test

author: EEVblog
author_rank: Unknown
timestamp: May 26, 2012, 03:08:25 am
content: Quote from: IanB on May 26, 2012, 01:15:48 amTrue. But 1mohm resistance is pretty high when you are talking huge conductors and massive bolt interconnects etc.QuoteI don't know about "ridiculously" expensive, but certainly a bus bar approach with the terminals directly bolt (or weld) connected as I mentioned would be the key.Once you have that bus bar and connection that can carry your 1000A without much drop, then it's almost trivially easy to connect on as many smaller loads as required in parallel, and each one can be measured accurately, so you will know the total load current.If you wanted to take into account the drop on the bus bar and/or connectors then you'd need an additional diff amp to measure the drop, wired directly onto the bar at appropriate points. That's likely necessary with the original requirement of 1% accuracy.Here is an online calculator:http://www.bdbatteries.com/buss-bars.phpFor example, a 12" long copper buss bar 1" wide and 0.1" thick is 0.00008126 (0.081mohms) and drops only 81mV at 1000A.You'd probably have to worry more about your connecting leads and connections to do with the DUT more so than the design of the load itself.Dave.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 26, 2012, 08:25:54 am
content: Quote from: EEVblog on May 26, 2012, 03:08:25 amMy biggest worry would be the heat. 4 kW is enough to operate a 1000 C furnace, or heat your house.My second worry would be the electro magnetic induction. Turn 1000A on, and ... Or maybe even just the ripple of the regulation. I would run the numbers here first.And I have to agree, this is not something I would recommend as a final year project. To many things to juggle in the air for a beginner. One of those things "if you have to ask how to do it you are not qualified to do it".

author: Allen
author_rank: Contributor
timestamp: May 26, 2012, 07:07:57 pm
content: I have another question for you guys. The datasheet for one of the MOSFETs I am considering using says that the maximum operating junction temperature is 150 degrees C. Do you guys think that is a safe number, or do you think I should shoot for a lower limit?

author: T4P
author_rank: Super Contributor
timestamp: May 26, 2012, 07:53:33 pm
content: Quote from: Allen on May 26, 2012, 07:07:57 pmOf course not. that would shorten the life of everything.maybe 80C would be better but it's still scalding

author: Allen
author_rank: Contributor
timestamp: May 26, 2012, 10:11:52 pm
content: I am looking at a datasheet of another MOSFET that gives a thermal resistance from case to heat sink (RthCS) of .15 K/W. Does this number assume a dry contact? Can't I significantly reduce this thermal resistance by using thermal grease?Data sheet:http://ixapps.ixys.com/DataSheet/DS100133A%28IXFK-FX230N20T%29.pdf

author: IanB
author_rank: Super Contributor
timestamp: May 26, 2012, 10:21:13 pm
content: Quote from: Allen on May 26, 2012, 10:11:52 pmOn their datasheet they are going to quote the best achievable value under optimistic conditions. If you could reduce the thermal resistance by different mounting arrangements they would quote that lower number on the datasheet. They are trying to sell product so they will always cast it in the best possible light.

author: amspire
author_rank: Super Contributor
timestamp: May 26, 2012, 11:36:25 pm
content: Quote from: Allen on May 26, 2012, 10:11:52 pmThe spec will always be for an optimal thermal connection so to get close, you will need a very thin smear of heatink compound - just enough to fill the surface irregularities but not enough to add a definite layer between the mosfet and heatsink. Heatsink compound has poor thermal conductivity, so its performance depends on the fact that you are using as little as possible. If you are going for a lot of power, you will have to do your own tests to work out what power you can dissipate.There are other factors like how well the heatsink surface is machined and how thick the heatsink is at the point of mounting.Looks like the devices are good for about 50A (perhaps more if you can keep voltage drops down elsewhere in the circuit) which at 4.2V is 210W, so if you can get a good case to heatsink contact, you could possibly run the heatsink temperature at the mosfet at 100 degC. There will be big thermal drop across the heatsink so this may equate to an average heatsink temperature of 80 - 90 degC.The other thing about using these devices is that they have a pretty massive gate capacitance (over 30nF) which is offset by the fact you do not have to change the gate voltage by much to change the current. You may have to beef up the driver from the opamp to gate to get the speed you need - again do some tests.At under $7 each, the price is very good. Impressive.Richard.

author: codeboy2k
author_rank: Super Contributor
timestamp: May 27, 2012, 11:01:54 pm
content: Quote from: Bored@Work on May 26, 2012, 08:25:54 amI think it's a great final year project. It makes the student think really hard about the real problems, and the pitfalls, and all the tiny little details that will make or break this project. Yes, it's a lot of current, and 4KW is a lot of power, but it's nothing special. It just needs lots and lots of good engineering, and he's seeking help to understand the problem better and how to overcome these. I think the report should detail the design of a 4KW load, but actually build a smaller 1KWmoduleand follow Dave's idea of having it bolt onto a bus bar. Additional modules can be added later to achieve 4KW. A 1KW oil cooled module with oil pump and radiator fan is not that hard. Youcouldair cool it, but I would not do it that way myself, too much volume of air required and the heatsink is going to be huge.As you say, he should run the numbers. Working at these power levels makes a student appreciate engineering more, and will make a better graduate.I'm tired of the cheesy final year projects that involve a micro making a tune, or building an air band usings MEMS devices taped to your hands, or robots that follow a line. This project has some meat to it

author: EEVblog
author_rank: Unknown
timestamp: May 27, 2012, 11:28:06 pm
content: Quote from: codeboy2k on May 27, 2012, 11:01:54 pmYeah, I think it's a good meaty project.It's begging to be broken down into a huge bus bar and separate modules having off it. And no one said it has to all be in the same case. I'd have a separate case and heat sink for each module, with their own temp controlled fan, bolted on the end of the extruded fin heatsink like the BK 8500 load teardown.Designing your 1000A shunt isn't too hard.Build and perfect one module (say 50-100A) and see where the issues are and perfect it.With consideration for the control system that can control 10-20 modules or something of course, would be your next step.Then when it's ready, mass duplicate the modules, and test progressively until the system gives you issues.And that alone can give you excellent data to graph the issue with the increase in current/power.And even if you don't achieve the final 1000A or 4kW, then you've easily got enough good material to pass the final report.Dave.

author: Allen
author_rank: Contributor
timestamp: May 28, 2012, 12:48:30 am
content: Quote from: EEVblog on May 27, 2012, 11:28:06 pmThanks Dave and Codeboy for your support. This was my first pick of many projects because I though it would encompass most of the aspects of equipment design from beginning to end. I knew that the current and power levels would provide some challenges, but I have already learned a lot and expect to learn much more. And funny that Codeboy should mention it as one of the projects is the "robot following the line".I am going to go ahead and order a IXFK230N20T MOSFET (http://ixapps.ixys.com/DataSheet/DS100133A%28IXFK-FX230N20T%29.pdf) and a small 1% 10 mOhm shunt resistor and build a module and test it in the 0.5-3A range. I'll convince the sponsor buy the bigger, more expensive current sense resistor later after I can demonstrate some good results. Then I can test it in the higher current ranges.If the single module works well, I'll use 20 of them in parallel. I also like Dave's suggestion about the bus bar. I think I'll have two copper bus bars (one for each terminal) which I will use to connect all of the modules in parallel.

author: electroguy
author_rank: Regular Contributor
timestamp: May 28, 2012, 05:49:06 am
content: Quote from: IanB on May 26, 2012, 01:15:48 amHeaps of reasons. If i had to guess, one would be testing telco/solar/UPS batteries? These come in 2V and capacities range all the way up to 3000AH (or more)so if you have a bunch of these batteries (say 100 or 200 pcs, or more, as found in industrial UPS's), even the 4.2KW load will take a while to drain them. Imagine if you only had a low power load, you could be waiting for weeks for the batteries to drain!Draining is the easy part, I'd be more interested in seeing how long it takes him to charge them back up!

author: Bored@Work
author_rank: Super Contributor
timestamp: May 28, 2012, 08:09:24 am
content: Quote from: codeboy2k on May 27, 2012, 11:01:54 pmThe thing is, when you claim 4 kW, 1 kA, you have to deliver that in the end, at least where I studied. And it should not just run for a few seconds. No "look I made a module, and if you add X of them you might get 4 kW, 1kA". You deliver what you claim, or else ...Especially, the modules do not scale linear. Build one, with a heatsink and a fan, run it at ambient temperature and it might work. Bundle a few of them in close proximity, and they heat up each other.Oh, and to develop and finally deliver you have to set up a test rig and instrumentation, generating a few typical power scenarios - tadaa, your problems instantly doubled.

author: M. András
author_rank: Super Contributor
timestamp: May 28, 2012, 10:13:43 am
content: if you are looking for ixys fets, they have a whole range of linear fets built for programmable loads, not cheap but could work more reliable then standard switching fets,just picked the first from the listhttp://ixapps.ixys.com/DataSheet/DS100200%28IXTH-TT75N10L2%29.pdf,"POWER DEVICES > DISCRETE MOSFETs > N-Channel: Linear "Extended FBSOA" Power MOSFETs > L2™ Linear Power MOSFETs" i would gladly see the picture or documented phases of this project

author: EEVblog
author_rank: Unknown
timestamp: May 28, 2012, 01:27:05 pm
content: Quote from: Bored@Work on May 28, 2012, 08:09:24 amThat might be a tad unfair compared to say the line following robot project which can be done with couple of transistors and some LDR's.A project like this one is a big time and maybe big money investment, with lost of serious incremental build and test engineering work.To fail because you only got 900A instead of 1000A continuous would be a very poor reflection on the teacher/school IMO. Especially so if the student showed all the engineering skills that went into developing such a beast. With a project like this, one poor choice (not necessarily through lack of poor engineering) up front could easily pip you at the post come 1000A. I'd like to see the professor develop it and see how he/she gets on...I've always found that if you did the engineering, and the learning, then you get the pass. Failure can be more important than success in a project like this.You could write half your paper on why the system design crapped out at 900AQuoteThat's why I mentioned there was likely to be plenty of nice juicy data and analysis to be had during this incremental process.Perhaps even the most interesting part of the entire project.QuoteYep. Although perhaps if this is a set project as it seems it was, the school might have something in place already for this perhaps?I'd want to know that up front though.Dave.

author: EEVblog
author_rank: Unknown
timestamp: May 28, 2012, 01:40:33 pm
content: Quote from: Bored@Work on May 28, 2012, 08:09:24 amOooh, a picture just popped into my head...A load that looks like one of the classic Cray supercomputers, with 10 or 20 separate modules in circle fanned out from a central core that contains the bus barEach one sucking air from the central column and out each "spoke".Add a nice couch over the top for comfort!Dave.

author: T4P
author_rank: Super Contributor
timestamp: May 28, 2012, 01:58:57 pm
content: The most important thing that any professor must evaluate is documentation because it shows that the student knows what he's doing and what he's done and why he can't manage to hit the written target if so.Not some shitty light following robot that the student doesn't know what he's doing or rather doing it only for the easy pass ( sadly they do really do that and graduate with a degree ... but clearly they fail in the actualy world )

author: amspire
author_rank: Super Contributor
timestamp: May 28, 2012, 02:08:50 pm
content: Quote from: Bored@Work on May 28, 2012, 08:09:24 amThat problem is very easy to solve with a simple design.Just put the loads in rack mount cases with air coming in the front and exiting the rear. Racks housings allow you to direct the air from the rear into exhaust ducting. A nice big exhaust fan would be a good idea. Last thing you want is a 4KW heater dumping hot air inside a room, particularly if it is an air conditioned room.Then every rack unit will get room temperature air in the front, regardless of how many units you have.I have seen far more then 20KW dissipated in a single rack without any problem. Blade server racks can easily have over 10KW of power in a single full height rack without any of the processors in the blades suffering heating from other adjacent processors.A 3U high case is 5.25 inches high. This is high enough to contain 3 heatsink channels in a 3U case with a 120mm fan on the front of each. Each channel could probably take 100W each side. That would be 600W per 3U case. 7 cases would make up the 4200W with no thermal problems. That is a total of 7 x 3U = 21U. Probably can save money with a smaller heatsink channel and 80mm fan. That would probably allow 4 channels in one case - or three with comfort.Half height racks are typically 22 U. That leaves a 1U space for a central controller and measurement box - easily big enough. The heatsink lengths would not need to be long - perhaps 100 to 150mm, so the cases would not need to be any more then 300mm deep which is short for a rack. This means lots of room at the rear for a nice hefty exhaust fan at the rear. The rack can be on castors, and for experimental use, some flexible airconditioning conduit just goes from the top of the rack out a window or door.Richard.

author: T4P
author_rank: Super Contributor
timestamp: May 28, 2012, 04:29:23 pm
content: And just think about the prices for Racks and the 19" 3U rack units themselves ... they are phenomenal

author: amspire
author_rank: Super Contributor
timestamp: May 28, 2012, 05:44:53 pm
content: Quote from: DaveXRQ on May 28, 2012, 04:29:23 pmI have seen excellent 2nd hand racks with all the doors for under $600, and cases for $220.This is a big project. What would you suggest instead?

author: IanJ
author_rank: Unknown
timestamp: May 28, 2012, 07:06:48 pm
content: Hi,I think this is a great project.......it has some real engineering in it rather than simply following a datasheet example or two. Best of luck to you, and pls keep us posted of your progress.Ian.

author: Allen
author_rank: Contributor
timestamp: May 28, 2012, 09:44:53 pm
content: I took the BJT and feedback capacitor ideas from another thread (https://www.eevblog.com/forum/beginners/mosfet%27s-current-at-low-vds-%28related-to-cc-dummy-load%29/15/). Let me know what you guys think of the below design for a single module. Also, do you think with this design, I would be OK with having the negative op-amp rail going to ground, or would I be better off providing a negative rail? The op-amp is the OP07Chttp://www.ti.com/lit/ds/symlink/op07c.pdf.

author: amspire
author_rank: Super Contributor
timestamp: May 28, 2012, 09:55:16 pm
content: OP07s need a negative supply. -3v or more. There are lots of other opamps that don't. Also, it is easy to find low offset opamps that do not need any offset adjustment.

author: mAJORD
author_rank: Contributor
timestamp: May 29, 2012, 12:09:51 am
content: Aweosme.I'm working on a similar project myself.. only targetting 1.5-2KW, and with higher voltages.As mentioned, you'll discover pretty quickly the true capabilties of the FET's you're looking at once you dig into the datasheet properly, and add your overhead for a realistic Case-HS Tr. Forget the 1000W ratings with any sort of ambient cooling, no matter how good it is, you can't keep a case @ 25c! and you don't really want a 175c junction temperatue either.The L2 (Linear) models are definatly hard to come by in the higher ratings. Digi-key only have a 60pc price break on them.it wouldn be interesting to know how the other switching series handle linear opertion at high power. they dont actually quote any SOA in the datasheet unless I've missed it.These models are cheap enough though so I'm probably going to try and find out. It may turn out they're still more suitable than the cheaper, lower power L2 series. They're only $12 after all

author: Allen
author_rank: Contributor
timestamp: May 29, 2012, 12:41:37 am
content: I don't have that MOSFET yet, but I hooked up the previous circuit using a MOSFET that I have on hand and it oscillated horribly. The oscillations were so bad, the op-amp was showing an offset voltage of >5 mV. Then, I did away with the emitter follower and changed the output resistor to a 1k and the circuit now runs stable. The new circuit is shown below. I recorded the following results from the circuit I setup.Note that current sense resistor is 1%, MOSFET is PSMN027-100PS, current sense voltage measured with Fluke 87-V, sink current measured with Fluke 289.With the equipment I currently have available at home, I can only measure up to about 2 amps, but so far, the results look promising. Looks like the design is good down to about 400 mA per module.

author: IanJ
author_rank: Unknown
timestamp: May 29, 2012, 06:57:06 am
content: Quote from: AcHmed99 on May 28, 2012, 11:26:49 pmSimilarly, i've been using an LM8272 to drive an IXTK series N-type Mosfet directly with no buffer. The 8272 is designed with high capacitive loads in mind.Ian.

author: amspire
author_rank: Super Contributor
timestamp: May 29, 2012, 07:51:42 am
content: I am puzzled about your instability problems.I am wondering if the problems were not caused by the wiring of the test circuit, and not the circuit itself.I have just been playing around with the following in LTSpice. The opamp is an auto-zero offset opamp. The allows for a low value current sense resistor (0.5milliohms) so that even at 100A, the power dissipation is only 5W. You really want to minimize the heat in that current sense resistor, as temperature of that resistor is the key to accuracy. If you can drop to 0.1 milliohms, that would really get the heat down.The transistor is a darlington, just to minimize load on the opamp. The FET was the one with the biggest gate capacitance I could see in LTSpice, but I tried adding an external 30n cap across the gate and it did not affect stability.Not only does the circuit appear very stable, but it settles to a large change in current ref input voltage within 80uS which is not too bad.The keys to stability are first you do need a 4 wire-type connection to the shunt resistor as much as possible. If you can get a shunt that actually has the 4 wire connections that will be the best. The second big point is the opamp supply much be totally floating with respect to the load current supply. The regulator circuit, particularly the ground, has to be immune to wiring voltage drops. So the regulator ground is connects to the load circuit only at the current shunt resistor's lower sense connection, and there are no other earth loops back through alternative ground connections.If you still get oscillations, it is worth tracking them down.In your current circuit, the compensation cap is 100n - that just sounds like it is too extreme. It is rather like fixing a slight bounciness in your cars suspension by filling the backseat with a ton of bricks. It is a fix, but not a good fix, and the problem that caused the instability is still there.Richard.

author: Allen
author_rank: Contributor
timestamp: May 29, 2012, 08:16:29 am
content: Quote from: amspire on May 29, 2012, 07:51:42 amI double and triple checked the wiring to ensure it was correct. The shunt resistor I am using is a 2 wire that I modified to be a 4 wire by soldering on some additional wires. I used both a 100 nF and a .5 nF cap as I don't have a 1 nF, but it made no difference. I considered using a very low current sense resistor value, but I'm afraid that the sense voltage will be swamped by circuit noise.I took a quick look with the oscilloscope and was getting either noise or oscillations in the 1.15 MHz region if I remember correctly. I'll hook it back up when I get a chance and report on all of the node signals.

author: poorchava
author_rank: Super Contributor
timestamp: May 29, 2012, 08:22:45 am
content: Maybe you could try adding a capacitor across inverting input and gnd to limit bandwidth? This could generally improve stability. I tend to add such capacitor to any opamp project (even if i'm 90% sure this footprint will remain unpopulated).As for controlling different modules, I'd try some simple MCU+DAC (like MCP4811 or 4812 if you need two voltages). Put same firmware into all modules, but program individual address into EEPROM. Or if you are going to use i/o-rich MCU just use 6-8 of these and either puldowns+jumpers or or resistors connected to either gnd or vcc. I would think about using larger number of lower power modules rather than few bigger ones. This would reduce the cost of uber-high-power transistors and shunts. I mean that you could calculate if it wouldn't be more economical to make say 100x45W module rather than 10x450W module or 5x900W module. Also any PCBs, including some speciality stuff like thick copper will be much cheaper if you order 100n units as compared to - let's say - 5 units (actually 100 smaller modules gets rid of the majority of the problems concerning high current through pcb tracks, you could probably do it on standard 35micron copper)

author: amspire
author_rank: Super Contributor
timestamp: May 29, 2012, 10:50:40 am
content: Quote from: Allen on May 29, 2012, 08:16:29 amI think you will find that the OP07 is not the easiest opamp for unity gain. If you look at the data, you see a bump in the unity gain curve (about an extra 25% gain) just under 1Mhz. That suggests for me that it is not far from instability at unity gain, so the extra phase shifts of the transistor and mosfet sends it over the edge.This is an opamp that would be far easier to stabilize at a gain of 10 or more.An opamp will a phase shift closer to 90 degrees at unity gain will be much easier to stabilize.Richard.

author: EEVblog
author_rank: Unknown
timestamp: May 29, 2012, 12:08:49 pm
content: Quote from: poorchava on May 29, 2012, 08:22:45 amMake sure you get cheap bare tin plated and not solder mask over bare copper (SMOBC). That way you get extra bonus trace thickness for free!Dave.

author: Allen
author_rank: Contributor
timestamp: May 29, 2012, 05:52:39 pm
content: Quote from: amspire on May 29, 2012, 10:50:40 amRemember, with that configuration, when the non-inverting input receives a voltage of about 10 mV, the output of the op-amp itself is around 4 V so the gain is really something like 400. Is there another op-amp you might suggest? I think the amp will need to be able to take at least 12 V supply voltage and have less than 250 uV input offset or maybe even a chopper amp. Also, because I will be using so many op-amps, I'm hoping to get them for under $2 each in quantities of 30 or so. Some people have made some op-amp suggestions before, but they were in the $5 range.

author: T4P
author_rank: Super Contributor
timestamp: May 29, 2012, 06:29:23 pm
content: TLC277I'm not too terribly sure if it's okay.Check the datasheet to see if it's good for you

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 29, 2012, 06:32:03 pm
content: why not to use this ? :http://www.ebay.com/itm/50-PCS-OP07CD-SOP-8-OP07C-OP07-operational-amplifier-/290719749402?pt=LH_DefaultDomain_0&hash=item43b03f551a#ht_989wt_926

author: IanJ
author_rank: Unknown
timestamp: May 29, 2012, 07:18:07 pm
content: Quote from: Allen on May 29, 2012, 08:16:29 amNot sure if anyone else has suggested, but try a small ferrite bead (tube) on the gate of the mosfets, as close to the fet as possible.Usually used to help combat oscillations when paralleling mosfets a ferrite bead can also be used with a single mosfet. High frequency noise on the gate is blocked, eliminating any tendency for oscillations.Ian.

author: Allen
author_rank: Contributor
timestamp: May 29, 2012, 07:54:32 pm
content: I just ordered a few parts to play with:A 1%, 1 mOhm, 4 terminal resistor.http://www.vishay.com/docs/30179/wslp2726.pdfSome 1 nF caps.http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3101_GoldMax.pdf/$file/F3101_GoldMax.pdfAn OPA27GP, internally compensated, unity-gain stable op-amp.http://www.ti.com/lit/ds/symlink/opa27.pdfSome 20k trim pots for offset nulling.http://www.bitechnologies.com/pdfs/67.pdfAnd the IXFK230N20T MOSFET.http://ixapps.ixys.com/DataSheet/DS100133A%28IXFK-FX230N20T%29.pdfI should get those in a couple of days. I'll give them a try and report back.

author: amspire
author_rank: Super Contributor
timestamp: May 29, 2012, 10:27:03 pm
content: Quote from: Allen on May 29, 2012, 05:52:39 pmNo. If you are oscillating at 1.15MHz, you are interested in the AC gain at 1.5MHz. Forget the DC levels. The transistor emitter follower and the mosfet source follower have a gain slightly less then 1, so the opamp has a gain slightly more then 1 for an oscillation.If you look at the feedback loop, 10mV in results in 10mV across the shunt resistor - that is a unity gain feedback loop. If you had a circuit that meant that 1mV in resulted in 400mV across the shunt resistor - that would be a gain of 400.I saw your have ordered some OPA27s and that is still the wrong opamp choice. I will do a post in a moment on how to choose the right kind of opamp. The OPA27 is a brilliant opamp - except not for your circuit. I am not saying you cannot get the OPA27 to work, but it will never work as well as a more optimal opamp for this purpose, and since you are going to be paralleling many active load modules together, you really want rock solid stability.Edit: I had written OP27. It is the OPA27 that have been ordered that are the ones that are not great. More in a new post.Richard.

author: IanB
author_rank: Super Contributor
timestamp: May 29, 2012, 11:13:05 pm
content: Quote from: amspire on May 29, 2012, 10:27:03 pmI imagine you will certainly want to design the circuit to minimize any cross-talk between the modules.

author: amspire
author_rank: Super Contributor
timestamp: May 30, 2012, 03:46:52 am
content: Finding An Opamp For A Constant Current Load CircuitI mentioned how I thought that OP07 and OPA27 opamps were not good choices for constant current load.Going back to opamp basics, you want an opamp to have as much gain as possible. The higher the gain, the lower the gain errors. But the gain will fall off with frequency, and at some point, the input-output phase shift will be 180 degrees. If the gain is still positive, the opamp can be come an oscillator.So an ideal opamp has one single resistor-capacitor type roll off that dominates, so you have a 6dB/Octave (or 20dB/decade) roll-off. The idea is that all other parts of the opamp circuit should be so fast that they do not even start to roll-off until below the opamp's unity gain point. In terms of phase shift, a single RC roll off becomes a constant 90 degree phase shift. This idea opamp is very stable as it always has a 90 degrees phase margin from instability. The other great thing is that this phase shift is spot on 90 degrees for every opamp regardless of frequency, voltage, input offset, etc (we are still talking the ideal opamp here) so if external compensation is required, the opamp behaves totally predictably.In the real world, for every opamp, the designers have pushed a few limits to make the opamps specs as good as possible. So when you look for an opamp, you need to look for one with the design choices that suit the application.If we look at the OPA27 data, we see there is no open loop phase information, but here is the gain versus frequency plot:The fall-off should be a ruler-straight 20dB/Decade. Above 200KHz though, the fall-off rate increases, so you know for sure that ugly things are happening with the phase shifts between 200Khz and 10Mhz. Even the slightest deviation usually means big diversions from the ideal 90 degrees shift.The designers have made tho opamp stable at unity gain - just. This is an opamp probably optimized for really great performance as a x10 or x100 amplifier, but it is unknown and unspecified for its phase performance near unity gain. With this opamp, even if you get one module to be stable, the next one might not be stable.Now if we look at the OP27 instead of the OPA27, we see an improved situation:The deviation is much smaller from ideal and the phase shift is specified (at +/-15V at least). The unity gain phase margin is 70 degrees which is good, and that means that even if the mosfet driver circuit adds a 35 degrees phase shift, the circuit will still be stable. The phase shift plummets to 180 deg at a gain of about -7dB, so that indicates that the designers have pushed the design limits to get maximum gain and speed. Definitely, the OP27 is much better for a constant load regulator then an OPA27 or OP07.Now if I go to a Microchip MCP6V26 opamp (2uV offset 5.5V maximum supply voltage, 6.5V absolute maximum), we see the Open Loop performance.This looks like an excellent amplifier for unity gain. The gain rolls off at an ideal 20dB/Decade to well below unity gain, and the phase shift shows no bumps to indicate the designers have added zero's to stabilize the opamp. The IC is specified for a 60pF capacitive load, and the 180 deg phase shift happens at a gain of about -12dB. This would be a great IC to use at a price well below $2. Since it is an auto-zero opamp, you need to give the opamp a few hundred uSecs on startup to zero the offset before the output regulation is accurate to 2uV across the shunt resistor. The 2MHz maximum frequency is a help too - lower unity gain frequencies make the design easier, and 2MHz would still mean a fast response is possible. This IC an go up to 5.3V at 3mA output, so if you have a transistor emitter follower, plus a 1mOhm shunt resistor, you have about 4.5V left to drive the MOSFET. If you can source mosfets that only need 4.5V or less to reach the desired current, it can be used.Back to my comments about the opamp maximum frequencies. Adding compensation to unity gain opamps is not great, as at best, as capacitor across the opamp can reduce the gain from about 1.1 to 1.0. The effect of any compensation is it slows down the opamps ability to respond dynamically - like a change in the load voltage.The way to design this circuit is aim for inherent stability without compensation, then add parts to improve stability. If you can design the driver and mosfet circuit to have a phase shift of less then 35 degrees at the maximum opamp frequency, then the circuit will probably be inherently stable. Both the emitter follower and source follower will be naturally fast, so the speed limit will be due to the time constant of the transistor drive into the mosfet gate capacitance. If the gate capacitance is 10nF, and the source follower gain is above 0.9, then the effective gate capacitance is less then 1nF. It is then all about getting the RC time constant from the transistor output to the mosfet input to be fast enough to to get the transistor/mosfet phase shift to less then the 35 degrees (or whatever you choose).If you can manage this, then you can probably get the circuit to be stable even with no compensation. That would be a great starting place.Hope that helps.Richard.

author: amspire
author_rank: Super Contributor
timestamp: May 30, 2012, 04:19:23 am
content: One more point related to the speed of the opamp compensation. If the load is at 0V, the FETS will be driven hard on. If the load then jumps to 4V, the constant current load will initially have a massive current surge, until the opamps has time to adjust. The size of this surge will be limited by the circuit resistance, the mosfet ON channel resistance and the circuit inductance. We can be talking over 10,000 amps easily.The more compensation, the slower the adjustment. With a minimal compensation, this surge pulse width could be a few microseconds wide. With heavy compensation (like the 100nF cap) it could be milliseconds wide. The size of this surge will be completely independent of the current limit settings.This also probably means you do not want more gate voltage available then you really need. A gate drive of 12V is probably excessive.Richard.

author: Allen
author_rank: Contributor
timestamp: May 30, 2012, 10:18:03 pm
content: What do you think of the potential of this op amp? NJMOP07D, offset voltage: 150 uV, Phase at unity gain: ~110 degrees.http://semicon.njr.co.jp/njr/hp/fileDownloadMedia.do?_mediaId=101

author: amspire
author_rank: Super Contributor
timestamp: May 31, 2012, 12:48:22 am
content: That is a op07 made by Japan radio with slightly lower specs then the TI version.My problem with the OP07 is that odd things do happen at unity gain. The Japan Radio specs are very brief, but if you look at the TI specs, the unity gain response curve has a hump just below 1MHz. For some odd reason, the Japan radio specs only give the curves for a x100 amplifier (instead of the open loop gain) and that can hide irregularities in the curves. If you go back to the OP07 curves, print out the page and put a ruler along the open loop gain curve. What do you see happening to the curve near 0dB gain? Form memory, TI do not publish a proper open loop phase versus frequency graph at all.Ultimately, you may want to just test for yourself. Get several different opamps, connect them up with the non-inverting input to ground, and the inverting input connected via a 100K resistor to the output. Connect a signal generator via a 1uF cap to the inverting input, and measure your own gain versus phase curves using a dual channel scope near unity gain. One probe on the inverting input and one on the output - both in x10 mode. If you do your own tests, you see the stuff the IC manufacturers do not want to publish. It is not a hard test to do at all.My concern with the OP07 is that the designers may have added a zero near the unity gain frequencies to improve stability or to extend the bandwidth a little. It would show up in the phase curves as a flattening or even upward bump. That alone doesn't make the chip bad, but it is a factor that will vary from chip to chip, with temperatures, with common mode voltages, etc. In general, the less fiddling the designers have had to do the easier the chip will be to use.The OP07s will need a dual supply. The microchip opamp I mentioned only needs a single supply as the inputs and outputs are rail to rail. The microchip opamp is also cheaper and the offset without adjustment will be probably over ten times better then the "OP" family of chips with adjustment. The catch is you need Mosfets that will give the maximum current you need at under 4.5V.Richard.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 31, 2012, 10:19:28 am
content: heyso ,(1) .I choosed the op07 because of he's low ofset (I can trim it compare with my meter )so I don't need something that came already at he's offset(2) . I run a little test , 2X-IRFP064N's in paralel ( no current shearing resistor but 2 -20AWG wiers , each is about 50mohm ) , and at 4.5V I'm getting 13A on about 9.7V . and I don't think it will do (for me anyway ) I plan something on the middle range [0-10A@up to 30V- 150W- HStemp would have been 20C above ambient at full load-or just saying 2CPU's HS- each one at 75W will last 30+years](3) . im afraid I can't run the test , I have no scope and no DDS (etc ) -at the momentcan you give me a name of high speed ,low offset (below 10uV -for high current sensing with "low" resistance shunt ) as for the cost , I'd like to don't go above 5$ per unit , but it's up to the productthank you in advance !

author: amspire
author_rank: Super Contributor
timestamp: May 31, 2012, 11:56:17 am
content: Quote from: eevblogfan on May 31, 2012, 10:19:28 amYou can definitely try it.QuoteYou cannot successfully parallel IRFP064N. They will not share properly, and the sharing will get worse as they get hotter. You really need to use two single IRFP064N's instead.This is not a sensitive gate mosfet, so you cannot use is with a low voltage mosfet, like I suggested. Typical gate voltage will be around 5V.QuoteSpeed often isn't important. It is very hard to beat the Microchip opamp for offset and performance. As I said, it is much better then an adjusted OP07 as the temperature coefficient is much better. Price is well under $2. Sure you cannot find a sensitive gate mosfet? I admit I haven't looked.Richard.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: June 01, 2012, 04:49:43 am
content: thank you richard !what do you mean by "sensitive gate mosfet" ?lower vgs(th) or lower gate charge ?I know I cant parallel them like this , I only had few lm324's and others so I drove the gates via the e-3610A , (at about 3.8V the IRFP064N began to draw current ) in order to not kill them ( fast current surge->output high for the gate -> low RDS(on)-> short -> bangwhat sort of mosfet you'll recommend for such a use/abuseBTW , I discover the problem I had with the huge heat sink , the thermal contact was atrocious , the terminal shrews weren't god at all , they was "pressed" upwards , so I took a bass bar and connected the 2 shrews to the edges , and via the bass bar I pressed the mosfets , I left it to work at 200W (without fan - ) ,it was 62C (HS temp ) and about 80-85C the surface of the mosfet ( I think the junction is like 20C more ) , the heat sink should handle (with fan ) 333W at only 20C above ambient ! ( and with a better fan , oh ->much more ! )when you say I can try the op07 , do you serious or joking ? (I can't figure out -sorry ...P )I'm asking because there's ebay seller selling them for 20$ word wild shipping for 50 pcs , is it worth something ?and why can't I use npn/pnp transistor on the output of you're op amp ? and if we are talking about chopper op amps , why that one (I'm trying to understand your considerations )hank you in advance !

author: amspire
author_rank: Super Contributor
timestamp: June 01, 2012, 05:17:14 am
content: Quote from: eevblogfan on June 01, 2012, 04:49:43 amYes. There are mosfets designed to be driven from 5V circuits, and so their gate voltages are specified to be less then this.QuoteEverything blows if you take them outside their limits.QuoteYou can try them.OP07 has no open loop phase graphs, so all I can go on is the open Loop gain and the unity gain response graphs that suggest to me that it may not be an easy opamp to use at Unity gain, but the spec sheets are vague. If I was considering using them regardless, I would have to do my own tests.QuoteThey may be good, but are they genuine or not? It is hard to know when you buy parts below normal market prices on ebay. The cheapest price I could see at Mouser was 36c if you got a reel of 2500 SMD OP07s.If you go for the cheap ebay ones, you will definitely have to test them to check they perform to the OP07 specs before you start building many modules.QuoteI would really try and get the circuit stable before compensation, just so with an extra capacitor across the opamp, you may be able to make it even better.To get it stable, I would need to know what phase margin I have with the opamp (not specified for the OP07) and I would then attempt to design a transistor driver + mosfet circuit that has a low enough phase shift to keep the total below 170 degrees total phase shift at the opamps maximum frequency. It all comes down to what gate capacitance the mosfet has. What the effective gate capacitance in source follower mode (it is hugely decreased - depending on the mosfet transconductance). What is the emitter resistor on the NPN transistor - the lower, the less the phase shift.If you learn how to use LTSpice, you can work on this at no cost. It is possible to get models into LTspice for devices not in the default library, and it allows you to work at any voltage and any current without anything blowing up, and without needing sources of massive current. There is sure to be a model for the OP07, and if it is a good model, it may help you understand how it works at unity gain.That is where I would start.Richard.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: June 01, 2012, 05:32:35 am
content: thank you richardI've orderd the MCP6V26 , throu digikey (it's my first order in digikey , so I hope I did well)can you tell me(either draw) what is the "apropriate" circuit for the mcp6v26 + the IRFP064N ?thank you in advance !

author: amspire
author_rank: Super Contributor
timestamp: June 02, 2012, 05:16:08 am
content: Try this:I have attached the LTSpice file and it is worth learning how to use it. Saves a lot of money and time, although models are never perfect.I don't have time to do a thorough search for the best mosfet, so I found one (IRL3803V - TO220 package) that has a 1000-off budgetary price from International Rectifier of less then $1. A low gate voltage and a decent Safe Operating Area. It is a 30V mosfet and you have not specified any maximum voltage. There are always tradeoffs in choosing devices and if you go for 200V mosfets, you may find that you loose out in other parameters.http://www.irf.com/product-info/datasheets/data/irl3803v.pdfYou will not get 100A out of it. I would start off with 20A and see if you can get to 30A or more, but that all comes down to heatsink efficiency. Before you start with fanciful heatsink efficiencies, see what is practical, and make sure the mosfet can still handle the power at the expected heatsink temperature. You may end up finding that 20A per mosfet is a maximum realistically achievable amount of power with the mosfet I have chosen. If you want a different mosfet, I will leave it to you. The IRFP064N is going to be marginal for gate voltages with the MCP6V26 opamp. You may be able to look at a circuit with a PNP driver transistor so you can get up to 6.2V gate drive for the IRFP064N , but there will be new design issues with that.The circuit looks pretty stable without the 100p cap, so it looks very promising. Perhaps R2 should be bigger - like 10 ohms, and perhaps R1 should be smaller - like 47 ohms for more speed. Again, I will leave that to you.If you look at the mosfets that could do 100A, there will be a way to get them to work, but if you can easily get a 20A module to work with much better performance, the smart thing may be to just use fifty 20A modules. At 20A, you only need a 1 milliohm current shunt that can handle 0.5W or more (1W would be great), so you can look at the cheaper surface mount current shunts.Richard.

author: Allen
author_rank: Contributor
timestamp: June 03, 2012, 05:19:09 am
content: I want to use a similar model in Pspice, but I can't find a Pspice model of the MOSFET I want to use. I found a similar one from the same company, but the Ciss capacitance is much lower (9,000 pF vs 28,000 pF). Do you think this will make much of a difference in stability analysis? Or should I just add a capacitor at the gate using the difference in capacitance?Here is the one in Pspice:http://ixapps.ixys.com/DataSheet/92802.pdfHere is the one I want to use:http://ixapps.ixys.com/DataSheet/DS100133A%28IXFK-FX230N20T%29.pdf

author: amspire
author_rank: Super Contributor
timestamp: June 03, 2012, 06:27:41 am
content: Quote from: Allen on June 03, 2012, 05:19:09 amMosfet's are made from lots of standard cells in parallel. So there could easily be a lower current mosfet that looks like it is made from similar cells. Then you may be able to simulate the one you want by paralleling the smaller mosfets in PSpice. Since all the parallel mosfets all use the same model, then gate voltages will match so they will share current perfectly in the simulation.But keep in mind that those big IXYS mosfets will be more difficult to drive, more difficult to make stable, and getting rid of heat from a single super power device is much harder then getting rid of heat from many smaller devices. I would suspect the only way to get rid of 400W-500W heat from a single device is a carefully designed water cooled heatsink.I have a suspicion that a fan cooled solid aluminum heatsink will just not do it at all. With that much heat coming from a small contact area, even aluminum cannot conduct the heat away fast enough.The trouble is that the temperature drops in the solid aluminum from the mounting point to the fins will produce huge temperature drops. The end result is you can only get perhaps a third the power from the mosfet that you were hoping for. Lower heat from multiple devices on a heatsink results in far more uniform heat distribution across the heatsink giving much better performance from the heatsink.Lets say a heatsink costs $50. You can get $40 worth of performance out of the heatsink with 8 mosfets spread evenly, compared to perhaps $15 worth of cooling performance with 2 big mosfets on the same heatsink. Anything you think you are saving is wasted on inefficient heatsinks, and wasted space in the case. You pay much more for less final power.If you are planning on getting 400W from a single mosfet, you may find it a very hard and slow job. You may never succeed in the time and budget that you have. Getting 80W each from five times the mosfets will be a much more achievable and realistic solution, and you will be able to use standard fan cooled heatsink extrusions.Richard.

author: amspire
author_rank: Super Contributor
timestamp: June 03, 2012, 06:57:55 am
content: With regards to stability, the lower the value of the shunt resistor, the easier it will be to achieve stability as the gain loss from the mosfet gate to the shunt resistor is much higher.If you can stick to 1mOhm shunt resistors, then I think you should be able to get stability from OP07s and even the OPA26 opamps without too much difficulty. With 10 milliohm shunt resistors, you get ten times less error from offset voltages, but stability is harder to achieve.

author: Allen
author_rank: Contributor
timestamp: June 03, 2012, 08:08:54 am
content: Serious question for you. How do to tolerate Pspice? Every time I come back to it and try to give it another chance, I am suddenly reminded of why I hate it so much. No matter what I do, I can not run an error-free simulation. Like "Error - missing node". What does that even mean? How is it that I can have a voltage source connected to a resistor and get an error?

author: amspire
author_rank: Super Contributor
timestamp: June 03, 2012, 08:26:05 am
content: Quote from: Allen on June 03, 2012, 08:08:54 amA while since I have used PSpice. Did you define a ground node? Spice always has to have a reference node (node 0 or ground) other wise it cannot solve anything.

author: Allen
author_rank: Contributor
timestamp: June 03, 2012, 08:33:53 am
content: Quote from: amspire on June 03, 2012, 08:26:05 amYes, there is a ground node. Even the simplest of circuits shown below will not run without an error.

author: amspire
author_rank: Super Contributor
timestamp: June 03, 2012, 08:44:13 am
content: Is the ground properly attached? I prefer to drag it away from the wire, and add another wire to connect it.Also what kind of simulation did you try? Did you just do a DC operating point simulation or a transient simulation? If you tried a transient simulation, is the battery a Spice Voltage Source or just a fixed voltage battery. Transient simulation probably need one valid voltage source.Can you look at the Spice netlist, as it will probably show the problem.

author: fmaimon
author_rank: Unknown
timestamp: June 03, 2012, 03:56:40 pm
content: Quote from: amspire on June 02, 2012, 05:16:08 amWhy not use a pnp transistor instead of R1? Shouldn't this be faster than a resistor? And, if you go to SMD, there are some with both NPN and PNP in the same package, like the BC846BPN.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: June 03, 2012, 07:26:27 pm
content: thank you richard .the voltage I aimed for is 60-80V ( the 4X15V power supply should be able to be connected in series ) ,I have tested CPU heat sink and the "nominal" power handling of it is 76W , (at 20 C above ambient ) wich is great ! ( at 50C ambient I'll be able to work with 70C heat sink and about 80+C junction of the mosfets ) so no worries hereI've learnd about transformers , and I know that mine micro wave's transformer can give me 474.3684W , while all I whant is 300W , I think he would have ran quite cold ,I can then add another channel , like 0-60V 0-1A , or +-0- 50V each at 0.75A or many other helpful "lower" power channels .I can't run this circuit , I cant find the modals.BTW , can it be ? , I have orderd 2X20,000 count , 20V panel meters , and they are too good to bo true , photoes :sorry for the hp's screen , it's the lighting in the roon , not the cemra or the screen.can this happen ? that's eror within the whole range ? it's a "cheap" thing , how??//BTW , I have fixed this thing awile ago . the seller claim it's oparet at 5V , correct , but the meter has 7805 inside , so the voltage is actually 5V to 30V no problem , how can I run it with commen ground without an arror ? (I mean for some sort of hack )sorry for the OT

author: amspire
author_rank: Super Contributor
timestamp: June 03, 2012, 11:45:54 pm
content: Quote from: eevblogfan on June 03, 2012, 07:26:27 pmThe numbers sound good. If you are aiming at 80W-150W with a cpu heatsink, that does sound reasonable.QuoteDon't Microwave Oven transformer's have a secondary voltage of about 2000V?QuoteWhat can't you run? If you download LTSpice, then the spice project I sent will run, but I have to admit that the MCP6V27 model I found has convergence problems with LTSpice. The LTSpice project file I sent includes the MCP6V27 and mosfet model as .SUBCKT modules.I added a LTSpice below for a IRFP064N circuit with PNP driver, but it has worse operating point convergence problems. Anyway, you can see the circuit and have a go.QuoteI have noticed that a lot of cheap meters are calibrated accurate to the last decimal point in production, so it does not surprise me. The issue is how much will it drift in one or two years? For some reason, the initial calibration of a lot of cheap Chinese meters and multimeters is very often better then some quality brand name devices such as low end Fluke multimeters.Richard.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: June 04, 2012, 04:44:16 am
content: well actually , they were totaly out, they claim to 0.3% (or something like that ) , but they were about 1.4% or even worse ! ,but then I recive my 3478A , I compare them , tweaked my tongue close my left eye . and now it's about 0.03% max ( +1 Digit ) within all the range ! , (I know it has adrift. maybe within 4 month I would have to call it agine , but for that price range I got what I paied for ( I have burned 2 times both of the refulators and replaced them with to-220 ones , and I have re connected all of the LED dispay's pins from the beggining ( the pads whered out ) but now I am quite shure I have done with it:Pthe simulation works grate ! , with 400uV ofset I can livedigikey wanted 36.62$ ! for the op amp , (in the visa I have like 20$ - therefor they diceline that order)I don't use the 2000V winding and neither the 220V (aluminum) winding , I took all out , now I'll wing my own 220V ad I should , 440windings for the 220V and 36winding per 15V channel ( it's like 1.928V per winging ) 42/21.78=1.928374Vyes . the CPUs can handle even 200W . but at 72-76 C above ambient tempthank you in advance !

author: Allen
author_rank: Contributor
timestamp: June 04, 2012, 04:59:45 am
content: Quote from: amspire on June 03, 2012, 06:57:55 amJust a quick update. I changed the shunt resistor to a 1 mOhm and the feedback capacitor to a 1 nF as shown in the below circuit. I ran the circuit for a few minutes and it was completely stable and quite accurate after adjusting the null offset. I have been using both of my power supplies to power the circuit so I had to use a makeshift DC source to serve as a current source. Unfortunately, after cranking up the current, the makeshift source burned out before I had a chance to record any results. I'll have to go pickup a proper high current source.

author: amspire
author_rank: Super Contributor
timestamp: June 04, 2012, 05:27:18 am
content: Quote from: Allen on June 04, 2012, 04:59:45 amTo test, what you want to do is put get a bank of big electrolytic capacitors across the source, and instead of feeding a DC voltage in, make the current reference a pulsed waveform - something like 1mS at 50mV, 999mS at 0V. If you do not have a waveform generator, you can build a NE555 circuit to do this. You will need to get your hands on a scope. This way, you can fully test your circuit for everything (except for thermal capacity), and 1000 amp pulses would only need 1A average from the source supply.Stay within the RMS ripple capacity of the electrolytics. Otherwise, you will be scraping bits of electrolytic capacitor off the roof. 1000A for 0.1% of a cycle is about 31A RMS ripple.Richard.

author: amspire
author_rank: Super Contributor
timestamp: June 04, 2012, 05:39:46 am
content: Quote from: eevblogfan on June 04, 2012, 04:44:16 amSounds like you ordered the opamp evaluation board instead of the opamp. The one off Digikey prices for MCP6V26 is $1.66Richard.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 04, 2012, 06:09:14 am
content: You can order from Microchip directly, I think they send from Asia somewhere, from Aus it is only $15 postage for something small. It only took about 4 days last time I ordered.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: June 04, 2012, 08:52:48 am
content: heycan somone tell me , assuming I want to use the op07C in the ltspice , how ? , I mean , what do you do ?assuming just for fun , if I'll connect lm324+2XIRFP064N's+ 50mohm shunt per mosfet , can I run that circuit without kill the mosfets ? ( I will put some 100-120 mohm resistor in order to reduce the max peak current )my goal is to run that at 10-15A @ 12V power supply with resolution-accuarcy of 0.01A ( for now I think it will do doun to 200-400mA ) , should I ?note : to my contry I "must" pay 40$ no matter what ammount ( anyway , the lowest is 40$ , so I dont know whats going on here ... )thank you in advance !

author: Bored@Work
author_rank: Super Contributor
timestamp: June 04, 2012, 11:23:58 am
content: Quote from: eevblogfan on June 04, 2012, 08:52:48 amYou download the SPICE model from the manufacturer's website. Then you follow the instruction for adding a 3rd party SUBCKT model in the LTSpice help - FAQ section. Yes, I know, reading and following instructions is sooooo old school, but trust me, it works.

author: amspire
author_rank: Super Contributor
timestamp: June 04, 2012, 03:23:42 pm
content: Quote from: Bored@Work on June 04, 2012, 11:23:58 amThat's exactly what I did in the LTSPICE files I have posted here. The Help was actually helpful! Just take a look at how I added the MCP6V27 models.Richard.

author: Allen
author_rank: Contributor
timestamp: June 05, 2012, 03:26:17 am
content: OK guys. I bought a new power supply and finally got some good results. And thanks to amspire for the idea of using a large source capacitor and pulse testing. So I hooked up the below circuit and input 1 ms, 50 mV square pulses at 10 Hz. Below shows the oscilloscope capture.The top waveform is the input wave and the lower waveform is the voltage across the 1 mOhm current sense resistor which shows I am getting 50 A pulses. I even cranked it up for a minute and was getting 200 A, pulses, although I don't plan on running these modules with that much current.I'd say these are some very promising results. As you can see, there is some overshoot. I can reduce the the overshoot by adding resistance to the feedback, but this also increases the delay between the start of the input and the start of the current rise. There is still a 48 us delay between the input and output rise which I am pretty sure is because of the low slew rate of the op-amp.

author: electroguy
author_rank: Regular Contributor
timestamp: June 05, 2012, 04:40:16 am
content: Allen, thanks for posting that up! It's good to see real life test results.I agree, this looks promising, as long as you work out your thermal solution, looks like your on the way to success!If it is not too much trouble, can you take a few screenshots of your results with 0.1n (instead of 1n). Also, how much resistance did you try in the feedback as you suggested? can you take another screenshot with some resistance, even if it is a small one, say 10R or 100R, just curious to see some screen captures. Thanks!

author: amspire
author_rank: Super Contributor
timestamp: June 05, 2012, 05:23:57 am
content: Quote from: Allen on June 05, 2012, 03:26:17 amBefore you try for anything to good, lab power supplies often only respond to changes in about 10uS to 50uS for older supply designs. 48uS response time for the opamp is not that far out, and with a 1milliohm shunt resistor, you should be able to reduce the 1nF cap to 100p.Also, the question I have is how much of the waveform is real, and how much is current induced into the scope probe-ground loop due to the large fast change in current?It looks as if the current goes negative on the falling edge, and is there actually enough inductance in the source leads to allow the current to go negative? Connect up one probe across the current hunt, and the other with ground and probe tip both to the lower end of the shunt resistor ground, and look at the difference.It may be that performance is better then it looks initially on the scope.Richard.

author: Allen
author_rank: Contributor
timestamp: June 05, 2012, 06:09:08 am
content: OK. Got a few captures for you.First, I don't have a 100 pF capacitor so I'll have to order some. I did have a 500 pF cap that I hooked up but didn't see much of a change. Here is the capture with the smaller capacitor as shown in Figure 1.Fig. 1. 500 pF feedback capacitor (C1).Electroguy, the last post showed results using no feedback resistor, but I tried a few for you to look at. The effects using feedback resistances of 10, 100, 1k, and 10 k resistors are shown Figures 2, 3, 4, and 5 respectively.Fig. 2. 10 Ohm feedback resistor.Fig. 3. 100 Ohm feedback resistor.Fig. 4. 1k Ohm feedback resistor.Fig. 5. 10k Ohm feedback resistor.Amspire, Figure 6 shows the results of connecting the probes as requested. Waveform 1 shows the probe in which the ground and probe tip are both connected to the negative terminal of the current sense resistor.Fig. 6. Probe 1 connected to current sense neg. terminal.

author: Allen
author_rank: Contributor
timestamp: June 07, 2012, 04:20:44 am
content: I took some measurements for accuracy which are shown in the table below. I think these results look pretty good. Over 0.5 A, the error is well under 1%. My highest current power supply only goes up to about 3.2A continuous so that is all I can test up to right now. I will get a larger heat sink and I should have access to the university's design lab soon which will allow me to test the module using much higher currents.Note, I am still using a 1 mOhm current sense resistor.

author: electroguy
author_rank: Regular Contributor
timestamp: June 07, 2012, 04:27:51 am
content: well done mate!

author: EEVblog
author_rank: Unknown
timestamp: June 07, 2012, 04:34:39 am
content: wow, nice results.What tolerance/type current shunt resistor did you use?Dave.

author: Allen
author_rank: Contributor
timestamp: June 07, 2012, 05:01:32 am
content: Quote from: EEVblog on June 07, 2012, 04:34:39 amI used a 1%, 1 mOhm, 4 terminal, surface mount current sense resistor with wires soldered to the terminals so I could stick it into a breadboard.http://www.mouser.com/ProductDetail/Vishay/WSLP27261L000FEA/?qs=H%252bW4FWGGyHcxqFU1TLZ1EQ%3d%3dI also used the OP07 op-amp like you showed in your programmable load teardown, but I tweaked the null offset to really get some good numbers.

author: amspire
author_rank: Super Contributor
timestamp: June 07, 2012, 05:47:28 am
content: Quote from: Allen on June 07, 2012, 05:01:32 amI really like the design of those resistors. Surface Mount. 7W 75 ppm/C, and the construction allows them to expand and contract in the board. Price is excellent for something like this.Good find. Richard.

author: EEVblog
author_rank: Unknown
timestamp: June 07, 2012, 09:49:23 am
content: Quote from: amspire on June 07, 2012, 05:47:28 amYeah, nice.Looks ideal for a precision 50A CC module.Perfect one of those modules with their own heatsink/fan assembly and then just bolt 20 onto your bus bar.It's coming along nicely.Dave.

author: Allen
author_rank: Contributor
timestamp: June 07, 2012, 09:17:45 pm
content: I'm still not set on this OP07 op amp yet. I wanted to check again with you guys and re-evaluate the op-amp situation.I have not done thermal tests yet, but I might have to use 40 modules (105 W each). If that is the case, I would like each module to have a range from 2.5A - 25A. Using a 1 mOhm current sense resistor and wanting a 1% tolerance, I calculate I will need, at most, (2.5)(.01)(.001) = 25 uV input offset voltage.And to drive the MOSFET I have chosen, including the voltage drop across the emitter follower, I am going to need at least 5.5 volts out.Do you guys think I should use the $0.75 OP07 (150 uV offset), and adjust it with a trim pot or maybe calibrate it in software, or should I get something like the $2 OPA177 (25 uV offset) and be done with it? I really don't like the idea of having to adjust 50 or so trim pots, but software calibration should be pretty easy.I don't mind spending extra on a good op-amp, but there are probably going to be about 50 of them and I don't want to just wast a bunch of money on something that is easily correctable in software.

author: akis
author_rank: Frequent Contributor
timestamp: June 08, 2012, 02:54:05 pm
content: Quote from: amspire on May 30, 2012, 03:46:52 amWould the the LM4562 and LT1361 qualify?

author: Allen
author_rank: Contributor
timestamp: September 09, 2012, 10:20:43 pm
content: Just a quick update. I used the high current power supply at the university and tested a module at 40 amps with the MOSFET mounted on a large heat sink with fans. The module was stable through the entire range and the junction temperature settled at about 76 degrees C. Next I am going to put five modules on the same heat sink and run 200 amps through them to make sure the temperature is not too high. I am expecting a junction temperature of about 100 - 110 degrees C.

author: Allen
author_rank: Contributor
timestamp: September 09, 2012, 10:53:48 pm
content: I decided to perform a stability analysis on the original circuit as shown below.I modeled this circuit as shown below. The summation junction and transfer function block (H(s)) represents the op-amp. The RC low pass filter formed by the output resistance of the op-amp and the gate capacitance of the MOSFET is represented by the F(s) transfer function. The gain (k) block is defined as the transconductance of the MOSFET multiplied by the resistance of the current sense resistor. The transfer function of this system is given by T(s) = HFk/(1+HFk).An op-amp with two poles can be represented by the following transfer function where g is the DC gain and P1and P2are the two poles:H(s) = gP1P2/((s+P1)(s+P2))The transfer function of the RC lowpass filter is given below where R is the op-amp's output resistance and C is the MOSFET gate capacitance:F(s) = 1/(2*pi*RCs+1)Plugging these into the system transfer function gives the following:T(s) = gkP1P2/(2C*pi*R*s3+ [2C*pi*R(P1+P2)+1]s2+ (2CP1P2*pi*R+P1+P2)s+(gk+1)P1P2)Performing the Routh test gives the following stability requirement:4C2P1P2(P1+P2)pi2R2-2C*pi*R(gkP1P2-P12-2P1P2-P22) + P1+ P2> 0This plot of the OP07 frequency response gives the following: P1=1 Hz, P2=1MHz, and g = 5.6e5. The output resistance of the op-amp (R) is 60 ohms and the gate capacitance of the MOSFET (C) is 28 nF. Plugging these into the above equation gives the following stability requirement:k < 1.95With a maximum MOSFET transconductance of about 200, this gives a maximum current sense resistance of about 9.7 mOhms. This is why my original design using 100 mOhm resistor was unstable and the new design using a 1 mOhm resistor is stable.Here is a simulink step response simulation of this system using a 1 mOhm resistor:And here is the actual step response from the oscilloscope which appears to be pretty close to the simulated result.

author: GK
author_rank: Super Contributor
timestamp: September 10, 2012, 07:22:51 am
content: Hi. Here is an alternative (loop gain bode plot) sim. Using your values I get a phase margin of only 40 degrees with a 1mR source resistor, which explains the huge transient overshoot.I'd be looking for either an alternative op-amp with a lower GBWP, or at perhaps implementing a discrete (bipolar transistor) driver stage with a significantly lower than 60 ohm Z-out between the OP07 and the mosfet gate, to push the "2nd" pole that is degrading your phase margin higher up in frequency.

author: GK
author_rank: Super Contributor
timestamp: September 10, 2012, 01:14:35 pm
content: Oh never mind; I just read through the previous page of posts.

author: GK
author_rank: Super Contributor
timestamp: September 10, 2012, 02:02:17 pm
content: OK, just some playing around before I go to bed. I have taken your "original" circuit and have configured the op-amp as a unity gain inverting stage instead of a unity gain voltage follower. The feedback resistors are R4 (Rf) and R3 (Ri). However R5 has been added to increase the noise gain to 3. This has the effect of lowering the unity loop gain frequency of the OP07 by itself from ~900 kHz to ~300 kHz. Phase lead compensation is then added in the form a a zero by C3, to effectively cancel out the pole formed by the 60 output resistance of the OP07 and the 28nF of MOSFET input capacitance. As you can see, this results in a simpler circuit and almost ideal loop gain plot with over 80 degrees of phase margin and a vastly improved transient response. The only downside is that bipolar op-amp supplies will be required and the input control voltage need to be negative.