BIG active DC load project: IGBTs? isolated communications? - Page 1
Forum section: electronics / projects

author: TheMG
author_rank: Frequent Contributor
timestamp: July 21, 2015, 01:14:00 am
content: Yeah, I know, *another* dummy load thread... but I think this one is a bit different, read on....Well, I'm getting tired of jury rigging light bulbs, resistors, and whatever else I can think of in terms of load when I need to test batteries or power supplies.Time to get moving on my long outstanding project to build an active load.I'm thinking a bank of 6 40A/200W loads, isolated from one another. That way I could easily test multi-output power supplies, or put several loads in the bank in parallel when I need to dissipate some more substantial amounts of power.Initially when I first thought of doing this project, I was going to put a bunch of MOSFETs in parallel to be able to dissipate the required power. However, that would require a substantial mounting surface area on the heat sink (keeping it on the safe side, I'd need at minimum 6 devices per load channel).Now I'm thinking... what if I were to use a substantially larger device, capable of dissipating all 200W in a single device? That would reduce the amount of heatsink real-estate required and simplify the circuitry and construction.I've been looking at IGBTs and on paper it looks like something like this could do the trick:http://ixapps.ixys.com/DataSheet/DS100505B%28IXXN110N65B4H1%29.pdfI know these are normally used in switching applications, but it looks to me like it could be used semi-linearly instead. Actually, linearity is not really a concern so much as being able to accurately control current flow through the device. Anyone foresee any problems with attempting this?While this simplifies things, it does add considerable cost as these don't come cheap (about $30 per IGBT on Digikey). Guesstimating with the cost of other major components such as the current sensing resistors, breakers, binding posts, LCD display, isolated DC-DC converters, PCB manufacturing, rackmount case, etc. I'd be looking at slightly upward of $600 for the whole project.Now the question is... is it still worth doing at this price point? Or can I get something already made commercially at this price level? I looked around a bit and can't find anything under a few thousand $$$. Looks like it's still worth doing as a project?Anyways, the second thing I've been debating is how I will get all 6 of these isolated loads communicating with the main controller. Obviously I'll need something like optocouplers to achieve isolated data communications, but I'm wondering what kind of communications protocol I should use to do so? I'm thinking of possibly connecting all the modules to a common data bus, using some sort of serial communications and addressing scheme for each module.But how exactly to accomplish this, I'm not sure. Should I use an existing protocol such as I2C? SPI? RS232? Roll my own?I'm most likely going to be using Microchip PIC 8-bit micros throughout the design (for each load as well as the main controller), as that's what I'm most familiar with. I'll still have to brush up on my programming skills though, it's been a long time since I've done any kind of project using microcontrollers.Anyways, the way I'm hoping this is going to work, is that each channel will have opamps forming a negative feedback loop to keep a constant current flowing through the IGBT, sensing the current through a shunt resistor. A DAC controlled by the channel MCU will set the desired reference voltage for the feedback loop, and two ADCs will provide digital current and voltage readouts, which will be shown on the main LCD display to the user, and also used to implement protections and advanced modes of operation in software. The main MCU will provide the user interface, consisting of pushbuttons and LCD display on the front panel, and will send commands to each channel MCU to set the desired mode of operation and operating parameter. It will also poll each channel to gather current and voltage information for display and logging purposes, as well as monitor the status of each channel for proper operation and any errors (such as overtemp, overcurrent, overpower). There will be provisions for the channels to be independently set, tracking, or paralleled. Constant resistance and constant power as well as pulsed load modes. This will all be done in software.I plan to start by prototyping one single channel, and see how it goes from there.Anyways, if any of you have any feedback/ideas/suggestions, greatly appreciated!

author: ovnr
author_rank: Frequent Contributor
timestamp: July 21, 2015, 01:32:50 am
content: First, because it's the most glaring: Really? You're worried about toolowpower density? Getting rid of 1200W will take a LOT of heatsinking. Unless you intend to use a watercooled setup with a small waterblock, I see no reason you should complicate your life by going for IGBTs as opposed to MOSFETs. Also,you can get MOSFETs that can also dissipate 200Win a reasonably safe way - hell, there are5kW-dissipation-rated (500V/497A) bricksif you're thus inclined - yours for only $350.Also, consider that the Vce(sat) of IGBTs will restrict you when testing low voltage circuits.Isolation: If you're just transmitting the setpoint to each device and reading back values, just run plain RS232 over isolators - either optos or RF/capacitive isolators (look on Digikey for "Digital isolators"). Lowest pin count - I2C is not very suited for isolated setups, and SPI requires more pins and more fuss.Also, consider the total system and implementation cost vs just buying a pile of Maynuo loads.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 21, 2015, 02:33:51 am
content: Quote from: ovnr on July 21, 2015, 01:32:50 amLet's just say I've got more than a few substantial heat sinks in my collection. As well as some fans capable of moving some serious air. Some of those heatsinks although they can dissipate a lot of heat have been designed with a very small contact/mounting surface which limits the number of devices I can attach to a single heatsink. I would use one of these heatsinks for each channel (200W per sink), each with its individual fan.I do have some larger heatsinks with more mounting area but they have a very wide fin spacing and were designed primary for passive cooling. They would not be efficient for forced air cooling dissipating large amounts of heat.Quote from: ovnr on July 21, 2015, 01:32:50 amThere's what I get for not having enough sleep. For some reason I couldn't find these when I was searching Digikey which falsely led me to believe that MOSFETs didn't come in such large packages.Thanks for pointing that out!Quote from: ovnr on July 21, 2015, 01:32:50 amI had thought about that. It looked as if it was manageable as I have no need or intention of testing anything below 3.3V (minus of course any wire resistance losses).But now that you've brought to my attention that high power MOSFETs do exist, it doesn't look like this is going to be much of a concern anymore.Quote from: ovnr on July 21, 2015, 01:32:50 amI guess RS232 would be doable as long as I can make sure the devices can share the bus without conflict. As long as only one transmits at a time I suppose it should be ok. You're right it would make things simply with only a TX and an RX isolator with no clock needed. The PICs do have built in RS232 so it would be easy to implement software wise.Quote from: ovnr on July 21, 2015, 01:32:50 amTwo of the M9711 (150W 150V 30A) and one of the M9714 (1200W 150V 240A) would meet my needs, but we're talking around $2700 plus whatever taxes, shipping, etc would be.Cheaper than others I've looked at. If this were a professional lab that price would be a bargain, but for a hobby lab still more than I'm willing to pay.If I can build one that meets my needs for under $1000, it'll be worth it, and with those MOSFETs which are much cheaper than the IGBTs I can most definitely bring that down to under $500 in parts. Heck, I could probably get $500 and offset the entire cost if I sold half my heatsink collection as scrap copper and aluminum (yeah, I've accumulated a few too many over the years).Plus, I get the enjoyment and experience of building something, which I wouldn't get from just buying something "off the shelf".

author: krish2487
author_rank: Frequent Contributor
timestamp: July 21, 2015, 07:09:21 am
content: And not to mention, IGBT such as what you mentioned are a disaster to use as a linear resistor!It creates hotspots on the IGBT die which causes rapid punchthrough and breakdown.Its a spectacular sight watching it blow up though!!!Secondly, there is no such thing assemi-linearlyin analog electronics. Its like saying someone is slightly pregnant.You either operate it in linear mode or as a switch. period.Companies like keysight, Maynuo, BK Precision use paralleled banks with individual op-amps, control loops and ballast resistorsbecause that is the proper way to do it.Not to sound rude but,I highly doubt that any individual can come up with a revolutionary way to do it when corporations with much higher budget, talent and resources stick to a traditional way of doing it.

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 21, 2015, 07:12:09 am
content: krish2487You took the words right out of my mouth.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 21, 2015, 07:23:27 am
content: As communication I'd suggest CAN bus. It's robust and easy to isolate using TI's ISO1050.Checksums and retransmits are done by hardware. And for such simple project, the frame-size of CAN does not limit your capabilities.Say, each unit has a set of messages, with a programmable offset (by dip switch for example).It regularly reports it's status, say: temperature, voltage, current, timer. And receive master commands on a different identifier.But beware, CAN bus uses Identifiers describing the content of the message. Stick to that concept.You can create each load-node with a simple mcu reading some i2c/spi fancy-ADC and fancy-DAC to control the analog loop.Fun part is that you can control the system from your pc (using kvaser), direct plot to screen and generate reports.As stated above, such mosfet igbt bricks are for switching (included pwm) only. You can create a resistor ladder network using them. Or switch shunts if you want to change range.

author: dom0
author_rank: Super Contributor
timestamp: July 21, 2015, 07:26:00 am
content: Quote from: TheMG on July 21, 2015, 02:33:51 amYou basically meant to say "I got a ton of 2000s CPU fans"?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2015, 07:27:18 am
content: Quote from: krish2487 on July 21, 2015, 07:09:21 amWell, no. You can operate it in a "fuzzy logic" sort of mode, where it goes between cutoff ("0"), linear (0.0-0.99..), and saturation ("1"). Maybe that saturation is even detected, which causes the next stage to begin to pull in, and so on. A cascade of fixed range, or resistor limited, current sinks could do the job with a very scaleable circuit. Although it doesn't save you any heatsink space, because you still need each and every device capable of dissipating its full linear load.Tim

author: krish2487
author_rank: Frequent Contributor
timestamp: July 21, 2015, 07:30:50 am
content: If it is not either in cutoff mode or saturation mode, then no matter what fancy name you choose to give for it it is still linear in operation.Does not change the nature of operation.Quote from: T3sl4co1l onTodayat 05:27:18 PMWell, no. You can operate it in a "fuzzy logic" sort of mode, where it goes between cutoff ("0"), linear (0.0-0.99..), and saturation ("1"). Maybe that saturation is even detected, which causes the next stage to begin to pull in, and so on. A cascade of fixed range, or resistor limited, current sinks could do the job with a very scaleable circuit. Although it doesn't save you any heatsink space, because you still need each and every device capable of dissipating its full linear load.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2015, 07:39:13 am
content: Quote from: krish2487 on July 21, 2015, 07:30:50 amNonlinear is linear?News to me...I think the word you mean is "analog", though.Tim

author: krish2487
author_rank: Frequent Contributor
timestamp: July 21, 2015, 07:40:16 am
content: My bad.Analog is the correct term

author: Kalvin
author_rank: Super Contributor
timestamp: July 21, 2015, 08:06:19 am
content: PWM might be a nice solution for the communication. Optical isolation is easy. If the PWM-drive signal is disconnected, the load will be turned off automagically. If you want to measure only a load voltage, the PWM-output might be a simple solution as well. Converting PWM to a DC is a simple task.If you need more versatile solution an optically isolated SPI-bus or I2C-bus might be usable. Of course, it shouldn't be too difficult to make an optically isolated, multidrop bus with a small MCU, on-chip UART and RS-232 level translator.

author: nctnico
author_rank: Super Contributor
timestamp: July 21, 2015, 09:10:42 am
content: Quote from: krish2487 on July 21, 2015, 07:09:21 amThat is the problem with using big devices. Their large silicon dies are made for switching, not operating in the linear area. The best devices are the ones intended for use in linear applications.QuoteI don't agree. They make one size fit most solutions.If you use transistors you can create an ideal current source which works much better than any control loop. Ideal for testing power supplies because an ideal current source is very likely to make a poorly designed PSU oscillate. If you make it fancy you can servo-out the temperature dependance using a slow control loop.If you use parallel resistors and switch these using MOSFETs you can create extremely steep pulses, dissipate a lot of heat using a small heatsink + fan and the solution will be extremely robust / reliable.However if the topic starter still wants to go the MOSFET route it is better to buy a readily made DC load.

author: TopLoser
author_rank: Unknown
timestamp: July 21, 2015, 09:24:04 am
content: I could do you a good price on this:https://www.eevblog.com/forum/buysellwanted/fs-%28uk%29-bk-precision-bk8514-electronic-load-1200w-120v-250a/

author: dom0
author_rank: Super Contributor
timestamp: July 21, 2015, 09:27:38 am
content: I suppose by ideal current source you mean the typical common-emitter circuit with constant ib? I measured Ri's of a few dozen k? at low ic ... getting steadily lower with rising ic. Much worse than a good op-amp current source. However the bandwidth of those will (usually) be lower.Or do you mean the classic two-transistor current source? That one is actually very good.

author: krish2487
author_rank: Frequent Contributor
timestamp: July 21, 2015, 10:54:38 am
content: Correct, they do.What I meant was that if you were to pick up any vendor's, any series, any model electronic load - It will always have multiple mosfets sharing the load and not a single one.My entire reply was based onanypedigreed dc load. Even what dave has reviewed and tore down has IIRC 8 or 12 fets sharing the entire 300 watt load.Likewise with the control loops - I meant that each FET has its own op amp,ballast resistor and current sense resistor meant for controlling the drain currentand not a single op amp driving all the FETs (much like what many newbies would end up doing)Quote from: nctnico onTodayat 07:10:42 PM[size=78%].

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 21, 2015, 11:15:31 am
content: As already noted by others, you don't really want to use IGBTs in the linear region (or if you want to be picky, the square law region of the Vgs->Ic transconductance curve). This restriction is also true for modern MOSFETs that are optimized for switching (which is most of them, really); what you want to look for is the word "lateral" in the description of the MOSFET (or that it is optimized for linear/amplifier applications, natch).One other tip is to not parallel lateral MOSFETs directly - use an op-amp and current sense resistor for each one.

author: krish2487
author_rank: Frequent Contributor
timestamp: July 21, 2015, 12:01:37 pm
content: Also, apart from the current sense resistor, it also makes sound sense to use a "ballast resistor" or a current sharing resistor to force equal current through the parallelled mosfets. Ensures that there is no noticeable current imbalance.Quote from: MagicSmoker onTodayat 09:15:31 PM

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 21, 2015, 01:27:51 pm
content: A custom electronic load I designed a couple of years ago (airborne application) used special SOT-227 IXYS MOSFETs designed for linear operation. Each of the six FETs had it's own sense resistor and op-amp control loop. All sensed currents were summed to indicate total current. Two of these modules were paralleled (800W each) and another control loop forced active balancing between the two banks. Both MOSFET banks were electrically isolated (to 2KV) from each other and the control circuitry. This required transfering analogue signals across an isolation barrier with high accuracy and temperature stability. The temp coefficient of the setpoint was the biggest headache.I chose to use a special high power resistor in series with the MOSFET's to reduce the thermal stress on the devices. It also meant that even when the MOSFETs were fully ON the current was limited. I've never had so many control loops in one circuit.Luckily there was PLENTY of air for cooling the massive heatsinks.I discovered that you cannot simply use switching MOSFETs for linear operation if they dissipate appreciable power. They are simply not reliable, hence the expensive IXYS devices.I also discovered that using silicon to dissipate power is expensive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2015, 06:33:58 pm
content: Quote from: MagicSmoker on July 21, 2015, 11:15:31 amI don't know that true lateral DMOS is available anymore, but there are "linear" parts available that still offer the full (power limited only) DC SOA. They're more expensive than regular, amp for amp -- but that's fine, because you'll only ever be able to use them at a fraction of that current anyway; basically, what you're paying for is sheer die area.Apparently, SuperJunction types typically offer full DC SOA too (e.g., Infineon CoolMOS; if the datasheet doesn't say what kind of design it is, check the Coss vs Vds curve: it distinctively drops very suddenly around 20-50V, and rebounds slightly in the 100s V). These have the highest current capacity of any HV part, so I shouldn't think they would be any good at DC. But worth a try I suppose.Tim

author: TheMG
author_rank: Frequent Contributor
timestamp: July 22, 2015, 12:30:28 am
content: Quote from: krish2487 on July 21, 2015, 07:09:21 amI don't remember using the word "revolutionary" anywhere.This is in no way whatsoever intended to revolutionize anything, it's not going to be a product for sale or anything, ever, and I have zero intention in turning it into a commercial endeavor.Simply trying to build something to perform a particular task, on a limited hobbyist budget. Otherwise, I'd go and purchase a commercially made load and be done with it, but the cost of such units is prohibitive.Quote from: Jeroen3 on July 21, 2015, 07:23:27 amSo you're saying none of these large MOSFET "bricks" are any good for linear applications? What sort of problems are we talking about? Difficulty in driving/controlling it properly, or catastrophic failures?Quote from: blueskull on July 21, 2015, 07:25:18 amWouldn't scalability be a problem with such an approach? Might be ok at higher currents, but when testing small power supplies there would be very large and short duration current pulses. Substantial inductance and capacitance would have to be incorporated as filtering in the load itself, but high capacitance attached to the power supply under test might be undesirable.Quote from: dom0 on July 21, 2015, 07:26:00 amI do have a few of those yes but I've also got much larger heatsinks that were removed from old scrapped broadcast transmitters and stuff like that (thanks to the analog to DTV broadcast transition), and the power dissipation in some of those was on the order of several hundreds of watts per power amplifier and heatsink.Being able to dissipate the heat is the least of my concerns. I've done the calculations, and it's not going to be a problem, by a fairly wide margin.Quote from: nctnico on July 21, 2015, 09:10:42 amMy wallet tends to disagree.Quote from: TopLoser on July 21, 2015, 09:24:04 amThanks but that's more than twice what I'm willing to pay. £1100 is currently about $2215 CAD (more likely around $2300 at the bank's exchange rate) plus your "arm and a leg" shipping cost to northern Canada.Far too much for my hobbyist budget.Anyways... I guess I might pick up one of those large MOSFETs and give it a try. If it blows up spectacularly or fails to perform to my expectations I'll only be out about $30.I have in the past jury rigged small single-device 50W loads using switching type MOSFETs. Never had any issues, but maybe it's a different story with the larger "bricks"?Or I suppose I could scrap the idea of a large device and go with paralleled smaller devices. PCB and circuit design would be more complex but I suppose I could do it.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 22, 2015, 03:54:34 am
content: Quote from: AcHmed99 on July 22, 2015, 02:56:40 amThanks for the link, that's a very good read indeed.I'll have to do more research into it, but by the looks of it, MOSFETs designed for high power linear audio amplification could potentially be suitable candidates, as audio applications are very demanding in terms of FSOA limitations and thermal performance.Actually I might even look around a bit and see what transistors some of the commercial electronic loads employ in their designs. Why reinvent the wheel if someone else has already done all the work?Also I guess the good news is that using multiple parallel devices reduces the cost of some other parts, such as the current sensing resistors, which go up in price quite quickly when you get into higher currents, but using several smaller ones is much cheaper. Also, opamps are dirt cheap so giving each MOSFET its own control loop isn't going to be that bad.Oh, and I found a monster heatsink in my collection that is going to give me all the surface area required to mount plenty of devices on!

author: krish2487
author_rank: Frequent Contributor
timestamp: July 22, 2015, 04:05:19 am
content: Quote>[size=0px]I don't remember using the word "revolutionary" anywhere.[size=0px]This is in no way whatsoever intended to revolutionize anything, it's not going to be a product for sale or anything, ever, and I have zero intention in turning it into a commercial endeavor.Simply trying to build something to perform a particular task, on a limited hobbyist budget. Otherwise, I'd go and purchase a commercially made load and be done with it, but the cost of such units is prohibitive.[size=0px]As I ve said before, I apologize if I came across as rude. I did not mean to take a dig at you.My only point was this - MNCs take the tried and tested path for a reason. The commercially available units are optimized heavily - either to a price point or a performance point. Our trying to match up to their performance is going to end up more expensive than simply buying an COTS unit.It is perfectly fine to want to do it ourselves as a hobbyist. Just that there will be compromises on certain aspects.Quote>[size=0px]So you're saying none of these large MOSFET "bricks" are any good for linear applications? What sort of problems are we talking about? Difficulty in driving/controlling it properly, or catastrophic failures?[size=0px]Just this,they cant be used as variable resistors. It is simply a square peg in a round hole. Paralleling as mentioned by me and others in other replies is the way to do it.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 22, 2015, 04:57:12 am
content: Quote from: krish2487 on July 22, 2015, 04:05:19 amOh, there will definitely be compromises. It will most definitely not have anywhere near the accuracy of the COTS units, nor the same response speed in anything but the constant-current mode of operation. It won't have all of the fancy data-logging features and other advanced modes of operation. I don't expect it to come anywhere near what the COTS units can do.But for what I need it for, that will be perfectly fine.Quote from: krish2487 on July 22, 2015, 04:05:19 amYes that does in fact sound like the most sane option now that I have more information on the subject.I actually underestimated how much heatsink "real estate" I have. Looks like I can cram 12 TO-220 devices on each one (individual heatsink and fan for each 200W load). I'd need a bit of space for a temperature sensor and thermal cutoff so realistically up to 10 TO-220 devices. That splits up the power dissipation to a nice easy 20W per device. I've already measured these heatsinks performance at 0.2 degC/W (with cooling fan going full blast). That would give a sink temp of about 65 degC with 25 degC ambient.Note the transistors on the heatsink are not the ones I plan on using. I just got these out of one of my junk bins for a test fit/mockup.Or... I can go with one of these monster heatsinks (and a couple 100 CFM muffin fans) and mount all 6 loads on the same one. That's 12 pounds of aluminum right there. 9V batteries for size comparison. I still have the 3U rackmount chassis that this monsters were mounted in, and have 3 more where that came from. While I have not yet measured these heatsinks I have little doubt one would be able to get rid of 1200W.

author: krish2487
author_rank: Frequent Contributor
timestamp: July 22, 2015, 05:03:28 am
content: The first option is a disaster.It is not only about cooling the device using heatsinks, but also having enough surface area around the heatsink such that the heatsink itself cools.The way the first pics shows the devices setup, I doubt if you can dissipate 20watts total among all the devices.The second option seems fair enough for a total 300 - 400 watts dissipation.Though I suspect it might have to be force cooled using fans.It is a whole different ball game usin heatsinks for switching applications vs linear applications. Most of the power what we are trying to "load" is wasted as heat unlike switching applications. Look at the teardown pics of any COTS eload posted here in the forums. You ll see they use a single heatsink with 4 devices mounted at extreme ends on both sides. and then a fan for force cooling. All for dissipating 150 watts of power.PS: You should really be looking at TO-247 or TO-3P footprints rather than TO-220. The increased die size relaxes the requirement on heatsinking.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 22, 2015, 05:19:21 am
content: Quote from: krish2487 on July 22, 2015, 05:03:28 amThere's more to it than shown in the picture. That is not intended to be a convection cooled heatsink.What I haven't shown is that there's a Delta model BFB1012H centrifugal blower that mounts to that all-copper heatsink (opposite to the device mounting surface). That thing pulls a significant amount of air through those densely packed fins. It gets rid of heat in a hurry, trust me. It's LOUD, but it does the business.As I said, I've measured its performance to approximately 0.2 degC/W (sink-to-air).Quote from: krish2487 on July 22, 2015, 05:03:28 amYou're right. I really had no intention of using TO-220, but I just showed it as an example. I don't have any TO-247 in my junk bin for a test-fit (nothing that a simple ruler can't fix though).

author: krish2487
author_rank: Frequent Contributor
timestamp: July 22, 2015, 05:21:17 am
content: Hmmm, looks like you have thought it through.All the best!!

author: Jeroen3
author_rank: Super Contributor
timestamp: July 22, 2015, 05:29:40 am
content: Are you not going to electrically isolate the heatsink?

author: TheMG
author_rank: Frequent Contributor
timestamp: July 22, 2015, 05:35:58 am
content: Quote from: Jeroen3 on July 22, 2015, 05:29:40 amIf I end up using those small copper heatsinks, that'll be rather easy to do as there is a plastic bracket that supports both the heatsink and the fan. The plastic bracket and fan are bolted through to the chassis, and electrically speaking the heatsink is thus isolated from the chassis and the other heatsinks.If I use the large aluminum heatsink, then I'll have to use mica spacers or silpads for isolation. Obviously, this would degrade the thermal characteristics somewhat compared to direct metal-to-metal mounting with thermal grease, and probably the electrical isolation won't be quite as good.So there is definitely a benefit to using smaller individual heatsinks instead of one or two massive ones.

author: rs20
author_rank: Super Contributor
timestamp: July 22, 2015, 06:01:18 am
content: Quote from: TheMG on July 22, 2015, 05:35:58 amI'm probably missing something, but don't all the MOSFETs in a dummy load have a common drain? And since the drain is the tab, it's fine for them to share and electrical connection to the heatsink?Yeah, the sources need to be kept separate from the separate current sense/ballast resistors, and the gates are independently driven -- but the drains are all connected directly to the + terminal at the input, right?EDIT: Whoops, forgot the bit where the OP mentioned wanting multiple separate channels on the dummy load.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 22, 2015, 06:19:41 am
content: Quote from: rs20 on July 22, 2015, 06:01:18 amYou'd be correct. The issue is that I wish for the six individual load "channels" to be electrically isolated (so that power supplies with a positive and negative output can be tested). So if more than one load/channel is mounted to the same heatsink, then there needs to be some method to isolated them.

author: dom0
author_rank: Super Contributor
timestamp: July 22, 2015, 07:10:51 am
content: QuoteCatastrophic failure. The high current density in vertical FETs leads to the Spirito effect, which will quickly destroy the FET. The only solution is to use FETs rated for DC operation and have a large security margin even to that rating.Alternatively one can use BJTs (which have similar limits with the second breakdown), but the high control power becomes undesirable quickly.Since this is a DC application the thermally best and electronically w/o relevant impact solution is to isolate the heatsinks to avoid the additional thermal resistance of anything electrically isolating but thermally somewhat conductive.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 22, 2015, 09:11:01 am
content: Quote from: TheMG on July 22, 2015, 05:35:58 amTry aluminum oxide insulators instead. Their thermal conductivity is not that bad. Isolation scales with the thickness.With those mosfet bricks you will need a gate driver or you'll kill a lot of them. Those drivers do not support linear usage.

author: rs20
author_rank: Super Contributor
timestamp: July 22, 2015, 11:53:33 am
content: Quote from: Jeroen3 on July 22, 2015, 09:11:01 amWhy? I thought gate drivers were there to speed the mosfets from off to on as quickly as possibly, without drifting slowly through the "linear region". Since we'retryingto be in the "linear region", do we need such strong gate current source/sinks?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2015, 12:06:41 pm
content: Quote from: rs20 on July 22, 2015, 11:53:33 amThat's what he's getting at.Rather than FUD,A close look at the datasheet reveals: nothing, of course. The SOA goes down to 10ms, which is useful (some MOSFETs, and IGBTs especially -- if they give SOA > 10us at all -- are already cutting out considerable SOA by this time scale), but not a guarantee of DC operation.To be sure, you'd have to test it yourself, at the highest voltage you're looking at, and slowly ramp current until it pops.Besides the high cost and poor availability, those Microsemi modules have terrible performance, not only for the old dies used, but the wiring as well: they are built without any regard at all for stray inductance. I can't imagine an application you'd actually want to use them for. They're only slightly faster than similar sized IGBTs (especially recent generation), and those are quite a bit more efficient.You can stand to save even more money by buying commodity level TO-247s and connecting them in parallel (for switching applications, I mean), and get operation out beyond 400kHz easily.Tim

author: Jeroen3
author_rank: Super Contributor
timestamp: July 22, 2015, 12:17:16 pm
content: Those bricks actually contain many paralleled transistors or mosfets. And as you'd probably already know from mosfets, the characteristics differ too much to reliable work in linear mode when the gates are tied.If you look at the Vgs-Id curve of a normal mosfet you see major differences between -40C, 25C and say 60C.If you take a random known-brand brick:http://www.semikron.com/dl/service-support/downloads/download/semikron-datasheet-skm-180-a020-21911460It says in the SOA curve: Not for linear use. Also, they don't have a Vgs/Id curve.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 22, 2015, 12:23:47 pm
content: If you want the current sink to have a decent frequency response then you will still need to drive the MOSFET gates with a fairly strong amplifier because they basically look like big and non-linear capacitors. Also, most op-amps hate driving capacitive loads directly and will complain loudly (in the form of destructive oscillation) if attempted. A quick-n-dirty workaround to both issues is to buffer the output of the op-amp with a push-pull emitter follower but don't include the buffer in the feedback loop. Precision/accuracy suffers but stability should be more than good enough for everything except demanding ATE/pin-driver and laser diode applications.P.S. - Exicon and Semelab both reputedly make true lateral construction MOSFETs; mainly aimed at the audiophile (phool?) crowd.

author: tomizett
author_rank: Newbie
timestamp: July 22, 2015, 12:54:35 pm
content: Have you given serious consideration to BJTs? I'd imagine there are more choices for BJT designed for this kind of application than there are of MOSFET. Granted, you need to worry about substantial base drive, but a three stage Darlington should be sufficient and shouldn't give you too much trouble with stability given that you've no ambitious requirements on bandwidth.As we've already discussed, the ideal FETs would be the big lateral devices made for audio like the BUZ900 - you can still get them, but they're expensive. Perhaps you could be lucky enough to find a large MOSFET amplifier being sold as faulty to be a donor of parts?I reckon that BJTs might well prove to give more options and be more cost effective though.Just my 2p worth anyway!

author: TopLoser
author_rank: Unknown
timestamp: July 22, 2015, 01:12:34 pm
content: I've got this damn thing on my desk, shall I gently open it up and see what's inside?http://uk.farnell.com/b-k-precision/bk8514/dc-load-electronic-120v-240a-1200w/dp/1799870

author: c4757p
author_rank: Super Contributor
timestamp: July 22, 2015, 01:24:52 pm
content: Quote from: tomizett on July 22, 2015, 12:54:35 pmOr a single FET as the base driver, no need for multiple stages. Almost a simulated IGBT, if you will.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2015, 04:35:29 pm
content: Quote from: Jeroen3 on July 22, 2015, 12:17:16 pmThis issue is pertinent to switching as well -- you want all the gates to turn on at the same time, otherwise one hogs all the switching loss and explodes. So they always use matched dies in these modules. The thermal matching is also quite good (better than you can do between discrete devices on a heatsink, because they're bonded in common to a massive copper plate), so that their linear performance -- assuming you wished to sacrifice few of these monsters to verify their SOA -- shouldn't be too bad.IGBTs are sensitive to imbalance and tempco in saturation as well, but they are also assembled from matched sets (and designed with enough internal resistance) that this isn't a problem for switching modules.QuoteBizarre, they give a DC SOA, which contradicts their immediate statement. Which one did they mean?Tim

author: dom0
author_rank: Super Contributor
timestamp: July 22, 2015, 05:11:51 pm
content: QuoteThe most likely cause of a faulty amplifier is destroyed output devices, so that's probably a waste of money.

author: tomizett
author_rank: Newbie
timestamp: July 22, 2015, 09:04:27 pm
content: Very true, although it's not normally all of the output devices which have failed... it's a gamble though, so I really only meant if you could get one dirt cheap.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 23, 2015, 05:31:40 am
content: Hmmm those linear MOSFETs sure don't come cheap.But, it looks like I can get away with using switching MOSFETs if I take it easy on them and spread the power dissipation over several devices taking care to leave a large safety margin in terms of dissipation and temperatures.Actually, I had another look at the teardown Dave did of a BK Precision 300W load. They're using IRFP250N which from the datasheet looks like a fairly ordinary switching MOSFET. No DC FSOA curve given in the datasheet.They are, however, using 8 devices to dissipate the 300W, thereby limiting dissipation to 37.5W per device. If there was a published DC FSOA curve available... they'd probably be under it by a wide margin.

author: krish2487
author_rank: Frequent Contributor
timestamp: July 23, 2015, 07:06:05 am
content: Actually you can push the power dissipation to twice that without breaking a sweat.toploser did a teardown of the 1200 Watt version here :https://www.eevblog.com/forum/testgear/bk-precision-bk8514-electronic-load-1200w-120v-240a-teardown/They use 16 devices total for 1200 watts which means 75 watts per device. But yes, huge ass heatsinks with lots and lots of surface area are a must.

author: TopLoser
author_rank: Unknown
timestamp: July 23, 2015, 08:08:48 am
content: Quote from: krish2487 on July 23, 2015, 07:06:05 amActually there are 32 of the devices in the 1200 watt load, 37.5 watts per device. Same kind of rating as the 300 watt version of the load.

author: oldway
author_rank: Super Contributor
timestamp: July 23, 2015, 09:41:17 am
content: In my opinion, its a wrong solution to dissipate so much power in semi-conductors...It should be shared with dissipation in high power resistors.Low cost high power resistors of low ohms value can easily be make with Inox MIG welding wire.

author: krish2487
author_rank: Frequent Contributor
timestamp: July 23, 2015, 10:04:12 am
content: Quote from: oldway onTodayat 07:41:17 PM

author: Siwastaja
author_rank: Super Contributor
timestamp: July 23, 2015, 10:11:48 am
content: Quote from: oldway on July 23, 2015, 09:41:17 amIt can be quite complex to design a system which would dissipate considerable amount of power in the resistor over large range of voltages. Sure, for a fixed operating point, even a resistor-only solution works, but for everything else, the broader the operating range, the more the semiconductors are taking the hit, because you need to design it so that the resistors won't start limiting the current at any setting (FETs fully on).Price for silicon dissipation has also dropped and continues to do so, while the cost for resistive elements stays the same or even goes up. So it's important to confirm the ideas using today's numbers; for example, ten-year-old data is useless.The same goes for the cost of aluminum vs. silicon; by increasing die area (by properly paralleling larger number of devices), the die-to-case and case-to-heatsink thermal resistance decreases, which in turn allows the designer to increase the heatsink-to-ambient thermal resistance, saving weight and cost in heatsinking. This idea would have been ridiculed just 20-30 years ago. Although, I have to say, the difference is notthathuge in linear applications; this remark is more important when it comes to a typical switching application. But anyway, in 2015, don't even try to get maximum power from those semiconductors; just put in more than you absolutely need; it'll make everything easier.

author: oldway
author_rank: Super Contributor
timestamp: July 23, 2015, 10:27:57 am
content: QuoteIf you associate power resistors with semi-conductors and regulation, any variation of the resistance does not matter as it is compensated by the regulation...You don't need nichrome, inox wire is good enough.Quotewith association of semi-conductors and power resistors, there is no steps, it is a linear adjust as with semi-conductors only.QuotePower dissipated by semi-conductors or by power resistors has the same effect, then this is nonsense.QuotePower resistors can reach higher temperature than semi-conductors, then it use more reduced space.QuoteYou think about power resistors bank without electronic control and without association with power semi-conductors...I mean about sharing power dissipation between semi-conductors and power resistors, not to use only power resistors.QuoteAbsolutely not...Able to do CC, CV, Cr as well as an electronic load with only semi-conductors.

author: krish2487
author_rank: Frequent Contributor
timestamp: July 23, 2015, 10:45:38 am
content: My bad, I went off on a tangent ranting about only resistor banks.However,I mentionedventilationand notpower dissipationwrt heaters.The Power Devices along with the heatsink singularly act as a huge variable resistor. The key point being such extremely huge heatsinks are used to increase the surface area for heat dissipation. A similar rated heater or resistance(s) will have a smaller area.This allows for a more power dense configuration vis-a-vis heater including a fan for blowing air.[size=0px]Power resistors can reach higher temperature than semi-conductors, then it use more reduced space. You lost me here.Dissipate 300 watts in a COTS Eload vs resistance/heaters, then measure which occuppies lesser volume while maintaining reliability and being safe.[size=0px]You think about power resistors bank without electronic control and without association with power semi-conductors...I mean about sharing power dissipation between semi-conductors and power resistors, not to use only power resistors.Again, trying to dissipate equal power in semiconductors and resistances will lead to an increase in volume of the enclosure, and at best is a waste of space while requiring additional forced cooling requirements.

author: oldway
author_rank: Super Contributor
timestamp: July 23, 2015, 10:51:30 am
content: For example, this is a 100W 1R7 power resistor home made with 0.8 Inox welding wire...price = peanuts !

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2015, 11:08:59 am
content: Quote from: Siwastaja on July 23, 2015, 10:11:48 amNot really. Pretty simple given some thought.I did the design for these guys,http://www.bumblebeebatteries.com/it's not available yet, but send them an e-mail saying "I heard you guys are working on a battery discharger / DC load dump" and you can probably help nudge it alongTim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2015, 11:13:41 am
content: Quote from: oldway on July 23, 2015, 10:27:57 amPrecisely. If everything's in a loop, you don't much care about what's happening, as long as the total is correct. And you can verify that based on whether the loop is in the linear range or hitting the rails.Our design saves cost and size by minimizing heatsink area and maximizing resistor use.Power resistors can run considerably hotter than semiconductors (250C+!), so the area required for dissipating that heat is much smaller.Resistors are more expensive than power transistors, but far cheaper and smaller than the heatsinks, and thermal interface materials, and mounting hardware, and machining and assembly time, required for a purely transistor regulated solution.Tim

author: oldway
author_rank: Super Contributor
timestamp: July 23, 2015, 11:18:25 am
content: This is my idea about the solution (dynamic load 30V 10A but can be applied for greater power):Control would be quite simple:From the current request information, first would T7 became conducting (linear mode).If current is not enough, T7 stay fully conducting and T6 became conducting (also linear mode) ...and so on...Must have a current and power limitation.A relay must open input if voltage is higher than max voltage.EDIT: T6 on schematic is also a TIP35C, not a TIP32C

author: krish2487
author_rank: Frequent Contributor
timestamp: July 23, 2015, 11:46:41 am
content: @oldwayThe schematic looks interesting.I am assuming you intended to put a resistance even for T1??Secondly, why switch on one transistor after other?? and why not FETS??FETs can be made to thermally balance the current. BJTs will cascade and breakdown.Why not share the current simultaneously among all of them?Even with FETS,why share load only after one FET is likely operating near the end of our "comfort zone"?Third, The resistance you showed. Granted, I am nitpicking here - how repeatable is it?And how did you arrive at the power rating of the resistance? Stay inductance, capacitance, tolerance, tempco?

author: oldway
author_rank: Super Contributor
timestamp: July 23, 2015, 12:24:15 pm
content: Quote from: krish2487 on July 23, 2015, 11:46:41 am

author: krish2487
author_rank: Frequent Contributor
timestamp: July 23, 2015, 12:30:46 pm
content: I stand corrected. This is what happens when I dont pay attention to the finer details of the schematicIt really is a neat way of implementing eloads.The last part of my question really was about the 100 watts 1E7 resistor. How did you arrive at the power rating, resistance value etc.[size=78%]

author: oldway
author_rank: Super Contributor
timestamp: July 23, 2015, 12:42:46 pm
content: This homemade wound resistor with 0.8mm inox wire can withstand 5A with natural cooling. (experimental value, the resistor is not red hot with this current).With forced cooling, 100W (7.67A) is on the low side.

author: digsys
author_rank: Unknown
timestamp: July 23, 2015, 01:02:31 pm
content: Check out -https://www.eevblog.com/forum/testgear/heat-sink-in-bk-precision-8500-electronic-load-teardown-eevblog-281/msg582515/#msg582515and pix at -www.pbase.com/digsys/eevblogThe MJL1302A / 2STA1943 Power transistors have a pretty much flat hFe of 100+ across most the Ic range. They are designed for Class_A amps.With 12 + 12 TO247 packages, 0.33R 3W ceramic Emitter resistors AND the special thermal pads, each device has incredibly low heat for what it's pushing.As I say, I made these years ago, and have pushed 2.5KW peaks. Next year I intend to slave 10x units to do 20KW peakIf you want details of the LV driver circuit, let me know

author: BravoV
author_rank: Super Contributor
timestamp: July 23, 2015, 01:13:42 pm
content: Quote from: digsys on July 23, 2015, 01:02:31 pmPlease, I'm interested.If possible the whole circuit, and make another separated thread for it.TIA

author: digsys
author_rank: Unknown
timestamp: July 23, 2015, 01:58:27 pm
content: Quote from: BravoVHiya old mate. Those electronic bridge controllers are still one of my most favourite ICS :-) and we go through 100sAs per ccts, The "ideal" part here is the TS922ID dual 80mA R/R IO LV op amp. If you supply it at around 3.0V (or less), you have very little heat / loss.The Power transistors only need app 1.5V max and can drive down to 0.8V. Make sure you have at least 2KV isolation between the driver side andnetwork / bus side. Those transistors go to 250VDCYou could have feedback on every transistor, but with the 2x thermal sensors on board, I only get app 5% variation MAX, which is fine by me.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2016, 12:26:32 am
content: Bump!For anyone doing battery conditioning or high voltage load testing, this should be of interest:http://hybridautomotive.com/collections/all/products/prolong-intelligent-dischargerDisclosure: I designed the guts, so...Tim