High power (~few kilowatts) DC/DC converter - Page 1
Forum section: electronics / projects

author: jaromir
author_rank: Unknown
timestamp: February 16, 2018, 10:31:09 am
content: Hello,I'm working on a project (physics experiment) where I need to make bigger DC/DC converter than I'm used to.I have roughly 100V DC from massive lead-acid battery, now I need to generate 40-60V, digitally adjustable, with current demand roughly 50A. I can expect the thing to scale up, so perhaps more like 100A in future revisions. The output doesn't need to be isolated from input.Linear regulator is probably out of question, so PWM buck regulator seems like obvious choice. My knee-jerk reaction was venerable TL494, with massive MOSFET, in configuration like thishttps://snag.gy/R8vKgS.jpg- with NFET and isolated driver (TLP250) to achieve higher gate potential than drain has.This simple configuration is probably not exactly good for high power output. I assume high losses on diode, at least few tens of watts. Schottky diodes for higher voltages are not exactly common, and usually exhibit forward voltage similar to fast silicon rectifiers.TL494 seems to be able to proper drive half-bridge with two transistors - that should bring the losses down, somehow.Output choke has to be probably massive too, I can't find many read-to-go chokes in this current range, so I probably need to order custom one.Is that a good way? Is there a better way to do this?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 16, 2018, 11:01:38 am
content: For a one-off build it makes no sense to roll your own. You want to be doing physics, not dicking about with DC/DC converter design.Some mains SMPSUs would probably do it 80VAC ( =113VDC) is a fairly common minimum input, so not far off.e.g. a quick search finds this, which is pretty close - more searching with more specialist PSU distributors will probably find something closer to what you needhttps://www.digikey.co.uk/product-detail/en/delta-electronics/IMA-S1000-48-YYPLI/1145-1208-ND/581888848V is a standard input for DC/DC converters, so a pair of these with inputs in series may be an option.

author: jaromir
author_rank: Unknown
timestamp: February 16, 2018, 11:22:35 am
content: I'm not doing physics, I was asked to do the converter. This project is after "one-off quick prototype" phase.The need for adjustable output (at least 40-60V) is vital. It's part of bigger setup (digital control, measurement of sensor values, logging, etc...) I can do those things on my own, but the big DC/DC is out of my usual experience, so I asked here for general directions.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 16, 2018, 11:33:13 am
content: Quote from: jaromir on February 16, 2018, 11:22:35 amMany off-the-shelf PSUs and DC-DC converters have output voltage control, some digital. I'd be surprised if you can't find either one or a combination of multiple off-the-shelf units to do this job

author: max_torque
author_rank: Super Contributor
timestamp: February 16, 2018, 01:07:13 pm
content: IME,"one-off quick prototype"does not go hand in hand with"kW power DC/DC"and"No previous experience"Buy something off the shelf., It'll be cheaper, better, and mean you can concentrate on doing the stuff where you can add value to the project.......

author: Siwastaja
author_rank: Super Contributor
timestamp: February 16, 2018, 02:00:34 pm
content: Quote from: max_torque on February 16, 2018, 01:07:13 pm... unless, of course, you want to learn to do these things. There's no other way to gain that experience.The project goal is realistic, for example, done as a multiphase buck converter, using any modern current-mode controller IC (TL494 doesn't offer the same level of robustness).This being said, when I was in a similar situation the first time, I just did it - had I been writing to forums like this, I would have got the same "don't do it" reply from everyone.It's easy to dismiss project opportunities like this, and say: learn when you have "free time" etc. blah blah. The truth is, for many of us, weneeda real project, with a real timeline, with someone who trusts you and with whom you can discuss, to get it going. It's going to be a bit stressful to learn doing something new when results are expected in little time, but I don't regret working like that a least bit. Of course you need to keep realistic and have some understanding of your skills, but how can you know if you never try?I promised my first actual medium power DC/DC converter system to be finished in one month, and it took two. Off-the-shelf product cost was about $200k. My time was worth almost nothing at that time, and BOM was around $500. Take these chances, and learn.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 16, 2018, 02:04:45 pm
content: If the object is learning, fine, but it will take an interterminate amount of time and cost a lot more than an off-the shelf solution.The OP appears to be looking for a solution and has little experience, so it seems that getting something working is probably more important than learning to produce a lot of smoke in the process of maybe getting something working.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 16, 2018, 02:16:05 pm
content: My point is, this kind of experience really comes handy later.It's always the same."We need this specific interesting thing. Oh, it's a one-off""Ah, a one-off, then it's not economically viable to learn how to do it.""Hey, we need this same kind of thing! It's a one-off""Ah, a one-off again, then it's not economically viable to learn...""Hey, we need this product designed, it's a nice three-year project timeline, well paid, well managed.""Ah, sorry, I don't have the experience required."And so on.Guess what, just by simply saying "YES!" to some challenges, just ahandful of times, I have reached a situation where I can pick any interesting project I want because:1) I have gathered so much experience there's no need to learn everything from scratch anymore2) I know what I can and what I cannot learn, and I can generate a pretty good estimate on how much time it takes, so I can safely pick up new challenges, as well.And yes, you'll surely sometimes fail, and sometimes can't deliver, and end up giving overoptimistic schedules. And sometimes that sucks. But guess what, again - this happens all the time, even in professional settings where everybody's very qualified.

author: ale500
author_rank: Frequent Contributor
timestamp: February 16, 2018, 02:29:32 pm
content: Keysight 6692A 0..60 V up to 110 A.The questions you should be asking yourself are more like:Which efficiency do we need ?What short circuit behavior should it have ?Which is the admissible ripple the load can tolerate ?Which load and source regulation do we need ?And things like that.And not: with this bunch of components can I get something that produces 2kW @ ~ 50 V ?...

author: jaromir
author_rank: Unknown
timestamp: February 16, 2018, 02:46:41 pm
content: For this project I may use some off-the shelf modules, that is good suggestion.On the other hand, I considered this as good opportunity to learn something new. I'm not particularly time constrained, but I lack experience building higher power SMPS. That being said - I'm not completely inexperienced in power supplies and electronics in general. Apart from quite a bit of low-power (less than 50W) SPMS in past, I've built 350W (35V/10A) adjustable buck converter in past with two MOSFETs in half-bridge, regulated by fast PID on DSP.Taking output power one order of magnitude higher probably brings a lot of usual devils in details. That's why I asked here for directions.Quote from: ale500 on February 16, 2018, 02:29:32 pmThe efficiency requirements are not strictly dictated. The better efficiency, the less heat to dissipate.Quote from: ale500 on February 16, 2018, 02:29:32 pmCurrent limit to - say - 50A. After defined time in CC mode (few seconds), power supply should shut down.Quote from: ale500 on February 16, 2018, 02:29:32 pmAgain, no strict requirements, anything under few volts will be just fine.Quote from: ale500 on February 16, 2018, 02:29:32 pmAlmost pure resistive load, with requirement to keep within a few percent of set voltage.Thanks for asking those questions. I'll check also the Keysight power supply you pointed out.

author: daqq
author_rank: Super Contributor
timestamp: February 16, 2018, 02:55:35 pm
content: Well, you could build a load of small units and then hook them up parallel. Ask what's the biggest converter unit that you can make with mundane parts, then make a lot of them and tie them together in parallel. You might end up saving quite a lot.Or, if you have access to a lead acid battery (in particular, if you can access the individual cells), the simplest you can do is connect between the ground and to the cell that's nearest to your desired voltage. That's a coarse regulation, any fine adjustments could be done using even a linear regulator.Or, if you want an off the shelf-ish solution, take a peek at Vicors offering - they have awesome modules that can be connected in many ways.http://www.vicorpower.com/dc-dc-converters-board-mount/high-density-dc-dc-converters

author: max_torque
author_rank: Super Contributor
timestamp: February 16, 2018, 04:48:34 pm
content: I'm all for learning, and pushing ones design and skill boundaries. But i'm also in favour of getting the job done. If the project is to develop a DCDC then great, learn away, but if it's just an obsticle in the process of doing something else, and especially if it could de-rail that main project if it fails to work as expected, then no, buy something in!!IME, one thing most engineers are very bad at is learning which battles to fight, and which to run away from....

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 16, 2018, 05:25:40 pm
content: Quote from: daqq on February 16, 2018, 02:55:35 pmYou'd want to do series, not paralell, as you need specific paralleling/balancing support on the converter, whearas any (isolated) converter can be run in series.

author: nctnico
author_rank: Super Contributor
timestamp: February 16, 2018, 05:36:52 pm
content: Quote from: mikeselectricstuff on February 16, 2018, 05:25:40 pmParallel is the way to go. That way you can create a multi-phase converter with reduced ripple as a bonus. That is pretty much a standard approach for high current DC-DC converters. Putting converters in series adds a lot of complexity due to needing isolated outputs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2018, 05:40:39 pm
content: Don't use a voltage mode controller. Use current mode.You must sense inductor current.This is easy enough with a TL494. You will have a hard time sensing shunt resistors on top of 50V, but you don't want to anyway: use current transformers. One in series with the transistor, one with the diode.To measured "pulsed DC" with a CT, you connect a diode in series with the secondary, so that forward current is directed into the burden resistor, while in the off state, the voltage is allowed to freewheel. Since the currents are exactly interleaved (diode conducting when transistor is not), you can use the same burden resistor, the currents sum, and voila: you have effectively transformer-coupled the DC inductor current.Take this current, filter it slightly (it will be noisy, because the currents don't perfectly sync up), and use one of the TL494 error amps to regulate output current.Add another op-amp (say TLV2371 or TL431) to set the current setpoint, based on voltage feedback. In this way, you get regulated voltage output, from an inherently current limited system, that is inherently stable (because the current loop is only concerned with the inductor time constant, and the voltage loop is concerned with the filter capacitor and current loop time constants), with fast reaction to source and load changes.Finally, add logic to disable the TL494 (on a per-cycle basis, or maintained) to implement desat detection (kind of optional), UVLO (preferably including high side gate drive UVLO), current limit timeout, and external faults.Speaking of gate drive, a bootstrap driver is acceptable here, or you can build one fully isolated, using optoisolators (or other logic isolators -- there are many options these days, with reasonable performance) and a DC-DC converter.You can probably get away with handling full power with a single transistor and diode. It will be easier with two or three in parallel. Bonus points for duplicating the current loops, so that each inverter pair has its own inductor, current sense and current control, and all those channels (which are transconductance amplifiers: voltage in, current out) are driven from the same common voltage error amp. Multiple channels has the advantage that you can synchronize each one to a different phase, so the input and output ripple partially cancels out. Two phase would be particularly advantageous for a ~50% duty cycle application.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 16, 2018, 05:41:25 pm
content: Quote from: nctnico on February 16, 2018, 05:36:52 pmI was talking in the context of using off-the shelf converter modules or SMPSUs, not designing from scratch.

author: jbb
author_rank: Super Contributor
timestamp: February 16, 2018, 08:15:29 pm
content: When you say 100V in, what's the actual range? Lead acids sometimes need to have a balance charge of up to around 2.4V per cell, and then there should be some consideration for surges and cable inductance. I seem to recall 110V banks being a quasi-standard (55 cells, so max terminal voltage around 132V)...Quote from: jaromir on February 16, 2018, 02:46:41 pmNothing wrong with a module - could save you a lot of time and trouble.Quote from: jaromir on February 16, 2018, 02:46:41 pmHow did you find that experience? Was it 'this is interesting' or 'kill me now?' I'm guessing you deployed a gate driver chip, synchronous rectifying FET and current sensor? Do you feel happy with how the response turned out in the end?A DSP controller would be reasonable for a converter this size. If you do decide to try DSP control, I suggest you allow for at least 2 phases (perhaps 3 or 4), each phase shifted for interleaving, and use isolated gate drivers to avoid the pain of ground loops.Quote from: jaromir on February 16, 2018, 02:46:41 pmDoes the current limit need to be very accurate, or just a rough limit?The LTC7810 is a new part from LT. It's a 150V in, 60V out (absolute max) two phase synchronous buck controller. It supports interleaved operation right out of the box (very good for reducing ripple). Could be a good fit for this problem.

author: jaromir
author_rank: Unknown
timestamp: February 16, 2018, 08:20:08 pm
content: Quote from: T3sl4co1l on February 16, 2018, 05:40:39 pmThat is great suggestion, thanks for that!Quote from: T3sl4co1l on February 16, 2018, 05:40:39 pmDidn't think of desaturation detection. How much is this optional?Though it's relatively simple to implement with few passives, comparator and flip-flop, I found drivers with this feature, like TD352. Is this as simple as wiring the driver for desaturation detection?Quote from: T3sl4co1l on February 16, 2018, 05:40:39 pmConsidering I'd use single diode and transistor, how to choose its ratings? When I did previous low-power designs, I just used devices that had a lot of headroom, it wasn't that much of problem for a few amps. Here, things are getting a bit more complicated.For transistor, I can find 200V/200A/5mOhm MOSFETs in TO-247, also screw mount SOT-227 with similar ratings. My gut feeling tells me to use the SOT-227 device - as the TO-247 package just seems just too small. Perhaps two of them to get the headroom, though one may do the job on itself. Should I search for devices with more headroom?What about switching frequency? Higher brings more switching losses, but I can save on inductor and filtering. My first try would be roughly 30-50kHz. Should I aim higher, or lower?Thank you for your detailed and informative response, it was really very helpful.

author: jaromir
author_rank: Unknown
timestamp: February 16, 2018, 08:41:35 pm
content: Quote from: jbb on February 16, 2018, 08:15:29 pm8x12V batteries, containing 6 cells each. That is 48 cells by 2,4V max, 115,2V. The voltage doesn't drop under 90V and usually keeps under 110V.Quote from: jbb on February 16, 2018, 08:15:29 pmYour guess is right. IR2110 was gate driver, dsPIC30F2020 did the PWM drive and PID control synchronous with PWM switching frequency (~700kHz). Current was sensed via resistor with amplifier.At first I didn't have properly rated transistors, so it was horrible experience. After blowing two pairs of relatively cheap transistors I got it right, it was fun afterwards, no more components were harmed. That was like 10 years ago.Quote from: jbb on February 16, 2018, 08:15:29 pmIt doesn't need to be accurate, just to prevent supply and its surrounding from doing too much harm when something goes wrong.Quote from: jbb on February 16, 2018, 08:15:29 pmThat is interesting part. The maximal output voltage is a bit on the edge, though; thanks for the tip.

author: jaromir
author_rank: Unknown
timestamp: June 09, 2018, 09:05:24 pm
content: Returning back to this topic - after a long hiatus I finally made the converter using IXGN200N60B3 IGBT as switcher, using IDW100E60 as freewheeling diode, followed by massive 50uH/100A choke and 2.2mF/100V capacitor. There is microcontroller on board, sensing the output voltage and setting output PWM via P-I voltage regulation loop (and overcurrent fault mode). PWM signal is driven to IGBT via TLP250 driver and floating 15V supply. I added current transformer and comparator with reference voltage to shut down PWM once the choke current reaches critical value.Everything worked on a first try, I had no problems with regulation from zero to almost maximal input voltage. I ran it at 30 Amps at 60V into 2 Ohm resistor load - that is 1,8kW; apparently being limited more by load resistors than switcher itself. No semiconductors fried by now.The only problem was test load able to dissipate such amount of power. Now I have stacked four 500W resistors in huge aluminium cases and big fan providing airflow - that is barely enough for a 10 minutes of running time at 30A. Hot weather doesn't help much, too.It was far easier than I expected, thanks to your input here, guys.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 09, 2018, 10:12:10 pm
content: Quote from: mikeselectricstuff on February 16, 2018, 05:25:40 pmActually, just never parallel modules, even if they support it. I learned that the hard way. I have some Keysight N6700 mainframes that support multiple modules. These are supposed to be super high end power supplies. We put two modules in parallel, which is an explicitly supported configuration. It turns out that rather than having the two slaved modules run with the same PWM duty cycle with a single feedback loop, they just parallel them and use the same programming voltage. This means that at low load all the power is handled by one module (actually, the second module *sinks* some significant current). If you have a surge current that crosses the load limit of one PSU, it will run into current limiting for *milliseconds* causing a huge voltage drop until the other supply kicks in to make up the difference. I contacted keysight and they said basically that this is how they work, and that when paralleling modules you should not run them below the current for a single unit. Which is incredibly stupid. _incredibly_stupid_.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 10, 2018, 10:46:02 am
content: Quote from: ejeffrey on June 09, 2018, 10:12:10 pmJust a good example that a well-regarded high-end brand product can be total and utter broken-by-design shit with faked specifications. It should be simple: if you don't have a feature for paralleling that actually works, don't claim you have one.Similarly, I have learned to question any marketing claim or datasheet value, regardless of the reputation of the brand.My advice is to request your money back every time this happens. I do it with the broken Chinese Ebay items, and hell yes, there is even more reason to do it with the real brands when they only deliver the Chinese Ebay experience.