Help with power electronics - Page 1
Forum section: electronics / projects

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 04, 2015, 04:22:41 pm
content: Hi,I'm looking to start a project where I would require a adjustable power supply from around ~100 V to ~1KV (from ~600V 3 phase rectified). The upper voltage limit is 1.2 KV where the transistor will crap out. The prototype converter should be able to handle ~20 KW. The ripple is not important (tho I would not go for a thyristor + transformation approach), however the efficiency is. Also the polarity of the output is not important and it doesn't have to be isolated. I have worked with a buck converter ~ 15KW before.My questions are:- Any body know any good books on the subject. I have found some linear/ analgo devices datasheets for design of ~15V at ~2 A switchers. I have a feeling that it does not scale up to 20 KWMaybe some /wide/ output voltage/efficiency graphs.- I'm looking at the buck-boost converter and SEPIC converter. Does anybody know of any other topologies that I should take a look at?- Would I be better of first using a 60 Hz transformer to boost to ~1KV and then just use a buck for down converter? (cost/efficiency)- I'm also looking at a isolated full-bridge topology, but I don't think I can achieve the voltage swing I need (the efficiency would be bad and ripple would be large)- right?- Would building a scaled down test model help me at all? With the buck I have build, I had a problem that only appeared at 600V impute voltage and I fixed it by rewire the power part of the electronics.(Got ride of some interference.)Any thoughts are appreciated.P.S. All above voltages are DC.

author: oldway
author_rank: Super Contributor
timestamp: September 04, 2015, 07:50:31 pm
content: 3 phase breaker / 3 Phase variac / 3 phase transformer with fuses / 3 phase bridge rectifier

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 04, 2015, 09:12:28 pm
content: Thats a lot of copper and iron = expensive, but the main problem is that I need to adjust the output DC voltage quickly as part of regulation. So a switch mode power supply is a must.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2015, 10:06:21 pm
content: I'd suggest,Input PFC (optional)600-700VDC link (or ~800 if PFC'd)Multiphase Forward ConverterAverage current mode controlEach phase designed as a transconductance stage, so they can be paralleled easily.Voltage error amplifierAdjustable current/voltage limit (optional)Assorted safety/management/startup/interface controlsFor each phase: IGBT H-bridge, probably phase shift PWM, or a quasi/resonant configuration (e.g., LLC)Consider using snubbers to save on switching loss and EMIQuasi-resonant / lossless snubbers may be of interest as wellFor extreme efficiency, use huge MOSFETs (CoolMOS or SiC) and synchronous rectification. For even extremer: consider inductive gate drive ("lossless", minus internal gate resistance that is).I'd suggest making a module of modest capacity (2-5kW?) and stacking them. Bring along a sync signal, so they can be phase shifted, just like cylinders in a V8 engine (or whatever).Good luck, and don't kill anyone. Arc flash is nasty. Respect it.Tim

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 05:46:20 am
content: Quote from: MarkoAnte on September 04, 2015, 09:12:28 pmThe first thing to do before you start a project is to give the product specifications.Your description contains no serious specification.NB: if isolation is not required, transformer may be a low power auto-transformer to increase voltage up to DC 1KV (rectified)It will be a low impedance power supply and regulation is perhaps not needed.You can use a 3 phase motorized variac to make the regulation.The great advantages of this solution are its simplicity and reliabilityI have used a motorized three-phase variac of 120kVA for testing transformers and busbar.1KV 20KW variable output power supply is a professional project.If you need to ask help on a forum, you better not to try to deal with such voltages and powerFrom another topic "Hi, what would you suggest for a first Spectrum Analyzer"You said:QuoteThat's not a project for a student nor a hobbyist, it is far too dangerous !

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 10:47:29 am
content: @T3sl4co1lPFC would be nice, maybe as a later upgrade. I afraid I don't follow you completely with the Multiphase Forward Converter. Is this what you meant?Do you mean I should make (e.g.) 4 5 KW forward converters, with the transformer designed so that at ~90% duty cycle it would give 1KV. Then parallel them, phase shift their PWM so that the ripple is improved whilst making sure whit current sensing that all are equally loaded?How do you make a forward converter with a H-bridge? Wouldn't that just make it a h-bridge converter (or a resonant, zero voltage || zero current switched converter).@oldwayI did not give serous specification because I don't have the whole project worked out 100%. And a lot can happen in 2 years. Finished my studys, worked for 1 year. When I worked I made a induction heater from 0 and I have worked whit multiply Mhz induction heaters (using power tubes rated @12KV and @200KW) and developed a process of matching impedance of 3 different LC resonant circuits to minimise heating of components. So 1KV @20 KW.It doesn't matter if its 600V at 0.5KW or 1000V at 20KW. You have to respect it the same way. E.G. Powering it up with a variac, form a safe distance after you have finished a voodoo dance to the goods of power electronics.

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 01:06:12 pm
content: QuoteThis is nonsense...You should at least have previous specifications...Or you don't know what project it is ?QuoteYes, it does matter...Both are dangerous but there are things that happen with high energy circuits that does not happen with lower one.Questions you asked in your first post prove that you don't have knowledge nor experience to deal with such a project...but after all, if you want to risk to kill yourself, that's your problem.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 01:13:59 pm
content: Don't worry I put my safety squints on and have my dear old mother on speed dial. So its k.

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 01:35:34 pm
content: Quote from: MarkoAnte on September 05, 2015, 01:13:59 pmYou have been advised, in my opinion, you are irresponsible and unconscious.At a minimum, you should specify:- 20KW power is the maximum power (1000V 20A to 100V 20A) or it is the power for all output voltage levels (1000V 20A to 200A 100V)?- Current limiting? Adjustable?- Control: Static: How much % Dynamic: special requirements?- Minimum eficiency at full power ?- Mains current: special requirements? Power factor ?, harmonics ?- Dimensions? special requirements?Why not use a SCR 6 pulses bridge with an auto-transformer ? It would be a very simple project.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 02:37:47 pm
content: - 20KW power at all output voltage levels.- Adjustable voltage, current limited only as a fault condition.- Adjustable voltage form 100V to 1KV. No special requirements on ripple on the current or voltage. Also the speed of regulation is not super important but it has to be faster then a motor controlled variac.- Minimum eficiency at full power - hard to define as depending on the conditions it may draw 20KW at 100V or at 1KV. So that the efficiently will be all over the place.- Mains current no issue will put a mains filter in before the converter.I don't like the idea of a 6 pulsed bridge as I have made it and the efficiently and the ripple on the current or voltage are terrible at eg 100V. Also to filter it you need big metal chokes whit large caps. With the buck converter I build I was way happier.

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 05:19:08 pm
content: QuoteThat make the project almost impossible to realize with SMPS technology, as output circuits will have to be sized for 200A !Ferrite transformers and inductors will be very big.I would do this with a 3 Phase transformer with primary triangle, secundary hexaphase (or zig zag) with neutral with 3 taps and 3 x 6 SCR's all with commun cathode.Lower Taps and 6 associated SCR's would be phase controlled from 100 to 400V with 200A current limit, the mid taps and 6 associated SCRs would be phase controlled from 400V to 700V with 50A current limit and the higher taps with associated 6 SCR's would be controlled from 700V to 1000V with 29A current limit.For protecting primary (as power of 20KW may be exceeded in some conditions) I would provide a 3 phase thermal trip in the primary.I do not recomand to make a not isolated power supply with such a high voltage.Output waveform will stay very good even with 100V.When SCR's from mid tap begin conducting, scr's from the lower tap stay fully conducting, and when the scr's from higher tap begin conducting, all other scr's stay fully conducting.Current information coming from TC's.Lower tap: 3 TC's 200/1A (each TC with 2 Wires from 180° phases passing in opposite direction to generate AC information)Mid tap : 3 TC's 50/1Ahigher tap : 3 TC's 50/1A

author: DanielS
author_rank: Frequent Contributor
timestamp: September 05, 2015, 06:24:10 pm
content: Quote from: oldway on September 05, 2015, 05:19:08 pmHence the other posters' suggestion to make it a load-sharing modular design with each module providing a much more manageable 40A or so.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 06:30:56 pm
content: @OldwayI have never seen something like that. Sounds elegant, but I'm not quite sure how to draw a schematic from your description.Shematic of 1 phasehttp://shrani.si/f/14/rT/2TOLxyXg/6phase.pngIn the 1 options all SCR have the common cathode. But at the higest tap the bottom 2 scr should be off?The 2 options is just wrong isn't it?

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 06:32:14 pm
content: Quote from: DanielS on September 05, 2015, 06:24:10 pmBut I would like to see how he would make a SMPSwith variable output voltage from 100V to 1KVeven with only 40A....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2015, 06:35:53 pm
content: Quote from: MarkoAnte on September 05, 2015, 10:47:29 amYes!QuoteH-bridge isn't a complete converter, it's a kind of inverter. It becomes a converter when paired with another on the secondary side, whether passive (diodes) or active (synchronous rectification).Half bridge could be used as well. I wouldn't suggest other topologies such as push-pull (peak voltage too high) or the current mode equivalents (requires constant current driver).Speaking of, it might be worthwhile to consider a rectifier that's a different style than the primary side inverter. (That is, H-bridge in diodes == FWB.) Full-wave doubler would be good, but has the problem of being a cap-input filter. This would require an LLC resonant, or something like that.QuotePerhaps you could tell us a bit about the 100V end of the problem, and what it's powering. I'm guessing it's not powering a tube oscillator, at least.Tim

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 06:42:16 pm
content: QuoteNot at all...It does not matter if the SCR's of the lower tap are conducting or not when the higher tap scr's are beginning conducting as the middle tap scr's are allready fully conducting...but for easier control circuits, it is a better solution.No need of any king of logic, only the control voltage and a voltage divider.Each scr's tap begin to conduct from a voltage level of the control voltage.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 06:50:51 pm
content: Its is a HF induction heater project. Where I would always work in near resonance and adjust the power via DC link. I have made a prototype that works form ~ 100V to ~ 550V adjusted with a buck converter. And I got nice results with it. The problems comes if you want to make it universal and it has to run at resonance with an empty inductor + if you make inductor a funny shape (required for e.g. heat treating a funny part). In testing the existing design I have found that the voltage span is kinda really needed.So the 1 option is correct. I'm not following you with the voltage divider. The only SCR I have seen were in a triac configuration on the primary of a 3 phase transformer and were being triggered form with a adjustable alpha so the secondary whent form 0 to 15 KV.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2015, 07:01:36 pm
content: Oh, is that all?It's my opinion that a wide range, fixed tuned system is a dumb idea. For, well, basically the entire purpose of this thread, right? For all the kVA of excess inverter and supply capacity you need, you might as well direct drive the inductor (which isn't an exaggeration for about half the operating range!).I normally figure on a factor of 2 (total, or +/- to each side), for a modest 2-4x increase in inverter/supply capacity. This directly determines the tap spacing, and the center of the desired operating range (frequency, inductance and Q) determines how many taps are needed, and where.It's way the fuck easier to wind a transformer, and tell the user to change a bolt and lug every so often, than to design and build,and attempt to sell, all that extra capacity.(Speaking of -- I've worked on a direct-drive supply before. The added selling point there is, not only is tuning kind of out of the picture to start with, but frequency -- and overtones -- can be adjusted in real time to control heating depth and pattern.)Tim

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 07:04:18 pm
content: If you want to have 200A at 100V and 20A at 1KV, you absolutely need to use a transformer with taps.Primary control with triacs is not possible in this case...NB: nor the solution of the variac !Everyone who has field experience with power electronics will say to you that phase primary control of transformers is not a reliable solution (risk of saturation of the transformer by DC current), even worse if you use triacs as they have a low dV/dT.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 07:15:15 pm
content: QuoteThat is why I didn't like the SCR suggestion at first.@ T3sl4co1lI don't understand what you are trying to say.QuoteAll the heaters I was followed the resonant frequency some more real time then others. Then the power was adjusted with frequency or with pwm of the main frequancy.

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 07:20:34 pm
content: This problem of risk of saturation does not occur when phase control is in the secondary of the transformer as I suggest.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 07:25:29 pm
content: What about a ball park estimate:- off efficiency?- transformer size?- cost compared to SMPS?

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2015, 07:42:02 pm
content: Quote from: MarkoAnte on September 05, 2015, 07:25:29 pmWhat do you mean with "off efficiency"...With no load, efficiency is allways = 0 ...With full load, you loose 2V on SCR, that's 2% with 100V...Efficiency of the transformer is high, let's say more than 90% (97% for a high quality transformer)Transformer size: more or less 50KVA's 3 phase transformer.I don't think it is possible to make such a project with SMPS , then no way to compare the cost.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 07:47:40 pm
content: @oldwayTy for the insight. I will think about it and simulate it.*off was a type. estimate of efficiency.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 05, 2015, 09:08:55 pm
content: Quote from: oldway on September 05, 2015, 06:32:14 pmWith a modular design, there is the option of series/parallel switching and each module only needs to be variable enough to cover the range between interconnect configuration changes. For a six modules configuration, you get 166V max per module at 1kV with the six modules in series, at 83V while still in 6x series for 500V, each module needs to provide 40A max. After switching in three pairs in series, each module is back to 166V but can now back off to 20A each. At 333V in the three pairs topology, each module needs to provide 30A at 111V which becomes 20A at 166V after switching to two triplets in series. At 166V in the serial triplets topology, we are at 40A and 83V per module which becomes 20A at 166V in full parallel configuration. At the final 100V/200A, each module only needs to provide 33A.There, a much more manageable 2:1 range for individual modules to cope with.Since his application is induction heating, there should not be much of an issue with power having to cut off for a few milliseconds while relays or other mechanical switches are being reconfigured. Or just put a bunch of high-voltage MOSFETs in parallel for shorter dead-time between range changes.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 05, 2015, 09:46:16 pm
content: I had a feeling this was a completely irrational thread, but, well... sometimes irrational is fun.One approach would be a buck current-fed full bridge with interleaved buck switches (and a bridge rectifier using 1.2kV SiC diodes on the secondary). Each leg of the full bridge needs to run at slightly more than 50% duty cycle but there are even a couple of chips out there that can do this (e.g. LM5041).A more conventional approach would be to use a fixed isolation stage (using, e.g., several two-switch forward or one or more full bridge converters) that also steps up the voltage then use a buck to step down voltage as needed. Both the buck switch and freewheeling diode need to be rated for the max output current and voltage (e.g. - 1.2kV and 200A) which makes them costly, but this isn't out of the realm of feasibility using, e.g., IGBT or SiC MOSFET modules. Oh, and don't forget that the inductor needs to have enough inductance to not allow the ripple current ratio (i.e. - "r") to get too high at the high voltage/low current end of the range, yet also not saturate at the low voltage/high current end of the range. Also note that you will either have to employ slew rate control of the buck switch and or quasi-resonant switching to keep EMI and dV/dt related problems under control.All that said, I don't see why there is a need for such a wide range of output voltage/current for the stated objective of supplying an induction heater in the first place; don't you have considerable freedom to set the characteristic impedance of the tank?

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 05, 2015, 10:25:04 pm
content: QuoteWell not really. At least I don't think. Here is what I observed with my current set up.You have a given object to heat in a given frequency to achieve a set heat penetration depth. (I'm looking at 1Mhz and above - if the planets aline). So with that your locked into a specific inductor shape. Then to achieve your resonant frequency you need a calculated capacitance. Then you can use a impedance matching transformer but there are limitation on what you can do with it. If you over do it then after you insert some material with iron you get no power out of the thing as the oscillations get dampend. But if you make it so that you get eg 15 KW of power on the inductor when the iron material is cold , when on max power, and then you reach Curie temperature you have to reduce the dc voltage by a lot or else boom-smoky goes the transistor. Then after the big drop in load, if you want to continue heating you have to start increasing the DC voltage. This goes if you want to work at a constant (max) current.Tho I have not experienced it its, supposedly heating of crystals (1-2 Mhz) is even more erratic. Now only done with tube oscillators.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2015, 11:40:56 pm
content: Quote from: MarkoAnte on September 05, 2015, 07:15:15 pmHey, voltage is voltage and current is current.You have a transformer in your circuit, right?Tim

author: oldway
author_rank: Super Contributor
timestamp: September 06, 2015, 05:15:38 am
content: @DanielS: Switching DC current is a poor solution...It seems that you don't have experience with DC power electronics. When you will see what happen when you try to open 40A 166V with relais or contactors, you will understand that it is not a good solution.I am absolutely sure that the solution I proposed works well, that this solution is no "technical adventure" and that I am able to project this power supply with 100% confidence.SCR solution will be far more reliable than all SMPS solutions you can find (if possible !)Your problem is that you don't know nothing about SCR's technology !

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 07, 2015, 07:20:02 am
content: @oldway Would it be possible, to use a "normal" transformer with taps and use full wave SCR controlled rectifiers to get the same 6 wave rectification. (Or maybe upgrade to 12 wave rectification with full wave rectifiers on a 6 phase transformer.) I know that means double the SCR and double the SCR loose. But I may make it back with a reduce size/weight/price of a normal transformer.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 07, 2015, 10:26:31 am
content: I made a quick cad drawing to clarify what we are talking about.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 07, 2015, 02:49:02 pm
content: Quote from: oldway on September 06, 2015, 05:15:38 amLooks like you missed or overlooked the memo about turning output power off before changing the module configuration. There is no current during the tap change if you turn off power by shutting down the PWM chips first.Since OP's application is induction heating, the voltage and current range is determined by the coil he wants to use and does not change by much during operation. Unless you are advocating that OP changes coils in a live circuit, then he already needs to shut down power before making any significant changes anyway.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 07, 2015, 03:14:42 pm
content: @DanielS:The thing is that by adjusting the DC link voltage you adjust the output power of the heater. And as I described before the regulation must be able to handle the goings on at the Curie temperature, meaning that a quick DC link voltage change is needed. If the application was not a HF induction heater but a universal "smiting" heater than it would be feasible to make it the way you describe as other power management approaches could be used and the DC voltage level only used for fine tuning. But for universal low frequency heaters there are batter ways of going about things. With fewer components and less cost.On a side note, if anybody is interested:"quick" - depends on the type of work peace your heating as it can happen that the skin of the work peace has already surpass the Curie temperature but the bulk of the mass has not. So the transition is not that quick. But if the work peace is e.g. a pipe that has to be hard soldered then the change will be quick.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: September 07, 2015, 04:56:05 pm
content: @blueskullI was looking at something like that but I have a 3 concerns.1) I will have to make some kind of drivers for all the transistors.2) I will have to do some voodoo when I will start hard switching something and I will get elector-magnetic garbage spewing all over the place - I had this problem with my prototype buck converter (but it was a simple synchronous buck converter, so maybe I will not have so many problems with a more advanced design or maybe even more).3) The cost. Its why I like the thyristor idea. With 3*6 thyristors + a transformer I already get a usable DC voltage + a capacitor. But with a boost buck converter I first require a 3 phase rectifier then for 1 phase 4 switches. And the price for 1 SCR <<<< 1 1.7KV SiCMOSFET. And the thyristors can be fired with a simple impulse transformer while the MOSFETs and IGBTs can't. I have yet to inquire for a transformer price but the power chokes required in a boost buck converter aren't free also.Tho the buck boost option may have a smaller weight and space requirement. (But a 20KW HF induction heater is not something you pack with your phasenprufer in your toolbox nether.) The ripple is going to be significantly better with a buck boost option tho. I think.

author: oldway
author_rank: Super Contributor
timestamp: September 07, 2015, 10:19:44 pm
content: Quote from: MarkoAnte on September 07, 2015, 07:20:02 amYes, you can use full bridge SCR's rectifiers and 3 phase transformer, but, as you said, power losses on SCR's are twice higher....this can be a problem of efficiency at low voltage.The two drawings are allright...@Daniels: with 40A V to 166V ajustable units, look for the complexity of the comutations you will have to do.- from 100 to 166V, you will need 5 converters in //- from 166V to 332V, you will need 2 units in serie of 3 converters in //- from 332V to 498V, you will need 3 units in serie of 2 converters in //- from 498V to 664V, you will need 4 units in serie of 1 converter- from 664V to 830V, you will need 5 units in serie of 1 converter- from 830V to 996V, you will need 6 units in serie of 1 converterCRAZY !And the converters must be able to work with output voltage up to 1000V higher than ground...High isolation is required.@blueskull:Smaller size --> better EMI performance. Not sure at all...you will have to make an ajustable power supply from 100 to 1KV and able to feed 200A at 100V and 20A at 1KV.You probably will have to use a lot of units in //.Dimensions seems not to be a concern.SCR --> poor THD and PF --> your utility company will not be happy.: not right because;- you will need a bridge rectifier to feed your converters and you will also have poor THD and high peak currrent with high rms value...your utility company will also not be happy- full voltage range is divided in 3 subranges so phase delay in converters is limited and THD and PF stays acceptable.- SCR technology is a well know and reliable technology, with few components and none of them are working in critical situation.

author: moffy
author_rank: Super Contributor
timestamp: September 08, 2015, 01:03:33 am
content: Have to agree with oldway, having built a 1500v/130A variable PSU and a 100v/1500A variable PSU(3 sets of taps and 3 sets of 6 SCRS), a transformer with taps and SCRs is the best way to go.