I'm looking to build a power supply for a krypton arc lamp, and need a design that can deliver an adjustable 20-60 amps into a LOW resistance load (around 5 ohms). I'm a programmer for a living and this is my first power electronics project (previous electronics projects have mostly been microcontroller based stuff)
I've done quite a bit of reading, but am running into a brick wall scaling up some of the various designs that I've found online. I'm working with LTC's SWCad3 to test concepts, but I'm having problems with input currents spiking up way too high since my target output is so close to my current source (AC 220v 60A single phase rectified to 320vdc), and the 120hz line is just too slow to keep the input capacitor bank charged under heavy load without some serious high current spikes to recharge.
Any suggestions to smooth out the current draw from the line would be MUCH appreciated !!
John, such a power supply is rather tricky. The power level is rather high, and the nonlinear negative dynamic impedance load doen't make it simpler. You'll have to use a switching approach. Not recommended.
Must agree here. At this power level , I'd recommend you get a professional power supply designer to tackle this. DNA , you around :0) If you still decide to tackle this yourself , I recommend some eye protetion of some sort and prepair yourself for lots of bangs and smoke :0( Rob
Ohh I'm quite prepared for a bit of mess :) Even got a few spare arc tubes just in case :) And a welding helmet for eye protection :)
As stated... I'm planning a current mode buck regulator, though it has been suggested that I'll need a boost circuit in front of it to smooth out the line input current surges (to push up the voltage so that charge is transfered into the caps more effciently ??) . I've already got a spice model created that is close (need to work on the control loops a little more, and a better soft start circuit., and beat these 120+ amp surges that I'm getting)
"nonlinear negative dynamic impedance load" -- I'm not an EE so I could use a bit of explanation I'll probably need to understand that as I'd like to create a spice model of the arc tube for the later stages of testing. (resistance decreases as current increases ?? am I even close ??)
(cummon guys !!! so far everyone who has responded, here and in alt.lasers [because this is for a KW output ND:YAG laser] has had nothing to hand out but warnings !! I'm committed, it will get built !! )
We are using AC/DC converters and special DC constanter with 300 V/DC
30 up to 60 Amps for nano or micro perforation machines since more then 20 years.
There are several companies very well equiped - have a look to
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or
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Such AC/DC STABLIZED power supplies are to purchase by around 4000 -
6000 bucks by one year guarantee so that it makes not sense to invest several 10,000 bucks in developments for NEW primary AC switching and secondary DC regulated units.
Let me know if you need more or specific details - I'm glad to help and support you.
The only detail that could possibly help me at those kinds of prices is "do you give free samples ??" :) :)
I makes very little sense to invest that kind of money when I've got less than $500 into this laser so far, and I dont think its going to cost me more than about $1000 more to bring it online. I'm not being funded by a corporation with deep pockets :) In any case, for that kind of money, I could afford to pump this rod with 808nm laser diode bars and skip the massive current requirements that the krypton arcs are forcing me to deal with. ($6,000 times the 3 supplies I need pays for a LOT of laser diode bars !!)
And I seriously doubt its going to cost me 10K or better to to design my own -- even if I count my time invested -- I'm pretty close to something workable now with less than a week invested, and between the folks here and what I'm finding online, working out the rest of the issues shouldnt take all that long. And besides -- I get to learn something new in the process !!
[Flame On] So save the commercial hype for someone that doesnt want to be bothered with the nitty gritty details !! [Flame Off]
dude, your insane trying to build a 6Kw. lamp supply from scratch, those lamps blow if ya sneeze. I know I just blew one trying to build a 2 kW version. I have one left.
contact bruce @ contact @ second source lasers do com, he has two eski lamp supplies in stock. BTW the series 6 pins and two connector bodies you asked for shipped yesterday.
It's a mountain all right.. I'm very new in smps.. From near scratch, my little 130Watt (90% eff) converter project has gone on for > 2 months. I had to read on: mosfet switching losses trace inductances fast power diodes feedback control theory control topologies smps simulation paralleling power devices reducing EMI creepage ground planes capacitor losses magnetics design line filtering and ov protection snubbers
But, I got it working. :) Just have to fix some ringing.... Score: smps: 1 quality of life: 0 :) D from BC
and OF COURSE I'm insane.. even more so than you think because I need 8KW !! (7mm ID bore needing 195-205 volts !!)
However -- the design is progressing -- I've got a model up and running that (without current regulation) stabilizes at a very smooth 48 amps into the tube (presuming 5 ohms tube resistance) after startup... its got a bit more ripple voltage than I would like, and its spiking the AC input hard, but at least I got the soft start and main current path worked out
having a real problem with the current sense -- trying to use the switch model to trigger when the ISense voltage passes a preset threshold, but I must have it modeled wrong some how because it is not switching when the tube current spikes. (I know -- its a kludge, but I'm too lazy to add in the full pwm driver until I got everything else close to working as intended) I'll prolly see what I can do about putting at least a comparator in and see if that helps
At this high output current, I would probably thinking about a 2 phase parallel current mode output stage (or pair of stages. This is a fairly simple way to divide the current load equally between two parallel paths, while having something like 80% or 90% of the current ripple cancel. Toy can work with one of the halves, till you pretty much get it working at half current and then build a second one. The trick is to come up with the dual complementary triangle wave to drive the error amplifiers, so that the output pulses are always almost precisely 180 degrees out of phase. I have made fairly high current, low ripple current mode laser supplies this way, though the voltage was much lower, and the load better behaved.
I think you may need a more continuous current sense, rather than a peak trip.
I know I need continuous feedback -- but I'm still trying to get a handle on the main current path before I add complexity -- just wanted something simple in there to provide SOME regulation so I'd have an idea if I was getting close.
I have made a deciscion over the last 24 hours -- I'm going with a 2 phase boost front end for active PFC and an internal 400v DC bus. One phase will be a non-inverting boost for the positive halves of the 2 line inputs, the other will be inverting for the negative halves, eliminating all the problems that I'm having because my negative input to the buck is a half wave pulse instead of a solid ground.
I'm gonna spend today modeling the non-inverting boost supply.. see if I can get that working -- nice thing is -- it doesnt have to be strongly regulated... -- the not-so-nice thing is I'm going to have to do the full PWM setup because crude regulation methods wont work to drive the boost switch.
hehehe at this point I would kill for a PWM controller spice model generic enough to be used for this :) Gonna have to search around and see whats out there !!
Oh..... OK, fiddle fiddle fiddle. Try one of these,
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That's two stages operated in parallel with the clock waveforms shifted. The idea is sort of that the current error amplifiers receive the same demand signal from a single or master voltage error amplifier and program the same current in their respective power stages. You have to figure out how to tie the available parts in the ICs together to do the job.
One thing you are forced to do is use current transformers to recreate the inductor current. At high powers this would be done anyway but at low powers you would use a sense resistor in the return lead to the input rectifier. Unfotunately when you try to operate things in parallel you can't do that any more.... they just won't connect together proper like.
I've added some source sense resistors to the 'mosfets' but not implemented a current limit with them. Problem is that the drain current transformer is operated at high duty cycles and might saturate. You lose the signal and things go bang. Resistors don't, lose signals, so it's a kind of 'oh shit!' protection.
The peak limit is simplified a bit. It really needs slope compensation but the IC does not make that easy to add, that's why those transistors were in the previous one. Here I've just thrown in some appropriate current sources.
I've diddled it to do 1KW per section from a 220V line..... I suppose you will need 8 of them because you are some sort of cross dressing freak. There are ways of synchronising these ICs by shoving a pulse up their oscillator. I'm sure if you read some data sheets you'll get some ideas.
In the old days a 4017 Johnson Counter clocked at N (stages) times the desired base frequency with the reset pin on the appropriate tap did the job. These day I suppose you would use a PIC.
Really all you are getting is something to practice your sums on. You will probably find some of the values I've left in there are sub-optimal. You can try putting in 'real' components to get some guesses about power losses. I'd recommend you grab hold of some SiC diodes for the main boost diodes.
I've already located inexpensive isolated current sensors with models for 50 or 100 amps and 3000v isolation, so I can easily do high side current sensing -- just gotta do a spice model for it, which looking at your models has helped me figure out (a voltage source defined via a formula referencing the load current)
I'm seriously looking at the PFC chip you mentioned, and some others (I've already tried to model out one using the LT1248 but its not working right yet)
You want fruit pie ?? here is fruit pie -- what would it take if I wanted a true earth ground on the output of this mess -- I've considered running seperate boosts on the poitive and negative outputs from the bridge, using the isolated current sensor to get EVERYTHING out of the ground path.. one of them inverting to get a positive voltage (add more in pairs if 2 wont get me enough current at reasonable ripple)
is that fruit pie enough for you ?? or is there a better way ??
Ermmmmm...... Current sense transformers, something like
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Each PFC section needs to control its own inductor current otherwise they won't share the total load. It's tied up with the volt-second balance, inductors integrate the voltage placed accross them as the current through them. If there is a mismatch in the drive to the inductors then one of them will hog the current.
The local current error amplifiers force them to share it but you need separate sense signals for each stage so one honking big current sensing thing on the input won't do. You also need a reasonable frequency response from the devices if you want to compensate the current loops correctly.
Ahaaaa, LT1248, shows you how crap I am for not checking if they did one or if LTSpice had the model. I knew they did but there is something wrong with my head. I suppose I'm fixated on TI because I grew up with Unitrode. It's all much of a muchness and once you get happy with one you can see that a lot of the internals are the same in all the others.
If you really really need to ground the output of your beast....... Uhm, right, your side of the pond has strange electricity with something like
110V balanced either side of ground. My head hurts but I think you might only get away with it if you use half wave rectification which is going to be dirty and waste half of the capacity of your supply.
Since you seem to enjoy pain you might as well go the whole banana and dangle a transformer isolated coverter of the end of your PFC stages. Something like a half or full bridge. That will make any (well most) concerns about earth disappear.
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