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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
just started reading this thread, if the final output required is still "15kv and 250khz at about 500 watts" 1) you should look at Radio Transmitter circuits 2) 500W/15kV = 33.33mA rms, load impedance c450kOhm 3) at 250kHz 450kOhm is equivalent to just 14pF 4) I would like to see a 15kVrms secondary that has less than 14pF self-capacitance So to put it mildly, making the output transformer will not be a simple task.
Since your intention seems to be the C-W multiplier start with a rough idea of components and cost 5) higher frequency (e.g. 250kHz) allows low pF and cost capacitors 6) lower frequency (e.g. 60Hz) allows slower cheaper rectifiers. 7) if you need to series up capacitors and diodes to achieve the required voltage per stage, you may as well reduce the input voltage and increase the number of stages. I think that the cost and effort of the multiplier makes changes to the design of the ac/ac inverter worthwhile. I haven't done any analysis, but I suspect 20 to 100 kHz, 1 to 10 kVrms would give lowest overall cost. I'd check eBay etc. for 'bulk' capacitors (voltage rating and current vs. frequency) diodes (voltage and current vs frequency =?/recovery time)
Although there are concerns at 250kV that may not be noticed at 25kV experimenting with lower voltage and frequency (in my case 230Vac 50Hz input to 15-stage multiplier) gives a 'feel' for what causes various failures! Anyway, I'd have a think about the C-W before starting the inverter.
I have since revised the frequency requirements on the advice of Ash and I can get away with a 60khz frequency for the AC rather than 250khz.
How did you arrive at the 14pF self capacitance of the transformer? Would a 60khz transformer be much more attainable? Would less power affect the self capacitance?
Hey Ash,
I was hoping I could stop corona with the epoxy potting of all the components in the voltage multiplier. I don't know how feasible this is or if all the air can be eliminated.
This does sound quite a bit more complicated than I first thought it would be.
That is one of the reasons I wanted to farm out the making of the transformer. I figured the transformer company would know what transformer to use and the number of windings to use for the frequency, power and voltage specs I have.
If need be on the first time out I can lower my power requirements, perhaps 250 watts but I don't know from what you are saying if that would affect the leakage inductance and stray capacitance or if after losses I would have any output left.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Robert, I was trying to give an idea of some of the problems involved with HV SMPS's that don't occur with most 'normal' SMPS's. There are several programs out there that will virtually design a 'normal' SMPS for you, but they don't take into account some of the issues relating to HV units, like extra insulation which results in more space between windings etc.
As I pointed out earlier, the type of copmpany you mention don't usually wind HV stuff. There may be some specialist ones that do, but they will be expensive.
Whatever you build, you will be able to get the 15kV, but you may need to run at a lower frequency than you'd planned if the resonant frequency is lower than hoped for, this means less power.
The other point worth mentioning here is that we, on this forum, tend use use the biggest ferrite we can get, in order to keep the 'volts per turn' as high as possible, and therefore, the number of secondary turns to a minimum. The combination of fewer windings and larger 'winding window' tends to keep stray capacitance to a minimum, however, at the expense of leakage inductance, but if one of these is low, the product of both will be fairly low.
Now you obviously want to run at as high a frequency as possible, so that you can use smaller capacitors in your multiplier. I was just pointing out the issues involved here.
It will be possible to pot everything in epoxy, especially if you can suck most of the air out with a small vacuum pump (virtually any old refrigeration pump will do for this).
Now, What I'm saying is that you will be able to operate any SMPS you build here at the designed voltage, however you 'may' need to reduce the frequency. If this turns out to be the case, you can then modify it and hopefully run it at a higher frequency, if needed.
It is for this reason that I suggest winding a prototype bobbin or two so that you can run some tests, before considering farming the work out to a company. You need to be sure it will work before paying out for any quantity of windings.
I'm just trying to outline the issues involved, and how I would choose to go about solving them.
EDIT: At least, I think that if you use a controller like a TL494, with PWM, you can adjust the PWM, and run at a lower frequency, if required. Otherwise it may go into saturation, unless you have another means of reducing input voltage.
As I said before, you need to wind some bobbins and then measure the resonant frequency of the leakage inductance and stray capacitance, and then you'll have a 'starting point'.
Where would you suggest getting the ferrite bobbins? I don't mind shipping from the UK if you have an online store you like to order the ferrite bobbins from. What type of bobbin would you suggest?
How would I measure the resonant frequency of the transformer once wound? What kind of tool is used to do this?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Robert Francis wrote ...
Hey Ash,
Sorry for being so pig headed.
Where would you suggest getting the ferrite bobbins? I don't mind shipping from the UK if you have an online store you like to order the ferrite bobbins from. What type of bobbin would you suggest?
How would I measure the resonant frequency of the transformer once wound? What kind of tool is used to do this?
Thank you,
Bob
Don't worry about it, I probably didn't explain it very well.
Ok, so the bobbin is like a plastic thing that fits over the ferrite and has the windings on it. Generally, the ones that the ferrite suppliers supply aren't really suited to HV applications, so we generally have to make our own. You might get lucky, but a 'purpose designed' one will generally be much better anyway.
Before we get to that bit, we have to decide on the ferrite cores. The two main points here are 'a sufficient winding window' and 'sufficient cross sectional area of ferrite'.
The winding window has to be large enough to accomodate the primary, the secondaries and sufficient insulation.
The The ferrite has to have sufficient cross sectional area to support the 'volts per turn' that we want to use at the frequency chosen without saturating.
Time for some maths and a few sketches.
I'll post some more details regarding maths, etc if required in the morning, but we generally use pairs of 'C' cores, sometimes paired up, because 'E' cores, etc. don't usually have a large enough winding window.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Robert, my intention was to highlight the difficulties of self- and stray-capacitance. at 250 kHz 15 kVrms, a 14 pF capacitance would draw as much current as the load (450 kOhm=R=X=1/(2.pi.F.C) so your transformer operating in a non-resonant mode should have less than 14 pF self-capacitance. 'allowable' capacitance is inversely proportional to both frequency and voltage so for example changing from 250 kHz to 60 kHz would allow 4.167 x 14 pF = 58 pF = achievable ..I would still have difficulty making a reliable transformer for 60 kHz 15 kVrms !
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Ok, I've found a couple of links , one to a thread on 'resonant frequency' of these things and one on 'volts per turn'
I'm assuming that most of the maths, etc. that you'll require is 'in there somewhere', however don't hesitate to ask if you are unsure.
Those two links also have links to other stuff, if I remember correctly, and cover the subjects in quite a bit of detail, as I was pretty much in the same situation as you when I started them.
I read through those threads and I am wondering... I am building the SMPS to feed a voltage multiplier. Maybe it makes more sense to build the same transformer that you did since it works. If I up it to 50kv and use 2 50kv 2,000pF caps in series in the multiplier I can get away with a low frequency, much lower than 60khz, between 2-7khz depending on capacitor farad size.
Or, I am only going for around 500 watts, maybe I should try for a flyback transformer. I bought one on ebay it was only $20 but it was only 30kv (no idea if it had a rectifier diode built in) and the ones on ebay don't seem to specify voltage, do you know where I could find an AC output 50kv flyback ready to go?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Robert Francis wrote ...
Hey Ash,
I read through those threads and I am wondering... I am building the SMPS to feed a voltage multiplier. Maybe it makes more sense to build the same transformer that you did since it works. If I up it to 50kv and use 2 50kv 2,000pF caps in series in the multiplier I can get away with a low frequency, much lower than 60khz, between 2-7khz depending on capacitor farad size.
Or, I am only going for around 500 watts, maybe I should try for a flyback transformer. I bought one on ebay it was only $20 but it was only 30kv (no idea if it had a rectifier diode built in) and the ones on ebay don't seem to specify voltage, do you know where I could find an AC output 50kv flyback ready to go?
OK, first, I haven't yet built the 50kV one, I was distracted last year by a few other things (like building a stepper motor controled winding machine, which isn't really required for this, anyway).
The conclusions I came to were to push the Bmax as much as possible, to reduce the number of secondary windings, and to use the biggest ferrite available to reduce capacitance.
Most AC flybacks from old TV's are rated for around 15-16kHz, and only a milliamp or two. Any losses in the multiplier would leave very little output, if any at all.
There is a transformer built by Newton Brawn that is described in one of the threads I linked to. I think he posted it around the 22 feburary, from memory, I think it achieves around 13kV peak to peak. I can't remember the frequency or power, offhand, though.
The reason I linked to those threads is because all the theory you need is there (or is linked to), and your target of 15kV will be much easier than my proposed target of 50kV.
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