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 Posted: Mar 07, 2012 - 11:36 AM |
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Joined: Jun 27, 2005
Posts: 3448
Location: St.Petersburg, Russia
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| I have a little cute CRT tube named 3ЛО1И and I feel guilty of not giving it enough love. From what I know, it needs whereabouts of 500-800V on the accelerating anode, I don't think the current will exceed 100uA. I know how to make 800V from 220V with a regular isolating transformer and a diode multiplier. But I would really like to keep it small and wallwartsome, potentially nixieclockable, so I'm shooting for 12VDC->800VDC conversion. Maybe someone has done something of the sort recently, care to share your experience? I'm thinking of using an AVR to switch a trusty IRF630, like in a buck-boost converter, but with a switchmode transformer in place of the coil and feeding a 2x multiplier from the secondary. Worth trying, or there's a better way to do it? |
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Posted: Mar 07, 2012 - 12:04 PM |
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Joined: Oct 30, 2002
Posts: 5727
Location: The Netherlands
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| Push-pull or flyback converter with an appropriate transformer? But with a ratio of 1:66 this is going to be a big transformer with boatloads of windings on the secondary. Maybe 1:33 with a 2x multiplier. |
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Posted: Mar 07, 2012 - 12:41 PM |
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Joined: Jun 27, 2005
Posts: 3448
Location: St.Petersburg, Russia
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| Re: the amount of windings, we can talk pretty high frequencies here so maybe fewer windings. I know of no good way of estimating them other than by experimentation. Say, how do you calculate how many do you need on the primary on a toroid with such dimensions and such permeability at such frequency, like, why don't we just make it 1 winding on the primary and 33 on secondary? I guess at some frequency 1 winding will make enough Z for the power supply not to burn down? I remember the past full of sparks when I tried to make a switchmode power supply and my secondary was 1.5 windings. It worked, until it exploded, but I don't think the transformer was at fault. |
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Posted: Mar 07, 2012 - 01:13 PM |
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Joined: Oct 30, 2002
Posts: 5727
Location: The Netherlands
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I think you first need to determine at what frequency you want to operate it, and the amount current ripple allowed and from that an inductance can be calculated. From inductance you can determine the amount of windings needed on the primary side.
You might find good information in the datasheet of the LT3439 and the AN118 application note. |
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Posted: Mar 07, 2012 - 01:25 PM |
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Joined: Feb 06, 2009
Posts: 858
Location: TN
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| I have generated high voltages using a xenon trigger transformer. Very small. Not sure how well it will work at high repetition. |
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Posted: Mar 07, 2012 - 01:44 PM |
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Joined: Jun 27, 2005
Posts: 3448
Location: St.Petersburg, Russia
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Usually the inverters for the bulbs are incapable of producing sustainged voltages. They need some HV to ionize the gas and then the voltage is pretty low.
Thanks for the pointers jayjay, reading. |
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Posted: Mar 07, 2012 - 03:18 PM |
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Joined: Mar 27, 2002
Posts: 854
Location: Athens, Hellas
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1:33? Why use such high ratios? If, for example, you create 160V in the primary, the secondary will only need to have five times the primary turns.
Driving a flyback is easy: We turn the power stage fully on until the inductor's current exceeds a certain, pre-calculated value. Right after that we turn the power stage off, making the core nuts until it returns all the magnetic energy it has stored in its air-gap. The core will try to do anything to return the stored energy back: It will try to find the easier way, creating an opposite current either in the primary coil destroying the power stage or in the secondary one lighting up our load. When the core is fully (Discontinuous Mode) or partially (Continuous Mode) discharged, we turn the power stage on again and we repeat the above indefinitely until the generated voltage in the secondary reaches the desired value; then we stop and wait for that voltage to drop below another threshold in order to repeat everything. This is the secondary voltage regulation. We can monitor the secondary current if we need a constant current source. For safety reasons we should monitor both! In a more elegant way, we can avoid the so-called "hiccup" of the switching power supply if we try to regulate the output by changing on the fly the driving pulse width, or the frequency, or both, instead of brutally turning the whole thing on and off. Then, we have a so-called "green" SMPS! (I know how much you love that term  )
Now, windings of a few only turns give the inductor not much inductance, which requires higher switching frequencies in order to avoid saturating the primary coil and melt everything by skyrocketing the primary current; multi-turn windings have larger inductance and require lower frequencies to give the core the time to store the energy. Actually, we are not talking about a transformer because we do not collect the energy as we are pushing it to the primary winding; this is the Forward Topology. Instead, we collect the energy stored in the magnetic medium, which is the ferritic core. So, this is two coils strongly coupled magnetically together using the same ferritic core, where the primary creates energy that is stored in the visible air-gap of the ferrite core (or in the invisible countless tiny gaps created by the bonding material of the ferritic powder the ferrite cores are made of), and we discharge the coil's energy using the second coil (the "secondary"). This is the so-called Flyback Topology that energised the electron beam of the TVs, where it was firstly used commercially and, thus, became known; the electron beam that "flew back" to the next line of the field/frame during the blanking time after rendering the end of the previous line. 
So, we should know the power the flyback will need handle in order to choose the ferritic core material, properties and size. Knowing the type of the ferritic material, the maximum and the ripple current (I_max-I_min) of the primary inductor and the time of the current transitions (in other words, the switching frequency), and we can calculate the primary coil inductance per turn. An LCR meter will be proved to be valuable, if it exists, to verify the inductance of the masterpiece we created in order to calculate the exact frequency needed to drive it. For one-off's you might even need to trim the core gap with a fine file!
Anyway, regarding the boring details about formulas, snubbers and MOSFET driving, you can find almost everything you need to know in the manufacturer's application notes and data sheet of their power switching products. If I remember correctly, TopSwitch offers guides for the transformer design.
-George |
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Last edited by Giorgos_K on Mar 07, 2012 - 03:47 PM; edited 1 time in total
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Posted: Mar 07, 2012 - 03:45 PM |
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Joined: Feb 19, 2003
Posts: 2236
Location: Seattle, WA
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| Flyback or push/pull. Don't drive the transistor too fast otherwise you'll get huge spikes on your switch due to the transformer's leakage inductance. I'm a big fan of a high ratio transformer so that you can keep your primary switch's voltage rating low. |
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Posted: Mar 07, 2012 - 05:26 PM |
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Joined: Dec 01, 2003
Posts: 2507
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| Check out the little inverter modules used for florescent LCD backlight tubes. The high AC voltage is there, just add a rectifier! |
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Tom Pappano
Tulsa, Oklahoma
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Posted: Mar 07, 2012 - 08:42 PM |
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Joined: Jul 19, 2011
Posts: 477
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Tom Pappano has about the best idea of the lot in terms of ease of implementation. One caution for this approach is that these LCD CCFL drivers have high output impedances. The output voltage is designed to foldback after the CCFL tube "strikes". This may or may not effect you at 100 uAmp draw, it depends on the specific module. These inverter modules generally have two output voltages specified: the "starting" voltage and the "running" voltage. The starting voltage is always higher and is essentially the no-load voltage - and probably the specification you should be looking at for your application. The running voltage is the voltage the output will pull back to as it is loaded to the specified output current (after the bulbs "strike"). Generally these CCFL backlights take around 5-10 mA running current per bulb, so your 100 uA load will fall into the "no-load" or "starting voltage" mode of operation.
If you still want to build your own DC-kiloV converter, I'd recommend the diode multiplier approach. There is a little trick to getting the traditional diode multipliers to work from a DC supply rather than an AC one. If you want to pursue this path, let me know & I'll give you some guidance. |
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Posted: Mar 07, 2012 - 09:25 PM |
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Joined: Jun 27, 2005
Posts: 3448
Location: St.Petersburg, Russia
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| Thanks for all the advice. I'm a little bit worried that hacking a ready-made CCFL inverter may take me longer than building my own from scratch. I have experience making buck-boosts for the nixies and they are only a step away from flyback converters so I'm rather looking in that direction. Probably even push-pull or half-bridge to fully utilize the core. Anyway, at the moment I'm only lazily researching the options, any input is appreciated. |
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Posted: Mar 07, 2012 - 11:55 PM |
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Joined: Nov 01, 2005
Posts: 6354
Location: Hilversum - the Netherlands
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| Especially when the current consumption is low, a boost converter is IMO the way to go. Finally found time for the little scope-tube ? |
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Posted: Mar 08, 2012 - 01:08 AM |
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Joined: Jun 27, 2005
Posts: 3448
Location: St.Petersburg, Russia
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I just want it to glow
Do you think a boost converter will do? Somehow I have never seen a boost converter that goes over 200V, but I do not know why. Do you? |
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Posted: Mar 08, 2012 - 01:39 AM |
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Joined: Nov 01, 2005
Posts: 6354
Location: Hilversum - the Netherlands
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The 1000V+ schottky diode for the boost converter will be the hard part to find.
How about a boost converter to 200V (nixie-wise) followed by a multiplier: a push-pull-stage driven by a squarewave generator feeding a diode-capacitor matrix. Of course those should be flux-capacitors  But the diodes can be ordinary 1N4007's or so. |
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Posted: Mar 08, 2012 - 02:33 AM |
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Joined: Jun 15, 2008
Posts: 1779
Location: North Carolina USA
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Posted: Mar 08, 2012 - 02:57 AM |
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Joined: Feb 19, 2010
Posts: 509
Location: Montreal, QC, CA
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If you want wall-wart style, 120V AC to 800V DC is a lot easier to accomplish than 12V DC to 800V DC. If you're gonna plug it in the wall anyways why would you reduce voltage before amplifying it?
From 120V 60Hz AC, feed into a 2-stages Villard doubler with 68uF caps to get ~580V AC, and rectify to ~820V DC, 100mA. Wear gloves and shoes.
Also because each stage of those ladders only operate at twice the input voltage, and since regular 120V peaks at ~170V, all components before the rectifier only need to be rated for 400V. |
Last edited by hugoboss on Mar 08, 2012 - 03:04 AM; edited 1 time in total
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Posted: Mar 08, 2012 - 02:59 AM |
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Joined: Feb 19, 2010
Posts: 509
Location: Montreal, QC, CA
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Hmm funny how similar Villard cascades are to CW... Same? Was there a battle for a name that was won somewhere?
EDIT ok so from reading a bit more....
CW = Greinacher <> Villard Cascade, but are sometimes incorrectly refered to as.... Phew...
A CW cascade is a Villard cascade with an added peak detector on each stage... |
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Posted: Mar 08, 2012 - 07:46 AM |
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Joined: Oct 30, 2002
Posts: 5727
Location: The Netherlands
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Plons wrote:
The 1000V+ schottky diode
I don't think you need a Schottky diode , the 0.4V vs 0.7V drop difference is negligible at these voltages. |
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Posted: Mar 08, 2012 - 09:03 AM |
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Joined: Mar 17, 2005
Posts: 1313
Location: Stockholm, Sweden
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Hi, I did not read all the posts, but I have done a 5V to 800V DC/DC converter recently.
I used the LT3580 and schematics according to datasheet for -350V. I use dual LDT565630T-011 transformers.
I think 800V might be out of spec. Im running at 350V. But I have tested up to 800V, and it worked at least for a while. And I have a little higher current consumption than you. |
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Posted: Mar 08, 2012 - 11:27 AM |
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Joined: Jun 27, 2005
Posts: 3448
Location: St.Petersburg, Russia
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@hugoboss: because I want to use store-bought 12V (or 9V, or something) wallwart.
@dak664: lol
@AgwanII: good to know that those appnotes are scalable and thanks for the transformer reference.
@Plons: that could be a very neat idea, given that a good part of this circuit would be tried and true for me. Let's see, my typical nixie converter can probably put out ~200V at 30-40mA. Sounds like enough juice to quadruplify it and have enough for the second anode and deflection system.
But it bothers me that I could do all that in one stage by using a flyback or pushpull. And unknown transformer. Say, to begin the experimentation, I have a nice ferrite ring which is about 22mm OD. I could start experimenting with some random number, e.g. 5 windings on the primary and 350 on secondary and see how it goes. I would start right away but I do not have a wire that would physically fit 350 windings in that ring. Not to mention the questionnability of excitement of threading a loop of wire 350 times through a tiny hole. So 2-stage could win :) |
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