I need maybe +750 volts at low current, 15 mA maybe. Here are some attempts to recover some of the leakage inductance energy and keep the transformer affordable.
I got some quotes on PCB mountable HV bricks, and the prices were insane. Maybe I'll make my own, which is more fun anyhow.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On a sunny day (Wed, 15 Apr 2015 21:59:10 -0700) it happened John Larkin wrote in :
Yea, I dunno, potcore, 12 turns for 12V in, 325 turns for 325 V out, plus and - rectifier... Your sine osicllator (the one with the feedback in the emitter. Eficiency? Not many harmonics.
12 W out..Even for a TV output HV generator there is resonance in the flyback. so cap with resistor in series is wrong. +12 | | -------------------- )|( )|( HV multiplier | -------------------- |----------------||----
-- switch \ | | CS | | --- | | | \ / === ) hor deflection coil | --- | | | | | | /// /// /// ///
CS = S correction capacitor.
well, ehh, need coffee.
(D) stinks because you need a current limited switch. Ex:
(*Except not, as was discussed in another thread. :-p )
The switching feedback stuff still does its job, so it still behaves like a blocking oscillator. Obviously, this is a hybrid (B) and (D) topology circuit, which works great for the somewhat more modest ratio (1:5, so the primary saves a good 20% on secondary turns) and the capacitance isn't nasty. Efficiency is crappy because of the current limiting.
If you don't have the low leakage, your bigger concern will probably be capacitance, especially if you want to run at high frequencies (>200kHz?).
Your (C) is the most scalable, because you can stack N stages of secondary
For your thingy, you'd probably do best by locating a COTS transformer for something like a 240V/universal to 5V 3A transformer. Should be $10 or less. Drive that "backwards", with some sort of chopper (it could be a proper flyback controller as implied in your diagram, or something much crummier like an upscale equivalent of a 555, who cares), and maybe use the doubler arrangement. Or stack two 5W transformers. Leakage? Who cares, it's a 5/12V winding, use a 60V transistor and some RC or RCD damping/clamping. 750V is a lot for a single stage, but you could probably also convince a somewhat bigger (~20W?) transformer to do that kind of peak voltage... hence the suggestion of a 5V "secondary" (run it from 12 or even 24 instead!).
Added bonus: 2.5kV basic isolation, minimum. So you can stack them however you like, or connect the secondary to a line potential circuit without worries, or...
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Topology B works for me. UC3843 is my preference jelly-bean controller for that kind of app - cheap, easy, plentiful and the 5V ref always comes in handy (even supply small mcus from it!).
piglet
At low voltage with a modest load range, and no isolation, yuo could do some interesting things with a switched snubber in the lower voltage section of C. There are integrated controllers that might reduce parts count in the second low-voltage switch.
RL
If you want to do this on the cheap, a spare USB charging brick may have the parts you need.
The transformer should have three windings: primary, secondary, and circuit power. This is what you need for the classic blocking oscillator. Drive pulsed current into the original secondary (short thick) winding, use the circuit power (short thin) for feedback, and the original primary (long thin) for output.
High quality chargers may have another winding that doesn't seem to connect to anything. That's an RFI screen for the secondary. It's not likely useful here but grounding it might change the waveform slightly.
You should get 200V easily from a few volts. Tuning and a voltage doubler should hit 750V.
Crude regulation isn't too hard. The feedback winding produces a reverse bias voltage during the flyback phase that is proportional to the output voltage. You can rectify that pulse into a capacitor and use it to cut bias to the transistor.
-- I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.
I have always used CCFL transformers for such jobs. They cost very little and are small. But I could imagine them becoming less available in a decade or so because backlighting goes towards LED (which IME is also better).
Yes, I know, it is very decadent and not the manly thing to use such a mundane part ...
-- Regards, Joerg http://www.analogconsultants.com/
Not manly to use a multi-kilovolt transformer? It's OK as long as you don't call it a tranny, I guess.
Aren't CCFL tansformers usually run in sinewave mode? I can do that.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The usual method and a rather cheap one is a Royer oscillator. CCFL transformers usually come with a helper winding for the feedback. Just be careful because fluorescent backlighting is on the way out and some parts have been EOL'd or there are warning signs such as "not recommended for new designs".
-- Regards, Joerg http://www.analogconsultants.com/
Coilcraft has a series of "capacitor charging" flyback transformers, with turns ratios up to 10:1. It's not clear that they are rated to make 750 volts, but look like they would be OK with a stage or two of voltage multiplier on the output, easily done with some surface-mount dual diodes and a few caps. The flyback config is a heap simpler than a closed-loop Royer.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Right, beats me why no working voltage is specified.
Royers aren't really closed loop. All it is is just a couple of cheap transistors going in a see-saw fashion. The feedback winding maintains oscillation.
Cockroft-Walton sure is another viable option given that you only need 15mA.
Yet another option would be flyback transformers for international input voltage ranges or 600V industrial rails but operated "in reverse".
-- Regards, Joerg http://www.analogconsultants.com/
Coilcraft seems generally reluctant to discuss breakdown voltages.
I meant that I'd have to close a voltage feedback loop around the Royer oscillator. That would probably be a buck pre-switcher. So, switcher controller, mosfet, diode, filter cap, dual-fet Royer oscillator: another inductor, resonating cap, two more mosfets, transformer.
I can do a closed-loop flyback with a 75 cent controller chip and a 25 cent mosfet. One transformer, no inductors.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Which for me is a reason not to use a products on some designs. If mission critical or for a very safety-regulated market there has to be a good breakdown spec and a guaranteed working voltage. Else no release.
I've had cases where I literally had to obtain signed paperwork and submit it. Not just datasheets. Avago/HP used to be very good at that which is why I generally preferred their parts when crossing isolation barriers. Right now I am doing a 5kV rated design with Analog Devices parts, also a good company when it comes to safety-related data.
Ok, you need another inductor but other than that it's just a FET to PWM the Royer. That's not very elegant but usually gets the job done.
-- Regards, Joerg http://www.analogconsultants.com/
The current application isn't isolated, and will be powered by DC, a wart or a customer 24 volt supply. If the transformer arcs, nothing dangerous will happen. But I'd get Coilcraft to declare the insulation rating before I used their part. They sort of imply it will make 300 volts DC.
I once asked them how much AC voltage could appear across an inductor, and the reply was that their inductors have no AC voltage rating!
Too many parts!
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
One day an FAE visited the office; I asked about isolated parts. They don't have any (and don't recommend their multi-winding parts for more than they're expecting -- which is 50-100V or whatever, enough to arrange the windings for your favorite buck/boost/SEPIC/etc. application, and no more). They could do a custom, but then you could ask any of a hundred manufacturers, and probably get a better quote, assuming you're willing to quote that much, wait for samples, and commit to production of 1000+ for that price. Other than that, yeah, you'll be wanting off-the-shelf.
They have good RF and low power (converter rather than isolator) parts though, and their prices are almost half of anyone else's because they're factory direct.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Not unusual, unfortunately; on another occasion, I asked [a different manufacturer] if their part would be safe up to 700V peak (industrial converter application). They said it's rated for 1000V.
Ah, but that was 1000V coil to core (one of those bobbin types). What about across the winding? Do you have really slick enamel on there that I don't know about? Are there simply enough layers not to worry about it?...
But then, industrial converters are probably enough of a niche market, even for simple parts like inductors, that getting a custom is probably a better idea. And then you can insist on ratings.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Given that I have no formal safety issues, I could just test a couple of transformers to destruction and see what sorts of margins exist.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Don't get me started. I just had to replace an inductor because of excessive core loss. It even had a hint of a core loss number but it seemed to be way understated. Anyhow, upon firing up the prototype I touched all the parts to make sure nothing gets too hot. Phsss ... OUCH ... had a blister on my middle finger for a while. That is real fun when mountain biking and pulling the handle for the rear brake.
One of the super low cost switchers I designed in the mid 90's uses a CD40106 as PWM driver. Only 1/6th of that chip, the other five sections are sub-let for other purposes. It doesn't get a lot cheaper than that. They still produce them. The feedback is via a BJT that tugged on the input which pulled down the duty cycle. The "reference" is the 0.6V Vbe of this BJT.
-- Regards, Joerg http://www.analogconsultants.com/
If you ever need a good split bobbin design at reasonable cost try Renco USA. I have very good experience with them.
-- Regards, Joerg http://www.analogconsultants.com/
Problem is, past a certain limit high voltage exposure can be gradually damaging to parts. You almost never know where exactly where that limit is unless the manufacturer states a working voltage. Meaning it all work for quite a while until one fine day ... tssst ... z-t-t-t-t ... TSK ...
*PHUT*-- Regards, Joerg http://www.analogconsultants.com/
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