Forums

some flyback switchers

Started by John Larkin April 16, 2015
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.

https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG

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
<jlarkin@highlandtechnology.com> wrote in
<e4guiadtlrn7qr62ssladjdmg0mvt2u49f@4ax.com>:

>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. > >https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG > >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.
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. http://www.earlytelevision.org/damper.html well, ehh, need coffee.
> > >-- > >John Larkin Highland Technology, Inc >picosecond timing laser drivers and controllers > >jlarkin att highlandtechnology dott com >http://www.highlandtechnology.com > >
"John Larkin" <jlarkin@highlandtechnology.com> wrote in message 
news:e4guiadtlrn7qr62ssladjdmg0mvt2u49f@4ax.com...
>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. > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG
(D) stinks because you need a current limited switch. Ex: http://seventransistorlabs.com/Images/Deadbug_Sch.png The 2N4401 is in a "ring of two*" so it's a current limited switch. (*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 + diode to get the right effect. That's what they do in FBTs: each secondary is a single layer (partly why the cores are so tall, as C-cores go), with a single HV diode. Maybe six of those stack up to get your 30kV+ DC from a converter running at 15kHz or more. (Trinitrons go up past 100kHz, but the deflection is separate from the HV and I'm not sure what frequency the HV runs at. Lesser multiscan or non-multiscan monitors probably went up to 50kHz or so.) 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
On 16/04/2015 05:59, John Larkin wrote:
> I need maybe +750 volts at low current, 15 mA maybe. Here are some > https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG
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
On Wed, 15 Apr 2015 21:59:10 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

>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. > >https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG > >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.
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
In article <e4guiadtlrn7qr62ssladjdmg0mvt2u49f@4ax.com>,
 John Larkin <jlarkin@highlandtechnology.com> wrote:

> 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. > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG > > 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.
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.
On 2015-04-15 9:59 PM, John Larkin wrote:
> 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. > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG > > 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. >
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/
On Sun, 19 Apr 2015 08:23:22 -0700, Joerg <news@analogconsultants.com>
wrote:

>On 2015-04-15 9:59 PM, John Larkin wrote: >> 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. >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG >> >> 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. >> > >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 ...
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
On 2015-04-19 4:59 PM, John Larkin wrote:
> On Sun, 19 Apr 2015 08:23:22 -0700, Joerg <news@analogconsultants.com> > wrote: > >> On 2015-04-15 9:59 PM, John Larkin wrote: >>> 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. >>> >>> https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG >>> >>> 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. >>> >> >> 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 ... > > 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. >
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/
On Mon, 20 Apr 2015 09:47:25 -0700, Joerg <news@analogconsultants.com>
wrote:

>On 2015-04-19 4:59 PM, John Larkin wrote: >> On Sun, 19 Apr 2015 08:23:22 -0700, Joerg <news@analogconsultants.com> >> wrote: >> >>> On 2015-04-15 9:59 PM, John Larkin wrote: >>>> 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. >>>> >>>> https://dl.dropboxusercontent.com/u/53724080/Circuits/Power/Flybacks.JPG >>>> >>>> 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. >>>> >>> >>> 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 ... >> >> 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. >> > >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".
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