HV flyback design theory?

Hi all,

I apologize for the probably too stupid questions, but it's a totally new
electronic territory for me and I don't seem to be finding the needed
theory articles and/or books to help me.

I would like to re-use some old BW TV's HV transformer to power the CRTs in
X-Y mode (all the yoke changes and drivers are already done). I'd just need
the Anode supply and the G2/G4 supplies that are usually obtained with
a few taps on a "low" voltage secondary winding on the HV transformer.

Now, what would be the step to design a proper driver circuit around a
given transformer?
I have quite a lot of test equipment and I can measure inductances at
various frequencies, I have pulse/function generator (HP-8116A), HV probe
and most of what it's probably needed, but can't find a good text about
this subject.
Of course I don't want to copy a typical horizontal yoke driver plus
flyback driver circuit. I've tried to use a big-ish air-cored inductor
in place of the horizontal yoke coil and it kind of worked well enough
but the inductor was getting hot quite soon. Also the typical TV driver
circuit has no voltage feedback, probably since the horizontal yoke is the
big load there and CRT's anode load won't be changing much the power budget
of the converter.
My next attempt will probably be designing a PWM driver based on the
old UC3843 (current mode, with a voltage feedback from a low voltage
secondary). But I'm afraid there're too many unknowns still for me to
make a good design.
Is there any way to get the right knowledge to design such a supply?

Thanks in advance.
Frank

 

[John Robertson]

On 2020/04/30 1:38 p.m., frank@invalid.org wrote:

Hi all,

I apologize for the probably too stupid questions, but it's a totally new
electronic territory for me and I don't seem to be finding the needed
theory articles and/or books to help me.

I would like to re-use some old BW TV's HV transformer to power the CRTs in
X-Y mode (all the yoke changes and drivers are already done). I'd just need
the Anode supply and the G2/G4 supplies that are usually obtained with
a few taps on a "low" voltage secondary winding on the HV transformer.

Now, what would be the step to design a proper driver circuit around a
given transformer?
I have quite a lot of test equipment and I can measure inductances at
various frequencies, I have pulse/function generator (HP-8116A), HV probe
and most of what it's probably needed, but can't find a good text about
this subject.
Of course I don't want to copy a typical horizontal yoke driver plus
flyback driver circuit. I've tried to use a big-ish air-cored inductor
in place of the horizontal yoke coil and it kind of worked well enough
but the inductor was getting hot quite soon. Also the typical TV driver
circuit has no voltage feedback, probably since the horizontal yoke is the
big load there and CRT's anode load won't be changing much the power budget
of the converter.
My next attempt will probably be designing a PWM driver based on the
old UC3843 (current mode, with a voltage feedback from a low voltage
secondary). But I'm afraid there're too many unknowns still for me to
make a good design.
Is there any way to get the right knowledge to design such a supply?

Thanks in advance.
Frank

Years ago I made what I call my Raster-Hack to do just what you are
looking for - running an XY video game (Star Wars) monitor from a raster
HV as the XY monitor's HV circuit was toast and there was (at the time)
no replacement for the xformer.

https://www.flippers.com/vid-tips.html#RasterHack

I caution you that whatever you decide to power your monitor to beware
of the X-Ray potential if you get near the picture tube's rated maximum
voltage. That was one reason I used the raster chassis to drive the
tube, it had built in X-ray over-voltage sense and shutdown built in and
would thus be safe for home use.

So, depending on the size and type of your XY tube, you may be better
off finding a good video game chassis and using it for your project.

At least if it is a one-off...

John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

[Bill Sloman]

On Friday, May 1, 2020 at 6:38:52 AM UTC+10, fr...@invalid.org wrote:

Hi all,

I apologize for the probably too stupid questions, but it's a totally new
electronic territory for me and I don't seem to be finding the needed
theory articles and/or books to help me.

I would like to re-use some old BW TV's HV transformer to power the CRTs in
X-Y mode (all the yoke changes and drivers are already done). I'd just need
the Anode supply and the G2/G4 supplies that are usually obtained with
a few taps on a "low" voltage secondary winding on the HV transformer.

Now, what would be the step to design a proper driver circuit around a
given transformer?

You need to characterise the transformer. You need the inductance of each winding, the mutual inductance between each pair of windings, and the stray capacitance associated with each winding (which can be hard to measure since getting current flowing through one capacitance means having current flowing through all the others).

The transformer equation is

V1= L1.dI1/dt + M.dI2/dt

V2 = M.dI1/dt + L2.dI2/dt

where M is the mutal inductance of the two coils and less than the square root of the product of L1 and L2

M = k. (L1.L2)^0.5

where k can be as high as 0.999 for high-permeability cores, and can get down to 0.98 for gapped ferrite cores.

LTSpice lets you use these numbers to model coupled inductors.

The self-capacitance of a single layer winding can be quite low - around 1pF. The self-capacitance of a multiplayer winding can be quite a lot higher. If you got some idea of the dimensions of the windings and the wire gauge used you can generate useful rough estimates. High voltage transformers can be quite bad.

Peter Baxandall seems to have invented the class-D oscillator while working up an inverter to generate high voltage photomultiplier voltages (around 1kV) from 12V rails. It doesn't show up in his paper

http://sophia-elektronica.com/0344_001_Baxandal.pdf

but I worked for a guy who had trained under him, and that was the story I got from him, along with tales of dropping aluminised ping-pong balls from the bomb-bay of a Canberra bomber flying up in the stratosphere. Peter was working at the UK Royal Radar Establishment at Malvern at the time.

--
Bill Sloman, Sydney

 

[Tim Williams]

Replace the yoke with an equivalent R+L (or maybe even not much R) and drive
it at roughly the expected frequency. You can use a 3843, sure, get some
nicely regulated A2 with a big fat resistor divider, without needing the
tricks/hacks they used back in the day. Trace the circuit, use the original
primary winding and voltages. If it's offline, that's probably 160VDC at
15kHz. Easy enough, if a bit of a pain if you're using a low voltage supply
instead (use a boost? add external winding? isolator for SELV control to
offline side drive?).

Bonus points for tracing the circuit to find what HV regulation feedback it
had in the first place. Usually there's a focus/screen divider chain that
returns to a not-quite-ground pin that can be used for sensing.

Incidentally, a lot of Trinitons did this already, a separate HV supply,
since the horizontal sweep range was so huge. Some multisync monitors
managed all in the same circuit (impressive!). Last one I had, still have
the service docs for, HV supply was a MOSFET of all things, driving what
looks to be a fairly ordinary flyback transformer, with a BA9756 controller.
Not current mode, really weird circuit; it's parafeed through a fixed value
inductor (not an RFC, its value is significant). Well, I suppose that's as
good an indication as any about the first thing I said -- the inductance
effectively in parallel accounts for the lack of yoke, storing energy that
the transformer wouldn't otherwise. (FBTs aren't gapped very much,
remember. Air gap is what stores the energy.)

And if you are using a different supply voltage, there's always the
self-excited oscillator on some turns on the back leg of the core. May be
harder to regulate though. Mind that, if you do use the same kind of
circuit (3843 + such), you MUST deal with the leakage inductance of the
shitty coupling from primary being outside the main windings. A resonant
drive is preferred for this reason.

Even with good coupling, note the secondary is significantly resonant by
itself; combined-HV multisync monitors pushed over 100kHz fundamental, with
10% or so of that being retrace -- the waveform is roughly class E, i.e.
flyback but with a resonant hump instead of a square clamped pulse. That
means the self-resonant frequency was around a MHz. Which itself is
impressive for such high voltages; they used special winding techniques to
achieve it. Regular TV FBTs won't, and will have a lower SRF (again, likely
near a useful harmonic of sweep). A resonant load means awful performance
for hard-switched flyback; it's best to take advantage of it instead.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

<frank@invalid.org> wrote in message news:r8fd0l$jl6$1@dont-email.me...

Hi all,

I apologize for the probably too stupid questions, but it's a totally new
electronic territory for me and I don't seem to be finding the needed
theory articles and/or books to help me.

I would like to re-use some old BW TV's HV transformer to power the CRTs
in
X-Y mode (all the yoke changes and drivers are already done). I'd just
need
the Anode supply and the G2/G4 supplies that are usually obtained with
a few taps on a "low" voltage secondary winding on the HV transformer.

Now, what would be the step to design a proper driver circuit around a
given transformer?
I have quite a lot of test equipment and I can measure inductances at
various frequencies, I have pulse/function generator (HP-8116A), HV probe
and most of what it's probably needed, but can't find a good text about
this subject.
Of course I don't want to copy a typical horizontal yoke driver plus
flyback driver circuit. I've tried to use a big-ish air-cored inductor
in place of the horizontal yoke coil and it kind of worked well enough
but the inductor was getting hot quite soon. Also the typical TV driver
circuit has no voltage feedback, probably since the horizontal yoke is the
big load there and CRT's anode load won't be changing much the power
budget
of the converter.
My next attempt will probably be designing a PWM driver based on the
old UC3843 (current mode, with a voltage feedback from a low voltage
secondary). But I'm afraid there're too many unknowns still for me to
make a good design.
Is there any way to get the right knowledge to design such a supply?

Thanks in advance.
Frank

 

[Jasen Betts]

On 2020-04-30, frank@invalid.org <frank@invalid.org> wrote:

Hi all,

I apologize for the probably too stupid questions, but it's a totally new
electronic territory for me and I don't seem to be finding the needed
theory articles and/or books to help me.

I would like to re-use some old BW TV's HV transformer to power the CRTs in
X-Y mode (all the yoke changes and drivers are already done). I'd just need
the Anode supply and the G2/G4 supplies that are usually obtained with
a few taps on a "low" voltage secondary winding on the HV transformer.

G2 and G4 are "screen" and "focus" right? usually those are tapped of the
trippler.

Now, what would be the step to design a proper driver circuit around a
given transformer?

Use the transformer and circuit you already have, Find a different way to drive the yoke.

--
Jasen.

 

Tim Williams <tiwill@seventransistorlabs.com> wrote:

Replace the yoke with an equivalent R+L (or maybe even not much R) and drive
it at roughly the expected frequency. You can use a 3843, sure, get some
nicely regulated A2 with a big fat resistor divider, without needing the
tricks/hacks they used back in the day. Trace the circuit, use the original
primary winding and voltages. If it's offline, that's probably 160VDC at
15kHz. Easy enough, if a bit of a pain if you're using a low voltage supply
instead (use a boost? add external winding? isolator for SELV control to
offline side drive?).

The original TV is a 220V AC/12V battery 11" black and white design. All was
powered with a regulated 11V rail.
I have traced the original flyback circuit, it used germanium PNP driver and
a germanium diode. It doesn't appear to have any voltage feedback. One low
voltage secondary tap was brought to a TCA511 pin to probably sense the end
of flyback period.
I've tried to substitute the horizontal coil with another similiar inductance
air-wound coil, but it became quite hot and anyway the HV wasn't quite right.
The 11" CRT needs 11kV nominal anode supply and I was getting a bit less than
8 kV with that method. G2/G4 supplies were however correct more or less
(no loading, just the tube capacitances, no filament supply when I did the
test).
So one thing that was really puzzling is: how come I get the correct low
voltages and the HV is so much off?

Bonus points for tracing the circuit to find what HV regulation feedback it
had in the first place. Usually there's a focus/screen divider chain that
returns to a not-quite-ground pin that can be used for sensing.

G2/G4 are derived from a single secondary tap, rectified by a BA158 diode
filtered by 22nF to ground then divided by a couple of 2M2 trimmers to
set the two voltages.
It was 1972...

Incidentally, a lot of Trinitons did this already, a separate HV supply,
since the horizontal sweep range was so huge. Some multisync monitors
managed all in the same circuit (impressive!). Last one I had, still have
the service docs for, HV supply was a MOSFET of all things, driving what
looks to be a fairly ordinary flyback transformer, with a BA9756 controller.
Not current mode, really weird circuit; it's parafeed through a fixed value
inductor (not an RFC, its value is significant). Well, I suppose that's as
good an indication as any about the first thing I said -- the inductance
effectively in parallel accounts for the lack of yoke, storing energy that
the transformer wouldn't otherwise. (FBTs aren't gapped very much,
remember. Air gap is what stores the energy.)

One approach I've attempted was to use an astable 555 circuit, tuned to
approximately 16 KHz, then I've used that secondary tap that went to the
TCA511 IC to derive a voltage feedback to make the 555 "wait" for the
transformer flux to go back to zero before starting a new cycle. That
produced nice driver collector and base waveforms, no ringing almost, but
also way too low HV (around 6kV), that was with no replacement yoke coil
and experimenting with capacitor values in parallel to the flyback diode.

So are you saying that the transformer alone can't store enough energy
without the original yoke coil?

And if you are using a different supply voltage, there's always the
self-excited oscillator on some turns on the back leg of the core. May be
harder to regulate though. Mind that, if you do use the same kind of
circuit (3843 + such), you MUST deal with the leakage inductance of the
shitty coupling from primary being outside the main windings. A resonant
drive is preferred for this reason.

I've seen self oscillating circuits, for example the HV supply for electrohome
G05 XY monitor. It needs a driver's base secondary winding though, that I
might add to the transformer, but well, how do you calculate the correct
turn ratio? Coupling would also be way less than perfect and so on...
Also the G05 has voltage feedback from a 90V secondary and it uses this
feedback to regulate the DC input to the primary side, clever...

Is there a way to properly design a resonant drive without using an added
secondary? Maybe using that low voltage tap I already have? Though that would
be referenced to ground.

Even with good coupling, note the secondary is significantly resonant by
itself; combined-HV multisync monitors pushed over 100kHz fundamental, with
10% or so of that being retrace -- the waveform is roughly class E, i.e.
flyback but with a resonant hump instead of a square clamped pulse. That
means the self-resonant frequency was around a MHz. Which itself is
impressive for such high voltages; they used special winding techniques to
achieve it. Regular TV FBTs won't, and will have a lower SRF (again, likely
near a useful harmonic of sweep). A resonant load means awful performance
for hard-switched flyback; it's best to take advantage of it instead.

I definitely can see multiple peaks in the driver's collector waveform, no
matter how I tried to drive it. Is there a good way to properly measure this
secondary resonant frequency? Aren't all the windings contributing to resonance
(and their loads too)?

Thanks a lot!
Frank

 

Really, just drive it. If in the US, or who cares just measure the inductance and make it resonate at about 70KHz. The you can regulate by frequency easily. Not a problem but most of the transistors that handle that will be Asian origin, if that is a problem, if so you can feed a smaller winding. Then European styles get cheaper as well as US ones. Hell I haven't bought one in a while so don't take that to the bank necessarily, but voltage is one of the prime things for transistors.

Get you a signal generator that does square waves. Get a piece of metal for a heatsink and a variable DC power supply. Got your transistor at what they intended as the primary (doesn't have to be but better if it is) and can pulse it with the generator and gradually bring up the voltage.

I approach it this way because you got a "bunch" or whatever of them. There was never any implication they were all the same. therefore it can be adapted. But if they are all the same once adapted you know what you have to do and it is a piece of cake.

I the end if they are the same you got OK, 22KHz, 55 volts and it should pull XX mA. (those are just off the top, don't use them just figure it out for yourself)

NOW, that is licked,l what about the focus voltage and possibly a high G2 voltage required ? Divide it down ? High resistors have a high price,l so it all depends on what you want to sink into this.

If you got low focus CRTs, and they exist down to zero volts, but can be as high as 25% of the anode, the lower voltage ones are advantageous in the design but in my experience the lose their focus as they age.

You can stack your own, but it really isn't much cheaper. You have to connect them and then they need to BE somewhere. The big square ones cost a fortune.

There are ways but to go into that we need to get into the parameters of the CRT.

We can find out by the number. And IF your flybacks are all the same we can figure out what they do, many have resistors built in for focus and G2.

And if you want to control the intensity which you do, those voltages are much more manageable. You don't want to burn the screens. Now if you control it by G1 you get near infinite impedance, if you use K you get to use a bit lower voltage but then there is a load. Not a huge load but something.

You work with what you got.

 

[Bill Sloman]

On Friday, May 1, 2020 at 5:44:12 PM UTC+10, fr...@invalid.org wrote:

Tim Williams <tiwill@seventransistorlabs.com> wrote:

I definitely can see multiple peaks in the driver's collector waveform, no
matter how I tried to drive it. Is there a good way to properly measure this
secondary resonant frequency? Aren't all the windings contributing to resonance (and their loads too)?

All the windings are coupled together. Just drive the secondary with a sine wave - through a resistor (or some other) impedance and see a what frequency the voltage across the coil peaks.

If the resistor is too low you will get a very broad flat-topped peak, too high and you won't see much voltage swing across the coil.

Compare the phase of the sine wave across the coil with the phase of the drive waveform - they will be in-phase at resonance.

There's not a lot of point in trying to work out where the parallel capacitance is - most of it is going to be between the turns of the secondary winding.

--
Bill Sloman, Sydney

 

Bill Sloman <bill.sloman@ieee.org> wrote:

All the windings are coupled together. Just drive the secondary with a
sine wave - through a resistor (or some other) impedance and see a what
frequency the voltage across the coil peaks.

ok, I've driven the smallest secondary tap with a 2k2 carbon comp resistor,
sine wave output at about 3V.The peak is definitely at 36 KHz
Though, if I drive the primary and measure the secondary voltage on the
same smallest tap, I get the highest amplitude at 38 KHz, I'm not sure
about this difference. Driving the primary with the same sine wave and
measuring the anode output, through a 1Gohm probe not to load it too much,
I get a peak again at 38 KHz (87V DC with 690mV RMS drive on the primary).
How can we explain this resonance difference?

If the resistor is too low you will get a very broad flat-topped peak, too
high and you won't see much voltage swing across the coil.

Compare the phase of the sine wave across the coil with the phase of the
drive waveform - they will be in-phase at resonance.

they remain pretty much in phase on a broad range around the resonance

There's not a lot of point in trying to work out where the parallel
capacitance is - most of it is going to be between the turns of the
secondary winding.

so this 36 / 38 KHz is the self resonance of the high voltage secondary?

Thanks
Frank

 

[Tim Williams]

<frank@invalid.org> wrote in message news:r8gk05$s6v$1@dont-email.me...

The original TV is a 220V AC/12V battery 11" black and white design. All
was
powered with a regulated 11V rail.
I have traced the original flyback circuit, it used germanium PNP driver
and
a germanium diode. It doesn't appear to have any voltage feedback.

Ah...

Probably didn't even have regulation, on account of B&W not giving a crap.
Or possibly some sneaky primary side or boost feedback, who knows.

One low
voltage secondary tap was brought to a TCA511 pin to probably sense the
end
of flyback period.
I've tried to substitute the horizontal coil with another similiar
inductance
air-wound coil, but it became quite hot and anyway the HV wasn't quite
right.

How similar? Hot means low efficiency. Maybe you were shorting something
out. What did the voltage and current waveforms look like?

Keep in mind they liked to use coupling caps everywhere in those things,
too, guess I should add that. Could well be that the FBT is designed for
full bipolar flux swing, not unipolar switching directly into it. Which
means it should still work at about twice the frequency without caps, which
will need even less (about half) the inductance in parallel to keep the same
peak current.

One approach I've attempted was to use an astable 555 circuit, tuned to
approximately 16 KHz, then I've used that secondary tap that went to the
TCA511 IC to derive a voltage feedback to make the 555 "wait" for the
transformer flux to go back to zero before starting a new cycle. That
produced nice driver collector and base waveforms, no ringing almost, but
also way too low HV (around 6kV), that was with no replacement yoke coil
and experimenting with capacitor values in parallel to the flyback diode.

So are you saying that the transformer alone can't store enough energy
without the original yoke coil?

Yeah, that's the right kind of waveform. Mind if the transistor is BJT, it
needs an antiparallel ("damper") diode (the original will either be right
beside it, or integrated with the HOT). MOSFET has body diode, is fine.
What's missing then is either enough on-time to charge to the required
energy, or low enough inductance to do the same in the first place.

And yep, that's exactly what missing the yoke should do.

I've seen self oscillating circuits, for example the HV supply for
electrohome
G05 XY monitor. It needs a driver's base secondary winding though, that I
might add to the transformer, but well, how do you calculate the correct
turn ratio? Coupling would also be way less than perfect and so on...

Because of poor coupling, a push-pull circuit is best. The reactive energy
of the primary and leakage inductances are "stirred" into the supply, just
as the damper diode clamps resonant energy in the quasi-resonant case.
Whereas a single-switch circuit might have to dump all that energy into a
snubber.

The base winding can be in the same place. Just put on two CT windings, one
with few turns and wire size that doesn't matter, and another with a few
more turns and a few amps worth of wire size.

Typical guess would be a few volts per turn, so out of a 12V supply, 3 or 4
turns (for each half of the primary) should do. Base off voltage should be
above -6V peak (to avoid breakdown, causing runaway operation), so about
half the turns or 1-2 will do there. Connect the CTs to supplies and you're
off running. The base winding is usually supplied with a resistor, the
current setting on-time or power output or the like.

Alternately, use MOSFETs with the gates driven from opposite drains, no need
for a separate drive winding. Usually a series diode and pull-up resistor
is used to limit gate on voltage.

Example:
https://www.seventransistorlabs.com/Images/HVPower1.png
This was a custom transformer so the voltage isn't incredible, but the
circuit overall is actually a bit relevant...

The series inductor to the primary CT is generally beneficial, but it
depends on what type of circuit one is building. Baxandall (as BS will
regale you) needs it. Royer doesn't (commutation driven by transformer
saturation).

You may find you need much higher supply voltage (or fewer turns) to get the
desired peak output voltage this way. And if it's resonating at a much
higher frequency (100s kHz), well, that's just what it's going to do; if you
need to keep PRF below 20kHz say to avoid overheating diodes and stuff,
you'll need to set up a gate circuit to do that.

Also the G05 has voltage feedback from a 90V secondary and it uses this
feedback to regulate the DC input to the primary side, clever...

Is there a way to properly design a resonant drive without using an added
secondary? Maybe using that low voltage tap I already have? Though that
would
be referenced to ground.

Random windings and taps have random ratings. Bad idea. Stick to known
primary.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[John S]

On 4/30/2020 8:39 PM, Bill Sloman wrote:

On Friday, May 1, 2020 at 6:38:52 AM UTC+10, fr...@invalid.org wrote:
Hi all,

I apologize for the probably too stupid questions, but it's a totally new
electronic territory for me and I don't seem to be finding the needed
theory articles and/or books to help me.

I would like to re-use some old BW TV's HV transformer to power the CRTs in
X-Y mode (all the yoke changes and drivers are already done). I'd just need
the Anode supply and the G2/G4 supplies that are usually obtained with
a few taps on a "low" voltage secondary winding on the HV transformer.

Now, what would be the step to design a proper driver circuit around a
given transformer?

You need to characterise the transformer. You need the inductance of each winding, the mutual inductance between each pair of windings, and the stray capacitance associated with each winding (which can be hard to measure since getting current flowing through one capacitance means having current flowing through all the others).

The transformer equation is

V1= L1.dI1/dt + M.dI2/dt

V2 = M.dI1/dt + L2.dI2/dt

where M is the mutal inductance of the two coils and less than the square root of the product of L1 and L2

M = k. (L1.L2)^0.5

What is M for 3 coils?

 

Tim Williams <tiwill@seventransistorlabs.com> wrote:

Ah...

Probably didn't even have regulation, on account of B&W not giving a crap.
Or possibly some sneaky primary side or boost feedback, who knows.

this is as much as I tracked of the original circuit:
https://drive.google.com/open?id=12QyYICJwguBYYugYzlthaz2xRN7TZri8

The horizontal yoke is 88uH, the schematic has an error (not so relevant) as
R504 is a pullup on the TCA511 driver pin which is open collector, so it
is infacts on the other side of the 220 ohms resistor.

I've tried to substitute the horizontal coil with another similiar
inductance
air-wound coil, but it became quite hot and anyway the HV wasn't quite
right.

How similar? Hot means low efficiency. Maybe you were shorting something
out. What did the voltage and current waveforms look like?

I've found a big-ish 95 uH inductor, probably a power supply filter of some
sort and that was wired in place of the horizontal yoke coil and the linearity
coil was shorted out of circuit.
Voltage waveforms here:
https://drive.google.com/open?id=17aBdC87kRmNPmunUetfP2-y3SjAuxYRH

Top one is the driver transistor's collector (a BU124), 50V/div
bottom one is the transistor base, horizontal is 10us/div.
I didn't make more checks as the EHT was still too low to give me
confindence in tweaking the circuit further.

Keep in mind they liked to use coupling caps everywhere in those things,
too, guess I should add that. Could well be that the FBT is designed for
full bipolar flux swing, not unipolar switching directly into it. Which
means it should still work at about twice the frequency without caps, which
will need even less (about half) the inductance in parallel to keep the same
peak current.

well the original circuit seems enough simple, I don't think I've missed
a series capacitor to the primary.

So are you saying that the transformer alone can't store enough energy
without the original yoke coil?

Yeah, that's the right kind of waveform. Mind if the transistor is BJT, it
needs an antiparallel ("damper") diode (the original will either be right
beside it, or integrated with the HOT). MOSFET has body diode, is fine.
What's missing then is either enough on-time to charge to the required
energy, or low enough inductance to do the same in the first place.

And yep, that's exactly what missing the yoke should do.

well, I've played quite a little bit with the pulse width and repetion
frequency, but that didn't really give any good result.

Because of poor coupling, a push-pull circuit is best. The reactive energy
of the primary and leakage inductances are "stirred" into the supply, just
as the damper diode clamps resonant energy in the quasi-resonant case.
Whereas a single-switch circuit might have to dump all that energy into a
snubber.

The base winding can be in the same place. Just put on two CT windings, one
with few turns and wire size that doesn't matter, and another with a few
more turns and a few amps worth of wire size.

Typical guess would be a few volts per turn, so out of a 12V supply, 3 or 4
turns (for each half of the primary) should do. Base off voltage should be
above -6V peak (to avoid breakdown, causing runaway operation), so about
half the turns or 1-2 will do there. Connect the CTs to supplies and you're
off running. The base winding is usually supplied with a resistor, the
current setting on-time or power output or the like.

Alternately, use MOSFETs with the gates driven from opposite drains, no need
for a separate drive winding. Usually a series diode and pull-up resistor
is used to limit gate on voltage.

Example:
https://www.seventransistorlabs.com/Images/HVPower1.png
This was a custom transformer so the voltage isn't incredible, but the
circuit overall is actually a bit relevant...

thanks so much for the example! Should I use litz wire for the primary turns?
Can these turns be added over the existing primary? That would make them
quite far away from the ferrite. Or maybe it's better to try to "squish" them
between the end of the existing winding and the start of the top ferrite
horizontal segment?

The series inductor to the primary CT is generally beneficial, but it
depends on what type of circuit one is building. Baxandall (as BS will
regale you) needs it. Royer doesn't (commutation driven by transformer
saturation).

You may find you need much higher supply voltage (or fewer turns) to get the
desired peak output voltage this way. And if it's resonating at a much
higher frequency (100s kHz), well, that's just what it's going to do; if you
need to keep PRF below 20kHz say to avoid overheating diodes and stuff,
you'll need to set up a gate circuit to do that.

Should I expect that this resonating frequency be close to the current
resonance peak I've measured on the unmodified transformer?

Random windings and taps have random ratings. Bad idea. Stick to known
primary.

yes makes a lot of sense, and I think it's feasible on the old 1970's
high voltage transformers that weren't all potted.

Thanks again
Frank

 

Tim Williams <tiwill@seventransistorlabs.com> wrote:

Because of poor coupling, a push-pull circuit is best. The reactive energy
of the primary and leakage inductances are "stirred" into the supply, just
as the damper diode clamps resonant energy in the quasi-resonant case.
Whereas a single-switch circuit might have to dump all that energy into a
snubber.

The base winding can be in the same place. Just put on two CT windings, one
with few turns and wire size that doesn't matter, and another with a few
more turns and a few amps worth of wire size.

Typical guess would be a few volts per turn, so out of a 12V supply, 3 or 4
turns (for each half of the primary) should do. Base off voltage should be
above -6V peak (to avoid breakdown, causing runaway operation), so about
half the turns or 1-2 will do there. Connect the CTs to supplies and you're
off running. The base winding is usually supplied with a resistor, the
current setting on-time or power output or the like.

Alternately, use MOSFETs with the gates driven from opposite drains, no need
for a separate drive winding. Usually a series diode and pull-up resistor
is used to limit gate on voltage.

Example:
https://www.seventransistorlabs.com/Images/HVPower1.png
This was a custom transformer so the voltage isn't incredible, but the
circuit overall is actually a bit relevant...

4+4 turns on the first random HV transformer I have, 2xIRF640 (I don't have
the 644, but looks like it shouldn't matter much).
HV is at 8.6kV huge step forward!!!
The oscillation frequency went to 29.3 KHz.
Now, to increase the output, can I safely go to 3+3 turns?
The mosfets don't even get warm after one minute or so.
Here's one drain waveform:
https://drive.google.com/open?id=18tHnsTQeCGhZrISgkg8uDgI2rbPZ-UHy

The series inductor to the primary CT is generally beneficial, but it
depends on what type of circuit one is building. Baxandall (as BS will
regale you) needs it. Royer doesn't (commutation driven by transformer
saturation).

I've used a 150 uH inductor, doesn't even get warm, but I'm not loading
any supply other than the HV (connected to the CRT to have the DC smoothed
by the internal capacitor) with the 1Gohm HV probe.

You may find you need much higher supply voltage (or fewer turns) to get the
desired peak output voltage this way. And if it's resonating at a much
higher frequency (100s kHz), well, that's just what it's going to do; if you
need to keep PRF below 20kHz say to avoid overheating diodes and stuff,
you'll need to set up a gate circuit to do that.

I guess BA159 fast recovery will be ok for the low voltages?
As for the HV diode string, that's internal to the potted part of the flyback,
I hope it's not too upset for the 29 KHz now.

So, thanks a whole lot again! I was really scratching my head since many weeks
on the wrong circuits

Frank

 

[Bill Sloman]

On Friday, May 1, 2020 at 9:59:52 PM UTC+10, John S wrote:

On 4/30/2020 8:39 PM, Bill Sloman wrote:
On Friday, May 1, 2020 at 6:38:52 AM UTC+10, fr...@invalid.org wrote:
Hi all,

I apologize for the probably too stupid questions, but it's a totally new
electronic territory for me and I don't seem to be finding the needed
theory articles and/or books to help me.

I would like to re-use some old BW TV's HV transformer to power the CRTs in
X-Y mode (all the yoke changes and drivers are already done). I'd just need
the Anode supply and the G2/G4 supplies that are usually obtained with
a few taps on a "low" voltage secondary winding on the HV transformer.

Now, what would be the step to design a proper driver circuit around a
given transformer?

You need to characterise the transformer. You need the inductance of each winding, the mutual inductance between each pair of windings, and the stray capacitance associated with each winding (which can be hard to measure since getting current flowing through one capacitance means having current flowing through all the others).

The transformer equation is

V1= L1.dI1/dt + M.dI2/dt

V2 = M.dI1/dt + L2.dI2/dt

where M is the mutal inductance of the two coils and less than the square root of the product of L1 and L2

M = k. (L1.L2)^0.5


What is M for 3 coils?

Different. Basically, k is that proportion of the flux generated by the current in the first coil that threads the other coil.

k for two coils that are wound as twisted pairs of wire are going to be closer to one than k for two coils that are wound separately, on top of or next to one another.

For a three coil assembly you'd have M12 = k12.(L1.L2)^0.5,
M13 = k13.(L1.L3)^0.5, and M23 = k23.(L2.L3)^0.5 .

LTSpice lets you specify different k values for each pair of windings - if you want to. See the manual on "K mutual inductance".

--
Bill Sloman, Sydney

--
Bill Sloman, Sydney

--
Bill Sloman, Sydney

 

frank@invalid.org wrote:

Example:
https://www.seventransistorlabs.com/Images/HVPower1.png
This was a custom transformer so the voltage isn't incredible, but the
circuit overall is actually a bit relevant...


4+4 turns on the first random HV transformer I have, 2xIRF640 (I don't have
the 644, but looks like it shouldn't matter much).
HV is at 8.6kV huge step forward!!!
The oscillation frequency went to 29.3 KHz.
Now, to increase the output, can I safely go to 3+3 turns?
The mosfets don't even get warm after one minute or so.
Here's one drain waveform:
https://drive.google.com/open?id=18tHnsTQeCGhZrISgkg8uDgI2rbPZ-UHy

ok I've tried 3+3 turns as primary and the PC power supply that I'm using
tried to start but went immediately into shutdown. I could see a +12kV
indication at start.
I then had the idea of trying a 1 ohm resistor in series to the
positive supply to the transformer center tap, but that made the mosfets
(both) very hot and made something smoke. I'll investigate later what
smoked and I'll try someting like 3.5 + 3.5 turns.
I'm not using the voltage and current limiting circuit *yet*, I think that
would do the job of making the oscillator start and regulate the DC input,
but I would first measure what voltage can I get from a secondary tap,
then calculate a divider to serve as voltage feedback.
Any better idea?
Well, lots of experiments before I reach the goal, but I think I'm on the right
track.

Frank

 

Hi again,

Tim Williams <tiwill@seventransistorlabs.com> wrote:

Alternately, use MOSFETs with the gates driven from opposite drains, no need
for a separate drive winding. Usually a series diode and pull-up resistor
is used to limit gate on voltage.

Example:
https://www.seventransistorlabs.com/Images/HVPower1.png
This was a custom transformer so the voltage isn't incredible, but the
circuit overall is actually a bit relevant...

I have a few questions about this circuit, if you don't mind:

1) I don't have such a high Hfe BJT as the 2SD1273, I think a sziklai pair
in place of it should be fine, since it seems to me it will never be saturated
anyway, unless I'm missing something?

2) the 1/2 358 opamp has no DC negative feedback, that's well, unusual to
me, I would like to understand better that part.
Thanks

Frank

 

[Tim Williams]

<frank@invalid.org> wrote in message news:r8ltgm$dq9$1@dont-email.me...

I have a few questions about this circuit, if you don't mind:

1) I don't have such a high Hfe BJT as the 2SD1273, I think a sziklai pair
in place of it should be fine, since it seems to me it will never be
saturated
anyway, unless I'm missing something?

2) the 1/2 358 opamp has no DC negative feedback, that's well, unusual to
me, I would like to understand better that part.

This is what we call an error amplifier. Its output can indeed be saturated
+/- if the load is taking its sweet time to catch up, or if the input is
sudden and large. The 10k in, and 0.01 + 100k across it, sets the response
time and gain (compensation).

2SD1273 can of course be replaced with anything of suitable rating, Sziklai,
Darlington, MOSFET if you don't mind the lost Vgs(on); or even better, a
buck converter -- the purpose is simply to make voltage at the primary CT.
Or current at the CT more specifically, since the oscillator is current
mode. Which means a current-mode buck, and usually a relatively large
inductor so the current ripple is small, is ideal.

Eventually the voltage output responds to the error amp's output (changing
the oscillator's supply voltage), and the voltage divider closes the DC
feedback loop.

The feedback is also buffered, so there's an accurate x1000 output sense (I
don't have HV probes, as it happens).

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

Tim,
first of all thanks for all the information!

Tim Williams <tiwill@seventransistorlabs.com> wrote:

This is what we call an error amplifier. Its output can indeed be saturated
+/- if the load is taking its sweet time to catch up, or if the input is
sudden and large. The 10k in, and 0.01 + 100k across it, sets the response
time and gain (compensation).

well yes I figured out it would be closing the loop with that time constant
of 1ms, it was just the lack of DC path that was looking odd to me, but it's
just me
Nothing bad happens if the output saturates. New things are always welcome
for learning.

2SD1273 can of course be replaced with anything of suitable rating, Sziklai,
Darlington, MOSFET if you don't mind the lost Vgs(on); or even better, a
buck converter -- the purpose is simply to make voltage at the primary CT.
Or current at the CT more specifically, since the oscillator is current
mode. Which means a current-mode buck, and usually a relatively large
inductor so the current ripple is small, is ideal.

ok I understood, I think I'll just stick with a sziklai pair since that
will allow me to have a prototype sooner.
I'm still a long way from the needed 11 kV.

Eventually the voltage output responds to the error amp's output (changing
the oscillator's supply voltage), and the voltage divider closes the DC
feedback loop.

The feedback is also buffered, so there's an accurate x1000 output sense (I
don't have HV probes, as it happens).

yes I've noticed that, I won't be needing it since it's a bad idea to
wire a long string of resistors down from the CRT anode. I'll be using
a low voltage G2/G4 supply for feedback, if I can make it to work in the
first place that is.

Thanks
Frank