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the best way to stack flyback transformers secondaries?

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LAN_403

father dest

Tue Sept 10 2019, 06:34PM

Registered Member #54795 Joined: Mon Apr 06 2015, 11:55AM
Location:
Posts: 28

hi errbody.
can you help me a little bit with flyback transformers? in this video

there is 4 of them - two in parallel, another two in parallel too and those 2 groups in series, with primaries out of phase (like center tapped nst). so maximum voltage between secondary output of each transformer in each group and the core is no more than rated secondary voltage, about 8 kv. thats great, but total output is 16 kv only - i need 32. so i want to connect all secondaries in series, or two in series and and two groups in series too, as i did before while paralleling them, or some other variants. but this time some secondaries can see 2 or 3 their rated voltage, depending on the variant of their connection, which is bad.

so maybe you know the best way to stack transformers? which will result in minimum overvoltage on the secondary? or the minimum number of overvolted secondaries in total.

and there is one more problem here - flybacks work on something like 60 khz, so capacitive currents build up like crazy - i think primaries are acting like directly connected to the ground through transistors of the half bridge and mains, so probably i need not only configuration of secondaries connection that results in minimum overvoltage, but configuration that results in minimum secondary to core/primaries capacitance too? or both at the same time?
and maybe that capacitive high frequency stress on the dielectric of the secondary is even worse than just overvoltage, coz it heats dielectric up?

so there is 4 variants of interconnection as i see it in the pic below, with one wire of secondary connected to the core (inner wire - beginning of the winding) - core is that vertical line:

the variant on the right is the easiest one which is the worst one at the same time. so what variant is the best?
plz give your own vars & thougts.

Patrick

Sun Sept 22 2019, 06:19PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Putting flybacks in series isnt generally done. After 2 (or including 2) you get secondary to core arcing, which theres now way to insulate against. Even if you improved that, you tend to get secondary to primary/source/transistor arcing, which you cant improve.

Best is to move up to a better transformer, i have a flyback that puts out 52 kV using in native primary. new flybacks are better then scavenged ones. check ebay.

father dest

Mon Sept 23 2019, 03:43PM

Registered Member #54795 Joined: Mon Apr 06 2015, 11:55AM
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Posts: 28

Patrick wrote ...
i have a flyback that puts out 52 kV

with no current i guess, so no power also. while each of my four flybacks give about 200ma shorted on the secondary side, so around 600W from each, 2400W in total.
that's the difference.

Sulaiman

Mon Sept 23 2019, 04:18PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

What is the intended purpose of the high voltage ?
This will usually determine the design.

father dest

Mon Sept 23 2019, 04:35PM

Registered Member #54795 Joined: Mon Apr 06 2015, 11:55AM
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Posts: 28

Sulaiman wrote ...

What is the intended purpose of the high voltage ?

power source for some home x-rays, 100-300kv, 1-3kw.

Sulaiman

Tue Sept 24 2019, 02:16AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

300 keV is in the gamma ray range, 10 mA beam current is a lot of radiation,
sorry, I do not want to help with this project as I feel it is too hazardous.

jpsmith123

Tue Sept 24 2019, 03:01PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

So you've got four HV transformers that can produce 8 kv and you're looking to get a reliable +/- 16 KV to drive a full wave CW voltage multiplier, or something like that?

Can you just wind some ferrite core isolation transformers with secondaries insulated for high voltage? IOW each HV transformer will be driven by, say, a 1:1 ferrite transformer whose secondary is insulated?

father dest

Tue Sept 24 2019, 06:42PM

Registered Member #54795 Joined: Mon Apr 06 2015, 11:55AM
Location:
Posts: 28

Sulaiman wrote ...

I do not want to help with this project as I feel it is too hazardous.

who said you can help? you just lacking info for that.
"gamma ray" - LOL.
for your information - i already have 110kv 25ma working unit, it's just too weak for me.
and you can't help with that eather : )
"beam current" - LOL.

jpsmith123 wrote ...

So you've got four HV transformers that can produce 8 kv and you're looking to get a reliable +/- 16 KV to drive a full wave CW voltage multiplier, or something like that?

exactly - it's just the easiest way. my 110kv head does just that, it's simply not designed for continuous operation - not thermally, nor electrically.
so am doing other (much more beefier) one from scratch.

wrote ...
Can you just wind some ferrite core isolation transformers with secondaries insulated for high voltage? IOW each HV transformer will be driven by, say, a 1:1 ferrite transformer whose secondary is insulated?

i have enough leakage inductance in those 4 flybacks already - my concern is that any additional one can cap my power below 3-4kw, which i don't like.
and i am too lazy to wind another 8 windings - there must be easier way : D
i'll save this variant for backup plan tho.

jpsmith123

Tue Sept 24 2019, 07:48PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

If you use a big ferrite core maybe you could put the primary and four secondary windings on it? With a big core you wouldn't need many turns so I think leakage inductance would not be excessive.

BTW check out this power supply:

father dest

Wed Sept 25 2019, 03:34PM

Registered Member #54795 Joined: Mon Apr 06 2015, 11:55AM
Location:
Posts: 28

jpsmith123 wrote ...

If you use a big ferrite core

BTW check out this power supply:

that's all great and such, but, the question of this topic was "can you help me a little bit with flyback transformers? / maybe you know the best way to stack transformers?"
it wasn't about how to build high voltage supply per se, or how to build it in a proper way, etc.

it's just about those 4 flybacks, and nothing more, like "gamma rays" for example, LMAO.

so i guess the answer is number 2 from the left, coz all secondaries to core voltages would be no more than their rated voltage, the maximum would be 32kv between two primaries, but since primaries are wound by myself, i can isolate those as much as i want. which is not the case for the secondaries.

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