HVHF Transformer problem

[davoud]

Hello guys
I want to make a H.V.H.F Transformer (25Kv – 10ma).
So I want to turn a transformer.
Last time I made a Trans with fly-back circuit and I reached to this voltage but coils burned because of corona inside wire winds.
Next I wound coils and coated them with a type of high voltage strength Resin.
In this part I did wonder when I connect it to circuit board and turn it on.
1-Resonance frequency was changed and was very low (before curing we had 11.KHz and after curing with epoxy I had about 1 KHz).
2-When I connect it to rectifier and regulated it with capacitance there is not enough current for staying to this voltage and the Voltage dropped to 300v.

I don’t know where is the problem?!
I thought there wasn’t enough Flux, So I turned a greater primary coil but I hadn’t any Promising result.

I would be thankful for any suggestions.

 

[goldsmith]

Trans with fly-back circuit

Hi Davoud
What kind of flyback circuit ? it can be easily done with some simple methods .is accuracy important for you ? can you tell me more about your details , please ?

Next I wound coils and coated them with a type of high voltage strength Resin.
In this part I did wonder when I connect it to circuit board and turn it on.
1-Resonance frequency was changed and was very low (before curing we had 11.KHz and after curing with epoxy I had about 1 KHz).

How did you twist your wires ? i hope you didn't wound them like a usual transformer ? did you ?
the usual way to earn 25KV 10 ma is using an stage which can convert the voltage to the 5 KV or a bit less and then a multiplier . ( by the way , do you need DC voltage 25 kv ? )
More details can clear it more then you can get more helps .
Best Wishes
Goldsmith

 

[FvM]

As a first point, I agree with goldsmith that using a cascade rectifier is the most promising way to generate high DC voltages, if this is your intention.

In detail, your post misses most necessary information, so it's hard to give any helpful suggestions.
Some points that should be answered:
- intended output DC or AC, is 25 kV peak or rms voltage?
- transformer core data, did you calculate saturation flux and set number of turns respectively

Windings capacitance and self resonance frequency are surely a problem, your results are inplausible, however. Permittivity of expoxy resin isn't above 4, resonance frequency after potting the winding shouldn't be therefore lower than half of the in-air value. Otherwise, it's either a measuring fault or something else went wrong.

In many cases, it should be possible to reuse a TV HV transformer.

 

[davoud]

Dear friends,
Thanks for your kind answers.
As you wanted, additional information about transformer is:
1) Core : Dual U type (240mm*120mm)
2) Wire : primary : magnet wire 2mm
Secondary: magnet wire 0.20mm
3) Primary coil : 100 turns
4) Secondary coil including: 10 Stage and each stage 1900 turns
5) Insulation Gap between each two stage : 3mm
6) Bobbin type : P.T.F.E
And about circuit I use a simple fly back circuit by TL494 controller; however I put two potentiometers for tuning duty cycle and frequency.
I should pointed if I use cascade circuit then I need a big space for high voltage high current capacitors and rectifiers (I need continues DC Voltage 25Kv 10ma for air refinery).So I should build this voltage by the coil. Also I need DC RMS 25Kv for it.
AS I said problem happened after curing it by epoxy.
Thanks

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[FvM]

Also I need DC RMS 25Kv for it.

Positive or negative? Positive would be very easy with a standard TV "diode split" transformer.

Required number of turns is a calculus of core area, maximum flux and intended Vt integral. Flyback converters will ususally implement asymmetrical duty cycles and respective voltage ratios. For the present 19000 windings, I estimate about 25 kV*100µS possible voltage time integral, which sounds to be at least factor 10 high for a usual flyback design.

 

[davoud]

I need produce negative voltage. I connect one end of coil 1 to earth.

I estimate about 25 kV*100µS possible voltage time integral, which sounds to be at least factor 10 high for a usual flyback design.

Thus you think I should replace my switching topology to another , for example Full Bridge ?


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Also please clear me about equation that you used : voltage * cycle time ?

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At last I knew if power be higher than 100 watt there is suitable we use double forward or full bridge but i didn't know about related equations.

 

[FvM]

The schematic in your latest post implements what I called a "diode split" transformer. Filter capacitors should be added to take full advantage of the toplogy (reduced AC voltage between windings).

The time in my voltage time integral is the flyback pulse width, not the cycle time. I was considering an asymmetrical waveform.

 

[davoud]

Okay , very thanks.yes you are right it need filter capacitor.
Actually I was connecting a Big Capacitor to output but it does not help to rising voltage however it was damping ripple but droop voltage because of power consumption.

Dear FVM as I said I could produce 25Kv but that is not stable because of corona and after 5 minute all coil damaged. After curing coils I don't able to reach this voltage, I don't know where is my wrong .

 

[FvM]

My preference would be a rectifier cascade with a larger number of stages and moderate transformer output voltage that doesn't bring up partial discharge issues. And a handy switching frequency, e.g. 25 up to maximum 50 kHz, push-pull switcher, similar to standard SMPS, e.g. PC power supply.

 

[davoud]

Before I potting coils I could rise voltage at each stage to 4Kv but after curing there is just 232v per each coil at 700Hz.
I measure parameter of each coil . all of them are same . the result show at below :
1) 100Hz L = 500mH Dissipation = 1.05 Quality = 0.94 and ESR=330 ohm
2) 1KHz L= 490mH Dissipation = 0.117 Quality = 8.57 and ESR = 360 ohm
3) 10KHz L=505mH Dissipation = 0.02 Quality = 45.7 and ESR = 698 ohm
4) 100KHz L=36.9mH Dissipation = 2.36 quality = 0.45 and ESR = 50Kohm

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Push-Pull Okay good suggestion I work on it. Thank you

 

[FvM]

The measurement conditions are not clear.

I measure parameter of each coil

Means:
- single coil with core
- multiple coils with core
- or something different?

the 500mH number refers to an AL of about 140 nH for n=1900. Is this true?

The 100 kHz measurement is apparently near to self resonance frequency, of course LCR measurements are wrong in this case.

To determine transformer behaviour, leakage inductance between primary and each secondary winding is the more interesting parameter, resosnance of leakage inductance with windings capacitance is the actual limiting factor for high frequency operation.

 

[davoud]

Oh sorry
That measurement is for one stage without core.
parameters with core approximately are :
1) 100Hz L = 1167mH Dissipation = 0.451 Quality = 2.2 and ESR=331 ohm
2) 1KHz L= 1154mH Dissipation = 0.05 Quality = 19.7 and ESR = 369 ohm
3) 10KHz L=1268mH Dissipation = 0.012 Quality = 80.9 and ESR = 950 ohm
4) 100KHz L=118mH Dissipation = 0.14 quality = 7.00 and ESR = 51Kohm

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Dear FVM is there any way for measure leakage also for measurement capacitance of each coil ?

 

[FvM]

I don't understand the results, there should be a considerable core effect, so something is apparently wrong.

 

[davoud]

Measurement for all coil :
Without core
100 Hz L= 15.39H D= 0.341 Q= 2.93 ESR= 3.2 Kohm
1KHz L=15.34H D= 0.37 Q= 26.1 ESR = 3.5 Kohm
10KHz -------there is no ability to measure it's parameters
with core
100Hz L=1027H D=0.007 Q=1600 ESR=5Kohm
120Hz L=1437H D= 0.005 Q= 269 ESR = 5.7Kohm
1KHz -------there is no ability to measure it's parameters

 

[davoud]

I change switching circuit to push pull but still it not have a good result.
However if I disconnect secondary then it can make 30kv but when I connect any capacitance or load voltage droop to 300v.

 

[goldsmith]

I change switching circuit to push pull but still it not have a good result.
However if I disconnect secondary then it can make 30kv but when I connect any capacitance or load voltage droop to 300v.

Hi Davoud
Can you show us the schematic which are referring to ?
What kind of push pull ? with or without feedback ? if you have feedback path , then you shouldn't see any voltage drop when you are putting the load in outlet .
( I mean if you go through an standard Push pull switching converter as an increasing system , then you can have a feedback to reach your desired voltage as well . )
Good Luck
Goldsmith

 

[FvM]

If the output voltage is just collapsing with a small load, it's unlikely a feedback problem, presumed that the primary side is driven regularly. It rather looks like a problem of excess leakage inductance.

 

[davoud]

Hi Goldsmith, and thanks FvM,
Hear you can see my circuit :

I think my switching circuit have a good operation because monitoring at oscilloscope show a good signal on primary coil , but according to FvM post

It rather looks like a problem of excess leakage inductance.

this problem build by leakage.
Also I reviewed epoxy characteristics and i found this epoxy have a higher dielectric constant(6.5) and lower surface resistance (2.4*10e11) than the others.
also i measured capacitance between to coils,it is 50pf ! and it have impedance 3.5kohm it 100khz and 22kohm at 10khz.
So please guide me :
1- is this epoxy suitable for insulation coils ? high dielectric is good or not ?
2- is this impedance correct? i am confused ! (i measure it for all coils and many times but it is true value )
Any suggestions will be appreciated.