Author Topic: Flyback transformer power capability  (Read 543 times)

Offline MRMILSTAR

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Flyback transformer power capability
« on: December 07, 2020, 05:10:37 PM »
I know that it varies with the size of the flyback but does anyone know the "typical" amount of power that a flyback transformer can supply? I have two identical AC units which appear to be large (2.38" x 1" coil dimensions) from my limited experience with them. I am considering paralleling them just to be conservative. I can never find any specs on flybacks whether AC or DC.

This will be used to power the 14-stage CW multiplier that I am currently building. There are 28 capacitors with 1.7 nF each.
« Last Edit: December 07, 2020, 08:52:33 PM by MRMILSTAR »
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline Da_Stier

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Re: Flyback transformer power capability
« Reply #1 on: December 07, 2020, 07:49:42 PM »
A small warning at first, my answer might not be too helpful.  ;)

Now that that is out of the way, I always try to calculate the output current by looking at the input power and measuring the output voltage. By assuming, I'm not saturating the transformer and an efficiency of 80%, I calculate the current.

A typical ZVS driver that I like to use can draw around 50W at 12V on the input.
By using the top assumptions, that would give me 4mA at 10kV. However the flyback gets pretty hot pretty quick under these circumstances.

I can't guarantee that any of this is accurate, however the current that is needed in a TV is pretty darn small, as far as I know. 

Offline davekni

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Re: Flyback transformer power capability
« Reply #2 on: December 07, 2020, 09:39:11 PM »
AC flyback transformers are rare, so I have no idea what yours were originally designed for.  I think 50W is about right for a typical DC flyback for a 19" TV, around 2mA at 25kV.  (Da_Stier:  4mA at 10kV may be running hot due to higher current through the windings and diodes.  Or, more likely, ferrite losses due to sine-wave excitation.  Typical flyback drive has less average voltage for a given peak output voltage.)
David Knierim

Offline Da_Stier

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Re: Flyback transformer power capability
« Reply #3 on: December 08, 2020, 12:10:30 PM »
(Da_Stier:  4mA at 10kV may be running hot due to higher current through the windings and diodes.  Or, more likely, ferrite losses due to sine-wave excitation.  Typical flyback drive has less average voltage for a given peak output voltage.)

Makes sense, I guess it might have been a better idea to test it at the voltage it is designed for, thanks.

I asked a friend who used to work  a lot on CRT TVs, according to him, a smaller TV (like 15" or something like that) uses around 1.3 to 1.8mA.

Another source might be something like this:
https://www.genvolt.com/application-information/5147347e0e033/CRT-Power-Supply
It's a piece of testgear / a powersupply for work on CRTs.
It provides 1.3mA at 20 to 28kV.

So I guess the 50W still seems pretty realistic.

Offline SteveN87

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Re: Flyback transformer power capability
« Reply #4 on: December 08, 2020, 04:17:51 PM »
This thread from 4HV might be useful:

https://4hv.org/e107_plugins/forum/forum_viewtopic.php?165666

With DC flybacks, I've found that 150W seems to be about the limit for ones without internal capacitors or screen/focus outputs.

Offline hammertone

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Re: Flyback transformer power capability
« Reply #5 on: January 20, 2021, 04:37:14 PM »
Steve,

The only limit to the power that you can pass with a transformer is set by the copper losses, the core doesn't care, with a few limitations: (Ferrite core)
The core must be kept from saturation, this means that deltaB (fluxvariation) should be below ~0.4T, but if it is driven at a frequency higher than 20kHz,  the limit to the flux variation has to be lowered because the core will heat up. At 100kHz you may well have to lower the fluxvariation to 0.1T. So below 20kHz the core is *saturation limited*, above 20kHz it is *core loss limited*.

Look at it this way: the transformer has an ideal transformer, with the primary parallelled by the magnetizing inductance, like this:



It is the magnetizing inductance that saturates the core if too much current is passed through it, or it heats the core up, if the frequency is high enough, so as long as there are enough turns on the primary coil to produce enough inductance, so that the flux stays below a safe limit, then the ideal transformer will happily transfer all the power you want.
But at some point, the coils will get hot, start to smoke, burst in flames, ultimately, fuses will blow.

I highly recommend you read this fine piece by Dr. Ray Ridley, he is an expert on magnetics :

https://ridleyengineering.com/design-center-ridley-engineering/39-magnetics/281-106-custom-transformers%E2%80%93one-design-equation.html
 
Hope this helps,
Cheers, Finn Hammer

Offline SteveN87

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Re: Flyback transformer power capability
« Reply #6 on: January 20, 2021, 08:40:45 PM »
OK. Got it.

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Re: Flyback transformer power capability
« Reply #6 on: January 20, 2021, 08:40:45 PM »

 


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