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Old 13th November 2012, 07:58 PM   #1
vasko is offline vasko  Czech Republic
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Smile SMPS number of turns

I made this flyback for tube preamp.

Click the image to open in full size.


Problem is I have low voltage.

What I did:

- Input 16V

Primar:

- 8 turns of 3x1mm diameter wire

Secundar:

- 13 turns, should give 20V, it gives 11V, with 10k pot at max
- 110 turns, should give 200V, it gives 130V, with 10k pot at max

I did try other orientation of diode, it gave much less voltage. PCB wiring and mass is made properly for high frequency.

I can add more turns, but it doesn't make sense bearing in mind many calculations I saw in books and schematics.

What is going on?

Last edited by vasko; 13th November 2012 at 08:02 PM.
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Old 13th November 2012, 08:12 PM   #2
Elvee is offline Elvee  Belgium
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You didn't provide what are probably the most important data in this case: the core parameters and properties: Ae (effective area), material, Lf, air gap, etc.
That's probably where your problems hide
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Old 13th November 2012, 08:13 PM   #3
vasko is offline vasko  Czech Republic
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Its EI-33 core form 350W ATX.

Last edited by vasko; 13th November 2012 at 08:16 PM.
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Old 13th November 2012, 11:39 PM   #4
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Gap?

Are the windings closely interleaved?

Is IRF510 big enough for the amount of power you intend to draw, and is the 0.22 ohm shunt resistor small enough as well?

There should be a resistor in series with the 0.01 capacitor between COMP and FB, try 10k.
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Old 13th November 2012, 11:40 PM   #5
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You could increase the value of 470k to compensate ?

However you need to gap the core.
At least a 1mm gap is required to stop the transformer saturating.
Also if the transformer gets hot the gap will again help stop saturation.
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Old 13th November 2012, 11:44 PM   #6
vasko is offline vasko  Czech Republic
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Quote:
Originally Posted by Sch3mat1c View Post
Are the windings closely interleaved?
Yes.

Quote:
Originally Posted by Sch3mat1c View Post
Is IRF510 big enough for the amount of power you intend to draw, and is the 0.22 ohm shunt resistor small enough as well?
IRF510 is cold totaly.

Quote:
Originally Posted by Sch3mat1c View Post
There should be a resistor in series with the 0.01 capacitor between COMP and FB, try 10k.
100k is there, I put it.
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Old 13th November 2012, 11:49 PM   #7
vasko is offline vasko  Czech Republic
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Quote:
Originally Posted by nigelwright7557 View Post
You could increase the value of 470k to compensate ?
What to put? 1M?

Quote:
Originally Posted by nigelwright7557 View Post
However you need to gap the core.
At least a 1mm gap is required to stop the transformer saturating.
Also if the transformer gets hot the gap will again help stop saturation.
Core is cold.

There is no gap. Should I make it on all 3 pillars of 'E'? To put 1mm paper between?
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Old 14th November 2012, 12:02 AM   #8
AP2 is offline AP2  Italy
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what is the total current at output you need?
I not happy, just on preamp, with this topology. yes it is regulated but not in a good way. better unregulated in this case but very stable,low noise.
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Old 14th November 2012, 12:06 AM   #9
vasko is offline vasko  Czech Republic
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It's PCL8x preamp. So let's say 16V 0.5A for 2 filaments, and about 5W for anodes.

Let me see schematic what you have in mind?
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Old 14th November 2012, 08:36 AM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by vasko View Post
Its EI-33 core form 350W ATX.
Let us suppose the flyback operates just in critical mode, this means the On time will be 7µs, the peak current will reach 4.8A, requiring a primary inductance of 17µH.
If the material is typical of its application, the core will have a Al of ~=4000.
With 8t, this results in an inductance of 256µH. Way too much.
For 4.8A^ and 0.35T^, the minimum number of turns required is ~=2.5, so you are OK with your 8t, but you have to insert a total airgap of 0.666mm.
If the core is totally ungapped and you create it with a spacer, the spacer needs to be 0.33mm.
The IRF510 will work, but it is rather light for such an application, and this will lead to severe conduction losses when the circuit operates normally
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