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Old 13th February 2012, 02:32 PM   #1
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Default HELP with modifying ATX PSU and ferrite core calculations.

So I am studying the ATX power supplies for few days.
Now I have few questions.

1. TL494 is switching the 340 DC volts at some frequency lets say 50khz. Now my question is what voltage goes to the EI ferrite core at the primary winding. At that frequency I think there wont be 340V or am I wrong? As far as I know by changing the switching frequency you change the primary voltage.

2. I would like to rewind ATX PSU EI ferrite transformer (EI-33) so I can get 2x36V out of PC ATX. Now my question is how to calculate the number of windings. Should I use 220V as input parameter for the primary winding or should I use 340V because AC-DC will be 340V or should I use other voltage because of the switching frequency?

3. I know how to calculate standard laminated EI transformer, but can I use those calculations for these EI ferrite?

4. Can someone tell me the way to calculate EI ferrite cores for windings wire tickness etc etc.

5. How can I know at what frequency the PSU is working and is that frequency variable? What will happen if it is changed.

Thanks.

Last edited by DigiSoft; 13th February 2012 at 02:37 PM.
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Old 13th February 2012, 02:56 PM   #2
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1) The classic TL494 is normally operating at half bridge mode: the primary is connected between the two series transistor operating 180 deg out of phase, and the middle point of two eletro caps, via a bipolar 1uF to decouple DC in the winding and prevent saturation. The total primary voltage is the same as the peak rectified line voltage, in example for 220V supply, it is about 310VDC.

2) Unwind the xformer, you will find a half of primary, isolation, the secondaries, isolation, half primary. Simply unwound the secondary and scale the turns to match your desired voltage, in this case 3 times the 12V turns will give 36V.

3) It is equal, but the inductions are about 500 gauss.

4) There is a good book called "Switching Power supplies", from Abraham Pressman. I have one of them.

5) For large increase in frequency will give increased semiconductor looses, decreasing frequency will saturate the core and destroy power stage.

Have in mind that these topology of SMPS donīt support sudden changes in the output current, they are designed for steady loads. For Audio, it is better the Fly Back topology, I have one designed of my own since 12 years ago.

Good luck.
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Last edited by Osvaldo de Banfield; 13th February 2012 at 02:59 PM. Reason: typo
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Old 13th February 2012, 03:07 PM   #3
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Wow man you helped me so much. All the questions are answered THANK YOU SO MUCH. I wanted to modify ATX for my amplifier, you say that is bad idea? What if I use LARGE Capacitors at the output, won't this stabilize the sudden current drops little?

The total primary voltage is the same as the peak rectified line voltage, in example for 220V supply, it is about 310VDC.
This is what I needed to know.

Simply unwound the secondary and scale the turns to match your desired voltage, in this case 3 times the 12V turns will give 36V.
I was thinking about this but I was not that sure now I know I am on right track. THANKS.

BTW I know how to modify the other parts of the PSU to stand higher voltages. I'm changing capacitors, I disabled over-voltage circuit, I make new divider resistors for the TL494 etc etc but my problem is the core.

Last edited by DigiSoft; 13th February 2012 at 03:11 PM.
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Old 13th February 2012, 03:27 PM   #4
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Quote:
Originally Posted by DigiSoft View Post
I wanted to modify ATX for my amplifier, you say that is bad idea? What if I use LARGE Capacitors at the output, won't this stabilize the sudden current drops little?
No, augmenting output capacitors make things worse.

See: the rectangular high voltage is scaled inside the transformer, to the levels required at the output. Then, the bipolar squeare wave is full wave rectified, and integrated in a LC filter, designed to resonate at some frequency well below the switching frequency. (say 1KHz) Then, at the DC output, the IC senses the DC level and generate PWM accordingly to compensate it. In a sudden change in out voltage, the inductor momentarily "runs dry", the current in it goes zero during between pulses from transformer. The IC senses the drop out, and react to it, but the reaction canīt be instantaneously because of the time constant in the filter itself. Then, the voltage is reestablished. If you increase the capīs, decrease simultaneously the cut off frequency of the filters, so the drop will be more pronounced and more large in time.

The inverse situation also is true when the supply is suddenly unloaded.

And, in case of you audio material appears at the same frequency of the cut off frequency of the filter, not only there will be a high output ripple at the output, the SMPS all itself can make unstable and there exist risk of short circuit of the output transistors.

See it like a spring loaded by a mass. If you apply the resonant frequency to it, oscillations can be so large with risk of break the wire of the spring.

Remember that if you increase the output voltage, the L of the inductor will be incremented proportionally, and the cap decreased in the same amount. In example, if you use 100uH and 1000uF for 12V, for a 36V you will need 300uHy and 330uF.

Best regards.
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Last edited by Osvaldo de Banfield; 13th February 2012 at 03:30 PM.
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Old 13th February 2012, 03:43 PM   #5
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Then, at the DC output, the IC senses the DC level and generate PWM accordingly to compensate it
What if I make it unregulated?

So you are saying that the best way for audio is Fly Back?

Btw can you send me on PM skype, msn? something? Thanks....
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Old 13th February 2012, 03:57 PM   #6
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Quote:
Originally Posted by DigiSoft View Post
Then, at the DC output, the IC senses the DC level and generate PWM accordingly to compensate it
What if I make it unregulated?
The output voltage will go very high and uncontrollable.

So you are saying that the best way for audio is Fly Back?

No doubt. It is the only that supports large current variation because of it intrinsic wide dynamic range when used in current mode (UC3842 or simmilar).

Btw can you send me on PM skype, msn? something? Thanks....

Sorry, I had Skype but there is large time I do not run such "social networks", nor skype, nor facebook, nor tweeter, nor... Only Email and chat.
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Last edited by Osvaldo de Banfield; 13th February 2012 at 04:00 PM.
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Old 13th February 2012, 04:03 PM   #7
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Thanks man, at least I wont bother to modify ATX PSU for Amp. I'll use the parts from ATX PSU and make from ground up.

Can you show me some good circuits for 2x36-40V flyback?
I searched google but I found very little in practice. I search the forum but it came up with 100+ pages lol.
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Old 13th February 2012, 04:11 PM   #8
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Usually the seller donīt work about inside topology, but you quickly have an idea: search all type of SMPSīs that only have a unique transformer (and the line filters). IF near the diodes you see two or more cores of different styles, they arenīt Fly Back.

You can search an very old PC monitor from those that have power supply in a separate board and try to modify it. It surely is a Fly Back boundary mode type of SMPS.
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Old 13th February 2012, 04:30 PM   #9
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Usually the seller donīt work about inside topology, but you quickly have an idea: search all type of SMPSīs that only have a unique transformer (and the line filters). IF near the diodes you see two or more cores of different styles, they arenīt Fly Back.

You can search an very old PC monitor from those that have power supply in a separate board and try to modify it. It surely is a Fly Back boundary mode type of SMPS.
You are talking about the toroidal filters(the circle ferrites before the output capacitors LC circuit) or the EI cores (usually 3 of them 1 to separate high voltage from the driver circuit other to convert 220-12,5 and the last one is I think for current sensing)?
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Old 13th February 2012, 04:59 PM   #10
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At least, a normal SMPS must have a input line filter located near the main rectifier, and the bulk cap. The power transformer, and CAN have a third ferrite core with output LC filter. (Usualy a toroid, but may be a RM, EI or other core shape.). All topologies, including half, full bridge, forward converter (1 or 2 transistors) DO NEED an LC filter at its output, except very bad designs. The unique that no need for LC filter is the Flyback, and for same power, the core for the mutually coupled inductors (IN FB it is wrong to call "transformer"), is bigger than for any other one topology.

The best way to know how it works, is to place working and see the oscillogram placing the probe near the transformer, no needly to connect it. The wave is very different, but too long to explain in words.
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