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[SOLVED] multi-output flyback coverter output voltgae variation

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lok

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Hi all,

I have a tny266 flyback converter with two output secondary (4 turns each)....I need to get +5v and -5v ....Here we are taking a feedback with an opto isolator only from +5v ..After building the whole circuit Iam getting +5v and -7v...what would be the problem ??...I turned the transformer perfectly (4 turns each in secondary)....
 

leakage inductance is your problem.

Remove the two secondaries, twist the two wires together, and replace the four twisted together turns.

The two secondary windings will now be intimately coupled together, as the voltage difference between these two windings is very low, insulation between the twisted wires will be fine.
 

I don't understand clearly what you are mentioned ....I need to join the two winding for virtual ground...I have to start the winding from 1st pin and finish at 2nd pin(virtual ground) and start the next winding from 2nd pin and finish it to the 3rd. If I twisted together winding direction may not be correct. ..
Regards
lok
 

Twisted.jpeg
If you twist both wires together, you will have two starts at one end, and two finishes at the other end.

The connections are then exactly the same as before.
Nothing has changed, except the two windings are now twisted together instead of being separate.
 
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I have done it but there is not much change; now Iam getting -6.8V...In the design they are recommended Triple Insulated wire but Iam using magnetic wire....And the primary inductance needed is 1mH and Iam getting 15mH..for reducing it I placed some paper pieces(air gap) between the cores(E20) and now I have 1mH primary inductance.

Regards
lok
 
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what would be the problem ..I have no experience in flyback converter design....please give me some guidance....



Regards,
lok
 

A useful diagnostic strategy when you have an imbalance...

Is to switch things around, and see what happens. Try different things until...

You achieve equal balance. Or, perhaps until you make the imbalance worse, which will at least tell you something, and that's a positive thing if it helps you track down the problem.

Try switching the loads around.
Try switching the diodes around.
Try taking the positive supply from the other winding, and vice versa.
Etc.
 

What Warpspeed is saying, is that you require very tight coupling between the two secondaries to achieve a good cross regulation.

But even with this technique, if the load current in the negative winding is significantly lower than on the positive winding, the voltage will be higher because of reduced IR loss.

You have to apply both techniques: tight coupling and balanced load currents.
 

Another thought...

What kind of diodes are you using, and what are the two load currents?

For low voltages, shottky diodes are recommended, as they have a lower forward voltage drop and should improve cross regulation.
 

Iam using SB140 shotkey diode and my maximum output current(both) is 500mA..
 

I got -5v if one of the the secondary winding( where we taking feedback) is increased and other part is made no change...(7 turns in positive side and 4 turns in negative side).......how can I check the current is balanced or not??
 

Are you absolutely one hundred percent certain that both your diodes are connected the right way around ?

It will still work if one is in backwards, but the output voltages will be wrong.
 

One diode is connected backward direction( in the negative side) ...this is the circuit diagram....
flyback.jpg
 

.......how can I check the current is balanced or not??

Let me phrase it another way: are the loads balanced? Is the current consumption from the positive supply identical to the one on the negative supply, and most importantly, are you applying at least 10% of the load to the outputs?
 
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    lok

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Actually my circuit is not connected to load...it is open...when I changed diode sb140 to UF4001..and secondary winding change to Quadfilar way...now voltage reduced to -6.5v...
 
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Actually my circuit is not connected to load...it is open...
There my friend is your whole problem !

Try loading both sides with something greater than about ten percent of full load.
5v and 50mA perhaps about a 100 ohm 1 watt resistor on each output, and see what happens.
 

    V

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If the optofeedback is the only load causing the difference in outputs, then the load regulation is poor due to high source impedance perhaps due to core saturation.
 
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    lok

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I connected 2w (120 ohm resistor) ...now the output is perfect.....And another question If the +ve side has more load and negative side has less then , there will be voltage difference right??
 

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