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Self oscillating flyback converter

anditechnovire

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Ok
No, what is being said is the output would not stabilize unless there was a load. In a Joule Thief, the LED is the load, in your schematic, it is the 330 Ohm resistor across the output. If you remove it, the spikes in secondary voltage will make it rise much higher.

There is no output voltage stabilization at all in the circuit, it relies upon the feedback winding picking up the same signal, although not necessarily the same voltage as the output winding. If one produces more voltage, so will the other. When the rectified feedback voltage exceeds the Zener diode breakdown voltage it 'strangles' the transistor and limits the current it can pass and hence reduces the output. When running it will try to reach an equilibrium point where the transistor is conducting just enough that the Zener holds it back. Under increased load, there will be less voltage on the feedback winding so the Zebner will conduct less and the transistor will switch harder.

A much better system is not to monitor the feedback winding but to monitor the real output voltage, that way the losses in the transformer and rectifier are also taken into account. The problem that introduces is that the secondary is no longer isolated from the AC lines so it poses a risk of electric shock. That is why you will see an opto-coupler in most small power supplies of this kind. The output voltage is converted to current on the input side of the optocoupler (LED side) and its output side goes to the switching transistor. It works like the 1N4148 and Zener diode in your schematic
Ok thanks for your answer. So the flyback voltage on the secondary winding is dependent on the level of regulation on the feedback winding, not the turns ratio.
Anywhere, it a little bit uneasy to comprehend.
 

betwixt

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That is correct.
Think of the feedback winding being responsible for regulating the INPUT power to the transformer. That's why it can't compensate for losses in the transformer itself or the output rectifier circuit. The Zener knee voltage and number of turns on the feedback winding control the input power, you could change the secondary turns to get more or less output voltage. Be aware though that there is something of a balancing act here, if you want more voltage out you get poorer regulation as well.

Brian.
 

anditechnovire

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That is correct.
Think of the feedback winding being responsible for regulating the INPUT power to the transformer. That's why it can't compensate for losses in the transformer itself or the output rectifier circuit. The Zener knee voltage and number of turns on the feedback winding control the input power, you could change the secondary turns to get more or less output voltage. Be aware though that there is something of a balancing act here, if you want more voltage out you get poorer regulation as well.

Brian.
Ok thanks a lot. I'll try and follow it up.
Maybe it the circuit diagram that is looking intimidating. In case you have any equivalent circuit (maybe a common one with the similar regulation principle) you can show me.
 

Easy peasy

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That ckt will work to a degree - it just isn't a very good system overall - it relies heavily on the 330E resistors for operation ....
 

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