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I am designing an offline flyback converter using UC3844 pwm ic with output 5V/5A specification. But initially with no-load, the supply voltage is swinging between 10-16v meaning it is switching on and off continuously that is happening because the auxiliary winding is not able to provide enough current at no-load. It works fine when on load as it is supposed to be. I have increased the no. of turns to have increased voltage but to no avail.

Current Transformer spec: 63 turns 30swg, 1strand(pri) 3 turns 22swg, 5 strand(sec) 16 turns 35 swg, 1strand (aux)

Current schematic:(Removed D9 zener and a resistor of 10 ohms introduced D8 diode path. enter image description here

Another person having this problem, but no solution posted. http://www.eevblog.com/forum/projects/uc3844-flyback-converter-startup-problem/ Any help would be appreciated.

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  • \$\begingroup\$ A workaround I once implemented was a switched dummy load which regulated on the primary Vcc voltage. You need to pay careful attention if you have no load on your secondary and your cross regulation. If all the energy you are dumping in your primary winding end up in your primary Vcc, you are fine. Otherwise you are pumping up your secondary. A zener diode at your secondary comes to mind too. \$\endgroup\$ – winny Nov 7 '16 at 8:45
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Seems like flyback action cannot be initiated and/or completed.

Remember how flyback converters work: Energy is stored in the primary then is transferred to the "load".

No load, no transfer, no continuous flyback action.

The solution is simple: Put a "dummy load". I personally have no idea what value it should be. But, forcing the converter to work under %2-5 (or even %10) would be enough. For example, your converter's nominal output is 5V/5A. So, placing a resistor to draw at least 0.25A under no load can be enough: 5V/0.25A = 20R/2W.

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  • \$\begingroup\$ Yup, I did the same, put a resistor 24ohm, it was working fine. I see one disadvantage of flyback. Any workaround to it? \$\endgroup\$ – Div-lcr Nov 7 '16 at 7:40
  • \$\begingroup\$ Such a heavy dummy is really nonsense, you wasting 1W! I believe you just reverse auxilary winding phase (and possibly check turn ratio). Aux supply works better as forward converter \$\endgroup\$ – carloc Nov 7 '16 at 8:08
  • \$\begingroup\$ @Div-lcr Note that 20R comes from an assumption of %5-load thing. Maybe even %2 can be enough. Try 47R, 100R, 1k etc. until you get proper performance. \$\endgroup\$ – Rohat Kılıç Nov 7 '16 at 8:13
  • \$\begingroup\$ @carloc It's not an issue about aux winding phase. Anyway, wasting 1W may be nonsense on a 25W converter but I assure you, flyback, forward and even half-bridge converters need a dummy load. If you have a chance to take a look into a half-bridge or forward PC PSU then you'll see that there are dummy loads across each output. \$\endgroup\$ – Rohat Kılıç Nov 7 '16 at 8:18
  • \$\begingroup\$ As you like. :) \$\endgroup\$ – carloc Nov 7 '16 at 8:22

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